8.3 phOTOsynThesis
The proton gradient
Excited electrons from Photosystem II are used to
generate a proton gradient.
Once plastoquinone transers its electrons, the electrons are then passed
rom carrier to carrier in this chain. As the electrons pass, energy is
released, which is used to pump protons across the thylakoid membrane,
into the space inside the thylakoids.
A concentration gradient o protons develops across the thylakoid
membrane, which is a store o potential energy. Photolysis, which
takes place in the fuid inside the thylakoids, also contributes to the
proton gradient.
stroma Photosystem II cytochrome N ADP+
(low H+ concentration) reductase
2 H+ complex light Photosystem I
light Fd
NADP+ + H+
NADPH
Pq Pc
thylakoid space H2O 1 O2
2
(high H+ concentration) 2 H+
+2 H+
to Calvin
cycle
stroma thylakoid ATP synthase
(low H+ concentration) membrane
ADP +
P1 ATP
H+
Figure 5
Chemiosmosis
ATP synthase in thylakoids generates ATP using the
proton gradient.
The protons can travel back across the membrane, down the
concentration gradient, by passing through the enzyme ATP synthase.
The energy released by the passage o protons down their concentration
gradient is used to make ATP rom AD P and inorganic phosphate. This
method o producing ATP is strikingly similar to the process that occurs
inside the mitochondrion and is given the same name: chemiosmosis.
When the electrons reach the end o the chain o carriers they are passed
to plastocyanin, a water-soluble electron acceptor in the fuid inside
the thylakoids. Reduced plastocyanin is needed in the next stage o
photosynthesis.
393
8 M ETABOLI SM , CELL RESPIRATION AN D PHOTOSYN TH ESI S ( AH L)
Data-based questions: Evidence for chemiosmosis
One o the rst experiments to give evidence or protons diused into the space inside the thylakoids,
ATP production by chemiosmosis was perormed until the concentrations inside and outside were
in the summer o 1 966 by Andr Jagendor. equal. The thylakoids were then transerred, still
Thylakoids were incubated or several hours in darkness, into a solution o ADP and phosphate
in darkness, in acids with a pH ranging rom that was more alkaline. There was a brie burst o
3.8 to 5.2. The lower the pH o an acid, the higher ATP production by the thylakoids. The graph shows
its concentration o protons. During the incubation, the yield o ATP at three acid incubation pHs and a
range o pHs o the ADP solution.
1 a) Describe the relationship between pH o
ATP production /mol 3.8 ADP solution and ATP yield, when acid
incubation was at pH 3.8. [2]
b) Explain why the pH o the ADP solution
aects the ATP yield. [2]
4.8 2 Explain the eect o changing the pH o [2]
acid incubation on the yield o ATP. [2]
5.2 [2]
3 Explain why there was only a short burst
6.5 7.0 7.5 8.0 8.5 o ATP production.
pH of ADP solution
4 Explain the reason or perorming the
Figure 6 Results of Jagendorf experiment experiment in darkness.
uid in thylakoid uid outside Reduction of NADP
thylakoid membrane thylakoid
Excited electrons from Photosystem I are used to reduce NADP.
H2O Photosystem II
The remaining parts o the light-dependent reactions involve
2H+ 2e- Photosystem I. The useul product o these reactions is reduced NAD P,
plastoquinone which is needed in the light-independent reactions o photosynthesis.
1 O 2 Reduced NADP has a similar role to reduced NAD in cell respiration:
2 it carries a pair o electrons that can be used to carry out reduction
reactions.
plastocyanin electron
transport chain Chlorophyll molecules within Photosystem I absorb light energy and
pass it to the special two chlorophyll molecules in the reaction centre.
ferredoxin This raises an electron in one o the chlorophylls to a high energy level.
Photosystem II As with Photosystem II, this is called photoactivation. The excited
electron passes along a chain o carriers in Photosystem I, at the end
NADP o which it is passed to erredoxin, a protein in the fuid outside the
thylakoid. Two molecules o reduced erredoxin are then used to reduce
Figure 7 Summary of the light- NAD P, to orm reduced NAD P.
dependent reactions of photosynthesis
The electron that Photosystem I donated to the chain o electron carriers
is replaced by an electron carried by plastocyanin. Photosystems I and
II are thereore linked: electrons excited in Photosystem II are passed
along the chain o carriers to plastocyanin, which transers them to
Photosystem I. The electrons are re-excited with light energy and are
eventually used to reduce NAD P.
The supply o NADP sometimes runs out. When this happens the
electrons return to the electron transport chain that links the two
photosystems, rather than being passed to NAD P. As the electrons fow
394
8.3 phOTOsynThesis
back along the electron transport chain to Photosystem I, they cause ribulose
pumping o protons, which allows ATP production. This process is cyclic bisphosphate
photophosphorylation.
CO2
Carbon fxation
rubisco
In the light-independent reactions a carboxylase
catalyses the carboxylation of ribulose bisphosphate. 2 glycerate
3-phosphate
Carbon dioxide is the carbon source or all organisms that carry out
photosynthesis. The carbon xation reaction in which it is converted into 2ATP
another carbon compound is arguably the most important in all living
organisms. In plants and algae it occurs in the stroma the fuid that 2ADP +
surrounds the thylakoids in the chloroplast. The product o this carbon 2 phosphates
xation reaction is a three-carbon compound: glycerate 3-phosphate.
As so oten occurs in biological research, the details o the reaction were 2(NADPH + H+)
a surprise when they were discovered. Carbon dioxide does not react 2 N ADP+
with a two-carbon compound to produce glycerate 3-phosphate. Instead, 2 triose phosphate
it reacts with a ve-carbon compound called ribulose bisphosphate
(RuBP) , to produce two molecules o glycerate 3-phosphate. The enzyme Figure 8 Summary o carbon
that catalyses this reaction is called ribulose bisphosphate carboxylase, fxation reactions
usually abbreviated to rubisco. The stroma contains large amounts o
rubisco to maximize carbon xation.
The role o reduced NADP and ATP in the
Calvin cycle
Glycerate 3-phosphate is reduced to triose phosphate
using reduced NADP and ATP.
RuBP is a 5-carbon sugar derivative, but when it is converted to glycerate
3-phosphate by adding carbon and oxygen, the amount o hydrogen in
relation to oxygen is reduced. In sugars and other carbohydrates, the
ratio o hydrogen to oxygen is 2:1 . Hydrogen has to be added to glycerate
3-phosphate by a reduction reaction to produce carbohydrate. This
involves both ATP and reduced NADP, produced by the light-dependent
reactions o photosynthesis. ATP provides the energy needed to perorm
the reduction and reduced NADP provides the hydrogen atoms. The
product is a three-carbon sugar derivative, triose phosphate.
The ate o triose phosphate
Triose phosphate is used to regenerate RuBP and
produce carbohydrates.
The rst carbohydrate produced by the light-independent reactions o
photosynthesis is triose phosphate. Two triose phosphate molecules
can be combined to orm hexose phosphate and hexose phosphate can
be combined by condensation reactions to orm starch. However, i all
o the triose phosphate produced by photosynthesis was converted to
hexose or starch, the supplies o RuBP in the chloroplast would soon be
used up. Some triose phosphate in the chloroplast thereore has to be
395
8 M ETABOLI SM , CELL RESPIRATION AN D PHOTOSYN TH ESI S ( AH L)
TOK used to regenerate RuB P. This process is a conversion o 3 - carbon sugars
into 5-carbon sugars and it cannot be done in a single step. Instead a
To what extent is it acceptable to series o reactions take place.
adjust empirical evidence to conform
to theoretical prediction? As RuBP is both consumed and produced in the light-independent
reactions o photosynthesis, these reactions orm a cycle. It is called
One othe most amous experiments the Calvin cycle to honour Melvin Calvin, who was given the Nobel
in the history obiology is that othe Prize or Chemistry in 1 961 or his work in elucidating this process.
Flemish scientist Johannes Baptista For the C alvin cycle to continue indefnitely, as much RuB P must be
van Helmont, published in 1648. It produced as consumed. I three RUBP molecules are used, six triose
is regarded as the rst quantitative phosphates are produced. Five o these are needed to regenerate
biology experiment and also changed the three RuBP molecules. This leaves just one triose phosphate or
our understanding othe growth o conversion to hexose, starch or other products o photosynthesis. To
plants. At this time, plants were thought produce one molecule o glucose or example, six turns o the Calvin
to be soil-eaters. To test this idea, cycle are needed, each o which contributes one o the fxed carbon
van Helmont put 200 pounds (90 kg) atoms in the glucose.
odry soil in large pot and in it planted
a willow tree, which had a mass o Data-based questions: The eect o light and dark on carbon
5 pounds (2.2 kg). He attempted to dioxide fxation
keep dust out othe pot by covering
it with a perorated metal plate. He One o the pioneers o photosynthesis research was James Bassham.
watered the tree with rainwater or The results o one o his experiments are shown in fgure 9.
distilled water over a period ove Concentrations o ribulose bisphosphate and glycerate 3-phosphate
years. When the willow was reweighed were monitored in a culture o cells o the alga, Scenedesmus. The algae
at the end othis time it had increased were kept in bright light and then in the dark.
to 169 pounds (76 kg). Ater drying
the soil rom the pot he ound that it light dark
had remained almost unchanged in
mass, having lost only one eighth o relative concentration glycerate 3 - phosphate
a pound (about 50g). Removal osoil ribulose bisphosphate
rom willow roots is very difcult as
soil particles inevitably get stuck to the 0 100 200 300 400 500 600 700
roots. van Helmont's masses or the soil
beore and ater the ve-year period are Figure 9 Results of Bassham experiment
thereore surprisingly close. Some have
questioned whether van Helmont made 1 Compare the eects o the dark period on the concentrations
his data t pre-decided conclusions.
o ribulose bisphosphate and glycerate 3-phosphate. [2]
1 What evidence against the
hypothesis that plants are soil 2 Explain the change that took place in the 25 seconds ater the
eaters does van Helmont's
experiment provide? start o darkness, to the concentration o:
2 van Helmont concluded rom a) glycerate 3-phosphate [3]
his results that, 164 pounds o
Wood, Barks, and Roots, arose b) ribulose bisphosphate. [1 ]
out o water only. (164 pounds
is 73 kg.) This was not a new idea 3 Predict what the eect would be o turning the light back [2]
- 2000 years earlier the Greek on ater the period o darkness.
philosopher Thales had stated
that all matter arose rom water.
To what extent was van Helmont's
conclusion correct?
