Mineral Nutrition
fertility of soil. The important elements need to be replenished in crop fields are nitrogen, phosphorus and
potassium. These are grouped as nitrogenous fertilizers, phosphate fertilizers and potash fertilizers. These are
abbreviated as NPK. Common sources of NPK are ammonium chloride, ammonium sulphate, ammonium nitrate,
bone meal, calcium magnesium phosphate and nitrate of soda.
The common fertilizers that supplements NPK is nitrophosphate with potash in varying proportions. The
percentage of nitrogen, phosphorous and water soluble potassium are labelled on the bags as 17-18-9 or 15-15-15
and so on. The amount of fertilizer needed varies according to change in season, soil, nature of crop and other
climatic conditions.
Important Tips
• Woodward (1699) reported that plants grow better in muddy water as compared to fresh rain water.
• De Saussure (1804) first of all demonstrated that plants obtain minerals from soil through root system.
• Liebig for the first time discovered the presence of elements in plant ash.
• Liebig's law of minimum states that the productivity of soil depends upon the proportionate amount of that essential element which is
deficient in that soil.
• Tracer elements : These are radioactive isotopes of elements, which are used to detect various metabolic pathways in plants, e.g., C14,
N15, P32, S35, etc.).
• If dried plant parts are heated in silica crucible at 600°C, all organic substances vaporize and the remaining plant ash contains only
inorganic substances or mineral elements.
• Aeroponics : Growing plants in stands provided with fine mist of solution having all the required inorganic nutrients.
• Hydroponics developed by Geriche.
• Sodium (Na) regulates the transport of amino acids to the nucleus.
• Aluminium (Al) is accumulated in fern.
• Veledium (V) is required by alga Scenedesmus.
• Selenium (Se) is required by Atriplex and Astragalus.
• Iodine is required by marine alga Polysiphonia.
• The elements taken in the form of gas by prokaryotes only is nitrogen.
• Critical elements are the elements in which soil is generally deficient e.g. N, P and K. These are given in form of fertilizers.
• In addition to 16 essential elements, some plants require some more essential micronutrient elements such as
(i) Silica : Found in grasses and diatoms.
(ii) Sodium : Found in halophytes.
(iii) Cobalt : Found in ferns (e.g. Lycopodium), taking part in growth.
(iv) Nickel : Enzyme urease used it to hydrolyse urea by living organisms.
• In Rhizobium cobalt play an important role in nitrogen fixation and is an essential constituents of vitamin B12. It is used in 'cancer
therapy'.
• Cytozyme is a water soluble commercial preparation which contains essential mineral element for use as foliar spray.
• Khaira disease of rice and white bud of maize is due to zinc deficiency.
• Die back of Citrus and reclamation disease of cereals and legumes and exanthema in fruit trees are due to deficiency of Cu.
• Whiptail disease of cauliflower is caused by Mo deficiency.
• The symptoms produced by the deficiency of mineral substances are called 'hunger sign'.
• Mineral salt absorption is independent of water absorption.
• Maximum mineral salt absorption occurs by zone of elongation. No mineral salt absorption occurs by hair zone. Mineral salt absorption
occurs directly by cells of epiblema and not by root hair.
Chapter 18 304
Mineral Nutrition
• Mineral salts are absorbed mostly in form of ions i.e. anions and cations.
• Path of transport of mineral salts is xylem.
• Cytochromes act as anion carriers.
• Phytotron is the place or laboratory where plants can be maintained and studied under wide range of controlled conditions.
• Nif gene : Nitrogen fixing gene is nif gene. A cluster of 18 genes (nif gene) encode the protein required for nitrogen fixation in Klebsiella.
Special modes of nutrition.
Nutrition is an important characteristic of living organisms. Plants need energy for its various life activities.
Energy is provided by the oxidation of different foods. The method of taking in and synthesis of various types of
foods by different plants and animals is called nutrition.
Generally plants are autotrophic in their mode of nutrition, but there are some examples which are
heterotrophic in their mode of nutrition. These plants are unable to manufacture their own food due to lack of
chlorophyll or some other reasons, e.g., some bacteria, fungi, some bryophytes, pteridophytes and few
angiospermic plants also, but special mention is of angiospermic plants. There are 4 special modes of nutrition.
(1) Parasites
(2) Saprophytes
(3) Symbiotic plants
(4) Insectivorous plants
(1) Parasites : These plants obtain either their organic food prepared by other organisms or depend upon
other plants only for water and minerals with the help of which they can synthesize their own food. The living
organism from which the parasite obtains its organic food or water and minerals is called host. Any part of the
body of parasite is modified into a special organ called haustorium which enters into the cells of host and absorbs
food or water and minerals from the host.
A plant parasite may live on the root or stem of the host plant partially or totally. The total parasites remain
permanently attached to the host whereas the association of partial parasites is only short lived. Accordingly,
parasites can be classified into two categories :
(i) Total parasites.
(ii) Semiparasites or partial parasites.
(i) Total parasites : These plants never possess chlorophyll, hence they always obtain their food from the
host. They may be attached to branches, stem (stem parasites) or roots (root parasites) of the host plants.
(a) Total stem parasite : Cuscuta is a rootless, yellow coloured, slender Flower
stem with small scale leaves, which twines around the host. The parasite
develops haustoria (Small adventitious sucking roots) which enter the host plant Host
forming contact with xylem and phloem of the host. It absorbs prepared food, Parasite
water and minerals from the host plant.
Host
(b) Total root parasite : Total root parasites are common in the families
like Orobanchaceae, Rafflesiaceae, Balanophoraceae, etc. Orobanche, Rafflesia
and Balanophora are some of the common root parasites.
Orobanche is commonly known as broom rape. It has scale leaves and
pinkish or bluish flowers. The tip of the root of parasite makes haustorial
contact with the root of host and absorbs food from the host. Orobanche is
Chapter 18 305 AB
Fig : Cuscuta (dodder), a total parasite
A : Parasite coiled around host plant
B : Relationship between vascular
tissues of host and parasite
Mineral Nutrition
usually parasitic upon brinjal, tobacco. In Rafflesia (stinking corpse lily) another root parasite, vegetative parts of the
plant are highly reduced and represented by cellular filaments resembling fungal mycelium. These filaments get
embedded in the soft tissue of the host while the flowers emerge out in the forms of buds.
Balanophora occurs as a total stem parasite in the roots of forest trees.
(ii) Semiparasite or partial parasite : Such parasitic plants have
chlorophyll and, therefore, synthesize their organic food themselves. But they fulfill
their mineral and water requirements from their host plants. Like total parasites,
they grow on the stem and roots of the host plants and can be grouped into Parasite plant
following two categories :
(a) Partial stem parasites : The well known example of partial stem
parasite is Viscum album (mistletoe) which parasitizes a number of shrubs and Haustoria
trees. The mature plant of Viscum is dichotomously branched with green leaves
born in pairs attached on each node of stem. The shoots are attached to the host Host
by means of haustoria. The primary haustoria reaches upto cortex of the host
which runs logitudinally. It sends secondary haustoria which make connection with Fig : Viscum plant attached
the xylem of the host and absorb water and minerals Loranthus is another partial to the host stem (part of host
stem parasite. stem is cut open to show the
haustorium
(b) Partial root parasites : The common example of partial (semi-parasite) root parasite is Santalum album
(Sandal wood tree) which is an evergreen partial root parasite which grows in South
India. It grows on the roots of Dalbergia sisso, Eucalyptus. Like other partial
parasites, it also has green leaves and absorbs only minerals and water from the host
plants.
Similarly, Striga on roots of sugarcane and Thesium on the roots of grasses are
other partial root parasites.
(2) Saprophytes : These plants live upon dead organic matter and are
responsible for conversion of complex organic substances into simple inorganic
substances (minerals), e.g., some bacteria, some fungi (Yeast, Mucors, Penicillium,
Agaricus), few algae (Polytoma), few bryophytes (Buxbaumia, Hypnum and
Splanchnum), few pteriophytes (like Botrychium) and some angiosperms
(Monotropa and Neottia) also. AB
Monotropa, commonly known as Indian pipe, lacks chlorophyll and is Fig : Saprophytic plants
colourless or ivory white. It is found in Khasi hills and in the dense forests of Shimla. (A) : Neottia (Birds nest plant)
Monotropa, though usually referred to as a saprophyte, actually gets its nourishment (B) : Monotropa (Indian pipe)
from fungal mycelium which surround its roots. Such association between roots of higher plants and fungi is known
as mycorrhiza. Neottia (Bird's nest orchid) grows in the humus rich soil of
the forests. It has very few reduced leaves and thick pale yellow stem. The Upper cortex
roots lack root hairs and the nutrients are absorbed by mycorrhiza.
(3) Symbiotic plants : Sometimes two different species of organisms
spend much or all of their lives in close physical association, deriving mutual
benefit. Such an association is known as symbiosis and each organism is Algal cells
known as symbiont. Symbiotic association is so close that symbionts appear intermingled with
to be different parts of the same plant.
fungus hyphae
Chapter 18 306 Fruiting body
of fungus
Fig : A lichen thallus in T.S.
Mineral Nutrition
Symbiotic association may be between two higher plants or between a higher plant and a lower plant. Some
common examples of symbiosis are described below.
(i) Lichens : Lichens is a special group of plants, when an algae and fungi live together and are mutually
benefitted (algae provides food and fungi provides water minerals and protection of algae).
The fungus component of the lichens, called mycobiont, is generally a member of Ascomycetae or
occasionally a Basidiomycetae. The algal component of the lichen is known as phycobiont and is generally a
member of Chlorophyceae (e.g., Trebouxia) or Cyanophyceae (e.g., Nostoc, Gloeocapsa).
(ii) Mycorrhiza : It is a mutually beneficial association between a fungus and the root of higher plant. The
fungus absorbs water, salts (from organic matter) and Conifer tree Conifer roots
protects the plant from soil borne pathogens. In return, it infected by
gets shelter and nourishment from the plant. In such fungal hyphae
association the fungal mycelium forms a mantle over the
root surface and some of the hyphae penetrate between
cortical cells and metabolites are transferred in both
directions (i.e., from fungus to the root cells and vice- Fungus
versa). hypha
Usually the roots in the upper part of the soil, where
organic matter is abundant, are mycorrhizal, and the roots
penetrating deep in the soil are not associated with fungi.
Generally, mycorrhizal roots have few or no root hairs.
Water and minerals are absorbed by the fungus and Rootlet Root hairs Root cell
passed on to the host. The fungus digests starch grains
Fig : Micorrhizal roots
stored in the cortical cells of the host and uses the digested products in its own metabolism.
In some plants the mycorrhizal association is essential for normal growth and development. For example,
seedlings of orchids fail to survive if the soil is free from fungus. Pine seedlings grow poorly unless mycorrhizal fungi
are introduced in to the soil. Nodules
(iii) Root nodules of leguminosae : Members of the sub-
family Papilionaceae of the Leguminosae (e.g., pea, beans,
trifolium) harbour species of Rhizobium, a nitrogen fixing bacteria.
The bacteria form nodules in the roots. They fix elemental nitrogen
of the atmosphere and make it available to the plant in forms that
can be utilized. In turn they derive food and shelter from the Root hair B
leguminous plant. infected by
(iv) Myrmecophily : It is the symbiotic relationship between bacteria
ants and some higher plants. The ants obtain food and shelter from
the plant. They protect the plant (e.g., Mango) from other animals. Nodule
Bacteria
Root
In Acacia sphaerocephala the stipules are hollowed to function as ant AC
shelter. Leaflet tips (Belt's corpuscles) and rachis (extrafloral Fig : Symbiotic plants : A : A leguminous plant with
nectaries) possess feeding materials. A higher plant which is root nodules, B : A root hair infected with bacteria,
C : T.S. of a root nodule showing many bacteria
Chapter 18 307
Mineral Nutrition
benefitted by association with ants is called myrmecophyte. The term myrmecophily is also used for pollination by
ants.
(4) Insectivorous plants : These plants are Tentacle Endodermis
autotrophic in their mode of nutrition but they Leaf Palisade
grow in marshy or muddy soils, which are like tissue
generally deficient in nitrogen and in order to fulfil Secretory
their nitrogen requirement, these plants catch cells
small insects. The organs and specially leaves of
these plants are modified variously to catch the Drop of
insects. These plants have glands secreting secreted liquid
proteolytic enzymes which breakdown complex
proteins into simple nitrogenous substances, Vascular tissue
which inturn are absorbed by these plants. Some
of these plants are as follows :
(i) Drosera (Sundew) : It is a herbaceous
plant having spathulate or lunate leaves. The Fig : Insectivorous plant : Drosera (Sundew) Fig : One grandular tentacle
leaves are covered by glandular hair with a
swollen tip. The glands secretes a sticky purple juice which
shines like a dew drop in bright light sunshine, hence the name
sundew. These long special hair are generally referred to as C
'tentacles'. When an insect alights on the leaf, the tentacles
curve due to thigmonasty. The insect is killed and its proteins Door valve
are digested by pepsin hydrochloride. Similar tentacles are Leaf Bristle Outer wall
also found in Drosophyllum. segment Inner wall
(ii) Utricularia (Bladderwort) : It is submerged floating Bristles
aquatic herb which lack roots. Some of the species of Hairs
Utricularia also occur in moist soil. The leaves are dissected
into fine segments and appear like roots. Some of the leaf Single D
segments are modified into pear-shaped sacs called bladders bladder
B
or utricles. A
The bladders are triangular or semicircular structures Fig : Insectivorous plant : Utricularia (Bladderwort) A
having a single opening guarded by a valve. There are – Complete plant, B – One bladder, C – Part of leaf
numerous bristles near the mouth and digestive glands inside.
with several bladders, D – Internal structure of bladder
The bladders show special trap mechanism. The valve of the bladder Winged petiole
opens on the inner side. When small aquatic animalcules enter the
bladder along with water current, they get trapped inside. Their proteins
are digested enzymatically. When a bladder is full of undigested matter,
it degenerasis. Lid
(iii) Nepenthes (Pitcher plant) : They are commonly found in
tropical areas like Assam and Meghalaya (i.e. N. Khasiana). In this plant
the leaf base is winged, petiole is tendriller and the lamina is modified Tendrillar
into pitcher. The pitcher has a distinct collar at the mouth and the apex petiole
is modified into the lid. The undersurface of the lid has alluring
glands whereas the inner surface of pitcher is lined by numerous Pitcher
Fig : Insectivorous plant : Nepenthes
(Pitcher plant) A pitcher plant with pitcher
Chapter 18 308
A
Mineral Nutrition
digestive glands and several downward directed hair. The lid attracts insects which slide down into the pitcher. The
downward directed hair check their escape. The insect is killed and
its proteins are digested by pepsin hydrochloride. Other
insectivorous plants having leaf pitchers are Sarracenia, Cephalotus, Leaf
Lamina
Heliamphora, etc.
(iv) Dionaea (Venus fly trap) : It is a small herbaceous plant
found mainly in America. The plant has a rosette of radiating leaves. Winged
The petiole is winged and photosynthetic. The lamina is bilobed and petiole
the midrib acts like a hinge between the two lobes of the lamina.
Each lobe has 15-20 trigger hairs or bristles. These hairs are very
sensitive to nitrogenous substances. When an insect alights on the Fig : Insectivorous plant : Dionea (Venus fly trap)
leaf and touches the sensitive hairs, the two lobes of lamina fold
along the midrib. Thus the insect is trapped in between the lobes. Pepsin hydrochloride secreted by the digestive
glands, present in the upper part of the lobes digests the insect. The simple digested substances are absorbed by the
plant. Soon after the digested matter has been translocated to other parts
of the plant, the lobes of the lamina reopen.
(v) Sarracenia (Pitcher plant; Devil's boot) : This pitcher plant
is found in the temperate regions. It has a very reduced stem which bears
a rosette of leaves. The leaves are modified into pitchers. It can easily be
distinguished from Nepenthes on the basis of its trumpet-shaped sessile
pitchers. Contrary to Nepenthes, the pitchers of Sarracenia lack digestive
enzymes and here the insects are decomposed by bacteria.