396
8.3 phOTOsynThesis
4 Predict the eect o reducing the carbon dioxide concentration
rom 1 .0% to 0.003%, instead o changing rom light to
darkness:
a) on glycerate 3-phosphate concentration [2] 5 triose
phosphate
b) on ribulose bisphosphate concentration. [2]
3 ATP
RuBP regeneration 3(ADP + phosphate)
Ribulose bisphosphate is reormed using ATP. 3 ribulose
bisphosphate
In the last phase o the Calvin cycle, a series o enzyme-catalysed
reactions convert triose phosphate molecules into RuB P. Ater the Figure 10 Summary of RuBP
RuBP is regenerated, it can serve to fx CO2 and begin the cycle again. regeneration
Figure 1 0 summarizes the regeneration process.
Calvins lollipop apparatus
Developments in scientifc research ollow improvements in apparatus: sources o
14C and autoradiography enabled Calvin to elucidate the pathways o carbon fxation.
Sometimes progress in biological research suddenly to pump funnel for
becomes possible because o other discoveries. circulating adding algae
Martin Kamen and Samuel Ruben discovered 14C air and CO2
in 1 945. The hal-lie o this radioactive isotope
o carbon makes it ideal or use in tracing the light syringe for
pathways o photosynthesis. Figure 1 1 shows injecting H14CO3-
apparatus used by Melvin Calvin and his team.
At the start o their experiment, they replaced solenoid control algal suspension
the 12CO2 supplied to algae with 14CO2. They took valve for rapid in nutrient medium
samples o the algae at very short time intervals sampling
hot methanol to
and ound what carbon compounds in the algae kill samples rapidly
contained radioactive 14C. The results are shown
in fgure 1 2. The amount o radioactivity o each
carbon compound is shown as a percentage o the Figure 11 Calvins lollipop apparatus
total amount o radioactivity.
1 Explain the evidence rom the graph that 70
% radioactivity 60
convinced Calvin that glycerate 3-phosphate 50
is the frst product o carbon dioxide 40
fxation. [4] 30
2 Explain the evidence rom the graph or 20
the conversion o glycerate 3-phosphate 10
to triose phosphate and other sugar 0
4 8 12 16 0 1 2 3 4
phosphates. [4] seconds minutes
3 Using the data in the graph, estimate time after introducing 14C
how rapidly carbon dioxide can diuse glycerate-3-phosphate triose phosphate and
other sugar phosphates
into cells and be converted with RuBP to
malate and aspartate alanine
glycerate 3-phosphate. [2]
Figure 12 Graph showing Calvin's results
397
8 M ETABOLI SM , CELL RESPIRATION AN D PHOTOSYN TH ESI S ( AH L)
Chloroplast structure and function
The structure of the chloroplast is adapted to its function
in photosynthesis.
Chloroplasts are quite variable in structure but share certain eatures:
a double membrane orming the outer chloroplast envelope
an extensive system o internal membranes called thylakoids, which
are an intense green colour
small fuid-lled spaces inside the thylakoids
a colourless fuid around the thylakoids called stroma that contains
many dierent enzymes.
In most chloroplasts there are stacks o thylakoids, called grana. I
a chloroplast has been photosynthesizing rapidly then there may be
starch grains or lipid droplets in the stroma.
th y l a koi d
one thylakoid
granum a stack granum a stack
of thylakoids of thylakoids
Figure 13 Electron micrograph of pea chloroplast Figure 14 Drawing of part of the pea
chloroplast to show the arrangement
of the thylakoid membranes
Data-based questions: Photosynthesis in Zea mays
Zea mays uses a modied version o photosynthesis, reerred to as
C 4 physiology. The processes o photolysis and the C alvin cycle are
separated by being carried out in dierent types o chloroplast.
One o the advantages is that carbon dioxide can be xed even
when it is at very low concentrations, so the stomata do not need
to be opened as widely as in plants that do not have C 4 physiology.
This helps to conserve water in the plant, so is useul in dry
habitats.
The electron micrograph (gure 1 5) shows the two types o
chloroplast in the leaves o Zea mays.
One type (Chloroplast X) is rom mesophyll tissue and the other
(Chloroplast Y) is rom the sheath o cells around the vascular tissue
that transports materials to and rom the lea.
398
8.3 phOTOsynThesis
Chloroplast X
Chloroplast Y
Figure 15 Two types of chloroplast in Zea mays leaf
1 Draw a small portion of each chloroplast to show its structure. [5]
2 Compare the structure of the two types of chloroplast. [4]
3 Deduce, with a reason:
a) which type of chloroplast has the greater light absorption
capacity [2]
b) which is the only type of chloroplast to carry out the reactions of
the Calvin cycle [2]
c) which is the only type of chloroplast to produce oxygen. [2]
Diagram showing chloroplast structure
function relationship
Annotation of a diagram to indicate the adaptations of a
chloroplast to its function.
There is a clear relationship between the structure of the chloroplast
and its function.
1 Chloroplasts absorb light. Pigment molecules, arranged
in photosystems in the thylakoid membranes, carry out light
absorption. The large area of thylakoid membranes ensures that
the chloroplast has a large light- absorbing capacity. The thylakoids
are often arranged in stacks called grana. Leaves that are brightly
illuminated typically have chloroplasts with deep grana, which
allow more light to be absorbed.
2 C hlorop lasts p roduce ATP by p hotop hosp horylation. A
proton gradient is needed. This develops between the inside and
399
8 M ETABOLI SM , CELL RESPIRATION AN D PHOTOSYN TH ESI S ( AH L)
outside o the thylakoids. The volume o fuid inside the thylakoids
is very small, so when protons are pumped in, a proton gradient
develops ater relatively ew photons o light have been absorbed,
allowing ATP synthesis to begin.
3 Chloroplasts carry out the many chemical reactions of the
Calvin cycle. The stroma is a compartment o the plant cell in which
the enzymes needed or the Calvin cycle are kept together with their
substrates and products. This concentration o enzymes and substrates
speeds up the whole Calvin cycle. ATP and reduced NADP, needed or
the Calvin cycle, are easily available because the thylakoids, where
they are produced, are distributed throughout the stroma.
granum thylakoid membranes stroma containing
70S ribsomes and
naked DNA
inner outer starch grain
membrane membrane
chloroplast lipid droplet
envelope
400
QuesTiOns
Questions
1 a) State the meaning o the term metabolic intensity. The lamp was controlled by an
electronic timer, which switched it o at night.
pathway. [2] A light meter was placed against the side o
the ermenter, near the base, to measure the
Glucose phosphate (G6P) is converted to intensity o light passing through the liquid
pyruvate in one o the metabolic pathways in the ermenter. The maximum reading it
o cell respiration. This process happens could give was 1 ,200 lux. At the start o the
whether oxygen is available or not. experiment, a small quantity o Chlorella, a
type o algae, was added to the fuid in the
Figure 1 6 shows the concentrations o the ermenter. Figure 1 7 shows the light intensity
intermediates o this pathway in rat heart measured over the 45 days o the experiment.
tissue. The concentrations are shown as
a percentage o the concentrations in the
heart when it has been starved o oxygen.
400 a) The light intensity ollowed a similar
350 pattern, every day rom Day 1 2 onwards.
300
250 (i) Outline the daily changes in light intensity
200
150 over a typical day ater Day 1 2. [2]
100
50
G6P F6P FDP DHAP G3P 3PGA 2PGA PEP PYR
metabolic intermediate
Figure 16
percentage (ii) Explain these daily changes in light
light intensity/lux
in te n s ity. [2]
b) Each day there is a maximum light
intensity. O utline the trends in maximum
light intensity.
(i) rom Day 1 to Day 1 2 [1 ]
(ii) rom Day 1 3 to Day 38 [2]
b) Compared with concentrations during (iii) rom Day 39 to Day 45. [2]
oxygen starvation, state which metabolic
intermediate: c) Explain why the light intensity when the
light was switched on was lower at the end
(i) increased in concentration most [1 ] o the experiment than at the start. [3]
(ii) decreased in concentration most [1 ] d) Suggest reasons or the trend in maximum
(iii) did not change in concentration. [1 ] daily light intensity between Day 39 and
c) (i) The concentrations shown in Day 45. [3]
Figure 1 6 suggest that the rate o this
metabolic pathway has been greater
than is needed by the heart cells. 1000
800
Explain how the data in the bar chart 600
400
shows this. [2] 200
(ii) Because rate o the pathway has been 0
0
greater than necessary, the enzyme
Figure 17
catalysing one o the reactions in the
pathway has been inhibited. Deduce
which reaction this enzyme catalyses,
giving reasons or your answer. [3]
2 Water with mineral nutrients dissolved in 10 20 30 40
it was sterilized and then placed in a 2 dm3 time/days
ermenter. The temperature was kept at 2 5 C .
The ermenter was kept in natural sunlight,
but a lamp was also used to increase the light
401
8 M eTabOli sM , Cell RespiRaTiOn an D phOTOsyn Th esi s ( ah l)
3 At the start o glycolysis, glucose is Dt RQ
phosphorylated to produce glucose
6-phosphate, which is converted into ructose Lipid 0.71
6-phosphate. A second phosphorylation
reaction is then carried out, in which ructose Carbohydrate 1.00
6-phosphate is converted into ructose
1 ,6-bisphosphate. This reaction is catalysed Protein 0.74
by the enzyme phosphoructokinase.
Biochemists measured the enzyme activity Source: Walsberg and Wolf, Journal ofExperimental Biology,
o phosphoructokinase (the rate at which (1995) , 198, pages 213219.
it catalysed the reaction) at dierent
concentrations o ructose 6-phosphate. The Reproduced by permission of The Company of Biologists Ltd.
enzyme activity was measured with a low
concentration o ATP and a high concentration In an experiment to assess RQ values or house
o ATP in the reaction mixture. The graph sparrows, the birds were ed a diet o pure
below shows the results. mealworms (beetle larvae) or millet (a type
o grain) .
low ATP concentration
The graph below shows the RQ values o a
high ATP concentration house sparrow ed on a high carbohydrate diet
(millet) and a high lipid diet (mealworms) .
1.0
enzyme activity 0.9 millet
respiratory quotient mealworms
0.8
fructose 6-phosphate concentration
0.7
a) (i) Using only the data in the above
graph, outline the eect o increasing
ructose 6-phosphate concentration on 0.6
01234567
the activity o phosphoructokinase, at
time after feeding/h
a low ATP concentration. [2]
Source: Walsberg and Wolf, Journal ofExperimental Biology,
(ii) Explain how increases in ructose (1995) , 198, pages 213219.