(vi) Pinguicula (Butterwort) : It is a herbaceous plant having a AB
basal rosette of ovate leaves. The leaf margins are slightly curved in Fig : Sarracenia (Pitcher plant)
upward direction. The dorsal (upper) surface of leaf has two types of
glands stalked and sessile. The stalked glands secrete mucilage while
the sessile glands secrete digestive enzymes.
As soon as the insect sits on the leaf surface, it sticks to the mucilage
secreted by stalked glands. Meanwhile the margins of the leaf roll inward due to stimulation received by the insect.
Thus the insect gets enclosed
within the leaf. The protein Open
contents of the insect are digested leaf
by the enzymes secreted by the
sessile glands. The leaf reopens Glands Trigger hair
when the stimulation is over.
(vii) Aldrovanda (Water Closed leaf Closed leaf
flea trap) : It is also a rootless,
submerged aquatic plant (bog Open AB
plant) recalling the habit of leaf Fig : Aldrovanda vesiculosa
Utricularia. The leaves are bilobed Fig : Entire plant of Pinguicula A : A floating twig B : An open leaf
with long petioles. There are five
bristle like outgrowths associated
with the lamina. The leaf surface is
covered by visid stalked glands.
Chapter 18 309
Mineral Nutrition
The two halves of the lamina rise upward on stimulation by an insect, the midrib acting as hinge. The proteins of
the insect are digested enzymatically.
Modes of nutrition in plants
Autotrophic Heterotrophic
These are non-photosynthesizing plants which
These are photosynthesizing plants and obtain ready made food from other plants.
thus make their food by themselves.
Photosynthetic Chemosynthetic Parasitic
Use sunlight as a source of Use energy released in It obtain food only
energy to manufacture food. chemical reactions to from living plants
Examples : manufacture food
Most green plants. Examples : Total Partial
Green sulphur bacteria. Nitrifying bacteria.
Purple sulphur bacteria. Iron bacteria.
Purple non-sulphur bacteria. Hydrogen bacteria.
Sulphur bacteria.
Methane bacteria. Stem Root Stem Root
Example : Examples : Examples : Example :
Cuscuta. Orobanche. Viscum. Santalum.
Rafflesia. Loranthus
Balanophora.
Saprophytic Symbiotic Insectivorous
Obtain food from It is a close association These are green photosynthesizing
dead and decaying between two different plants but fulfill their nitrogen
organic matter. kinds of plants and both requirement from insects.
Examples : the plants obtain their food Examples :
Monotropa by different methods. Drosera
Neottia Examples : Utricularia
Agaricus. Lichens Nepenthes
Mycorrhiza Dionaea
Root nodules. Pinguicula
Myrmecophily. Sarracenia
Aldrovanda
Important Tips
• Term 'symbiosis' was given by De Bary.
• Rafflessia (largest flower in the world) was discovered by Sir Stamford Raffles from Java. Flower measures about a meter in
diameter, about 11 kg in weight, smell is like rotten fish, pollination by elephants and found on roots of Vitis and Cissus.
• Sapria himalayensis (largest flower in India), measures 15 cm – 30 cm in diameter.
• Insectivorous plants are example of predation (i.e. first killing and then eating).
• Cephalotus (Fly Catcher). A deep rooted carnivorous herb with a rosette of pitchers for trapping small animals.
• Cuscuta/Amarbel/Akashbel/Dodder : A dicot with no cotyledon (some workers consider it to have a single cotyledon). It is a
total stem parasite but initially grows on soil.
• Dischidia : The pitcher is without lid and is used only for storing rain water with some mud.
• Epiphytes are plants which live on other plants for space (shelter/support) only. They are therefore, called space parasites
• Bird of paradise flower is Sterilitzia reginae.
Chapter 18 310
Photosynthesis in Higher Plants
Introduction.
All living organisms require continuous use of energy to carry out their different activities. This energy directly
or indirectly comes from sun.
Photosynthesis is the only process on earth by which solar energy is trapped by autotrophic organisms and
converted into food for the rest of organisms.
In photosynthesis process, 'energy rich compounds like carbohydrates are synthesized from simple inorganic
compounds like carbon dioxide and water in the presence of chlorophyll and sunlight with liberation of oxygen'.
The process of photosynthesis can also be defined as "transformation of photonic energy (i.e. light or radiant
energy) into chemical energy".
Earlier, photosynthesis was considered to be reverse of respiration, i.e.,
6CO2 + 6H 2O Light → C6 H 12 O6 + 6O2
Chlorophyll
Above reaction gives an idea that O2 comes from CO2. But Ruben and Kamen (1941) experimentally
verified that source of liberated O2 in photosynthesis is H2O, not CO2.
Thus, overall reaction can be corrected as given below :
6CO2 + 12H 2 O Sunlight → C6 H12 O6 + 6O2 + 6H2O
Chlorophyll
About 90% of total photosynthesis in world is done by algae in oceans and in freshwater. More than 170
billion tonnes of dry matter are produced annually by this process. Further CO2 fixed annually through
photosynthesis is about 7.0 × 1013kg. Photosynthesis is an anabolic and endothermic reaction. Photosynthesis
helps to maintain the equilibrium position of O2 and CO2 in the atmosphere.
Historical background.
Before seventeenth century it was considered that plants take their food from the soil.
Van Helmont (1648) concluded that all food of the plant is derived from water and not from soil.
Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition
from air and light may also play a role in this process.
Joseph Priestley (1772) demonstrated that green plants purify the foul air (i.e., Phlogiston), produced
by burning of candle, and convert it into pure air (i.e., Dephlogiston).
Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of
plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air (Having
O2) during sunlight and phlogisticated air (Having CO2) during dark.
Jean Senebier (1782) proved that plants absorb CO2 and release O2 in presence of light. He also
showed that the rate of O2 evolution depends upon the rate of CO2 consumption.
Lavoisier (1783) identified the pure air (i.e., dephlogiston) as oxygen (O2) and noxious air (i.e.,
Phlogiston) produced by the burning of candle as carbon dioxide (CO2).
Chapter 19 311
Photosynthesis in Higher Plants
Nicolus de Saussure (1804) showed the importance of water in the process of photosynthesis. He
further showed that the amount of CO2 absorbed is equal to the amount of O2 released.
Pelletier and Caventou (1818) discovered chlorophyll. It could be separated from leaf by boiling in
alcohol.
Dutrochet (1837) showed the importance of green pigment chlorophyll in photosynthesis.
Julius Robert Mayer (1845) proposed that light has radiant energy and this radiant energy is converted
to chemical energy by plants, which serves to maintain life of the plants and also animals.
Liebig (1845) indicated that main source of carbon in plants is CO2.
Bousingault (1860) reported that the volume of CO2 absorbed is equal to volume of O2 evolved and
that CO2 absorption and O2 evolution get start immediately after the plant was exposed to sunlight.
Julius Von Sachs (1862) demonstrated that first visible product of photosynthesis is starch. He also
showed that chlorophyll is confined to the chloroplasts.
J.C. Maxwell (1864) developed 'wave model of light', leading to recognition that light is source of energy
in photosynthesis.
Theodore Engelmann (1884, 88) showed that chloroplast as the site of photosynthesis in the cell and
also discovered the role of different wave lengths of light on photosynthesis and plotted the action spectrum.
F.F. Blackmann (1905) proposed the 'law of limiting factor' and also discovered two steps of
photosynthesis i.e., light dependent and temperature independent steps and a light independent and temperature
dependent step.
He proved that photosynthesis is a photochemical and biochemical reaction. Photochemical reaction is light
reaction and biochemical reaction is dark reaction or carbon dioxide fixation.
Willstatter and Stoll (1912) studied structure of photosynthetic pigments.
Warburg (1919) performed flashing light experiment using green alga-Chlorella as a suitable material for
the study of photosynthesis.
Van Niel (1931) demonstrated that some bacteria use H2S instead of H2O in the process of
photosynthesis.
Emerson and Arnold (1932) proved the existance of light and dark reactions by flashing of light
experiment in photosynthesis.
Robert Hill (1937) demonstrated photolysis of water by isolated chloroplast in the presence of suitable
electron acceptor.
S. Ruben and M. Kamen (1941) used heavy isotope 18O and confirmed that oxygen evolved in
photosynthesis comes from water and not from CO2.
Melvin Calvin (1954) traced the path of carbon in photosynthesis (Associated with dark reactions) and
gave the C3 cycle (Now named Calvin cycle). He was awarded Nobel prize in 1961 for the technique to trace
metabolic pathway by using radioactive isotope.
Chapter 19 312
Photosynthesis in Higher Plants
Emerson, Chalmers and Cederstrand (1957) discovered Emerson effect.
Hill and Bendall (1960) proposed Z scheme and suggested that two photosystems operate in series.
Arnon (1961) discovered photophosphorylation and gave the term 'assimilatory powers'.
Peter Mitchell (1961) proposed chemi-osmotic coupling hypothesis.
Kortschak (1965) discovered the formation of C4 dicarboxylic acid in sugarcane leaves.
Hatch and Slack (1966) reported the C4 pathway for CO2 fixation in certain tropical grasses.
Huber, Michel and Deisenhofer (1985) crystallised the photosynthetic reaction center from the purple
photosynthetic bacterium, Rhodopseudomonas viridis. They analysed its structure by X-ray diffraction technique. In
1988 they were awarded Nobel prize in chemistry for this work.
Photosynthesis in higher plants.
(1) Chloroplast-The site of photosynthesis : The most active photosynthetic tissue in higher plants is the
mesophyll of leaves. Mesophyll cells have many chloroplast. Chloroplast are present in all the green parts of plants
and leaves. There may be over half a million chloroplasts per Granum Grana lamellae
square millimetre of leaf surface. In higher plants, the Stroma
chloroplasts are discoid or lens-shaped. They are usually
Stroma
lamellae
4-10µm in diameter and 1-3µm in thickness.
These are double membrane-bound organelles in the
cytoplasm of green plant cells. Chloroplast has two unit
membranes made up of lipoprotein. Outer membrane of Double
chloroplast is permeable and an inner one impermeable to
protons. Inside the membranes is the proteinaceous ground membrane Osmiophilic Fret Starch
substance called stroma, which contain a variety of particles,
droplets channel
Fig : Internal structure of a typical chloroplast
(Diagrammatic representation of sectional view)
osmiophilic droplets, dissolved salts, small double stranded
circular DNA molecules and 70S type ribosomes along with various enzymes. Inside the stroma is found a system of
chlorophyll bearing double-membraned sacs thylakoids or lamellae.
Thylakoids are flattened sacs arranged like the stacks of coins. One stack of thylakoids is called granum.
Different grana are connected with the help of tubular connections called stroma lamellae or frets. Grana are the
sites for light reaction of photosynthesis and consist of photosynthetic unit 'quantasomes' (Found in surface of
thylakoids). Photosynthetic unit can be defined as number of pigment molecules required to affect a photochemical
act, that is the release of a molecule of oxygen. Park and Biggins (1964) gave the term quantasome for
photosynthetic units is equivalent to 230 chlorophyll molecules.
(2) Chloroplast pigments : Pigments are the organic molecules that absorb light of specific wavelengths in
the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat
soluble and are located in the lipid part of the thylakoid membranes. There is a wide range of chloroplastic pigments
which constitute more than 5% of the total dry weight of the chloroplast. They are grouped under two main
categories : (i) Chlorophylls and (ii) Carotenoids
The other photosynthetic pigments present in some algae and cyanobacteria are phycobilins.
Chapter 19 313
Photosynthesis in Higher Plants
(i) Chlorophylls : The chlorophylls, the green pigments in chloroplast are of seven types i.e., chlorophyll a, b,
c, d, e, bacteriochlorophyll and bacterioviridin. H CH2 CHO
Of all, only two types i.e., chlorophyll a and C H CH3
chlorophyll b are widely distributed in green algae and H3C A B C2H5 B C2H5
higher plants. N N H Chlorophyll b
Porphyrin ‘head’ CH3
Chlorophyll 'a' is found in all the oxygen evolving 15 Å H Mg
photosynthetic plants except photosynthetic bacteria. H3C
Reaction centre of photosynthesis is formed of N NC
chlorophyll a. It occurs in several spectrally distinct D
forms which perform distinct roles in photosynthesis
(e.g., Chl a680 or P680, Chl a700 or P700, etc.). It directly H HE
takes part in photochemical reaction. Hence, it is CH2 H O
termed as primary photosynthetic pigment. Other
photosynthetic pigments including chlorophyll b, c, d | COOCH3
and e ; carotenoids and phycobilins are called CH2
accessory pigments because they do not directly |
take part in photochemical act. They absorb specific O=C
wavelengths of light and transfer energy finally to
chlorophyll a through electron spin resonance. |
Chlorophyll a is blue black while chlorophyll b is O
green black. Both are soluble in organic solvents like
alcohol, acetone etc. chlorophyll a appears red in |
CH2
|
CH
|
C– CH3
|
Phytol ‘tail’ (CH2)3
20 Å |
HC–CH3
|
(CH2)3
|
HC–CH3
|
(CH2)3
|
CH
reflected light and bright green in transmitted light as CH3 CH3
compared to chlorophyll b which looks brownish red in Chlorophyll a
reflected light and yellow green in transmitted light.
Fig : Chemical structure of chlorophyll a and b molecules
Chlorophyll is a green pigment because it does not absorb green light (but reflect green light) Chlorophyll a
possesses — CH3 (methyl group), which is replaced by — CHO (an aldehyde) group in chlorophyll b. Chlorophyll
molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail. The
magnesium atom is present in the central position of tetrapyrrolic ring. The four pyrrole rings of porphyrin head is
linked together by methine (CH=) groups forming a ring system. Each pyrrole ring is made up of four carbon and
one nitrogen. The porphyrin head bears many characteristic side groups at many points. Different side groups are
indicative of various types of chlorophylls.
Phytol tail is made up of 20 carbon alcohol attached to carbon 7 position of pyrrole ring IV with a propionic
acid ester bond. The basic structure of all chlorophyll comprises of porphyrin system.
When central Mg is replaced by Fe, the chlorophyll becomes a green pigment called 'cytochrome' which is
used in photosynthesis (Photophosphorylation) and respiration both.
Chlorophyll synthesis is a reduction process occurring in light. In gymnosperm seedlings, chlorophyll synthesis
takes place in darkness in presence of enzyme called 'chlorophyllase'. The precursor of chlorophyll is chlorophyllide.
Chapter 19 314
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Pigments Chemical Formula Distribution
Chlorophyll a C55H72O5N4Mg
All photosynthetic organisms except
Chlorophyll b C55H70O6N4Mg photosynthetic bacteria.
Chlorophyta, Euglenophyta and in all
Chlorophyll c C35H32O5N4Mg higher plants.
Brown algae (Phaeophyta), Diatoms
Chlorophyll d C54H70O6N4Mg and Pyrrophyta.
Chlorophyll e Not fully known Red algae (Rhodophyta).
Bacteriochlorophyll C55H74O6N4Mg
Chlorobiumchlorophyll Xanthophyta.
(Bacterioviridin)
Purple photosynthetic bacteria.
Green sulphur bacteria.
(ii) Carotenoids : The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8)
units. They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called
lipochromes due to their fat soluble nature. They are lipids and found in non-green parts of plants. Light is not necessary
for their biosynthesis. Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments.
They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into
harmless molecular stage. Carotenoids can be classified into two groups namely carotenes and xanthophyll.
(a) Carotenes : They are orange red in colour and have general formula C40H56. They are isolated from carrot.
They are found in all groups of plants i.e., from algae to angiosperms. Some of the common carotenes are α,
β, γ and δ carotene; phytotene, lycopene, neurosporene etc. The lycopene is a red pigment found in ripe
tomato and red pepper fruits. The β-carotene on hydrolysis gives vitamin A, hence the carotenes are also called
provitamin A. β-carotene is black yellow pigment of carrot roots.