6-phosphate concentration aect the Reproduced by permission of The Company of Biologists Ltd.
activity o the enzyme. [2]
b) (i) Outline the eect o increasing the a) Compare the RQ values or millet and
ATP concentration on the activity o mealworms between 1 hour and 6 hours
phosphoructokinase. [2] ater eeding. [2]
(ii) Suggest an advantage to living [1 ] The expected RQ value or house sparrows
organisms o the eect o ATP on metabolizing millet is 0.93. The expected
phosphoructokinase. value when metabolizing mealworms
is 0.75.
4 The respiratory quotient (RQ) is a measure b) Explain why the expected RQ values or
o the metabolic activity o an animal. It is
the ratio o CO2 produced to O2 consumed. In millet and mealworms are dierent. [2]
general, the lower the RQ value the higher the
energy yield. The RQ is dependent on the diet c) Suggest reasons or
consumed by the animal. The ollowing table
lists the typical RQ values or specifed diets. (i) the high initial RQ values or house
sparrows ed on millet; [1 ]
(ii) the rapid all in RQ values or house
sparrows ed on millet. [1 ]
402
9 PLANT BIOLOGY (AHL)
CInEtroLduLctioBn I O L O G Y
Plants are highly diverse in structure and Plants have sophisticated methods o adapting
physiology. They act as the producers in almost their growth to environmental conditions.
all terrestrial ecosystems. Structure and unction Reproduction in fowering plants is infuenced by
are correlated in the xylem and phloem o plants. both the biotic and abiotic environment.
9.1 Transport in the xylem of plants
Understanding Applications
Transpiration is the inevitable consequence o Adaptations o plants in deserts and in saline
gas exchange in the lea. soils or water conservation.
Plants transport water rom the roots to the Models o water transport in xylem using
leaves to replace losses rom transpiration. simple apparatus including blotting or lter
paper, porous pots and capillary tubing.
The cohesive property o water and the
structure o the xylem vessels allow transport Skills
under tension.
Drawing the structure o primary xylem vessels
The adhesive property o water and evaporation in sections o stems based on microscope
generate tension orces in lea cell walls. images.
Active uptake o mineral ions in the roots Measurement o transpiration rates using
causes absorption o water by osmosis. potometers. (Practical 7)
Design o an experiment to test hypotheses
about the efect o temperature or humidity on
transpiration rates.
Nature of science
Use models as representations o the real world: mechanisms involved in water transport in the xylem
can be investigated using apparatus and materials that show similarities in structure to plant tissues.
403
9 PLANT BIOLOGY (AHL)
water Transpiration
O2
Transpiration is the inevitable consequence o
Figure 1 gas exchange in the lea.
Plant leaves are the primary organ of photosynthesis. Photosynthesis
involves the synthesis of carbohydrates using light energy. C arbon
dioxide is used as a raw material. Oxygen is produced as a waste
product. Exchange of these two gases must take place to sustain
photosynthesis.
Absorption of carbon dioxide is essential for photosynthesis and the
waxy cuticle has very low permeability to it, so pores through the
epidermis are needed. These pores are called stomata. Figure 1 shows
that the problem for plants is that if stomata allow carbon dioxide to be
absorbed, they will usually also allow water vapour to escape.
This is an intractable problem for plants and other organisms: having gas
exchange without water loss. The loss of water vapour from the leaves
and stems of plants is called transpiration.
Plants minimize water losses through stomata using guard cells. These
are the cells that are found in pairs, one on either side of a stoma. Guard
CO2 cells control the aperture of the stoma and can adjust from wide open
to fully closed. Stomata are found in nearly all groups of land plants for
at least part of the plants life cycle. The exception is a group called the
liverworts.
Modelling water transport
Models o water transport in xylem using simple apparatus including blotting or
flter paper, porous pots and capillary tubing.
porous pot plant
water
Figure 2 Porous pots can be used to model evaporation Figure 3 Capillary tubes dipped into water with dye and
rom leaves. Water flls pores within the pot demonstrating mercury. Unlike water, there is no adhesion o mercury to the
adhesion to the clay molecules within the pot. As the water glass nor is there cohesion between mercury atoms, so the
is drawn into the pot, cohesion causes water molecules to be mercury does not climb the glass
drawn up the glass tubing
404
9.1 TransporT in The xylem of planTs
Figure 4 The ability of adhesive forces to result in the movement of water is demonstrated in this image. A folded paper towel with
one end immersed in water will transport water into an empty container by capillary action
Using a potometer
Measurement o transpiration rates using potometers. (Practical 7)
Mechanisms involved in water transport in the
xylem can be investigated using apparatus and
materials that show similarities in structure to
plant tissues.
Figure 5 shows a potometer. This is a device used Figure 5
to measure water uptake in plants. The apparatus
consists o a leay shoot in a tube (right) , a
reservoir (let o shoot) , and a graduated capillary
tube (horizontal) . A bubble in the capillary tube
marks the zero point. As the plant takes up
water through its roots, the bubble will move
along the capillary tube. The progress o the
bubble is being timed here, along with noting the
distance travelled. The tap below the reservoir
allows the bubble to be reset to carry out new
measurements.
Efect o humidity on transpiration
Design o an experiment to test hypotheses about the
efect o temperature or humidity on transpiration rates.
The rate o transpiration is difcult to measure directly. Instead, the
rate o water uptake is usually measured, using a potometer. Figure 6
shows one type o potometer.
To design an investigation you will need to discuss the ollowing
questions.
1 How will you measure the rate o transpiration in your
investigation?
405
9 PLANT BIOLOGY (AHL)
2 What biotic or abiotic actor will you investigate?
3 How will you vary the level o this actor?
4 How many results do you need, at each level o the actor that you
are varying?
5 How will you keep other actors constant, so that they do not
aect the rate o transpiration?
fresh shoot, cut under water and
transferred to apparatus under water
to avoid introducing air bubbles
reservoir from which water can be air tight
let into thecapillary tube, pushing seal
the air bubble back to the start of
the tube
tap
Figure 7 Longitudinal section through a capillary tube
rhubarb stem, Rheum rhaponticum. Cut xylem
vessels are coloured brown. Xylem vessels are scale calibrated in mm3
reinorced and strengthened with spiral bands air bubble moves along tube as
o lignin. Spiral bands allow xylem vessels to water is absorbed by shoot
elongate and grow lengthwise
Figure 6 Diagram o a potometer
Figure 8 Light micrograph o a vertical section Xylem structure helps withstand low pressure
o the primary wood or xylem o a tree showing
wood vessels with lignifed supporting The cohesive property of water and the structure of the
th i cken i n gs xylem vessels allow transport under tension.
The structure o xylem vessels allows them to transport water inside
plants very efciently. Xylem vessels are long continuous tubes. Their
walls are thickened, and the thickenings are impregnated with a polymer
called lignin. This strengthens the walls, so that they can withstand very
low pressures without collapsing.
Xylem vessels are ormed rom fles o cells, arranged end-to-end. In
owering plants, the cell wall material in some areas between adjacent
cells in the fle is largely removed and the plasma membranes and
contents o the cells break down (see fgures 7 and 8) . When mature,
these xylem cells are nonliving, so the ow o water along them must
be a passive process. The pressure inside xylem vessels is usually much
lower than atmospheric pressure but the rigid structure prevents the
xylem vessels rom collapsing.
406
9.1 TransporT in The xylem of planTs
Water molecules are polar and the partial negative charge on the
oxygen atom in one water molecule attracts the hydrogen atom in a
neighbouring water molecule. This is termed cohesion. Water is also
attracted to hydrophilic parts o the cell walls o xylem. This is termed
adhesion. As a result o the connections between the molecules, water
can be pulled up rom the xylem in a continuous stream.
Dt-bd qut: The Renner experiment
Figure 9 shows the results o an experiment and the rate caused by the leaves immediately
by the German plant physiologist Otto Renner
in 1 91 2. A transpiring woody shoot was placed beore the shoot top was cut o. [2]
in a potometer and the rate o water uptake was
measured. A clamp was attached to the stem to 4 The water in the potometer was at
restrict the fow o water up to the leaves. Later
on, the top o the shoot, with all o its leaves, was atmospheric pressure. The vacuum pump
removed. A vacuum pump was then attached to
the top o the shoot. generated a pressure o zero. Discuss what
the results o the experiment showed about
the pressures generated in the xylem by the
leaves o the shoot. [2]
Questions water uptake /cm 3 h1 stem clamped
20 shoot removed
1 Describe the eect o clamping the stem on 10
9
the rate o water uptake. [3] 8
7 vacuum pump
6
2 Explain the eect o cutting o the top o the 5
shoot on the rate o water uptake. [3] 4
3
2 1234
3 Calculate the dierence between the rate o 1
water uptake caused by the vacuum pump
time (hrs)
Figure 9 Results of the Renner experiment
Tension in leaf cell walls maintains the
transpiration stream
The adhesive property of water and evaporation generate
tension forces in leaf cell walls.
When water evaporates rom the surace o the wall in a lea, adhesion
causes water to be drawn through the cell wall rom the nearest
available supply to replace the water lost by evaporation. The nearest
available supply is the xylem vessels in the veins o the lea.
Even i the pressure in the xylem is already low, the orce o adhesion
between water and cell walls in the lea is strong enough to suck water
out o the xylem, urther reducing its pressure.
The low pressure generates a pulling orce that is transmitted though
the water in the xylem vessels down the stem and to the ends o
the xylem in the roots. This is called transpiration-pull and is strong
enough to move water upwards, against the orce o gravity, to the
top o the tallest tree. For the plant, it is a passive process, with all the
energy needed or it coming rom the thermal energy (heat) that causes
transpiration. The pulling o water upwards in xylem vessels depends
on the cohesion that exists between water molecules. Many liquids
407
9 PLANT BIOLOGY (AHL)
would be unable to resist the very low pressures in xylem vessels and
the column o liquid would break. This is called cavitation and it does
occasionally happen even with water, but it is unusual. E ven though
water is a liquid, it can transmit pulling orces in the same way as a
solid length o rope does.
Active transport of minerals in the roots
Active uptake of mineral ions in the roots causes
absorption of water by osmosis.
Water is absorbed into root cells by osmosis. This happens because the
solute concentration inside the root cells is greater than that in the
water in the soil. Most o the solutes in both the root cells and the soil
are mineral ions. The concentrations o mineral ions in the root can be
1 00 or more times higher than those in the soil. These concentration
gradients are established by active transport, using protein pumps in
the plasma membranes o root cells. There are separate pumps or each
type o ion that the plant requires. Mineral ions can only be absorbed by
active transport i they make contact with an appropriate pump protein.
This can occur by diusion, or by mass ow when water carrying the
ions drains through the soil.