C40 H 56 + 2H 2O Carotenase → 2 C20 H 29 OH
Carotene vitamin A
(b) Xanthophylls : They are yellow coloured carotenoid also called xanthols or carotenols. They contains
oxygen also along with carbon and hydrogen and have general formula C40H56O2.
Lutein a widely distributed xanthophyll which is responsible for yellow colour in autumn foliage. Fucoxanthin
is another important xanthophyll present in Phaeophyceae (Brown algae).
(iii) Phycobilins : These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae.
These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.
Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin. Phycocyanin
and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with
small granules attached with lamellae. Like carotenoids, phycobilins are accessory pigments i.e. they absorb light
and transfer it to chlorophyll a.
(3) Nature of light : Sunlight is a type of energy called radiant energy or electromagnetic energy. This energy,
according to electromagnetic wave theory (Proposed by James Clark Maxwell, 1960), travels in space as waves. The
distance between the crest of two adjacent waves is called a wavelength (λ). Shorter the wavelength greater the energy.
Chapter 19 315
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The unit quantity of light energy in the quantum theory is called quantum (hν), whereas the same of the
electromagnetic field is called photon. Solar radiation can be divided on the basis of wavelengths. Radiation of
shortest wavelength belongs to cosmic rays whereas that 10–14 10–12 10–10 10–8 10–6 10–4 10–2 1 102 104 106 cm
of longest wavelength belong to radio waves. Light
represents only one part of electromagnetic radiation. Cosmic X-rays Ultra Infrared Radio waves
Other parts include cosmic rays, X-rays, UV rays, infra- rays violet
Sound
red radiation and radio waves. A visible light has seven Gamma rays Solar rays
separated groups of more or less complete absorption. In Visible light
a spectrum of sunlight, bands of blending colours are
seen i.e., dark red at one end running through red,
orange, yellow, green, blue, indigo, violet and ending in λ=400 500 600 700 800 nm
darkest violet. Wavelengths in the violet portion of
Violet Blue Green Yellow Orange Red Infrared
spectrum are about 400 millimicrons (mµ) in length and at
other end of spectrum — the red portion — are much Fig : Electromagnetic spectrum of light
longer about 730mµ. In other words, visible light lies between wavelengths of ultra-violet and infra-red. The visible
spectrum of solar radiations are primarily absorbed by carotenoids of the higher plants are violet and blue. However, art
of blue and red wavelengths, blue light carry more energy.
Shortest wavelength → Longest wavelength
Maximum energy Minimum energy
Visible light : 390nm (3900Å) to 760nm (7600Å). Violet (390–430nm), blue (430–470nm), blue-green
(470–500nm), green (500–580nm), yellow (580–600nm), orange (600–650nm), orange-red (650–660nm) and
red (660–760nm) Far-red (700–760nm). Infra-red 760nm – 100µm. Ultraviolet 100–390nm. Solar Radiations
300nm (ultraviolet) to 2600nm (infra-red). Photosynthetically active radiation (PAR) is 400–700nm. Leaves appear
green because chlorophylls do not absorb green light. The same is reflected and transmitted through leaves.
Absorption and action spectra : The curve representing the 180 a
light absorbed at each wavelength by pigment is called absorption 160 b
spectrum. Curve showing rate of photosynthesis at different Specific absorption 140
wavelengths of light is called action spectrum. 120
Absorption spectrum is studied with the help of 100
spectrophotometer. The absorption spectrum of chlorophyll a and 80
chlorophyll b indicate that these pigments mainly absorb blue and 60
red lights. Action spectrum shows that maximum photosynthesis 40
takes place in blue and red regions of spectrum. The first action 20
spectrum of photosynthesis was studied by T.W. Engelmann (1882) 380 420 460 500 540 580 620 660
using green alga Spirogyra and oxygen seeking bacteria.
Wavelength, m µ
In this case actual rate of photosynthesis in terms of oxygen Fig : Absorption spectra of chlorophylls a and b
evolution or carbon dioxide utilisation is measured as a function of wavelength.
Mechanism of photosynthesis.
Chapter 19 316
Photosynthesis in Higher Plants
Before 1930 it was considered by physiologists that one molecule each of CO2 and H2O form a molecule of
formaldehyde (HCHO), of which 6 mols are polymerized to one molecule of glucose (a hexose sugar).
CO2 + H2O Light → HCHO + O2
Chlorophyll (Formaldehyde)
6CH 2O (or 6HCHO) Polymerisation → C6 H12O6
(Formaldehyde) (Hexose sugar)
However formaldehyde is a toxic substance which may kill the plants. Hence, formaldehyde hypothesis could
not be accepted.
On the basis of discovery of Nicolas de Saussure that "The amount of O2 released from plants is equal to
the amount of CO2 absorbed by plants", it was considered that O2 released in photosynthesis comes from CO2, but
Ruben proved that this concept is wrong.
In 1930, C.B. Van Niel proved that, sulphur bacteria use H2S (in place of water) and CO2 to synthesize
carbohydrates as follows :
6CO2 + 12H 2S → C6 H12O6 + 6H 2O + 12S
This led Van Niel to the postulation that in green plants, water (H2O) is utilized in place of H2S and O2 is
evolved in place of sulphur (S). He indicated that water is electron donar in photosynthesis.
6CO2 + 12H 2O → C6 H12O6 + 6H 2O + 6O2
This was confirmed by Ruben and Kamen in 1941 using Chlorella a green alga.
They used isotopes of oxygen in water, i.e., H218O instead of H2O (normal) and noticed that liberated oxygen
contains 18O of water and not of CO2. The overall reaction can be given as under :
6CO2 + 12H 18 O Light → C6 H12 O6 + 618 O2 + 6H2O
2
Chlorophyll
The fate of different molecules can be summarised as follows :
Light
6CO2 + 12H 2O chlorophyll → C6 H12O6 + 6H 2O + 6O2
Fig : Fat of different molecules
Modern concept of photosynthesis.
Photosynthesis is an oxidation reduction process in which water is oxidised to release O2 and CO2 is
reduced to form starch and sugars.
Scientist have shown that photosynthesis is completed in two phases.
• Light phase or Photochemical reactions or Light dependent reactions or Hill's reactions :
During this stage energy from sunlight is absorbed and converted to chemical energy which is stored in ATP and
NADPH + H+.
• Dark phase or Chemical dark reactions or Light independent reactions or Blackman reaction
or Biosynthetic phase : During this stage carbohydrates are synthesized from carbon dioxide using the energy
stored in the ATP and NADPH formed in the light dependent reactions.
Chapter 19 317
Photosynthesis in Higher Plants
• Evidence for light and dark reactions in photosynthesis : Evidences in favour of light and dark
phases in photosynthesis are :
Physical separation of chloroplast into grana and stroma fractions : It is now possible to separate
grana and stroma fractions of chloroplast. If light is given to grana fraction in presence of suitable H-acceptor and in
complete absence of CO2, then ATP and NADPH2 are produced (i.e., assimilatory powers). If these assimilatory
powers (ATP and NADPH2) are given to stroma fraction in presence of CO2 and absence of light, then
carbohydrates are formed.
Experiments with intermittent light or Discontinuous light : Rate of photosynthesis is faster in
intermittent light (Alternate light and dark periods) than in continuous light. It is because light reaction is much faster
than dark reaction, so in continuous light, there is accumulation of ATP and NADPH2 and hence reduction in rate
of photosynthesis but in discontinuous light, ATP and NADPH2 formed in light are fully consumed during dark in
reduction of CO2 to carbohydrates. Accumulation of NADPH2 and ATP is prevented because they are not produced
during dark periods.
Temperature coefficient studies : The temperature coefficient (Q10) is defined as the ratio of the velocity of
a reaction at a particular temperature to that at a temperature 10°C lower. For a physical process the value of Q10 is
slightly greater than one. In photochemical reaction the energy source is light and any increase in temperature is not
sufficient to cause an increase in the rate. Thus here also the value of Q10 is one. However, in case of chemical
reactions the value of Q10 is two or more i.e., with the rise of 10°C temperature, the rate of chemical reaction is
doubled. If the process of photosynthesis includes a hidden chemical reaction in addition to usual photochemical
reaction, its value of Q10 should be two or more.
Blackman found that Q10 was greater than 2 in experiment when photosynthesis was rapid and that Q10
dropped from 2 often reaching unity, i.e., 1 when the rate of photosynthesis was low. These results show that in
photosynthesis there is a dark reaction (Q10 more than 2) and a photochemical or light reaction (with Q10
being unity).
Q10 = Reaction rate of (t + 10)°C
Reaction at t°C
(1) Light phase (Photochemical reactions) : Light reaction occurs in grana fraction of chloroplast and in
this reaction are included those activities, which are dependent on light. Assimilatory powers (ATP and NADPH2)
are mainly produced in this light reaction.
Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media and Lamium
album are suspended in a test tube containing suitable electron acceptors, e.g., Potassium ferroxalate (Some plants
require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of
water. Under these conditions, no CO2 was consumed and no carbohydrate was produced, but light-driven
reduction of the electron acceptors was accompained, by O2 evolution.
4 Fe 3+ + 2H 2O ←→ 4 Fe 2+ + 4 H + + O2 ↑
Electron Electron Reduced
acceptor donor Product
The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is
known as Hill reaction.
Chapter 19 318
Photosynthesis in Higher Plants
Hill reaction proves that
(i) In photosynthesis oxygen is released from water.
(ii) Electrons for the reduction of CO2 are obtained from water [i.e., a reduced substance (hydrogen donor) is
produced which later reduces CO2].
Dichlorophenol indophenol is the dye used by Hill for his famous Hill reaction.
According to Arnon (1961), in this process light energy is converted to chemical energy. This energy is stored
in ATP (this process of ATP formation in chloroplasts is known as photophosphorylation) and from electron
acceptor NADP+, a substance which found in all living beings NADP*H is formed as hydrogen donor. Formation of
hydrogen donor NADPH from electron acceptor NADP+ is known as photoreduction or production of reducing
power NADPH.
Light phase can be explained under the following headings :
(i) Transfer of energy (ii) Quantum yield (iii) Emerson effect (iv) Two pigment systems
(v) Z-scheme (vi) Cyclic and non-cyclic photophosphorylation
(i) Transfer of energy : When photon of light energy falls on Photon Original
chlorophyll molecule, one of the electrons pair from ground or singlet state of light orbit
passes into higher energy level called excited singlet state. It comes back to –
hole of chlorophyll molecule within 10–9 seconds. –
CHL CHL
This light energy absorbed by chlorophyll molecule before coming back Ground state Excited state
to ground state appears as radiation energy, while that coming back from
excited singlet state is called fluorescence and is temperature independent. Fig : Photoexcitation of chlorophyll
Sometimes the electron at excited singlet state gets its spin reversed because
two electrons at the same energy level cannot stay; for some time it fails to molecule i.e. of its atoms
return to its partner electron. As a result it gets trapped at a high energy level.
Due to little loss of energy, it stays at comparatively lower energy level Excited second singlet state
(Triplet state) from excited singlet state. Now at this moment, it can
change its spin and from this triplet state, it comes back to ground state again Heat
losing excess of energy in the form of radiation. This type of loss of energy is
Excited first singlet state
Internal Triplet Chemical
conversion state reaction
Heat/radiation
↓
(Fluorescence)
called as phosphorescence. Up hill Heat/radiation
Down hill
When electron is raised to higher energy level, it is called at second ↓
singlet state. It can lose its energy in the form of heat also. Migration of (Phosphorescence)
electron from excited singlet state to ground state along with the release of e–
excess energy into radiation energy is of no importance to this process.
Somehow when this excess energy is converted to chemical energy, it plays a Ground state
definite constructive role in the process.
Fig : Movement of electron due to
photoexcitation of pigment molecule
(ii) Quantum yield
Rate or yield of photosynthesis is measured in terms of quantum yield or O2 evolution, which may be
defined as, "Number of O2 mols evolved per quantum of light absorbed in photosynthesis."
Chapter 19 319
Photosynthesis in Higher Plants
On the other hand quantum requirement is defined as, "Number of quanta of light required for evolution of
one mol of O2 in photosynthesis."
Quantum requirement in photosynthesis = 8, i.e., 8 quanta of light are required to evolve one mol. of O2.
Hence quantum yield = 1 / 8 = 0.125 (i.e., a fraction of 1) as 12%.
(iii) Emerson effect and Red drop : R. Emerson and C.M. Lewis (1943) observed that the quantum yield of
photosynthesis decreased towards the far red end of the spectrum (680nm or longer). Quantum yield is the number
of oxygen molecules evolved per light quantum absorbed. Since this decrease in quantum yield is observed at the
far region or beyond red region of spectrum is called red drop.
Emerson et al. (1957) further observed that photosynthetic efficiency of light of 680nm or longer is increased if
light of shorter wavelengths (Less than 680nm) is supplied simultaneously.
When both short and long wavelengths were given together the quantum-yield 0.10 Enhancement
Quantum yield
of photosynthesis was greater than the total effect when both the wavelengths 0.08 Red drop
were given separately. This increase in photosynthetic efficiency (or quantum 0.06
yield) is known as Emerson effect or Emerson enhancement effect.
E = Quantum yield in combined beam − Quantum yield in red beam 0.04 480 560 640 720
Quantum yield in far red beam 400 Fig : Red drop
(iv) Two pigment systems : The discovery of Emerson effect has clearly shown the existence of two distinct
photochemical processes, which are believed Photon Photon
to be associated with two different specific
group of pigments. One group of pigments Photosystem 1 Photosystem 2
absorbs light of both shorter and longer
wavelengths (More than 680nm) and another Carotenoids Exchange of Carotenoids
group of pigments absorbs light of only chl b excitation energy chl b
shorter wavelengths (Less than 680nm).
These two groups of pigments are known as chl a660–670 chl a660–670
pigment systems or photosystems. chl a670–680 chl a670–680
chl a678–687
Pigment system I or Photosystem I : chl a678–687 chl a685–695
The important pigments of this system are
chlorophyll a 670, chlorophyll a 683, chl a685–695
chlorophyll a 695, P700. Some physiologist
also include carotenes and chlorophyll b in chl a690–700
chl a705–710
710–715
720–735
pigment system I. P700 acts as the reaction P700 Reaction centre P680–682 Reaction centre
centre. Thus, this system absorbs both
wavelengths shorter and longer than 680nm. e– e–
Pigment system II or photosystem II : PC X ZQ
The main pigments of this system are
Fig : Distribution of pigments in the two photosystems or pigment systems
chlorophyll a 673, P680, chlorophyll b and
phycobilins. This pigment system absorbs wavelengths shorter than 680nm only. P680 acts as the reaction centre.
Chapter 19 320
Photosynthesis in Higher Plants
Pigment systems I and II are involved in non-cyclic electron transport, while pigment system I is involved only
in cyclic electron transport. Photosystem I generates strong reductant NADPH. Photosystem II produces a strong
oxidant that forms oxygen from water.
Comparison of photosystem I and photosystem II
S.No. Photosystem I Photosystem II
(1) PS I lies on the outer surface of the thylakoids PS II lies on the inner surface of the thylakoid.
(2) In this system molecular oxygen is not evolved. As the result of photolysis of water molecular oxygen is
evolved.
(3) Its reaction center is P700. Its reaction center is P680.
(4) NADPH is formed in this reaction. NADPH is not formed in this reaction.
(5) It participate both in cyclic and noncyclic It participate only in noncyclic photophosphorylation.
photophosphorylation.
(6) It receives electrons from photosystem II. It receives electrons from photolytic dissociation of water.
(7) It is not related with photolysis of water. It is related with photolysis of water.