Some ions move through the soil very slowly because the ions bind to
the surace o soil particles. To overcome this problem, certain plants have
developed a relationship with a ungus. The ungus grows on the surace
o the roots and sometimes even into the cells o the root. The thread-like
hyphae o the ungus grow out into the soil and absorb mineral ions such
as phosphate rom the surace o soil particles. These ions are supplied
to the roots, allowing the plant to grow successully in mineral-defcient
soils. This relationship is ound in many trees, in members o the heather
Data-based questions: Fungal hyphae and mineral ion absorption
Figure 1 0 shows the results o an experiment in b) Suggest a reason or the relationship. [1 ]
which seedlings o Sitka spruce, Picea sitchensis, were
grown or 6 months in sterilized soil either with c) Using the data in Figure 1 0, deduce
or without ungi added: C was the control with no
ungi added. The species o ungi added were: whether the eects o closely related ungi
on tree growth are the same. [2]
I = Laccaria laccata; II = Laccaria ameythestea; root dry mass (g) shoot dry mass (g)0.5
III = Thelophora terrestris rom a tree nursery; 0.4
IV = Thelophora terrestris rom a orest; 0.3
V = Paxillus involutus; VI = Pisolithus tinctorius. 0.2
0.1
1 a) Discuss the eects o the fve species o 0.0
0.1
ungi on the growth o the roots and 0.2
0.3
shoots o the tree seedlings. [4] 0.4
0.5 C I II III V IV VI
b) Explain the eects o the ungi on the
Figure 10 Results of Sitka spruce experiment
growth o tree seedlings. [2]
2 a) State the relationship between root
growth and shoot growth in the tree
seedlings. [1 ]
408
9.1 TransporT in The xylem of planTs
amily and in orchids. Most, but not all, o these plants supply sugars and
other nutrients to the ungus, so both the ungus and the plant beneft.
This is an example o a mutualistic relationship.
Replacing losses from transpiration
Plants transport water from roots to leaves to replace
losses from transpiration.
The movement o water rom roots to leaves is summarized in fgure 1 1 .
Water leaving through stomata by transpiration is replaced by water rom
xylem. Water in the xylem climbs the stem through the pull o transpiration
combined with the orces o adhesion and cohesion. Water moves rom soil
into roots by osmosis due to the active transport o minerals into the roots.
Once the water is in the root it travels to the xylem through cell walls (the
apoplast pathway) and through cytoplasm (the symplast pathway) .
water from xylem
1 cohesion water leaving
2 adhesion through stomata
1 root hair epidermal cell cytoplasm
2 soil particle
xylem cell water molecule
apoplastic movement
Casparian strip
water moves from
soil into roots
root hair absorbs symplastic xylem vessel
water from the soil movement and tracheids
Figure 11
Adaptations for water conservation
Adaptation of plants in deserts and in saline soils for water conservation.
Xerophytes are plants adapted to growing in uptake rom the soil and reducing the rate o
deserts and other dry habitats. There are various water loss by transpiration. Some xerophytes
strategies that plants can use to survive in these are ephemeral, with a very short lie cycle that
habitats, including increasing the rate o water is completed in the brie period when water is
409
9 PLANT BIOLOGY (AHL)
available ater rainall. They then remain dormant in these xerophytes are oten very similar to those
as embryos inside seeds until the next rains, o cacti. Some Arican species o Euphorbia or
sometimes years later. Other plants are perennial example, are difcult to distinguish rom cacti
and rely on storage o water in specialized leaves, until they produce owers.
stems or roots.
Marram Grass (Ammophila arenaria) is a xerophyte,
Most cacti are xerophytes, with leaves that are so i.e. it is a plant adapted or dry conditions. It has
reduced in size that they usually only consist o a rolled lea. This creates a localized environment
spines. The stems contain water storage tissue and o water vapour which helps to prevent losses o
become swollen ater rainall. Pleats allow the stem water. The stomata sit in small pits within the curls
to expand and contract in volume rapidly. The o the structure, which make them less likely to
epidermis o cactus stems has a thick waxy cuticle open and to lose water. The olded leaves have
and unlike most plant stems there are stomata, hairs on the inside to slow or stop air movement,
though they are spaced more widely than in leaves. much like many other xerophytes. This slowing o
The stomata usually open at night rather than in air movement once again reduces the amount o
the day, when it is much cooler and transpiration water vapour being lost.
occurs more slowly. C arbon dioxide is absorbed
at night and stored in the orm o a our-carbon
compound, malic acid. Carbon dioxide is released
rom the malic acid during the day, allowing
photosynthesis even with the stomata closed. This
is called Crassulacean acid metabolism. Plants such
as cacti that use this system are called CAM plants.
C4 physiology also helps to reduce transpiration.
reduced leaf
spine
swollen stem
Gymnocalycium baldianum (cactus) viewed Figure 13
from above
Saline soils are those that contain high
10 mm concentrations o salts. Plants that live in saline
Euphorbia obesa viewed from above soils are called halophytes. Halophytes have
several adaptations or water conservation:
swollen stem
the leaves are reduced to small scaly structures
5 mm or spines
Figure 12 Xerophytes the leaves are shed when water is scarce and the
Cacti are native plants o North and South stem becomes green and takes over the unction
America. Xerophytes in other parts o the world o photosynthesis when the leaves are absent
belong to dierent plant amilies. The adaptations
water storage structures develop in the leaves
410
they have a thick cuticle and a multiple
layered epidermis
they have sunken stomata
they have long roots, which go in search o water
they have structures or removing salt build-up.
9.1 TransporT in The xylem of planTs
Drawing xylem vessels
Drawing the structure of primary xylem vessels in sections of stems based
on microscope images.
Primary xylem vessels are visible in cross sections o xylem vascular
young stems such as in young Helianthus. Figure 1 6 bundle
shows a longitudinal section through a stem cambium
illustrating the structure o xylem. Primary xylem phloem
has a thin primary wall that is unlignifed and reely
permeable, plus lignifed secondary thickening o the epidermis
wall that is usually annular or helical. The thickening cortex
allows the xylem vessel to continue growing in length pith
because the rings o annular thickening can move
urther apart or helical thickening can be stretched so
the pitch o the helix is greater.
Once extension growth o a root or stem is complete
the plant produces secondary xylem which is much
more extensively lignifed. Secondary thickening o
its cell wall provides more strength but does not allow
growth in length.
Figure 14
thickenings of xylem vessel
wall impregnated with lignin
Figure 15 Light micrograph o a section through a young stem continuous tubular structure
rom a sunfower (Helianthus annuus) , showing one o the
many vascular bundles. The vascular bundles have an outer Figure 16 Structure o xylem vessels
layer o sclerenchyma tissue (crimson) . Next is the phloem
(dark blue) with phloem tubes, parenchyma and companion
cells. Then the xylem (red) and at the end o the xylem are
patches o bres (red) . In between the phloem and xylem is
the cambium (light blue)
411
9 PLANT BIOLOGY (AHL)
9.2 Transport in the phloem of plants
Understanding Applications
Plants transport organic compounds rom Structureunction relationships o phloem
sources to sinks. sieve tubes.
Incompressibility o water allows transport by Skills
hydrostatic pressure gradients.
Analysis o data rom experiments measuring
Active transport is used to load organic phloem transport rates using aphid stylets and
compounds into phloem sieve tubes at the radioactively-labelled carbon dioxide.
source.
Identication o xylem and phloem in
High concentrations o solutes in the phloem at microscope images o stem and root.
the source lead to water uptake by osmosis.
Raised hydrostatic pressure causes the
contents o the phloem to fow toward sinks.
Nature of science
Developments in scientic research ollow improvements in apparatus: experimental methods or
measuring phloem transport rates using aphid stylets and radioactively-labelled carbon dioxide were
only possible when radioisotopes became available.
xylem phloem Translocation occurs from source to sink
Figure 1 source Plants transport organic compounds rom sources to sinks.
412 water (leaf cell) Phloem tissue is ound throughout plants, including the stems, roots and
sucrose
leaves. Phloem is composed o sieve tubes. Sieve tubes are composed o
transpiration companion columns o specialized cells called sieve tube cells. Individual sieve tube
stream cell cells are separated by perorated walls called sieve plates. Sieve tube cells
are closely associated with companion cells (fgure 1 ) .
sieve plate
Phloem transports organic compounds throughout the plant. The transport
o organic solutes in a plant is called translocation. Phloem links parts o
the plant that need a supply o sugars and other solutes such as amino acids
sink to other parts that have a surplus. Table 1 classifes parts o the plant into
(root cell) sources (areas where sugars and amino acids are loaded into the phloem)
and sinks (where the sugars and amino acids are unloaded and used) .
Figure 2 shows the results o a simple experiment in which two rings o
water bark were removed rom an apple tree. The bark contains the phloem
tissue. The eects on apple growth are clearly visible.
companion Sometimes sinks turn into sources, or vice versa. For this reason the
cell tubes in phloem must be able to transport biochemicals in either
direction and, unlike the blood system o animals, there are no valves
or central pump in phloem. However there are similarities between
transport in phloem and blood vessels: in both systems a uid ows
inside tubes because o pressure gradients. Energy is needed to generate
9.2 TransporT in The phloem of planTs
the pressures, so the ow o blood and the movement o phloem sap are actvty
both active processes.
suc sk 1 State which the sources and
Photosynthetic tissues: Roots that are growing or absorbing which the sinks are in this part
mature green leaves mineral ions using energy from cell
green stems. respiration. of the apple tree. [2]
Storage organs that are unloading their Parts of the plant that are growing or
stores: developing food stores: 2 ) Compare the sizes of
storage tissues in germinating developing fruits
developing seeds the apples. [2]
seeds growing leaves
tap roots or tubers at the start of developing tap roots or tubers. b) Explain the conclusions that
the growth season. can be drawn from the sizes
of the apples. [4]
Table 1
Phloem loading Figure 2 Results of apple tree
ringing experiment
Active transport is used to load organic compounds into
phloem sieve tubes at the source.
The data in table 2 indicates that sucrose is transported in the phloem.
Sucrose is the most prevalent solute in phloem sap. Sucrose is not as
readily available or plant tissues to metabolize directly in respiration and
thereore makes a good transport orm o carbohydrate as it will not be
metabolized during transport.
Plants dier in the mechanism by which they bring sugars into the phloem,
a process called phloem loading. In some species, a signifcant amount travels
through cell walls rom mesophyll cells to the cell walls o companion cells,
and sometimes sieve cells, where a sucrose transport protein then actively
transports the sugar in. This is reerred to as the apoplast route.
In this case, a concentration gradient o sucrose is established by active
transport. Figure 3 shows that this is achieved by a mechanism whereby H+
ions are actively transported out o the companion cell rom surrounding
tissues using ATP as an energy source. The build-up o H+ then ows down
its concentration gradient through a co-transport protein. The energy
released is used to carry sucrose into the companion cell-sieve tube complex.
[outside cell] - high H+ concentration
H+
proton pump co-transporter
S
proton gradientlow H+
sucrose gradient
ATP ADP+P 413
H+ H+ S
[inside cell] - low H+ concentration
Figure 3 Movement of sucrose (S) across a sieve tube membrane
9 PLANT BIOLOGY (AHL)
In other species, much o the sucrose travels between cells
through connections between cells called plasmodesmata (singular
plasmodesma) . This is reerred to as the symplast route. Once the
sucrose reaches the companion cell it is converted to an oligosaccharide
to maintain the sucrose concentration gradient.
mesophyll cell sieve-tube
cell wall member
plasma membrane
plasmodesmata companion
(transfer) cell
mesophyll phloem symplast route
cell parenchyma cell apoplast route
Figure 4
Data-based questions: Carbohydrates in cyclamen
1 Choose a suitable presentation ormat to 2 Describe the trends in the data and suggest
display the data in table 2, including the reasons or them based on your knowledge o
standard error values. You can use graphing photosynthesis, the structure o disaccharides
sotware or you can draw graphs, tables, and polysaccharides and the transport and
charts or diagrams by hand. storage o carbohydrates in plants.
plant art mean carbohydrate content
(g g1 fresh ass standard error of ean)
Leaf blade
Vascular bundle in the leaf stalk, consisting of xylem sucrose glucose fructose starch
and phloem
Tissue surrounding the vascular bundle in the leaf stalk 1,312 210 494 62
Buds, roots and tubers (underground storage organs)
212 88 653 25
Table 2
5,757 479 1,303 <18
1,190 280 879
417 624 1,236 <18
96 714 1,015
2,260 120 370 152
926 41 242 242
Pressure and water potential diferences play
a role in translocation
Incompressibility of water allows transport by hydrostatic
pressure gradients.
The build up o sucrose and other carbohydrates draws water into the
companion cell through osmosis. The rigid cell walls combined with the
incompressibility o water result in a build-up o pressure. Water will
fow rom this area o high pressure to an area o low pressure.
414
9.2 TransporT in The phloem of planTs
At the sink end, sucrose is withdrawn rom the phloem and either
utilized as an energy source or such processes as growth or converted
to starch. In either case, the loss o solute causes a reduction in osmotic
pressure and the water that carried the solute to the sink is then drawn
back in to the transpiration stream in the xylem.
Dt-bd qut: Explaining water movement
Water potential is a measure o the tendency xylem vessel phloem sieve companion cell
o water to move rom one area to another. It elements elements H2O source cell
is represented by the variable w. It is defned
as the sum o solute potential and pressure H2O H2O sucrose
potential. H2O sink cell
w = -0.8 MPa solute potential w = -1.1 MPa
Pure water has a solute potential, s , o zero. p = -0.7 MPa decreases, high p = +0.6 MPa
Once solute is added, the value o the solute s = -0.1 MPa turgor pressure s = -1.7 MPa
potential becomes more negative. The more
negative the solute potential, the more likely bulk ow of
water and solute
water will be drawn rom another area with transpiration stream
higher solute potential; i.e., lower solute H2O
concentration.
Pressure potential (p) in a plant cell is the w = -0.6 MPa solute potential w = -0.4 MPa H2O
pressure exerted by the rigid cell wall that p = -0.5 MPa increases, lower p = +0.3 MPa sucrose
s = -0.1 MPa turgor pressure s = -0.7 MPa
limits urther intake o water. In a plant cell,
pressure exerted by the rigid cell wall limits
urther water uptake despite solute potential Figure 5
dierences.
1 Explain the movement o water rom point 3 Explain the movement o water rom point D
A to point C. [3] to point B. [3]
2 Explain the movement o water rom point C 4 Explain the movement o water rom point B
[3] to point A. [3]
to point D.
Phloem sieve tubes
Structurefunction relationship in phloem sieve tubes.
The unctions o phloem include loading o Sieve tube cells are closely associated with
carbohydrates; transport o the carbohydrates companion cells. This is due in part to the act
sometimes over long distances; and unloading o that the sieve tube cell and its companion cell
the carbohydrates at sinks. share the same parent cell. The companion cells
perorm many o the genetic and metabolic
Phloem is composed o sieve tubes. Sieve tubes are unctions o the sieve tube cell and maintain
composed o columns o specialized cells called sieve the viability o the sieve tube cell. Note the
tube cells. Unlike the vascular elements o xylem, abundant mitochondria in the companion cell
sieve tube elements are living, though they do have shown in fgure 6 to support active transport o
reduced quantities o cytoplasm and no nucleus. sucrose. The inolding o the plasma membrane
One reason that sieve cells need to be living is that seen in the companion cell image increases the
they depend on the membrane to help maintain the phloem loading capacity using the apoplastic
sucrose and organic molecule concentration that route. Plasmodesmata connect the cytoplasm o
has been established by active transport.
415
9 PLANT BIOLOGY (AHL)
companion cells with the sieve tube cells and Individual sieve tube cells are separated by perorated
have a larger diameter than plasmodesmata walls called sieve plates, shown in fgure 7. These
ound in other parts o the plant to accommodate are the remnants o cell walls that separated the
the movement o oligosaccharides and genetic cells. The perorated walls in combination with the
elements between the two cells. reduced cytoplasm means that the resistance to the
ow o phloem sap will be lower.
The accumulation o sucrose in the sieve tube
element-companion cell pair requires the presence
o active transport proteins or enzyme activity in the
companion cells to produce the oligosaccharides.
The rigid cell walls o the sieve tube cell allow or
the establishment o the pressure necessary to
achieve the ow o phloem in the sieve tube cell.
sieve tube companion
cell cell
branched
plasmodesma
Figure 6 Figure 7
activity Figure 8
anlysis of electron microgrph of phloem tissue
1 In the electron micrograph in fgure 8, identiy the
ollowing:
(i) sieve cells
(ii) the sieve plate
(iii) the companion cell
(iv) plastids with starch granules within the sieve cell
(v) plasmodesma
(vi) sieve cell cytoplasm
(vii) mitochondria within the companion cell.
2 I the scale bar in image represents 5m, then
estimate the width o the sieve tube cell in the region
o the sieve plate.
3 Suggest the evidence seen in the micrograph that
suggests that the sieve tube cell is living.
416
9.2 TransporT in The phloem of planTs
Experiments using aphid stylets
Analysis of data from experiments measuring phloem
transport rates using aphid stylets and radioactively-
labelled carbon dioxide.
Phloem sap is nutrient-rich compared with many other plant products
and the nutrients in it are small soluble molecules that do not need to
be digested. Despite this, the only animals to consume it as the main
part o their diet are insects belonging to a group called the Hemiptera
which includes aphids, whitefy, mealybugs and psyllids.
Aphids penetrate plant tissues to reach the phloem (p in the first picture
in figure 9) using mouth parts called stylets (st in the first picture) . If the
aphid is anaesthetized and the stylet severed (process about to occur in the
middle picture) , phloem will continue to flow out of the stylet (final picture)
and both the rate of flow and the composition of the sap can be analysed.
The closer the stylet is to the sink, the slower the rate at which the phloem
sap will come out.
Figure 10
417
9 PLANT BIOLOGY (AHL)
Data-based questions c) Aphids ingest larger volumes o phloem
sap than they need, to obtain sufcient
1 a) The only animals to consume phloem sugar or cell respiration. This is because
sap as the main part o their diet are they also need to obtain amino acids and
insects belonging to a group called the the concentration o amino acids in phloem
Hemiptera. The data in this question sap is low. Figure 1 1 shows the percentages
comes rom research into aphids. o individual amino acids in phloem sap
and the percentages in aphid protein. Nine
The sugar content o phloem sap is very o the amino acids cannot be synthesized
high oten greater than 1 mol dm3. in aphid cells and so are called essential
amino acids. The other amino acids can be
(i) Explain how plants increase the sugar synthesized rom other amino acids and so
are non-essential.
concentration o phloem sap to such
10
high levels. [1 ]
(ii) Explain how high sugar
concentrations cause a high pressure
to develop in the phloem. [2]
% oligosaccharides
Amino acid % in aphid proteinsb) Aphids only ingest a small proportion o1Non-essential
the sugar in phloem sap. The remainder amino acids
passes out in the aeces, which is a liquid 0 Essential
called honeydew. Because o the high sugar 0 1 10 100 amino acids
concentrations, phloem sap has a much Amino acid % in hloem sa
higher solute concentration than aphid cells.
Enzymes secreted into the aphid gut reduce Figure 11
the solute concentration o phloem sap by
converting sugars into oligosaccharides. (i) Evaluate phloem sap as a source o
Figure 1 1 shows the relationship between
the sucrose concentrations o phloem sap amino acids or aphids. [3]
ingested by aphids and the oligosaccharide
content o the honeydew.
(ii) Suggest reasons or the dierences in
100 amino acid content between phloem
80 sap and aphid protein. [2]
60 d) Specialized cells have been discovered in
aphids called bacteriocytes. These cells
40 contain bacteria called Buchnera, which
synthesize essential amino acids rom
20 aspartic acid and sucrose. Aspartic acid is
a non-essential amino acid that is ound
0 in much higher concentrations in phloem
0.00 0.25 0.50 0.75 1.00 sap than any other amino acid. When
aphids reproduce, they pass on Buchnera
Dietary sucrose concentration (mol) bacteria to their ospring.
Figure 10
(i) Describe the relationship between the (i) Explain how antibiotics could be used
sucrose concentration o phloem sap
ingested by aphids and the percentage o to obtain evidence or the role o
oligosaccharides in the honeydew. [3]
Buchnera in aphids. [2]
( ii) Suggest reasons or aphids (ii) Using the data in this question,
secreting enzymes to reduce the
solute concentration o the uid in discuss the reasons or ew animals
the gut.
[2] using phloem sap as the main part
o their diet. [3]
418
9.2 TransporT in The phloem of planTs
Radioisotopes as important tools in studying translocation
Developments in scientifc research ollow improvements in apparatus:
experimental methods or measuring phloem transport rates using aphid stylets
and radioactively-labelled carbon dioxide were only possible when radioisotopes
became available.
Carbon-1 4 is an isotope o carbon that is Figure 12
radioactive. Radioactively-labelled carbon
within carbon dioxide can be fxed by
plants during photosynthesis. It will release
radiation that can be detected either using
flm or radiation detectors. As the carbon
is metabolized, it will be ound in dierent
molecules within the plant. In other words,
both the ormation and movement o
radioactive molecules can be traced. Figure 1 2
shows a device known as a Geiger counter
measuring radiation levels in a crop o
sunowers. The sunowers in the picture
are being used or bioremediation o soil
contaminated with radiation.