(v) Z-Scheme of light reactions : When sunlight strikes the thylakoid membrane, the energy is absorbed
simultaneously by the 2NADP+ 2FADH2
antenna pigments of both PS 2NADPH2 2FAD+
I and PS II and passed on to 4e– 4e–
the reaction centers of both
photosystems. Electrons of Ferredoxin
both reaction center pigments Excited state If no NADP+ 4e– 4e–
are boosted to an outer available P700* A(Fe-S)
orbital and each P680+ Excited state 4H+
photoexcited electron is
e– ADP++ip
transferred to a primary ATP
electron acceptor. The Pheo Cyclic
photophosphorylation
transfer of electrons out of 4e– Excited high energy electrons e– 4e– Excited high energy electrons
the photosystems leaves the PPQQ
two reaction center pigments
missing an electron and thus, e–
positively charged. After Water splitting ADP++ip
center
losing their electrons, the ATP Cyt b6-f
complex
reaction centers of PS I and 2H2O
Mn2+ Noncyclic e–
4H++O2 4e–
PS II can be denoted as photophosphorylation PC
Released into e–
P700+ and P680+ atmosphere e– Reaction
center
respectively. Positively Reaction Light P700
center (photon) Antenna
charged reaction centers act Light P680 chlorophyll
(photon) Antenna hν molecules
as attractants for electrons, chlorophyll
hν molecules
which sets the stage for the
Photosystem I
Photosystem II
Fig : The Z-scheme of photosynthesis simplified diagram of the electron flow from
H2O to NADP+
Chapter 19 321
Photosynthesis in Higher Plants
flow of electrons between carriers.
In oxygenic photosynthesis, in which two photosystems act in series, electron flow occurs along three legs-
between water and PS II, between PS II and PS I and between PS I and NADP+ an arrangement which is described
as the Z scheme. The Z scheme as originally proposed by Hill and Bendall, 1960.
(vi) Photophosphorylation : Light phase includes the interaction of two pigment systems. PS I and PS II
constitute various type of pigments. Arnon showed that during light reaction not only reduced NADP is formed and
oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light
hence called photophosphorylation.
ADP + Pi Light → ATP .
Chlorophyll
(Where ADP = Adenosine diphosphate, Pi = Inorganic phosphate and ATP = Adenosine triphosphate).
When the light quantum is absorbed by various types of pigments (Like chlorophylls, phycobilins, carotenoids
etc.), it is transferred to reaction centre i.e. P700 in PS I and P680 in PS II. Electrons excite from reaction centres due
to funneling of energy. P700 gets photo excited and comes under first excited singlet state. As a result electron is lost,
which is accepted by an electron, acceptor in the way. After absorbing light, excited electron liberated from reaction
centre interacts with water.
4 H 2 O Light → 4 H + + 4OH −
Chlorophyll
4OH − + 4e − → 4OH
4OH − → 2H2O + O2
4H + + 2A + 4e − → 2AH 2
Another important aspect of light reactions is the formation of ATP and NADPH2 (Assimilatory power). H+
from water and electron from chlorophyll are made available to NADP to form NADPH2. The electrons are
accepted by NADP after passing through electron carriers. The carriers in the way undergo oxidation and reduction
and are arranged in accordance with their redox potential value.
Photophosphorylation is of two types Primary
acceptor
(a) Cyclic photophosphorylation : It involves
only PS I. Flow of electron is cyclic. When NADP is not Reducing Fd ADP+iP
available then this process will occurs. When the 2e–
photons activate PS I, a pair of electrons are raised to a ATP
higher energy level. They are captured by primary PQ
acceptor which passes them on to ferredoxin,
plastoquinone, cytochrome complex, plastocyanin and Oxidizing Cytochrome ADP+iP
finally back to reaction centre of PS I i.e. P700. At each Redox potential complex
step of electron transfer, the electrons lose potential
energy. Their trip down hill is caused by the transport 2 Photons 2e– ATP
chain to pump H+ across the thylakoid membrane. The Reaction PC
proton gradient, thus established is responsible for centre
P 700 Antenna
molecules
Fig : Cyclic photophosphorylation
Chapter 19 322
Photosynthesis in Higher Plants
forming (2 molecules) ATP. No reduction of NADP to NADPH+ H+. ATP is synthesized at two steps.Reducing
(b) Non cyclic photophosphorylation : It involves both PS-I and PS-II. Flow of electron is unidirectional.
Here electrons are not cycled back and are used in the reduction of NADP to NADPH2. Here H2O is utilized and O2
evolution occurs. In this chain high energy electrons released from 'P-680' do not return to 'P-680' but pass through
pheophytin, plastoquinone, cytochrome b6-f complex, plastocyanin and then enter P-700. In this transfer of
electrons from plastoquinone (PQ) to cytochrome b6-f complex, ATP is synthesized. Because in this process high
energy electrons released from 'P-680' do not return to 'P-680' and ATP (1 molecules) is formed, this is called
Noncyclic photophosphorylation. ATP is synthesized at only one step.
Primary
acceptor
Fd
Primary NADP +2e–
acceptor
PQ 2e–
Redox potential 2 Photons Cytochrome 2 NADPH
complex 2 Photons
Reaction 2e–
centre P 700
2e– ATP PC
H2O ADP+iP
Reaction
centre
Molecules Antenna
Oxidizing P 680 Antenna
Molecules
½ O2 Fig : Non cyclic photophosphorylation
2H+
Photons Photosystem I ATP
Photons Photosystem II NADP2H
O2
Fig : Final products of light reactions
Comparison of cyclic and noncyclic photophosphorylation
S.No. Cyclic photophosphorylation Noncyclic photophosphorylation
(1)
(2) No oxygen is given off (Anoxygenic). Oxygen is given off (Oxygenic).
(3)
No water is consumed. Water is used up.
(4)
(5) Only one light-trapping system (Photosystem I) Two light-trapping systems (Photosystem I and II) are
is involved. involved.
No NADPH synthesized. NADPH synthesized
Last electron acceptor is P700 Last electron acceptor is NADP.
Chapter 19 323
Photosynthesis in Higher Plants
(6) The system is found dominantly in bacteria. The system is dominant in green plants.
(7) The process is not inhibited by DCMU. The process is stopped by use of DCMU
(c) Pseudocyclic photophosphorylation : Arnon and his coworker (1954) demonstrated yet another kind
of photophosphorylation. They observed that even in absence of CO2 and NADP, if chlorophyll molecules are
illuminated, it can produce ATP from ADP and Pi (Inorganic phosphate) in presence of FMN or vit. K and oxygen.
The process is thus very simple and requires no net chemical change but for the formation of ATP and water. Arnon
called this oxygen dependent FMN catalysed photophosphorylation or pseudocyclic photophosphorylation which
involves the reduction of FMN with the production of oxygen. FMN is an auto-oxidisable hydrogen acceptor with
the effect that the reduced FMN is reoxidised by oxygen. Thus the process can continue repeatedly to produce ATP.
Since this process can be continuously self repeated, it appears that a single molecule of water should be
sufficient to operate pseudocyclic photophosphorylation to meet the requirement of ATP.
FMN + H2O Illuminated chloroplast FMNH2 + ½ O2
ADP+Pi ATP
(2) Dark phase : The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2
into carbohydrate is known as carbon fixation or photosynthetic carbon reduction (PCR) cycle or dark
reactions. The dark reactions are sensitive to temperature changes, but are independent of light hence it is called
dark reaction, however it depends upon the products of light reaction of photosynthesis, i.e. NADP .2H and ATP.
The carbon dioxide fixation takes place in the stroma of chloroplasts because it has enzymes essential for fixation of
CO2 and synthesis of sugar. The techniques used for studying different steps were Radioactive tracer technique
using 14C (Half life – 5720 years), Chromatography and Autoradiography and the material used was Chlorella
(Cloacal alga) and Scenedesmus (these are microscopic, unicellular algae and can be easily maintained in
laboratory).
The assimilation and reduction of CO2 takes place in this reaction by which carbohydrate is synthesized
through following three pathways :
(i) Calvin cycle (C3) (ii) Hatch and Slack cycle (C4) (iii) Crassulacean acid metabolism (CAM plants)
(i) Calvin cycle : Calvin and Benson discovered the path of carbon in this process. This is known as C3 cycle
because CO2 reduction is cyclic process and first stable product in this cycle is a 3-C compound (i.e., 3-
Phosphoglyceric acid or 3-PGA).
Calvin cycle is divided into three distinct phases : Carboxylation, Glycolytic reversal, Regeneration of RuBP.
Carboxylation : CO2 reduction starts with a 5-carbon sugar, ribulose-5-phosphate. 6 molecules of this
sugar react with 6 molecules of ATP (Produced in light reactions) forming 6 molecules of ribulose-1, 5-biphosphate
(RuBP) and 6 molecules of ADP. (equation i).
Ribulose -5 − phosphate + 6ATP Phosphopentokinase → Ribulose -1, 5 − biphosphate +6ADP .…. (i)
(6 mols)
(6 mols)
Chapter 19 324
Photosynthesis in Higher Plants
The reaction is catalysed by the enzyme ribulose biphosphate carboxylase (RUBISCO). Ribulose-1,5-
biphosphate (RuBP) (=Ribulose diphosphate) acts as CO2 acceptor and 6 mols of RuBP react with 6 mols of CO2
and 6 mols of water giving rise to 12 mols of 3-phosphoglyceric acid (a 3 carbon compound) (equation ii).
Ribulose -1, 5 − biphosphate + 6CO2 + 6H2O Carboxydismutase → 3 − phospho glyceric acid ..… (ii)
(6 mols) (12 mols)
Ribulose-1,5-biphosphate carboxylase
Glycolytic reversal : Carboxylation is followed by reactions that involve reversal of glycolysis part of
respiration.
12 mols of 3-phosphoglyceric acid react with 12 mols of ATP (Produced in light reactions) giving rise to 12
mols each of 1, 3-diphosphoglyceric acid + ADP (equation iii).
3 − phosphoglyceric acid + 12ATP Phosphoglyceric kinase → 1, 3 − diphosphoglyceric acid + 12 ADP …. (iii)
(12 mols)
(12 mols)
12 mols of NADP.2H formed in light reactions are used to reduce 12 mols of 1,3-diposphoglyceric acid
leading to the formation of 12 mols of 3-phosphoglyceraldehyde, 12 moles of NADP and 12 moles of phosphoric
acid (equation iv).
1,3 − diphosphoglyceric acid + 12 NADP.2H Triose phosphatedehydrogenase →
(12 mols)
3 − phosphoglyceraldehyde + 12NADP + + 12H3 PO4 …. (iv)
(12 mols)
In this way by the reduction of CO2, 12 molecules of 3-phosphoglyceraldehyde are formed. Out of these 12
molecules, 2 molecules go to synthesize sugar, starch and other carbohydrates and remaining 10 molecules are
recycled to regenerate 6 molecules of ribulose 5 phosphate.
Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone
phosphate (equation v).
3 − phosphoglyceraldehyde Phosphotriose → 3 − dihydroxyacetone phosphate ….. (v)
(1 mol) Isomerase (1mol)
One mol of 3-dihydroxyacetone phosphate react with 1 mol of 3-phosphoglyceraldehyde to form one
molecule of fructose-1,6-biphosphate (equation vi).
Phosphoglyceraldehyde + Dihydroxyacetone phosphate Aldolase → Fructose -1,6 − biphosphate …. (vi)
(1 mol) (1 mol) (1 mol)
One mol of fructose-6-phosphate and one mol of phosphoric acid is released from one mol of fructose-1,6-
biphosphate with the help of the enzyme phosphatase with utilizations of one mol of H2O (equation vii).
Fructose -1,6 − biphosphate Phosphatase → Fructose - 6 − phosphate + H 3 PO4 .... (vii)
(1 mol) H2O (1 mol )
Fructose-6-phosphate can be converted to other sugars (viz., glucose, sucrose, starch, etc.). In this way, the
atmospheric CO2 is used in the synthesis of carbohydrates.
Regeneration of RuBP : Both triose phosphates, i.e., 3-phosphoglyceraldehyde and dihydroxy acetone
phosphate, actively participate in the regeneration of CO2-acceptor ribulose-1,5-diphosphate. The sequence of
reactions are as follows :
Chapter 19 325
Photosynthesis in Higher Plants
(i) 3 − phosphoglyceraldehyde Triose phosphate → Dihydroxyacetone phosphate
(PGAL) Isomerase (DHAP)
[4 mols] [4 mols]
(ii) Dihydroxyacetone phosphate + 3 − phosphoglyceraldehyde Aldolase → Fructose - 1,6 − diphosphate
[2 mols] [2 mols] [2 mols]
(iii) Fructose -1,6 − diphosphate + 2H 2O Phosphatase → Fructose - 6− phosphate + 2H 3 PO4
[2 mols] [2 mols]
(iv) Fructose - 6 − phosphate + 3 − phosphoglyceraldehyde Transketolase →
[2 mols] [2 mols]
Xylulose- 5 − phosphate + Erythrose- 4 − phosphate
[2 mols] [2 mols]
(v) Erythrose- 4 − phosphate+ Dihydroxyacetone phosphate Transaldolase →
[2 mols] [2 mols]
(Produced in reaction (iv)) of the 4 mols produced in reaction (i)
Sedoheptulose-1,7 − diphosphate
[2 mols]
6 mols Ribulose-1,5-biphosphate
Carboxylation (ii)
(i) 6CO2 + 6H2O
6 mols ATP 12 mols 3-phosphoglyceric acid
12 ATP
6 mols ADP Ist Phosph- IInd Phosphorylation (iii)
orylation 12 ADP
Calvin
cycle 12 mols 1,3-diphosphoglyceric acid
6 mols Ribulose-5-phosphate 12 NADP.2H
(iv)
Reduction
Regeneration of ribulose- 12 mols 12 NADP++
5-phosphate 12H3PO4
by a series of complex 3-phosphoglyceraldehyde
reactions
10 mols
2 mols
(one mol) (one mol)
(v)
3-dihydroxyacetone
phosphate
(one mol) Enzymes
(vi)
Fructose-1, 6-biphosphate (i) Phosphopentokinase
H2O (one mol) (ii) Carboxydismutase
(vii) (iii) Phosphoglyceric kinase,
(iv) Triose phosphate dehydrogenase,
H3PO4 (v) Phosphotriose isomerase,
(vi) Aldolase,
Fructose-6-phosphate (vii) Phosphatase
(one mol)
Chapter 19
Fructose-1-phosphate
(one mol)
(one mol) Glucose-1-phosphate
(one mol)
3(vi2i) 6
Sucrose, starch and
other carbohydrates
Fig : Simplified diagram of Calvin cycle
Photosynthesis in Higher Plants
(vi) Sedoheptulose-1,7 − diphosphate Phosphatase → Sedoheptulose- 7 − phosphate + 2H 3 PO4
[2 mols] [2 mols]
(vii) Sedoheptulose- 7 − phosphate + 3 - p hosphoglyceraldehyde Transketolase →
[2 mols] [2 mols]
Ribose- 5 − phosphate + Xylulose- 5 − phosphate
[2 mols] [2 mols]
(viii) Ribose- 5 − phosphate Phosphopentose isomerase → Ribulose- 5 − phosphate
[2 mols] [2 mols]
(Produced in reaction (vii))
(ix) Xylulose-5-phosphate Phosphopentose epimerase → Ribulose- 5 − phosphate
[4 mols]
2 + 4 = 6 molecules of Ribulose 5 phosphate are formed during the changes from equation (viii) and (ix)
these molecule changed in Ribulose 1, 5 diphosphate (RuDP) by the consumption of 6 ATP. These RuDP again
ready for reduction of new molecules of CO2. Hence in this way regeneration of RuDP is going on. They are used in
next calvin cycle. In the overall reactions 18 ATP molecules and 12 NADPH2 molecules consumed and one
molecule of glucose (Hexose) is obtained (1 NADPH2 = 3ATP ∴ Total ATP consumed = 54 ATP). The whole
photosynthesis can be summarized in terms of equation which is as follows :
Light reaction : 12H 2O + 12NADP + 18 ADP + 18 Pi → NADPH − H + + 18 ATP + 6O2
Dark reaction : 6CO2 + 12NADPH − H + + 18 ATP → C6 H12O6 + 6H 2O + 12NADP + 18 ADP + 18Pi
Final equation : 6CO2 + 12H 2O → C6 H12O6 + 6H 2O + 6O2
(ii) Hatch and Slack cycle (C4 cycle) : Kortschak and Hart supplied CO2 to the leaves of sugarcane, they
found that the first stable product is a four carbon (C4) compound oxalo acetic acid instead of 3-carbon atom
compound. The detailed study of this cycle has introduced by M.D. Hatch and C.R. Slack (1966). So it is called as
"Hatch and Slack cycle". The stable product in C4 plant is dicarboxylic group. Hence it is called dicarboxylic acid
cycle or DCA-cycle. C4 plants are true xerophytic plants. Upper
The plants that perform C4 cycle are found in tropical (Dry epidermis
and hot regions) and sub-tropical regions. There are more than 900 Mesophyll
known species in which C4 cycle occurs. Among them, more than cells
300 species belong to dicots and the rest belong to monocots. The Chloroplasts
important among them are sugarcane, maize, Sorghum, Cyperus Xylem Vascular
rotundus, Digitaria brownii, Amaranthus, etc. These plants have Phloem bundle
"Kranz" (German term meaning halo or wreath) type of leaf
anatomy. The vascular bundles, in C4 leaves are surrounded by a Cells of bundle
layer of bundle sheath cells that contain large number of sheath with special
chloroplasts. The chloroplasts in C4 leaves are dimorphic (Two types of chloroplast
Chapter 19 327 Lower Stomata
epidermis
Fig : Cross section of leaf showing
“krantz” type of anatomy
Photosynthesis in Higher Plants
morphologically distinct types). The chloroplasts of bundle sheath cells are larger in size and arranged centripetally. They
contain starch grains but lack grana. The mesophyll cells, on the other hand, contain normal types of chloroplasts. The
mesophyll cells perform C4 cycle and the cells of bundle sheath perform C3 cycle.
CO2 taken from the atmosphere is accepted by phosphoenolpyruvic acid (PEP) present in the chloroplasts of
mesophyll cells of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA). This acid is
converted to another 4-C acid, the malic acid which enters into the chloroplasts of bundle sheath cells and there
undergoes oxidative decarboxylation yielding pyruvic acid (a 3-C compound) and CO2. CO2 released in bundle
sheath cells reacts with Ribulose-1,5-biphosphate (RuBP) already present in the chloroplasts of bundle sheath cells
and thus Calvin cycle starts from here. Pyruvic acid re-enters mesophyll cells and regenerates phosphoenol pyruvic
acid. CO2 after reacting with RuBP gives rise to sugars and other carbohydrates. Mesophyll cells have PEP
carboxylase and pyruvate orthophosphate dikinase enzyme while the bundle sheath cells have decarboxylase and
complete enzymes of Calvin cycle. In C4 plants, there are 2 carboxylation reactions, first in mesophyll chloroplast
and second in bundle sheath chloroplast.
Atmospheric CO2
Phosphoenol AMP+iPP ATP Pyruvic acid
(3-C acid)
pyruvic acid (iv) Pyruvic
(PEP) acid
(i)
Oxaloacetic (ii) Malic (iii)
acid acid CO2 RuBP
Calvin
NADPH NADP+ NADPH cycle
3-phospho-
NADP+
glyceraldehyde
Sucrose
Mesophyll cell Bundle sheath cell
Fig : Hatch-slack pathway (cycle)
Enzymes : (i) Phosphoenol pyruvate carboxylase, (ii) Malate dehydrogenase,
(iii) Decarboxylase, (iv) Pyruvate orthophosphate dikinase
C4 plants are better photosynthesizers. There is no photorespiration in these plants. In C4 plants, for formation
of one mole of hexose (glucose) 30 ATP and 12 NADPH2 are required. There is difference in different C4 plants in
mechenism of C4 mode of photosynthesis. The main difference is in the way the 4C dicarboxylic acid is
decarboxylated in the bundle sheath cells. The three categories of C4 pathways in C4 plants are recognised such as :
(a) Some C4 plants e.g., Zea mays, Saccharum officinarum, Sorghum utilise NADP+ specific malic enzyme for
decarboxylation. This mechanism of C4 pathway in these C4 plants is said to be of NADP+ –Me Type.
(b) Some C4 plants e.g., Atriplex, Portulaca, Amaranthus utilise NAD+ specific malic enzyme for
decarboxylation. This mechanism of C4 pathways in these C4 plants is said to be of NAD+ –Me Type.
(c) Some C4 plants e.g., Panicum, Chloris utilise PEP-carboxykinase enzyme. The mechanism of C4 pathway
in these plants is called as PCK-me-Type.
Characteristics of C4 cycle
Chapter 19 328
Photosynthesis in Higher Plants
(1) C4 species have greater rate of CO2 assimilation than C3 species. This is on account of the fact that
(a) PEP carboxylase has great affinity for CO2.
(b) C4 plants show little photorespiration as compared to C3 plants, resulting in higher production of dry matter.
(2) C4 plants are more adapted to environmental stresses than C3 plants.
(3) CO2 fixation by C4 plants require more ATP than that by C3 plants. This additional ATP is needed for
conversion of pyruvic acid to phosphoenol pyruvic acid and its transport.
(4) CO2 acceptor molecule in C4 plants is PEP. Further, PEP-carboxylase (PEPCO) is the key enzyme (RuBP-
carboxylase enzyme is negligible or absent in mesophyll chloroplast, but is present in bundle sheath chloroplast).
Differences between C3 and C4 plants
S.No. Characters C3 plants C4 plants
(1) CO2 acceptor
(2) First stable product The CO2 acceptor is Ribulose 1,5 diphosphate. The CO2 acceptor is phosphoenol-pyruvate.
The first stable product is phosphoglyceric acid. Oxaloacetate is the first stable product.
(3) Type of chloroplast All cells participating in photosynthesis have one type The chloroplast of parenchymatous bundle
of chloroplast. sheath is different from that of mesophyll cells.
Leaves have 'Kranz' type of anatomy. The bundle
sheath chloroplasts lack grana. Mesophyll cells
have normal chloroplasts.
(4) Cycles Only reductive pentose phosphate cycle is found. Both C4-dicarboxylic acid and reductive pentose
phosphate cycles are found.
(5) Optimum The optimum temperature for the process is 10-25°C. In C4 plants, it is 30-45°C.
temperature
(6) Oxygen inhibition Oxygen present in air (=21% O2) markedly inhibit The process of photosynthesis is not inhibited in
the photosynthetic process as compared to an air as compared to an external atmosphere
external atmosphere containing no oxygen. containing no oxygen.
(7) PS I and PS II In each chloroplast, photosystems I and II are present. In the chloroplasts of bundle sheath cells, the
Thus, the Calvin cycle occurs. photosystem II is absent. Therefore, these are
dependent to mesophyll chloroplast for the
supply of NADPH + H+
(8) Enzymes The Calvin cycle enzymes are present in mesophyll Calvin cycle enzymes are absent in mesophyll
chloroplast. chloroplasts. The cycle occurs only in the
chloroplasts of sheath cells.
(9) Compensation The CO2 compensation point is 50-150ppm. CO2 compensation point is 0-10ppm.
point
(10) Photorespiration Photorespiration is present and easily detectable. Photorespiration is present only to a slight degree
and difficult to detect.
(11) Net rate Net rate of photosynthesis in full sunlight (10,000- It is 40-80mg. of CO2 per dm2 of leaf area per h.
That is photosynthetic rate is quite high. The
12,000 ft.c) is 15-35mg. of CO2 per dm2 of leaf area
per h. plants are efficient.
(12) Saturation intensity The saturation intensity reached in the range of 100- It is difficult to reach saturation even in full
4000 ft.c. sunlight.
Chapter 19 329
Photosynthesis in Higher Plants
(iii) Crassulacean acid metabolism plants (CAM plants) : This dark CO2 fixation pathway proposed by
Ting (1971). It operates in succulent or fleshy plants e.g. Cactus, Sedum, Kalanchose, Opuntia, Agave, orchid, pine
apple and Bryophyllum helping them to continue photosynthesis under extremely dry condition.
The stomata of succulent plants remain closed during day and open during night to avoid water loss
(Scotactive stomata). They store CO2 during night in the form of malic acid in presence of enzyme PEP carboxylase.
The CO2 stored during night is used in Calvin cycle during day time. Succulents refix CO2 released during
respiration and use it during photosynthesis.
This diurnal change in acidity was first discovered in crassulacean plants e.g. Bryophyllum. So it is called as
crassulacean acid metabolism. The metabolic pathways are –
Acidification : In dark, stored carbohydrates are converted to phosphoenol pyruvic acid (PEP) by the
process of glycolysis. The opening of stomata in CAM plants in dark causes entry of CO2 in leaf. So, phosphoenol
pyruvic acid in presence of PEP carboxylase is converted to oxaloacetic acid (OAA). OAA is then reduced to malic
acid in presence of enzyme malic dehydrogenase with the help of NADH2. This malic acid (Produced by
acidification) is stored in vacuole.
Night Day
Carbohydrates Carbohydrates
Stomata open Phosphoenol Stomata
CO2 pyruvic acid close
(PEP) Calvin PGA
RuDP cycle
Oxaloacetic acid
(OAA) Pyruvic acid
NADPH+H+ CO2
NADPH+H+
NADP+ NADP+
Malic acid Malic acid
Fig : CAM synthesis
Deacidification : In light the malic acid is decarboxylated to produce pyruvic acid and evolve CO2. This
process is called deacidification.
The malate may be decarboxylated in two ways –
(a) In some CAM plants the malate is directly decarboxylated in the presence of NADP+ malic enzyme into
CO2 and pyruvate (ME-CAM plants).
(b) In other CAM plants, the malate is first oxidised to oxaloacetic acid by enzyme malate dehydrogenase
which is then converted into CO2 and phosphoenol pyruvate with the utilization of ATP by enzyme PEP
carboxykinase (PEPCK-CAM plants).
The CO2 produced by any above process is then consumed in normal photosynthetic process to produce
carbohydrate.
Chapter 19 330
Photosynthesis in Higher Plants
Characteristics of CAM pathway
(1) CO2 assimilation and malic acid assimilation take place during the night.
(2) There is decrease in pH during the night and increase in pH during the day.
(3) Malic acid is stored in the vacuoles during the night which is decarboxylated to release CO2 during the day.
(4) CAM plants have enzymes of both C3 and C4 cycle in mesophyll cells. This metabolism enable CAM plants
to survive under xeric habitats. These plants have also the capability of fixing the CO2 lost in respiration.
Photorespiration (Photosynthetic carbon oxidation cycle).
Decker and Tio (1959) reported that light CO2
induces oxidation of photosynthetic intermediates
with the help of oxygen in tobacco. It is called as Serine CO2
photorespiration. The photorespiration is defined by NH3
Krotkov (1963) as an extra input of O2 and extra
release of CO2 by green plants is light. NADH
Mitochondria NAD+ Glycine
Photorespiration is the uptake of O2 and release Glycine
of CO2 in light and results from the biosynthesis of
glycolate in chloroplasts and subsequent metabolism Serine
of glycolate acid in the same leaf cell. Biochemical Peroxysome Hydroxy Glyoxylic acid
mechanism for photorespiration is also called pyruvic acid
glycolate metabolism. Loss of energy occurs during H2O2
this process. The process of photorespiration involves
NADH 2H2O+O2
NAD+
the involvement of chloroplasts, peroxisomes and Chloroplast Glyceric acid 2O2
mitochondria. RuBP carboxylase also catalyses
another reaction which interferes with the successful Glycolic acid
functioning of Calvin cycle.
CaCldviyipRnchil(beoCus3lpo)hs3aetPeGCAOg3l2y-pcGehlryoiccseparhcicoidaAcAiDdTPP O2
Biochemical mechanism
(1) Ribulose bisphosphate carboxylase +O2 Glycolic acid
(RUBISCO), the main enzyme of Calvin cycle that 2-phospho
O2 glycolic acid Phosphatase
fixes CO2, acts as ribulose bisphosphate oxygenase Fig : The biochemical pathway of photorespiration
under low atmospheric concentration of CO2
(i.e., below 1%) and increased concentration of O2. In presence of high concentration of O2 the enzyme RuBP
oxygenase splits a molecule of Ribulose-1, 5-bisphosphate into one molecule each of 3-phosphoglyceric acid and 2-
phosphoglycolic acid.
Ribulose-1, 5- bisphosphate O2 → 2 Phosphoglycolic acid +3 Phosphoglyceric acid
(2) The 2-phosphoglycolic acid loses its phosphate group in presence of enzyme phosphatase and converted
into glycolic acid –
2 Phosphoglycolic acid + H2O → Glycolic acid + Phosphoric acid.
Chapter 19 331
Photosynthesis in Higher Plants
(3) The glycolic acid, synthesized in chloroplast as an early product of photosynthesis, is then transported to
the peroxisome. The glycolic acid reacts with O2 and oxidizes to glyoxylic acid and hydrogen peroxide with the help
of enzyme glycolic acid oxidase.
Glycolic acid + O2 → Glyoxylic acid + H2O2
The hydrogen peroxide is destroyed by enzyme catalase as follows :
2H 2O2 → 2H 2O + O2
(4) The glyoxylic acid is then converted to an amino acid-glycine by transamination reaction catalyzed by
enzyme glutamate-glyoxylate transaminase.
Glyoxylic acid + Glutamic acid → Glycine + α-keto glutaric acid
(5) The glycine is transported out of peroxisomes into mitochondria, where two molecules of glycine interact to
form one molecule each of serine, CO2 and NH3 –
2 Glycine + H2O + NAD+ → Serine + CO2 + NH3 + NADH
The CO2 is then released in photorespiration from mitochondria. The NH3 released during glycine
decarboxylation is transported to cytoplasm or chloroplast, where it is incorporated into synthesis of glutamic acid.
(6) The amino acid serine returns to peroxisome where it is deaminated and reduced to hydroxypyruvic acid
and finally to glyceric acid –
Serine + Glyoxylic acid → Hydroxypyruvic acid + Glycine Hydroxypyruvic acid → Glyceric acid
(7) The glyceric acid finally enters the chloroplast where it is phosphorylated to 3-phosphoglyceric acid, which
enters into C3 cycle –
Glyceric acid + ATP → 3-Phosphoglyceric acid + ADP + Phosphate.
Importance of photorespiration : The process of photorespiration interferes with the successful functioning
of Calvin cycle. Photorespiration is quite different from respiration as no ATP or NADH are produced. Moreover,
the process is harmful to plants because as much as half the photosynthetically fixed carbon dioxide (in the form of
RuBP) may be lost into the atmosphere through this process.
Any increase in O2 concentration would favour the uptake of O2 rather than CO2 and thus, inhibit
photosynthesis for this rubisco functions as RuBP oxygenase. Photorespiration is closely related to CO2
compensation point and occurs only in those plants which have high CO2 compensation point such as C3 plants.
It is absent in plants which have very low CO2 compensation point such as maize, sugarcane (C4 plants).
Photorespiration generally occurs in temperate plants. Few photorespiring plants are : Rice, bean, wheat, barley,
rice etc. Inhibitors of glycolic acid oxidase such as hydroxy sulphonates inhibit the process of photorespiration.
Unlike usual mitochondria respiration neither reduced coenzymes are generated in photorespiration nor the
oxidation of glycolate is coupled with the formation of ATP molecules. Photorespiration (C2 cycle) is enhanced by
bright light, high temperature, high oxygen and low CO2 concentration.
Differences between photorespiration, photosynthesis and true respiration
S.No. Photorespiration Photosynthesis True Respiration
(1) Occurs in green plants in light. Occurs in green plants in light. Occurs in all living organisms in light and dark.