Dt-bd qut: Radioactive labelling (1)
Source leaves were supplied with a pulse o d) Deduce, with a reason, whether the source
radioactively-labelled carbon and the time taken
or the radioactive carbon to be ound in sink lea is a growing or mature lea. [2]
leaves was measured by radiophotography. The
photosynthetic rate was varied, primarily, by 14C-translocation rate (g C dm-2 min-1) 5.0
altering the concentration o unlabelled carbon 4.0
dioxide. The experiment was carried out at three 3.0
dierent intensities o light (green squares are
20,000 lux; orange diamonds are 40,000 lux;
purple circles are 80,000 lux) .
a) Outline the relationship between 2.0
photosynthesis rate and translocation rate. [1 ] 1.0
b) (i) Deduce the relationship between light 0
0
intensity and translocation. [2]
Figure 13
(ii) Suggest whether this is a correlation or a 50 100 150 200 250
net photosynthesis rate (g C dm-2 min-1)
cause and eect relationship. [3]
c) Determine the ratio o translocation to net
photosynthesis at two dierent points on the
graph. [2]
419
9 PLANT BIOLOGY (AHL)
Data-based questions: radioactive labelling (2)
The distribution o radioactivity in leaves rom a (ii) Using fgure 1 4A, describe the location o the
sugar beet plant ( Beta vulgaris) was determined
1 week ater 14CO2 was supplied or 4 hours to sink leaves receiving the most photosynthate
a single source lea (labelled with an arrow in
fgure 1 4) . The degree o radioactive labelling is in relation to the source lea. [2]
indicated by the intensity o shading o the leaves.
Leaves are numbered according to their age; the (iii) Evaluate the hypothesis that leaves directly
youngest, newly emerged lea is designated 1 .
above and below the source lea are most
likely to receive photosynthate and that
pruning causes a rerouting o translocation
pathways to include lateral leaves. [3]
The purpose o the experiment was to determine A) 14 B) 10 14CO 2
the position o sink leaves in relation to the position 14CO 2
o source leaves. The hypothesis was that leaves
directly above and below the source lea are most 9 7
likely to receive photosynthate (the products 4
o photosynthesis) and that pruning causes a 11 6 12 52
rerouting o translocation pathways to include 3 7 8
lateral leaves. Figure 1 4A shows the distribution o 4
photosynthate in an intact plant. Figure 1 4B shows 1 3
the pattern ater several leaves have been removed. 8
1
5 69
11
2 15
13
(i) In fgure 1 4A, identiy the two leaves that 10
received the most photosynthate. [2] Figure 14
Identifying xylem and phloem in light micrographs
Identifcation o xylem and phloem in microscope images o stem and root.
Xylem cells are generally larger than phloem cells. Within one vascular bundle, phloem cells tend to be
closer to the outside o the plant in stems and roots.
Figure 15 Buttercup stem. Coloured scanning electron Figure 16 Light micrograph o a transverse section through
micrograph (SEM) o a transverse (cross) section through the stem o a sunfower (Helianthus annuus)
part o a stem o a buttercup, Ranunculus repens, showing a
vascular bundle. This is a typical dicotyledon stem. At the centre
is an oval vascular bundle embedded in the cortex cells o the
stem. Some cells contain chloroplasts (green) . The vascular
bundle contains large xylem vessels (centre right) which serve
to conduct water; the nutrient-conducting phloem is orange
420
9.2 TransporT in The phloem of planTs
Figure 17 Coloured scanning electron micrograph (SEM) of a section through a rootlet of a dicotyledon plant.
The vascular bundle consists of xylem (four yellow circles, centre) and phloem (beige) tissue. Xylem transports
water and mineral nutrients from the roots throughout the plant and phloem transports carbohydrates and plant
hormones around the plant. Surrounding the vascular bundle is a single layer of endodermis (orange) , then
cortex (brown) , which consists of parenchyma cells. The outermost layer (cream) is the epidermis
Figure 18 Light micrograph of a section through the stem of a maize plant (Zea mays) . Vascular
bundles (coloured clusters) can be seen containing xylem (larger openings, red/black) and
phloem (smaller openings, light blue) tissues
421
9 PLANT BIOLOGY (AHL)
9.3 Growth in plants
Understanding Applications
Undierentiated cells in the meristems o Micropropagation o plants using tissue rom
plants allow indeterminate growth. the shoot apex, nutrient agar gels and growth
hormones.
Mitosis and cell division in the shoot apex
provide cells needed or extension o the stem Use o micropropagation or rapid bulking up o
and development o leaves. new varieties, production o virus-ree strains
o existing varieties and propagation o orchids
Plant hormones control growth in the and other rare species.
shoot apex.
Plants respond to the environment by tropisms.
Auxin inuences cell growth rates by changing
the pattern o gene expression.
Auxin efux pumps can set up concentration
gradients o auxin in plant tissues.
Nature of science
Developments in scientic research ollow improvements in analysis and deduction: improvements in
analytical techniques allowing the detection o trace amounts o substances have led to advances in the
understanding o plant hormones and their eect on gene expression.
Growth in plants
Undierentiated cells in the meristems o plants allow
indeterminate growth.
The growth o a plant is an everyday phenomenon, but it is nonetheless
remarkable. Most animals and some plant organs undergo determinate
growth; that is, there is either a defned juvenile or embryonic period
or growth stops when a certain size is reached or a structure is ully
ormed. Growth can also be indeterminate when cells continue to divide
indefnitely. Plants, in general, have indeterminate growth.
Many plant cells, including some ully dierentiated types, have the
capacity to generate whole plants; i.e., the cells are totipotent. This
phenomenon is what sets plant cells apart rom most animals.
Growth in plants is confned to regions known as meristems. Meristems
are composed o undierentiated cells that are undergoing active cell
division. Primary meristems are ound at the tips o stems and roots. They
are called apical meristems. The root apical meristem is responsible
or the growth o the root. The shoot apical meristem is at the tip o the
stem. Many dicotyledenous plants also develop lateral meristems.
422
9.3 GrowTh in planTs
Role of mitosis in stem extension and dome of cells at centre
leaf development of apical meristem
Mitosis and cell division in the shoot apex provide youngest
cells needed for extension of the stem and development developing
of leaves.
leaf
Cells in meristems are small and go through the cell cycle repeatedly to
produce more cells, by mitosis and cytokinesis. These new cells absorb region of
nutrients and water and so increase in volume and mass. stem growth
developing
The root apical meristem is responsible or the growth o the root. bud
The shoot apical meristem is more complex. It throws o the cells
that are needed or the growth o the stem and also produces the Figure 1 Structure o a shoot apical meristem
groups o cells that grow and develop into leaves and owers. With
each division, one cell remains in the meristem while the other SAM
increases in size and dierentiates as it is pushed away rom the
meristem region. Figure 1 shows the shoot apical meristem o a FC P2
dicotyledonous plant. P1
Each apical meristem can give rise to additional meristems including Figure 2 In this image, the shoot apical
protoderm, procambium and ground meristem. In general these give meristem is labeled SAM. P1 and P2
rise to dierent tissues. For example, protoderm gives rise to epidermis, represent newly orming leaves (primordial
procambium usually gives rise to vascular tissue and ground meristem leaves) and FC reers to the ounding cells
can give rise to pith. The position o these tissues and some o the o a new lea that has yet to diferentiate
tissues they give rise to are shown in fgure 4. Chemical inuences also
play a large role in determining which type o specialized tissue arises lea primordia
rom unspecialized plant cells. Young leaves are produced at the sides
o the shoot apical meristem. They appear as small bumps known as
lea primordia.
Figure 3 This is a developing ower bud on a shoot apical Figure 4 apical meristem
meristem o a variety o the gunsight clarkia (Clarkia xantiana) procambium
plant. A shoot apical meristem is where new growth takes place axillary bud
in a owering plant. Floral buds (red) are developing between protoderm
lea axils (green) , surrounding the oral meristem dome (blue) ground meristem
423
9 PLANT BIOLOGY (AHL)
Plant hormones afect shoot growth
Plant hormones control growth in the shoot apex.
A hormone is a chemical message that is produced and released in one
part o an organism to have an eect in another part o the organism.
Auxins are hormones that have a broad range o unctions including
initiating the growth o roots, inuencing the development o ruits and
regulating lea development. The most abundant auxin is indole-3-acetic
acid (IAA) . IAA has a role in the control o growth in the shoot apex.
Among other eects, IAA promotes the elongation o cells in stems. IAA
is synthesized in the apical meristem o the shoot and is transported
down the stem to stimulate growth. At very high concentrations, it can
inhibit growth.
Axillary buds are shoots that orm at the junction, or node, o the stem
and the base o a lea. As the shoot apical meristem grows and orms
leaves, regions o meristem are let behind at the node. Growth at these
nodes is inhibited by auxin produced by the shoot apical meristem.
This is termed apical dominance. The urther distant a node is rom the
shoot apical meristem, the lower the concentration o auxin and the
less likely that growth in the axillary bud will be inhibited by auxin. In
addition, cytokinins, hormones produced in the root, promote axillary
bud growth. The relative ratio o cytokinins and auxins determine
whether the axillary bud will develop. Gibberellins are another category
o hormones that contribute to stem elongation.
Data-based questions: The acid growth hypothesis
The acid growth hypothesis o auxin eect is cellulose. As the cell wall becomes weak, turgor
that auxin stimulates the action o a proton (H+) pressure rom within the cell pushes the wall
pump. The pump moves protons out o the cell, outward, causing elongation.
leading to an increase in the acidity o the cell
wall. This leads to the activation o the protein When a shoot frst emerges, it has a protective
called expansin. Expansin plays a role in breaking sheath called a coleoptile. Oat coleoptiles were
and reorming the connections between cellulose bathed in a solution containing IAA. The pH o
fbres and the polysaccharides that cross-link the the solution surrounding the coleoptiles was
determined (see fgure 5) .
a) Suggest the eect o the application o IAA
240 on the pH o the solution surrounding the
6.0 200 coleoptiles. [3]
5.5 pH length
5.0
pH 160 b) Estimate the time where the change in length
elongation (m)
o the coleoptiles was the greatest. [1 ]
120
c) Outline the relationship between pH and
80 change in length. [2]
4.5 IAA 40 In a urther experiment, coleoptiles were
immersed in a pH 3 solution at time zero. The
0 frst arrow in fgure 6 indicates the point where
0 10 20 30 40 50 60 the coleoptiles were transerred to a pH 7
solution. The second arrow indicates the point at
time (min) which IAA was added.