(2) The primary substrate is Substrate is CO2 and H2O. Substrates are carbohydrates, fat and proteins.
glycolate formed from RuBP.
Chapter 19 332
Photosynthesis in Higher Plants
(3) Occurs in most of the C3 plants. Occurs in all green plants. Occurs in all living organisms.
(4) Intracellularly, the process occurs Occurs in chloroplasts. Occurs in cytosol and mitochondria.
in peroxisomes in association with
chloroplasts and mitochondria.
(5) The process increases with The process is inhibited with The process saturates at 2-3% O2 in the
increasing concentration of O2 and increasing concentration of O2. atmosphere and beyond this conc, virtually no
decreasing concentration of CO2. increase occurs.
(6) Hydrogen peroxide is formed H2O2 is not formed. H2O2 is not formed.
during this process.
(7) Phosphorylation does not occur. Photophosphorylation occurs. Oxidative phosphorylation occurs.
CO2 compensation point : In photosynthesis, CO2 is utilized in presence of light to release O2 whereas in
respiration, O2 is taken and CO2 is released. If light factor is saturating, there will be certain CO2 concentration at
which rate of photosynthesis is just equal to rate of respiration or photosynthesis just compensates respiration or
apparent photosynthesis is nil. It is called CO2 compensation point. Rate of photosynthesis is higher than that of
respiration during day time and ratio of O2 produced to that consumed is 10 : 1.
CO2 compensation point is very low in C-4 plants, i.e., 0 to 5 ppm whereas high CO2 compensation point is
found in C-3 plants, i.e. 25 to 100 ppm.
During compensation point there is no evolution of any gas.
Adenosine triphosphate (ATP).
A molecule of Adenosine is formed by reaction between a molecule of adenine (A nitrogenous base) and
sugar D-ribose (A pentose sugar). Adenosine is a nucleoside. Adenosine monophosphate (AMP = Adenylic acid) is
formed by condensation of a phosphate group at CH2OH site of 5th carbon atom of deoxyribose sugar.
With the formation of this bond (represented by –) between sugar and phosphate energy of 1500-1800
cal./mol is stored. This is low energy bond. When next group of phosphate is attached to AMP, Adenosine
diphosphate (ADP) is formed. In this bond 7300 cal./mol of energy is stored and this bond is represented by wavy
lines (~). This is high energy bond. In the same way when third phosphate group is attached to ADP, ATP is
formed. This third bond is also represented by wavy line (~) and the energy stored is equal to the second bond.
ATP
ADP
AMP
Adenosine
Adenine Ribose Three phosphate radicals
High energy bonds
NH2 N
N
N OH OH OH
N | ||
O CH2–O–P–O~P–O~P–OH
|| || ||
O OO
Chapter 19 OH OH
1500-1800 7300 7300
333Fig : Molecular structure of Adenosine Triphosphate (ATP)
Photosynthesis in Higher Plants
In photochemical reactions of photosynthesis 18 ATP molecules are synthesized. Out of these 18 molecules of
ATP, 6 react with ribulose monophosphate to form ribulose-1,5-biphosphate and the remaining 12 molecules react
with 12 mols of 3-phosphoglyceric acid to form 12 mols of 1,3-diphosphoglyceric acid. ATP synthesized in cyclic
and noncyclic photophosphorylation is utilized in dark reaction of photosynthesis.
Functions of ATP : In living cells energy yielding and energy consuming reactions take place continuously.
By release of energy from one substance (e.g., glucose) another substance, e.g., protein is synthesized. By release of
energy from proteins other activities of plants can be carried out. There is a mechanism of temporary storage of
energy in the cells. This is ATP. This chemical is extremely important for all living cells. Energy released as a result
of oxidation of carbohydrates, proteins and fats is utilized in the synthesis of ATP (from ADP and inorganic
phosphate). This method of synthesis of ATP in respiration is called oxidative phosphorylation which is essential
for various other synthetic activities, e.g., synthesis of carbohydrates, fats, proteins and osmosis, active absorption,
translocation of foods, streaming of protoplasm, growth, etc. In this way by taking out energy from one compound
and transferring it to another, ATP, functions as an intermediary compound of energy transfer. This is why
ATP is called as monetary system of energy exchange in living organisms.
Bacterial photosynthesis.
Like green plants, some purple and green sulphur bacteria are capable of synthesizing their organic food in
presence of light and in absence of O2, which is known as bacterial photosynthesis.
Van Niel was the first to point out these similarities. Oxygen is liberated in bacteria during process of
photosynthesis. Their photosynthesis is non-oxygenic. Because bacteria use H2S in place of water (H2O) as
hydrogen donor. Photosynthetic bacteria are anaerobic. Only one type of pigment system (PSI) is found in bacteria
except cyanobacteria which possess both PSI and PSII. Bacteria has two type of photosynthetic pigments.
Bacterial chlorophyll
Bacterio viridin
The photosynthetic bacteria fall under three categories :
(1) Green sulphur bacteria : They are autotrophic. The hydrogen donor is H2S and the pigment involved in
the process is chlorobium chlorophyll (Bacterioviridin) e.g. Chlorobium.
6CO2 + 12H 2 S → C6 H 12 O6 + 6H2O + 12S .
Chlorobium chlorophyll
(2) Purple sulphur bacteria : They are also autotrophic. The hydrogen donor is thiosulphate and the
pigment involved in photosynthesis is bacteriochlorophyll a. e.g., Chromatium.
6CO2 + 15H 2O + 3 Na 2 S2 O3 → C6 H12 O6 + 6H2O + 6NaHSO4 .
Bacteriochlorophyll a
(3) Purple non-sulphur bacteria : They are heterotrophic utilizing succinate or malate or alcohol.
e.g., Rhodospirillum, Rhodopseudomonas.
Chapter 19 334
Photosynthesis in Higher Plants
6CO2 + 12CH 3CHOHCH3 → C6 H12O6 + 12CH 3COCH3 + 6H 2O
Characteristics of bacterial photosynthesis are :
(1) No definite chloroplasts but contain simple structures having pigments called chromatophores (term
coined by Schmitz).
(2) Contain chlorobium chlorophyll or bacterio-chlorophyll.
(3) Use longer wavelengths of light (720-950nm).
(4) No utilization of H2O (but use H2S or other reduced organic and inorganic substances).
(5) No evolution of O2.
(6) Photoreductant is NADH2 (Not NADPH2).
(7) Only one photoact and hence one pigment system and thus one reaction centre, i.e., P890.
(8) Cyclic photophosphorylation is dominant.
(9) It occurs in presence of light and in absence of O2.
Chemosynthesis.
Some forms of bacteria obtain energy by chemosynthesis. This process of carbohydrate formation in which
organisms use chemical reactions to obtain energy from inorganic compounds is called chemosynthesis. Such
chemoautotrophic bacteria do not require light and synthesize all organic cell requirements from CO2 and H2O and
salts at the expense of oxidation of inorganic substances like (H2, NO3–, SO4 or carbonate). Some examples of
chemosynthesis are :
(1) Nitrifying bacteria : These bacteria oxidises ammonia to nitrites and release chemical energy. e.g.
Nitrosomonas, Nitrococcus etc.
NH + + 2O2 → NO2 + 2H 2O + energy
4
(2) Sulphur bacteria : Convert H2S to sulphur. e.g, Beggiatoa, Thiothrix and Thiobacillus.
H 2S + O2 → 2H 2O + 2S + energy
(3) Iron bacteria : Oxidises ferrous to ferric e.g. Ferrobacillus, Leptothrix and Cladothrix.
Fe 2+ → Fe 3+ + energy
(Ferrous) (Ferric)
(4) Hydrogen bacteria : e.g. Bacillus pentotrophus
H 2 [O] →(H 2O) + energy
(5) Carbon bacteria : Convert carbon monoxide to carbon dioxide. e.g., Carboxydomonas, Bacillus
oligocarbophilus.
CO [O] → CO2 + energy
Chapter 19 335
Photosynthesis in Higher Plants
Factors affecting photosynthesis.
Blackmann's law of limiting factors
F.F. Blackmann (1905) proposed the law of limiting factors according to which 'when process is conditioned to
its rapidity by a number of factors, the rate of process is limited by the pace of the slowest factor'. Blackmann's law
of limiting factor is modification of Leibig's law of minimum, which states that rate of process controlled by several
factors is only as rapid as the slowest factor permits. Theory of three cardinal points was given by Sachs in 1860.
According to this concept, there is minimum, optimum and maximum for each factor. For every factor, there is a
minimum value when no photosynthesis occurs, an optimum value showing highest rate and a maximum value,
above which photosynthesis fails to take place. The law can be explained best by the following illustration :
G Light intensity H
Photosynthesis E Medium F
Relative rate of C Low D
photosynthesis
Optimum
Minimum Maximum
Limiting factor B
Fig : The concept of A CO2 concentration
three cardinal points
Fig : Blackman’s law
of limiting factor
Light intensity provided to a leaf is just sufficient to permit it to utilize 5 mg of CO2. At 'A' no photosynthesis
occurs due to non-availability of CO2. If concentration is increased from 0 to 1 mg, rate of photosynthesis will
increase from 'A' to 'C'. Now even if the CO2 concentration is further increased to 5 mg rate becomes constant.
Further increase from 'C' to 'E' is possible only when light intensity is increased, which is at this time working as
limiting factor. Because the factor which is quantitatively smaller may not be limiting one, while a factor which is
relatively less than the amount actually required will act as limiting factor. That is why many modifications in name
have been suggested e.g. 'Law of relatively limiting factor' or 'Law of most significant factor'.
Factors : The rate of photosynthetic process is affected by several external (Environmental) and internal factors.
(1) External factors : These include light, temperature, CO2, water and oxygen.
(i) Light : The ultimate source of light for photosynthesis in green plants is solar radiation, which moves in the
form of electromagnetic waves. Out of the total solar energy reaching to the earth about 2% is used in
photosynthesis and about 10% is used in other metabolic activities. Light varies in intensity, quality (Wavelength)
and duration. The effect of light on photosynthesis can be studied under these three headings.
(a) Light intensity : The total light perceived by a plant depends on its general form (viz., height, size of
leaves, etc.) and arrangement of leaves. Of the total light falling on a leaf, about 80% is absorbed, 10% is reflected
and 10% is transmitted.
In general, rate of photosynthesis is more in intense light than diffused light. (Upto 10% light is utilized in
sugarcane, i.e., Most efficient converter).
Chapter 19 336
Photosynthesis in Higher Plants
Another photosynthetic superstar of field growing plants is Oenothera claviformis (Winter evening-primrose),
which utilizes about 8% light.
However, this light intensity varies from plant to plant, e.g., more in heliophytes (sun loving plants) and less
in sciophytes (shade loving plants). For a complete plant, rate of photosynthesis increases with increase in light
intensity, except very high light intensity where 'Solarization' phenomenon occurs, i.e., photo-oxidation of
different cellular components including chlorophyll occurs.
It also affects the opening and closing of stomata thereby affecting the gaseous exchange. The value of light
saturation at which further increase is not accompanied by an increase in CO2 uptake is called light saturation
point.
(b) Light quality : Photosynthetic pigments absorb visible part of the radiation i.e., 380mµ to 760mµ. For
example, chlorophyll absorbs blue and red light. Usually plants show high rate of photosynthesis in the blue and red
light. Maximum photosynthesis has been observed in red light than in blue light. The green light has minimum
effect. On the other hand, red algae shows maximum photosynthesis in green light and brown algae in blue light.
(c) Duration of light : Longer duration of light period favours photosynthesis. Generally, if the plants get 10
to 12hrs light per day it favours good photosynthesis. Plants can actively exhibit photosynthesis under continuous
light without being damaged. Rate of photosynthesis is independent of duration of light.
(ii) Temperature : The optimum temperature for photosynthesis is 20 to 35°C. If the temperature is increased
too high, the rate of photosynthesis is also reduced by time factor which is due to denaturation of enzymes involved
in the process. Photosynthesis occurs in conifers at high altitudes at 35°C. Some algae in hot springs can undergo
photosynthesis even at 75°C.
(iii) Carbon dioxide : Carbon dioxide present in the atmosphere is about 0.032% by volume and it is really
a low concentration which acts as limiting factor in nature. If we increase the amount of CO2 under laboratory
conditions and if the light and temperature are not the limiting factors, the rate of photosynthesis increases. This
increase is observed upto 1% of CO2 concentration. At the same time very high concentration of CO2 becomes
toxic to plants and inhibit photosynthesis.
(iv) Water : Water is an essential raw material in photosynthesis. This rarely, acts as a limiting factor because
less than 1% of the water absorbed by a plant is used in photosynthesis. However, lowering of photosynthesis has
been observed if the plants are inadequately supplied with water.
(v) Oxygen : Excess of O2 may become inhibitory for the process. Enhanced supply of O2 increases the rate
of respiration simultaneously decreasing the rate of photosynthesis by the common intermediate substances. The
concentration for oxygen in the atmosphere is about 21% by volume and it seldom fluctuates. O2 is not a limiting
factor of photosynthesis. An increase in oxygen concentration decreases photosynthesis and the phenomenon is
called Warburg effect. (Reported by German scientist Warburg (1920) in Chlorella algae).
This is due to competitive inhibition of RuBP-carboxylase by increased O2 levels, i.e., O2 competes for active
sites of RuBP-carboxylase enzyme with CO2. The explanation of this problem lies in the phenomenon of
photorespiration. If the amount of oxygen in the atmosphere decreases then photosynthesis will increase in C3
cycle and no change in C4 cycle.
Chapter 19 337
Photosynthesis in Higher Plants
(vi) Pollutants and Inhibitors : The oxides of nitrogen and hydrocarbons present in smoke react to form
peroxyacetyl nitrate (PAN) and ozone. PAN is known to inhibit Hill reaction. Diquat and Paraquat (Commonly
called as Viologens) block the transfer of electrons between Q and PQ in PS. II. Other inhibitors of photosynthesis
are monouron or CMU (Chlorophenyl dimethyl urea) diuron or DCMU (Dichlorophenyl dimethyl urea), bromocil
and atrazine etc. which have the same mechanism of action as that of viologens.
At low light intensities potassium cyanide appears to have no inhibiting effect on photosynthesis.
(vii) Minerals : Presence of Mn++ and Cl– is essential for smooth operation of light reactions (Photolysis of
water/evolution of oxygen) Mg++, Cu++ and Fe++ ions are important for synthesis of chlorophyll.
(2) Internal factors
(i) Protoplasmic factors : There is some unknown factor which affect the rate of photosynthesis.
These factors effect the dark reactions. The decline in the rate of photosynthesis at temperature above 30°C or
at strong light intensities in many plants suggests the enzymatic nature of this unknown factor.
(ii) Chlorophyll content : Chlorophyll is an essential internal factor for photosynthesis. The amount of CO2
fixed by a gram of chlorophyll in an hour is called photosynthetic number or assimilation number. It is
usually constant for a plant species but rarely it varies. The assimilation number of variegated variety of a species
was found to be higher than the green leaved variety. Emerson (1929) also found a direct relationship between
chlorophyll contents and photosynthetic rate in Chlorella.
(iii) Accumulation of products : The food is largely prepared in the mesophyll cells of the leaf from where it
is translocated to storage regions. If the rate of translocation becomes slower than the rate of manufacture, the
former declines due to accumulation of end products.
(iv) Structure of leaves : The amount of CO2 that reaches the chloroplast depends on structural features of
the leaves like the size, position and behaviour of the stomata and the amount of intercellular spaces. Some other
characters like thickness of cuticle, epidermis, presence of epidermal hairs, amount of mesophyll tissue, etc.,
influence the intensity and quality of light reaching in the chloroplast.
Significance of photosynthesis.
(1) Synthesis of food : Body of all living organism and their survival is dependent upon foods
(Carbohydrates, fats and proteins). They need energy for different life activities which is derived from foods. Green
plants are unique in the character that they are able to synthesize foods for all living beings.
(2) Purification of atmosphere : By oxidation of carbohydrates, fats and proteins CO2 is released along
with energy. Coal, petrol and many other type of oils release CO2 when they are used in different industries. CO2 so
released is added to the atmosphere and would have proved harmful to living organisms, but in photosynthesis
green plants take in CO2 and release O2 thus purifying the air.
(3) Conversion of radiant energy : It changes radiant energy into chemical energy. All organisms use
chemical energy for their activities.
Chapter 19 338
Photosynthesis in Higher Plants
(4) Plant products : A number of useful products are obtained from plants, they are synthesized by plants
through photosynthesis. Important plant products are fire wood, timber, oils, gums resins, rubber, cork, tannins,
alkaloids or drugs, fibres, etc.
(5) Productivity : Rising of photosynthetic capacity will reduce the effect of excess carbon dioxide
generation. It will increase crop productivity for feeding the rising human and cattle population. Therefore, methods
of photosynthetic enhancement are being studied.
Experiments. Blue
Green
Experiment : 1
Red
Ganong's light screen : This simple experiment confirms
that light is necessary for photosynthesis. It is a metallic structure
with a specific cut out. When a destarched leaf is covered by the
screen and placed in sunlight, photosynthesis occurs. The leaf is
then taken out, treated with ethanol and then with iodine. Only the
exposed parts of the leaf turn blue. The covered parts remain
unstained as no starch could be formed there due to non-
availiability of light.
Experiment : 2 Fig : Ganong’s light screen to study the effect of
Moll's half leaf experiment : This experiment is designed light on the photosynthesis
to prove that CO2 is necessary for photosynthesis. A plant is destarched by keeping in dark. A leaf of this plant is
half inserted in a vial or bottle containing some KOH solution through a split cork. If the leaf is detached, its petiole
should be dipped in a petridish containing water. The apparatus is kept in sunlight. After a few hours the leaf is
taken out and put in ethanol for removing chlorophyll. It is, then treated with iodine. The part of the leaf lying
outside the cork is stained blue confirming the occurrence of photosynthesis in that region. The part of the leaf that
lies inside the vial remains unstained because no photosynthesis occurs in that part due to non-availability of CO2.
The part which lies in between the cork pieces also remains unstained because it neither gets light nor CO2.
Split cork Inside Within Outside
cork
Wide mouth
bottle
KOH
solution
Potted plant
(destarched)
Unstained Stained
B Green or
A chlorophyllous Bluish
portion portion
Non-green or Colourless
portion
colourless
portion
Chapter 19 339 (A) (B)
Fig : Experiment to show that chlorophyll is
essential for photosynthesis (A) Variegated leaf
before experiment, (B) Variegated leaf after
treatment with iodine solution
Photosynthesis in Higher Plants
Experiment : 3
To show that chlorophyll is necessary for photosynthesis : Select a potted Croton or Coleus plant
having variegated leaves. Select a few young leaves and sketch the extent of the green as well as other colours of
these leaves on a piece of paper. Place the pot in the sunlight for a few hours and then take starch test. Only
chlorophyll containing cells give positive starch test.
Experiment : 4
To show that oxygen is evolved in photosynthesis by green plants : Water-weeds like Hydrilla or
Ceratophyllum are best for this experiment. Take some water-weeds and Sunlight
cut the bases of the plant and tie them with a thread. Put them in a Oxygen
beaker containing water and invert a funnel over them as shown in the
figure. Fill a test tube with water and invert it over the nozzle of the funnel Test tube
so that no air-bubble gets in. Expose the whole apparatus to light.
It is seen that some bubbles come out continuously and are Water
collected at the top of test tube by displacing the water.
Oxygen
On testing the gas it is found to be oxygen. This evolved oxygen bubbles
is produced by the green aquatic plant in the process of Funnel
photosynthesis.
Experiment : 5 Plant
To show that starch is formed in photosynthesis : Detach
a destarched green leaf from a plant. Boil the leaf in water and then Fig : Liberation of oxygen in photosynthesis by an
aquatic plant
boil in 70% ethyl alcohol for 15 minutes (In a water bath). The leaf
becomes colourless as chlorophyll gets dissolved in alcohol. Wash the
leaf with water and test it with iodine solution. It gives negative starch test. Now keep the plant in light for 8 hours
and again test it for starch. The leaf becomes bluish black or bluish-purple indicating the presence of starch.
Important Tips
• Photosynthetic Materials : 264 gm of CO2 and 216 gm of water give rise to 108 gm of water, 192 gm of O2 and 180 gm of glucose.
• Rubisco : Rubisco constitutes 16% of chloroplast protein. It is the most abundant protein on this planet.
• Actual reduction of CO2 to carbohydrates is independent of light, i.e., occurs in presence or absence of light, but production of
assimilatory powers (ATP and NADPH2) needs light and is light dependent.
• Willmott's bubbler is used to measure rate of O2 evolution or rate of photosynthesis.
• T.W. Engelmann (1882) experimentally verified that in monochromatic lights, photosynthesis is maximum in red light.
• Cyclic photophosphorylation is the most effective anaerobic phosphorylation mechanism.
• NADP (Nicotinamide adenine dinucleotide phosphate) was earlier called as TPN (Triphosphopyridine nucleotide),
• In green plants the hydrogen acceptor is NADP, but in bacteria it is NAD.
• No Emerson effect is seen in bacteria.
• NAD is considered to be the "Universal hydrogen acceptor".
• Non-cyclic photophosphorylation or Z-scheme is inhibited by CMU and DCMU.
• As Calvin cycle takes in only one carbon (as CO2) at a time, so it takes six turns of the cycle to produce a net gain of six carbons (i.e.,
hexose or glucose).
• Cytochromes : The terms was coined by Keilin (1925) though the biochemicals were discovered by Mac Munn (1866).
Chapter 19 340
Photosynthesis in Higher Plants
• Intensity of light can be measured by Luxmeter.
• Isolated chlorophyll 'a' in pure form emits red colour. It is called fluoresence.
• Phytochrome is a proteinaceous pigment found in low concentrations in most plant organs. Which absorbs red (PR or P660) and far red
(PfR or P730) light.
• Anthoxanthins and Anthocyanin pigments are also soluble in water and found in cell sap, due to which white, yellow and orange colour
produce in flowers.
Chapter 19 341
BOTANY
CHAPTERS 20-23
BOOKLET-4
Contents: Page No.
Chapter 20 Respiration in Plants
Chapter 21 Plant Growth and Development Part 1
Chapter 22 Plant Growth and Development Part 2
Chapter 23 Sexual Reproduction in Flowering Plants
Respiration in Plants
Introduction.
All organisms require continuous input of energy to carry on life process. These energy comes from cellular
activities. All the cellular activities can be grouped into two categories : anabolism (biosynthetic activities of the
cell) and catabolism (breaking- up process of the cell). The anabolic activities are endergonic (utilizes energy in
cellular activities), while the catabolic activities are usually exergonic (energy releasing process by oxidation of food
material). The sum of total catabolic and anabolic reactions occurring at any time in a cell is called metabolism.
Respiration is a vital process, includes the intake of oxygen. Chemically it is catabolic and brings about the
oxidation and decomposition of organic compounds like carbohydrate, fat, protein in the cells of plants and animals
with the release of energy. Oxidation of organic compounds by respiration, resulting in the release of chemical
energies water and carbon dioxide. The overall process may be states according to the following general equation:
C6 H12O6 + 6CO2 enzymes → 6CO2 + 6H2O+ energy
glucose carbondioxide Water (ATP)
In this reaction, six molecules of oxygen taken up and six molecules each of CO2 and H2O are formed with
energy derived from respiration of each molecule of sugar oxidation. The plant cell is able to do chemical work in
synthesizing energy- rich materials such as fat and hydrocarbon, osmotic work such as uptake and accumulation of
salt and mechanical work such as involved in growth.
Respiration
Definition of respiration : Cellular respiration is an enzyme controlled process of biological oxidation of
food materials in a living cell, using molecular O2, producing CO2 and H2O, and releasing energy in small steps and
storing it in biologically useful forms, generally ATP.
(1) Use of energy : Cellular activities like active transport, muscle-contraction, bioluminescenes,
homothermy locomotion, nerve impulse conduction, cell division, growth, development, seed germination require
energy. Main source of energy for these endergonic activities in all living organisms including plants, comes from the
oxidation of organic molecules. Reactions consuming
energy
Reactions releasing
energy
Glucose Inorganic ATP Synthesis of proteins, lipids, carbohydrates
Lipids phosphate P Osmotic work
Pi Growth, differentiation and development
Energy Active absorption
Energy
Proteins Cyclosis
Translocation
ADP
Fig. ATP cycle : ATP is an intermediate energy-transfer compound
between energy-releasing and energy consuming reactions
The energy released by oxidation of organic molecules is actually transferred to the high energy terminal
bonds of ATP, a form that can be readily utilized by the cell to do work. Once ATP is formed, its energy may be
utilized at various places in the cell to drive energy- requiring reactions. In these processes, one of the three
phosphate groups is removed from the ATP molecule. Thus the role of ATP as an intermediate energy transforming
compound between energy releasing and energy consuming reactions.
(2) Significance of respiration : Respiration plays a significant role in the life of plants. The important ones
are given below :
Chapter 20 342
Respiration in Plants
(i) It releases energy, which is consumed in various metabolic process necessary for life of plant.
(ii) Energy produced can be regulated according to requirement of all activities.
(iii) It convert in soluble foods into soluble form.
(iv) Intermediate products of cell respiration can be used in different metabolic pathways e.g.
Acetyl- CoA (in the formation of fatty acid, cutin and isoprenoids) ; α - ketoglutaric acid (in the formation of
glutamic acid) ; Oxaloacetic acid (in the formation of aspartic acid, pyrimidines and alkaloids); Succinyl- CoA
(synthesis of pyrrole compounds of chlorophyll).
(v) It liberates carbon dioxide, which is used in photosynthesis.
(vi) Krebs cycle is a common pathway of oxidative breakdown of carbohydrates, fatty acids and amino acids.
(vii) It activates the different meristematic tissue of the plant.
(3) Comparison between respiration and photosynthesis : Photosynthesis associated with
manufacturing of food, while respiration associated with releasing of energy from this food. Comparison between
respiration and photosynthesis is given below :
Photosynthesis Respiration
Occurs only in chlorophyll containing cells of plants. Occurs in all plant and animal cells.
Takes place only in the presence of light. Takes place continually both in light and in the dark.
During photosynthesis, radiant energy is converted into potential During respiration, potential energy is converted into kinetic
energy. energy.
Sugars, water and oxygen are products. CO2 and H2O are products.
Synthesizes foods. Oxidizeds foods.
CO2 and H2O are raw materials. O2 and food molecules are raw materials.
Photosynthesis is an endothermal process. Respiration is an exothermal process.
Stores energy. Releases energy.
It includes the process of hydrolysis, carboxylation etc. It includes the process of the dehydrolysis, decarboxylation, etc.
Results in an increase in weight. Results in a decrease in weight.
It is an anabolic process. It is a catabolic process.
Require cytochrome. Also require cytochrome.
(4) Exchange of gases in photosynthesis and respiration : Respiration is continually going on in all
living cells and oxygen is being continually absorbed and carbon dioxide liberate. The intake of oxygen (Liberated
by photosynthesis) and liberation of carbon dioxide (evolved in respiration) takes place through the stomata and
lenticels. The real process of respiration consists in the oxidation of organic substances which takes place in the
protoplasm of the living cells and the gaseous exchange is an outward manifestation and an accompaniment of
respiration. The intensity of gaseous exchange depends upon the intensity of respiration. It is comparatively rapid in
meristematic and growing tissues where the formation of new cells and cell wall material requires a large supply of
energy and is comparatively slow in mature cells due to the slowness of metabolic activities.
Chapter 20 343
Respiration in Plants
CO2
Photosynthesis
6CO2+6H2O+Energy
C6H12O6+6O2
Respiration Atmosphere
C6H12O6+6O2
6CO2+6H2O+Energy
Fig. Showing gas exchange due to
photosynthesis and respiration
Compensation point : It is that value or point in light intensity and atmospheric CO2 concentration when
rate of photosynthesis is just equivalent to the rate of respiration in photosynthetic organs so that there is no net
gaseous exchange. The value is 2.5- 100 ft candles/ 26.91-1076.4 lux in shade plants and 100-400 ft candles/
1076.4-4305.6 lux in case of sun plants. It is called light compensation point. There is, similarly, a
CO2 compensation point. Its value is 25-100 ppm (25-100 µl.l−1 ) in C3 plants and 0-5 ppm (0-5 µl.l −1 ) in C4
plants. A plant cannot survive for a long at compensation point because the nonphotosynthetic parts and dark
respiration will deplete organic reserve of the plant.
CO2 intake in photosynthesis balanced with CO2 release in respiration = Compensation point.
(5) Comparison between respiration and combustion : According Lavosier cell respiration resembles
the combustion (e.g., burning of coal, wood, oil etc.) in the breakdown of complex organic compounds in the
presence of oxygen and production of carbon dioxide and energy, but there are certain fundamental differences
between the two processes:
Differences between cell respiration and combustion
S.No. Characters Cell respiration Combustion
(i) Nature of process Biochemical and stepped process. Physico-chemical and spontaneous process.
(ii) Site of occurrence Inside the cells. Non-cellular.
(iii) Control Biological control. Uncontrolled.
(iv) Energy release Energy released in steps. Large amount of energy is released at a time.
(v) Temperature Remain within limits. Rises very high.
(vi) Light No light is produced. Light may be produced.
(vii) Enzymes Controlled by enzymes. Not controlled by enzymes.
Intermediates A number of intermediates are produced. No intermediate is produced.
(viii)
Phases of respiration.
There are three phases of respiration :
Chapter 20 344
Respiration in Plants
(1) External respiration : It is the exchange of respiratory gases (O2 and CO2) between an organism and its
environment.
(2) Internal or Tissue respiration : Exchange of respiratory gases between tissue and extra cellular
environment .
Both the exchange of gases occur on the principle of diffusion.
(3) Cellular respiration : It is an enzymatically-controlled stepped chemical process in which glucose is
oxidised inside the mitochondria to produce energy-rich ATP molecules with high-energy bonds.
So, respiration is a biochemical process.
Respiratory substrate or Fuel.
In respiration many types of high energy compounds are oxidised. These are called respiratory substrate or
respiratory fuel and may include carbohydrates, fats and protein.
(1) Carbohydrate : Carbohydrates such as glucose, fructose (hexoses), sucrose (disaccharide) or starch,
insulin, hemicellulose (polysaccharide) etc; are the main substrates. Glucose are the first energy rich compounds to
be oxidised during respiration. Brain cells of mammals utilized only glucose as respiratory substrate. Complex
carbohydrates are hydrolysed into hexose sugars before being utilized as respiratory substrates. The energy present
in one gram carbohydrate is – 4.4 Kcal or 18.4 kJ.
(2) Fats : Under certain conditions (mainly when carbohydrate reserves have been exhausted) fats are also
oxidised. Fat are used as respiratory substrate after their hydrolysis to fatty acids and glycerol by lipase and their
subsequent conversion to hexose sugars. The energy present in one gram of fats is 9.8 Kcal or 41kJ, which is
maximum as compared to another substrate.
The respiration using carbohydrate and fat as respiratory substrate, called floating respiration (Blackmann).
(3) Protein : In the absence of carbohydrate and fats , protein also serves as respiratory substrate. The energy
present in one gram of protein is : 4.8 Kcal or 20 kJ. when protein are used as respiratory substrate respiration is
called protoplasmic respiration.
Types of respiratory organism.
Organism can be grouped into following four classes on the basis of their respiratory habit -
(1) Obligate aerobes : These organisms can respire only in the presence of oxygen. Thus oxygen is essential
for their survival.