Figure 5
424
9.3 GrowTh in planTs
elongation elongation unpeeled segments in auxin
elongation
1.00
auxin
control
IAA 0.50
+KCN
10 0 10 20 40 60 80 100 0 30 60 KCN added
time (min) 0 min at arrow
Figure 6 continuous
Figure 7 KCN with auxin
90 30
d) Compare the eect o pH 3 on elongation third treatment group (the control) did not have
with the eect o pH 7 on elongation. KCN applied.
e) State the eect o the addition o IAA on f) State the eect o the addition o KCN on
elongation. elongation.
To test the hypothesis that active transport g) Based on the data, to what extent is there
plays a role in mechanism o action o auxin, a support or the conclusion that auxin
respiratory inhibitor (potassium cyanide, KCN) stimulates the active transport o protons out
was applied continuously to one treatment group, o the shoot and that these protons promote
and to a second treatment group at the arrow. A elongation.
Plant tropisms Figure 8 Fuchsia plant (Fuchsia sp.) growing
to the left towards a light source. This kind of
Plants respond to the environment by tropisms. directional plant growth in response to light
is known as phototropism
Plants use hormones to control the growth o stems and roots. Both the
rate and the direction o growth are controlled. The direction in which
stems grow can be inuenced by two external stimuli: light and gravity.
Stems grow towards the source o the brightest light or in the absence
o light they grow upwards, in the opposite direction to gravity. These
directional growth responses to directional external stimuli are called
tropisms. Growth towards the light is called phototropism and growth
in response to gravitational orce is called gravitropism.
Auxin infuences gene expression
Auxin infuences cell growth rates by changing the
pattern o gene expression.
The frst stage in phototropism is the absorption o light by
photoreceptors. Proteins called phototropins have this role. When they
absorb light o an appropriate wavelength, their conormation changes.
They can then bind to receptors within the cell, which control the
transcription o specifc genes. Although much research is still needed
in this feld, it seems likely that the genes involved are those coding or
a group o glycoproteins located in the plasma membrane o cells in the
stem that transport the plant hormone auxin rom cell to cell, called
PIN3 proteins.
Figure 9 A seed of Brassica napus 425
showing geotropism
9 PLANT BIOLOGY (AHL)
Intracellular pumps
Auxin efux pumps can set up concentration gradients
o auxin in plant tissue.
The position and type of PIN3 proteins can be varied to transport
auxin to where growth is needed. If phototropins in the tip detect a
greater intensity of light on one side of the stem than the other, auxin
is transported laterally from the side with brighter light to the more
shaded side. Higher concentrations of auxin on the shadier side of the
stem cause greater growth on this side, so the stem grows in a curve
towards the source of the brighter light. The leaves attached to the stem
will therefore receive more light and be able to photosynthesize at a
greater rate.
activity cell elongation
I AA
Design experiments to molecule
determine i potato eyes
positively or negatively Figure 10
gravitropic. Are they positively Gravitropism is also auxin dependent. The upward growth of shoots and
or negatively phototropic? Does the downward growth of roots occurs in response to gravity. If a root is
phototropism occur i the apical placed on its side, gravity causes cellular organelles called statoliths to
meristems are pinched o? accumulate on the lower side of cells. This leads to the distribution of
PIN3 transporter proteins that direct auxin transport to the bottom of the
cells. High concentrations of auxin inhibit root cell elongation so the top
cells elongate at a higher rate than the bottom cells causing the root to
bend downward. Note that the pattern of auxin effect is opposite to what
happens in the shoot. In the shoot, auxin promotes elongation but in the
root auxin inhibits shoot elongation.
vascular tissue
auxin
auxin
auxin
apical meristem
root cap
statoliths
PIN3
Figure 11
426
9.3 GrowTh in planTs
Micropropagation of plants
Micropropagation of plants using tissue from the shoot apex, nutrient agar gels
and growth hormones.
Micropropagation is an in vitro procedure that produces large
numbers o identical plants.
A stock plant is identifed that oten has some desirable
eature. Micropropagation depends on the totipotency
o plant tissues; i.e., their ability to dierentiate into any
unctional plant part.
Tissues rom the stock plant are sterilized and cut into Figure 12
pieces called explants. For most applications, the least
dierentiated tissue serves as the source tissue such as a
meristem. The explant is placed into sterilized growth media
that includes plant hormones. Inclusion o equal proportions
o auxin and cytokinin into the media leads to the ormation
o an undierentiated mass called a callus. I the growth
media contains a ratio o auxin that is greater than ten times
the amount o cytokinin, then this is called rooting media
and roots develop. I the ratio o auxin to cytokinin is less
than 1 0:1 , then this is called shoot media and shoots develop.
Once roots and shoots are developed, the cloned plant can be
transerred to soil.
Micropropagation is used for rapid bulking up
Use of micropropagation for rapid bulking up of new varieties, production of virus-free
strains of existing varieties and propagation of orchids and other rare species.
The international exchange o plant materials carries with
it the risk o transmission o pathogens. Micropropagation
techniques can be used to produce virus-ree strains o
plants. Viruses are transported within a plant rom cell to
cell through vascular tissue and via plasmodesmata. The
apical meristem is thereore oten ree o viruses.
Micropropagation can be used in the production o plants with Figure 13 Ophrys lutea
desirable characteristics, producing many identical copies o
an individual. The process is also much aster and takes up less
space than traditional methods o production. For example,
it is being used in the preservation o species such as orchids.
Oten the target o collection in the wild, the bulk production
o endangered varieties o orchids allows or wild replanting as
well as a method or commercial production. Further, the seeds
o orchids are difcult to germinate. Asexual reproduction is
oten more successul. Micropropagated plantlets can be stored
in liquid nitrogen a technique known as cryopreservation.
This is equivalent in unction to a seed bank.
427
9 PLANT BIOLOGY (AHL)
The loss o habitat or the orchid species Ophrys cultures was collected rom open felds. Once the
lutea ( fgure 1 3 ) in Malta, combined with their process o plantlet production is complete, the
normally low seed production and low rates o intent is to both replant the orchid back into the
successul germination identifed them as a target wild habitat as well as to maintain a stock o the
or conservation. The material needed to start the threatened species.
Genomics has improved understanding of the role of
plant hormones
Developments in scientic research ollow improvements in analysis and
deduction: improvements in analytical techniques allowing the detection o
trace amounts o substances have led to advances in the understanding o plant
hormones and their efect on gene expression.
Many o the classical experiments on the action o Brassica plant cells are relatively large and so
auxin such as those by D arwin and Went involved cellular activity is readily observable.
experiments with coleoptiles. Modern genomics
has opened up opportunities to understand Encoded protein Level of increase Level of increase
mechanisms and pathways in a way that would shaded vs bottom vs
not have been previously possible. lit ank top ank
Microarrays allow researchers to detect gene -expansin 3.9 2.2 3.9 2.9
expression. I a gene is being expressed, then putative oxidase 5.2 0.5 1.4 0.4
when the tissue is tested on the microarray, it will IAA-amido synthetase 1.6 0.3 1.7 0.3
cause uorescence. SAUR protein 1.3 0.5 1.4 0.2
BHLH transcription factor 1.7 0.2 2.0 0.9
In one such study, researchers ound that seven HD-zip transcription factor 1.9 0.3 2.3 0.4
genes are expressed at higher levels on the bottom IAA-amido synthetase 4.6 1.9 1.9 0.4
cells in gravitrophically stimulated cells and on the
shady side o phototrophically stimulated cells. Figure 14 The efect o light and gravity on the expression o
seven genes
The analysis o gene expression takes advantage
o the knowledge o model plants like Arabidopsis
thaliana and its close relative Brassica oleracea.
428
9.4 reproDucTion in planTs
9.4 rdt t
Understanding Applications
Flowering involves a change in gene expression Methods used to induce short-day plants to
in the shoot apex. fower out o season.
The switch to fowering is a response to the Skills
length o light and dark periods in many plants.
Drawing internal structure o seeds.
Most fowering plants use mutualistic Drawing ohal-views oanimal-pollinated fowers.
relationships with pollinators in sexual Design o experiments to test hypotheses
reproduction.
about actors aecting germination.
Success in plant reproduction depends on
pollination, ertilization and seed dispersal.
Nature of science
Paradigm shits; more than 85% o the worlds 250,000 species o fowering plants depend on pollinators
or reproduction. This knowledge has led to protecting entire ecosystems rather than individual species.
Flowering and gene expression Figure 1 The poinsettia is a short-day plant
Flowering involves a change in gene expression in the Figure 2 Red clover is a long-day plant
shoot apex.
429
When a seed germinates, a young plant is ormed that grows roots,
stems and leaves. These are called vegetative structures and the plant
is in the vegetative phase. This can last or weeks, months or years, until
a trigger causes the plant to change into the reproductive phase and
produce fowers. The change rom the vegetative to the reproductive
phase happens when meristems in the shoot start to produce parts o
fowers instead o leaves.
Flowers are structures that allow or sexual reproduction, thereby
increasing variety. They are produced by the shoot apical meristem and
are thereore a reproductive shoot.
Temperature can play a role in transorming a lea- producing shoot
into a fower-producing shoot, but day length is the main trigger, or
more precisely the length o the dark period. Some plants such as the
poinsettia ( Euphorbia pulcherrima) are categorized as short- day plants
because they fower when the dark period becomes longer than a critical
length, or example in the autumn. Other plants such as red clover
( Trifolium pratense) are long- day plants because they fower during the
long days o early summer when nights are short.
Light plays a role in the production o either inhibitors or activators
o genes that control fowering. For example in long-day plants, the
active orm o the pigment phytochrome leads to the transcription o
9 PLANT BIOLOGY (AHL)
a fowering time (FT gene) . The FT mRNA is then transported in the
phloem to the shoot apical meristem where it is translated into FT
protein. The FT protein binds to a transcription actor. This interaction
leads to the activation o many fowering genes which transorm the
lea-producing apical meristem into a reproductive meristem.
PR Photoperiods and fowering
RAPIDLY SLOWLY The switch to fowering is a response to the length o light
IN and dark periods in many plants.
RAPIDLY
FAR-RED IN RED Long-day plants fower in summer when the nights have become short
LI GH T IN LIGHT enough.
(660 nm)
(730 nm) OR WHITE Short-day plants fower in the autumn (all) , when the nights have
THE LIGHT become long enough.
( 40 0 -70 0
nm) Observations o fowering suggested that the trigger or this in some
DARK plants might be a particular day length, but experiments have shown
that it is the length o darkness that matters, not the length o daylight.