(2) Facultative anaerobes : Such organisms usually respire aerobically (i.e., in the presence of oxygen) but
under certain condition may also respire anaerobically (e.g., Yeast, parasites of the alimentary canal).
(3) Obligate anaerobes : These organism normally respire anaerobically which is their major ATP- yielding
process. Such organisms are in fact killed in the presence of substantial amounts of oxygen (e.g., Clostridium
botulinum and C. tetani).
(4) Facultative aerobes : These are primarily anaerobic organisms but under certain condition may also
respire aerobically.
Types of respiration.
Chapter 20 345
Respiration in Plants
On the basis of the availability of oxygen and the complete or incomplete oxidation of respiratory substrate,
the respiration may be either of the following two types : Aerobic respiration and Anaerobic respiration
Aerobic respiration
It uses oxygen and completely oxidises the organic food mainly carbohydrate (Sugars) to carbon dioxide and
water. It therefore, releases the entire energy available in glucose.
C6H12O6 + 6O2 enzymes → 6CO2 + 6H2O + energy (686 Kcal)
It is divided into two phases : Glycolysis, Aerobic oxidation of pyruvic acid
Glycolysis / EMP pathway
(1) Discovery : It is given by Embden, Meyerhoff and Parnas in 1930. It is the first stage of breakdown of
glucose in the cell.
(2) Definition : Glycolysis ( Gr. glykys= sweet, sugar; lysis= breaking) is a stepped process by which one
molecule of glucose (6c) breaks into two molecules of pyruvic acid (3c).
(3) Site of occurrence : Glycolysis takes place in the cytoplasm and does not use oxygen. Thus, it is an
anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.
(4) Inter conversions of sugars : Different forms of carbohydrate before entering in glycolysis converted
into simplest form like glucose, glucose 6-phosphate or fructose 6-phosphate. Then these sugars are metabolized
into the glycolysis. The flow chart that showing inter conversion of sugar are given below :
Starch UDPG Sucrose
+
UDP
Mannose Glucose Fructose Starch Galactose
+ATP
+ATP +ATP +H3PO4 +ATP
hexokinase hexokinase hexokinase Phosphorylase hexokinase
Mannose Glucose Galactose
6-phosphate 1-phosphate 6-phosphate
Isomerase Isomerase Phosphoglucomutase
Glucose Fructose Glucose
6-phosphate
6-phosphate 6-phosphate
+ATP
Phosphohexokinase
Fructose
1,6-phosphate
To glycolysis
Fig : Schematic conversion of complex carbohydrates before entering
into glycolysis
Chapter 20 346
Respiration in Plants
(5) Glycolysis cycle
Glucose (6c sugar)
ATP Hexokinase (– 1 ATP) 1. Phosphorylation
ADP 2. Isomerisation
Glucose-6-phosphate
(6c sugar)
Phosphoglucoisomerase
First phase : Fructose-6-phosphate
(6c sugar)
Phosphorylation of
glucose and its ATP Phosphofructokinase (– 1 ATP) 3. Phosphorylation
conversion into ADP 4. Cleavage
glyceraldehyde
Fructose-1-6-diphosphate
3-phosphate (6c sugar)
Fructose diphosphate aldolase
Dihydroxyacetone phosphate Lysis
(3c sugar)
Glyceraldehyde-3-phosphate
Phosphotriose isomerase
2×Glyceraldehyde-3-phosphate
(3 carbon)
2P(from H3PO4)
2NAD Glyceraldehyde phosphate 5. Phosphorylation
dehydrogenase and Dehydrogenation
2NADH+2H+
2×1.3-Diphosphoglycerate (3 carbon)
2ADP Phosphoglycerate (+2ATP) 6. Dephosph-
2ATP kinase orylation
Second phase :
2×3-Phosphoglycerate (3 carbon) 7. Rearrangement
Conversion of
glyceraldehyde Phosphoglycerate
3-phosphate into mutase
pyruvate and couple
2×2-Phosphoglycerate
formation of ATP Enolase
8. Dehydration
2×Phosphoenol pyruvate (3 carbon)
2ADP Pyruvate (+2 ATP) 9. Dephosphorylation
2ATP kinase Net gain = 2 ATP
2×Pyruvate (3 carbon)
Fig : Glycolysis: A molecule of glucose breaks into two molecules of pyruvate in nine steps. Enzymes that catalyze the
reactions 1-9 are sequentially listed on the right.
(6) Enzymes of glycolysis and their co-factors
Chapter 20 347
Respiration in Plants
S. No. Enzyme Coenzyme (s) and Activator (s) Inhibitor (s) Kind of reaction
cofactor catalyzed
(i) Hexokinase Mg2+ ATP4-, Pi Glucose 6-phopshate Phosphoryl transfer
(ii) Phosphogluco-isomerase Mg2- - 2-dioxyglucose Isomerization
6-phosphate
(iii) Phosphofructo-kinase Mg2+ Fructose 2, 6- ATP 4-, citrate Phosphoryl transfer
diphosphate, AMP,
ADP, cAMP, K+
(iv) Aldolase Zn2+ - Chelating agents Aldol cleavage
( in microbes)
(v) Phosphotriose isomerase Mg2+ - - Isomerization
(vi) Glyceraldehyde NAD - Iodoacetate Phosphorylation
3-phosphate dehydrogenase coupled to
oxidation
(vii) Phosphoglycerate kinase Mg2+ - - Phosphoryl transfer
(viii) Phosphoglycerate mutase Mg2+ 2,3-diphos - - Phosphoryl shift
phoglycerate
(ix) Enolase Mg2+ , Mn2+, Zn2+, - Fluoride+ phosphate Dehydration
Cd2+
(x) Pyruvate kinase Mg2+, K+ - Acetyl CoA, analine, Phosphoryl transfer
Ca2+
(7) Steps of glycolysis : Glycolysis consists of 9 steps. Each step is catalysed by a specific enzyme. Most of
the reaction are reversible.
(i) First phosphorylation : The third phosphate group separates from adenosine triphospate (ATP)
molecule, converting the latter into adenosine diphophate (ADP) and releasing energy. With this energy, the
phosphate group combines with glucose to form glucose 6-phosphate, The reaction is catalysed by the enzyme,
hexokinase or glucokinase in the presence of Mg2+. Thus, a molecule of ATP is consumed in this step. This
glucose 6-phosphate (phosphoglucose) is called active glucose.
Glucose + ATP Hexokinase → Glucose 6 − phosphate + ADP
Mg + +
(ii) Isomerisation : Glucose 6-phophate is changed into its isomer fructose 6-phophate by rearrangement.
The rearrangement is catalysed by an enzyme, phophoglucose-isomerase or phosphohexose isomerase.
Glucose 6-phosphate Phosphogluco Fructose 6-phosphate
isomerase
Fructose 6-phosphate may be formed directly from free fructose by its phosphorylation in the presence of an
enzyme fructokinase, Mg 2+ and ATP
Fructose + ATP Fructokinase → Fructose 6 − phosphate + ADP
Mg 2+
(iii) Second phosphorylation : Fructose 6-phosphate combines with another phosphate group from another
ATP molecule, yielding fructose 1, 6-diphosphate and ADP , The combination is catalysed by an enzyme
Chapter 20 348
Respiration in Plants
phosphofructokinase in the presence of Mg2+ and appears to be irreversible. This phosphorylation, thus,
consume another molecule of ATP. Excess of ATP inhibits phosphofructokinase.
Fructose 6 − phosphate + ATP Phosphofructo− → Fructose 1,6 − diphosphate + ADP
kinase, Mg 2+
phosphorylation reaction activate the sugar and prevent its excape from the cell. They go uphill, increasing the
energy content of the products.
(iv) Cleavage : Fructose 1,6-diphosphate now splits into two 3-carbon, phosphorylated sugars :
dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (3-PGAL), or glyceraldehyde 3-phosphate
(GAP). The reaction is catalyzed by an enzyme aldolase. DHAP is converted into PGAL with the aid of an enzyme
phosphotriose isomerase. Aldolase 3-phophoglyceraldehyde+Dihydroxyacetone phosphate
Fructose 1,6-diphosphate
Dihydroxyacetone phosphate Phosphotriose 3- phosphoglyceraldehyde
isomerase
(v) Phosphorylation and Oxidative dehydrogenation: In phosphorylation, 3-phosphoglyceraldehyde
combines with a phosphate group derived from inorganic phosphoric acid (H3PO4) found in cytosol, not form ATP,
forming1, 3-diposphoglycerate, or diphosphoglyceric acid. The reaction occurs with the aid of a specific enzyme.
(a) In dehydrogenation, a pair of hydrogen atom separate from a molecule of 3-phosphoglyceraldehyde. Their
separation releases a large amount of energy. A part of this energy is stored in newly formed phosphate bond of
1,3-diphosphoglycerate, making it a high energy bond. Separation of hydrogen is catalysed by an enzyme, 3-
phosphoglyceraldehyde dehydrogenase.
(b) As stated above, two hydrogen (H) atoms (2 proton and 2 electrons) separate from 3-
phosphoglyceraldehyde. Of these, one complete hydrogen atom (proton and electron) and one additional electron
are picked up by NAD+ which gets reduced to NADH. The remaining one hydrogen proton or ion (H+) remains
free in the cytosol.
2H + + 2e − + NAD + → NADH + H +
NADH is a high-energy substance, carrying the rest of the energy released by separation of hydrogen atoms
from 3- PGAL. Energy is actually released by transfer of electrons from 3-PGAL to NAD. The NADH provides
energy to convert ADP to ATP by passing its electrons over the electron transmitter system if oxygen is available.
The overall reaction is as under –
3 − PGAL + NAD+ + Pi 2 − 3−Phosphoglycer− →1,3 − diphosphoglycerate + NADH + H+
aldehyde dehydrogenase
(vi) Dephosphorylation or ATP generation (First) : High-energy phosphate group on carbon 1 of 1,3
diphosphoglycerate is transferred to a molecule of ADP, converting it into an ATP molecule. 1, 3-
diphosphoglycerate changes to 3-phosphoglycerate due to loss of a phosphate group. The reaction is catalysed by
an enzyme diphosphoglycerokinase. Formation of ATP directly from metabolites is known as substrate level
phophorylation.
1, 3-diphosphoglycerate +ADP Diphosphoglycero- 3-phosphoglycerate + ATP
kinase + Mg 2+
Chapter 20 349
Respiration in Plants
(vii) Isomerisation/ Rearrangement : The phosphate group on the third carbon of 3-phosphoglycerate
shifts to the second carbon, producing 2-phosphoglycerate. This change is aided by the enzyme
phosphoglyceromutase.
3-phosphoglycerate Phosphoglycero- 2-phosphoglycerate
mutase
(viii) Dehydration : 2-phosphoglycerate loses a water molecule in the presence of an enzyme, enolase and
Mg2+, and changes into phosphoenol pyruvate. The latter undergoes molecular rearrangement that transforms its
phosphate group into a high-energy phosphate bond.
2-phosphoglycerate Enolase Phosphoenol pyruvate +H2O
Mg 2+
(ix) Dephosphorylation or ATP generation (Second) : High-energy phosphate group of phosphoenol
pyruvate is transferred to a molecule of ADP with the help of an enzyme, pyruvate kinase in the presence of
Mg2+ and K+. This produces simple 3-carbon pyruvate and a molecule of ATP.
~ phosphoenol pyruvate +ADP Pyruvate kinase Pyruvate +ATP
Mg 2+, K+
All enzymes, reactants, intermediates and products of glycolysis are dissolved in the cytosol. Their interaction
depends on random collisions brought about by kinetic movements.
(8) Special features of glycolysis : The special features of glycolysis can be summarised as follows :
(i) Each molecule of glucose produces 2 molecules of pyruvic acid at the end of the glycolysis.
(ii) The net gain of ATP in this process is two ATP molecules (four ATPs are formed in glycolysis but two of
them are used up in the reaction).
(iii) During the conversion of 1, 3-diphosphoglyceraldehyde into 1, 3-diphosphoglyceric acid one molecule of
NADH2 is formed. As each molecule of glucose yields two molecules of 1,3-diphosphoglyceric acid, hence, each
molecule of glucose forms 2 molecules of NADH2.
(iv) During aerobic respiration (when oxygen is available) each NADH2 forms 3 ATP and H2O through
electron transport system of mitochondria. In this process ½ O2 molecule is utilized for the synthesis of each water
molecule.
In this way during aerobic respiration there is additional gain of 6 ATP in glycolysis
2ATP+ 6 ATP → 8 ATP
(net gain) (addition gain) (total net gain)
(v) Reaction of glycolysis do not require oxygen and there is no output of CO2.
(vi) Overall reaction of glycolysis represented by following reaction :
C6 H12O6 → 2C3 H 4 O3 + 4 H
Pyruvate
(vii) Total input and output materials in glycolysis :
Total Inputs Total Outputs
Chapter 20 350
Respiration in Plants
1 molecule of glucose (6 C) 2 molecules of pyruvate (2×3 C)
2 ATP 4 ATP
4 ADP 2 ADP
2 × NAD + 2× NADH + 2H+
2 Pi 2×H2O
Important Tips
• Lavosier (1783) found that respiration in animals involves intake of O2 and liberation of CO2. Dutrochet is belived to have used the
term of respiration for the first time, while book "cellular respiration" was written by Meldrum.
• Energesis : An old term of respiration.
• Glucose oxidation is very rapid process of complete oxidation of a glucose molecules takes only one second.
• Only 5% of total energy of glucose is released during glycolysis.
• Utility of phosphorylation during glycolysis : It traps glucose with in the cell as glucose 6-p is negatively charged.
• Splitting of fructose 1,6-diP into 3-PGAL and dihydroxyacetone P is called rate determining step of glycolysis.
• Glucose 6-phosphate called Rohinsonester, fructose 6-phosphate called Newberg's ester and fructose 1,6-diphosphate called Harden
and Young's ester.
• R.B.Cs gets energy only by glycolysis because they lacks mitochondria.
• Phosphofructokinase called regulatory enzyme of glycolysis, it is inhibited by high concentration of ATP and is stimulated by ADP and
Pi.
• Preparatory phase of glycolysis involves conversion of one molecule of glucose into two molecules of 3-PGAL and involves the
use of 2 ATP molecules, while pay-off phase of glycolysis involves conversion of 2 molecules of 3-PGAL into two molecules of
pyruvate and involves production of four ATP molecules. Preparatory phase causes activation of glucose, while pay-off phase involves
extraction of energy from the activated glucose.
• Formation of 1,3-diphosphoglyceraldehyde called non enzymatic phosphorylation.
Aerobic oxidation of pyruvic acid
(1) Oxidative decarboxylation/ Formation of acetyl CoA.
(2) Kreb's cycle/TCA cycle/Citric acid cycle.
(3) Electron transport system
(1) Oxidative decarboxylation of pyruvic acid : If sufficient O2 is available, each 3-carbon pyruvate
molecule (CH3COCOOH) enters the mitochondrial matrix where its oxidation is completed by aerobic means. It is
called gateway step or link reaction between glycolysis and Kreb's cycle. The pyruvate molecule gives off a molecule
of CO2 and releases a pair of hydrogen atoms from its carboxyl group (–COOH), leaving the 2 carbon acetyl group
(CH3CO–). The reaction is called oxidative decarboxylation, and is catalyzed by the enzyme pyruvate
dehydrogenase complex (decarboxylase, TPP, lipolic acid, transacetylase, Mg2+) . During this reaction, the acetyl
group combines with the coenzyme A (CoA) to form acetyl coenzyme A with a high energy bond (CH3CO~CoA).
Most of the free energy released by the oxidation of pyruvate is captured as chemical energy in high energy bond of
acetyl coenzyme A. From a pair of hydrogen atoms released in the reaction, to electrons and one H+ pass to NAD+,
forming, NADH+ H+ . The NADH forms 3 ATP molecules by transferring its electron over ETS described ahead.
Decarboxylation and dehydration :
Chapter 20 351
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