PFR
A pigment was discovered in leaves that plants use to measure the
phytochrome length o dark periods. It is called phytochrome and is unusual as it can
is located switch between two orms, PR and PFR.
in leaves
When PR absorbs red light o wavelength 660 nm it is converted
Figure 3 Interconversions of phytochrome into PFR.
When PFR absorbs ar- red light, o wavelength 73 0 nm, it is converted
to P . This conversion is not o great importance as sunlight contains
R
more light o wavelength 660 nm than 730 nm, so in normal
sunlight phytochrome is rapidly converted to PFR.
However, P is more stable than P , so in darkness P very gradually
R FR FR
changes into PR.
Further experiments have shown that PFR is the active orm o
phytochrome and that receptor proteins are present in the cytoplasm to
which PFR but not PR binds.
In long-day plants, large enough amounts o PFR remain at the
end o short nights to bind to the receptor, which then promotes
transcription o genes needed or fowering.
In short-day plants, the receptor inhibits the transcription o the
genes needed or fowering when PFR binds to it. However, at the end
o long nights, very little P remains, so the inhibition ails and the
FR
plant fowers.
Data-based questions: Sowing times for soybeans
Soybeans are rich in protein and are eaten both and pods containing beans develop rom them.
by humans and livestock. Ater germination, When they start to fower, soybean plants stop
soybean plants grow a series o sections o stem, growing more nodes and internodes.
with nodes between them. Leaves are produced
at the nodes. The stem sections are called Figure 4 shows the mean numbers o nodes o
internodes. Flowers are produced at each node soybean plants sown on dierent dates in Nebraska.
430
9.4 reproDucTion in planTs
1 Compare the growth o the soybean plants 22
20
sown on the dierent dates. [5] Mean number of nodes per plant18
1-May16
2 a) Deduce when the soybeans started to 16-May142-May
31-May1217-May
o w e r. [2] 15-Jun1030-May
30-Jun817-Jun
b) Deduce with reasons, the actor that 15-Jul6
30-Jul4Calendar date
triggers owering in soybeans. [3] 14-Aug2
29-Aug0
3 a) Explain the advantage, in terms o -2
soybean yields, o sowing the crop as
early as possible. [3]
b) Suggest two possible disadvantages o
sowing soybeans earlier than the dates
used in the trial. [2] Figure 4
Inducing plants to fower out o season
Methods used to induce short-day plants to fower out o season.
Flower orcing is a procedure designed to get The Siam tulip Curcuma alismatifolia is sold as cut
owers to bloom out o season or at a specifc owers. It normally produces owers seasonally
time such as during holiday time. Growers can during the rainy season where long-day conditions
manipulate the length o the days and nights to apply. Providing additional light in the middle o the
orce owering. night leads to owering in the o-season provided
that enough humidity and nutrients are provided.
Draw an animal pollinated fower
Drawing o hal-views o animal-pollinated fowers.
Figure 5 shows a ower o Prunus domestica. In stigma
the base o the ower are nectar-secreting glands, style
which attract insects, especially bees. The petals anther
are large and white, helping insects to fnd the petal
ower. The sepals protect the ower bud during
its development and at night when buds close. lament
The anthers produce pollen, containing the male sepal
gametes. The flaments hold the anthers in a ovary
position where they are likely to brush pollen
onto visiting insects. The emale part o the ower Figure 5 Structure o a plum fower
is called a carpel. It consists o a stigma, style and
ovary. The stigma is sticky and will capture pollen
rom the visiting insect. The stigma is held up
by the style. The ovary is located inside a small
rounded structure called an ovule.
431
9 PLANT BIOLOGY (AHL)
Figure 6 Honeybee pollinating common Mutualism between fowers and pollinators
mallow fower
Most fowering plants use mutualistic relationships with
pollinators in sexual reproduction.
Sexual reproduction in owering plants depends on the transer o pollen
rom the stamen to a stigma o another plant. Pollen is transerred between
plants via a number o strategies including wind and less commonly
water but most commonly by animals known as pollinators. Examples o
pollinators include birds, bats and insects such as butteries and bees.
Mutualism is a close association between two organisms where both
organisms beneft rom the relationship. Pollinators gain ood in the orm
o nectar and the plant gains a means to transer pollen to another plant.
Figure 6 shows a honeybee (Apis mellifera) covered in pollen ater visiting
a mallow ower (Malva sylvestris) . Figure 7 shows a purple-throated
carib hummingbird (Eulampis jugularis) that has a curved bill which is an
adaptation or extracting nectar rom the elongated ower o Heliconia bihai.
Figure 7 Purple-throated carib hummingbird Pollination, ertilization and seed dispersal
pollen grain pollen tube Success in plant reproduction depends on pollination,
containing ertilization and seed dispersal.
male gametes
The next process ater pollination is fertilization. From each pollen
surface of stigma grain on the stigma a tube grows down the style to the ovary. The pollen
through which the tube carries male gametes to ertilize the ovary. The ovary is located
pollen tube grows inside a small rounded structure called an ovule.
Figure 8 Pollen grain germinating on a stigma The ertilized ovule develops into a seed and the ovary develops into
at the start o the ertilization process a ruit.
Seeds cannot move themselves, but nonetheless they oten travel long
distances rom the parent plant. This is called seed dispersal and it
reduces competition between ospring and parent and helps to spread
the species. The type o seed dispersal depends on the structure o
the ruit dry and explosive, eshy and attractive or animals to eat,
eathery or winged to catch the wind, or covered in hooks that catch
onto the coats o animals.
Data-based questions: Factors afecting pollen development
Pollen grains sometimes develop when they are use ICT or you can draw graphs, tables, charts
placed in a drop o uid on a microscope slide. or diagrams by hand.
The composition o the uid and its temperature
aect whether this happens or not. Table 1 show 2 Describe clearly any trends that you have
the results o studies o pollen development in ound in the data. Try to explain each trend
plant species in Hong Kong. that you describe, using your biological
knowledge.
1 The data in table 1 is difcult to analyse in its
current orm. Choose suitable presentation 3 Identiy any weaknesses in the data obtained.
ormats to display the data clearly and allow Suggest how the investigation could have
you to identiy any signifcant trends. You can been improved.
432
9.4 reproDucTion in planTs
pt Dt m ot s tt ptg f Camellia
f gwth f ( d3) japonica g
g () tb .
( h1) ( d3) 0.30 tht dvd
0.46 22.5
Bougainvillea 44.00 41.8 0.75 0.60 23.0
glabra 0.75 13.0
0.90 0.0
Delonix regia 70.30 4.9 0.45 0.0
c tt
Leucaena 64.60 111.0 0.75 () m gwth f
leucocephala 71.50 69.9 0.45 tb f Bougainvillea
91.60 11.1 0.30 0.0
Bauhinia 86.82 50.6 0.45 1.0 glabra ( h1)
purpurea 2.5 33.6
5.0 25.1
Lilium 25.0 15.5
bulbiferum 10.8
0.0
Gladiolus
gandavensis
Table 1
Preserving habitats as a conservation measure
Paradigm shits: more than 85% o the worlds 250,000 species o fowering
plants depend on pollinators or reproduction. This knowledge has led to
protecting entire ecosystems rather than individual species.
The growth in number and nature o threats
to biodiversity in combination with scarce
resources being devoted to conservation means
that traditional conservation measures have
to be re- evaluated. Traditionally, the ocus
o conservation eorts was on populations
and species o particular concern. The close
association between such organisms as
pollinators and fowering plants suggests that it
is the ecosystem and biological processes which
must be protected.
The S aguaro cactus ( Carnegiea gigantea) is a Figure 9 Bat approaching a Saguaro cactus fower.
keystone species o the Sonoran desert. They
provide important perching and nesting sites
or birds such as red-tailed hawks and nesting
cavities or gilded fickers, gila woodpeckers,
el owls, purple martins and other birds. Once
the Saguaro ruit ripens, lesser long-nosed bats
( Leptonycteris yerbabuenae) , white- winged doves,
gila woodpeckers and other birds consume the
433
9 PLANT BIOLOGY (AHL)
ruits and disperse the seeds, which pass through The lesser long-nosed bat is listed as an
their guts intact. endangered species under the US Endangered
S pecies Act. However, invasive grasses,
The owers o the cactus bloom or just one development o the desert or human
evening each year. They attract lesser long- nosed habitation and changes in natural fre cycles
bats and mexican long-tongued bats ( Choeronycteris threaten the cactus. The survival o both bats
mexicana) to their nectar. The bats use their and their desert ood plants are threatened
elongated snouts to reach into the owers or by loss o habitat. Ensuring the uture o
nectar, covering their heads with pollen that they the Sonoran desert ecosystem depends on
then transer rom ower to ower as they y protecting the roles played by the bat, the cactus
rom cactus to cactus throughout the night. and seed-dispersing animals.
The structure of seeds
Drawing internal structure of seeds. ood storage tissue called endosperm. The scientifc
name or the seed coat is the testa. There is a small
A seed is a package containing an embryo plant and hole through the testa, called the micropyle. It is
ood reserves, all inside a protective seed coat. The located next to a scar where the seed was attached
embryo plant consists o an embryo root, embryo to the parent plant. Figure 1 0 shows the external
shoot and one or two cotyledons, depending and internal structure o a bean seed (Phaseolus
on whether the plant is monocotyledonous or vulgaris) . Figure 1 1 shows an annotated diagram o
dicotyledonous. The cotyledons are the embryo the same seed.
leaves and in many plants they contain the ood
reserves o the seed. In other seeds there is a special
testa
External structure
seed coat
( testa )
scar where seed
was attached to
the ovary
micropyle
embryo shoot Internal structure
embryo root
embryo root embryo shoot
( rad icle) (plumule)
seed coat cotyledon
one of two
in the seed
cotyledons Figure 11
Figure 10 Structure of bean seed ( Phaseolus
vulgaris) ; external structure (above) ; internal
structure (below)
434
9.4 reproDucTion in planTs
Germination experiment design
Design o experiments to test hypotheses about actors afecting germination.
The early growth o a seed is called germination. down starch in the ood reserves into maltose.
Some seeds do not immediately germinate, even Other enzymes convert the maltose into sucrose
i given the conditions normally required. This is or glucose. Whereas starch is insoluble and
called dormancy and it allows time or seeds to be immobile, sucrose and glucose can be transported
dispersed. It may also help to avoid germination rom the ood reserves to where they are needed
at an unavourable time. All seeds need water in the germinating seed. The embryo root and
or germination. Many seeds are dry and need shoot need sugars or growth, together with
to rehydrate their cells. Some seeds contain a amino acids and other substances released rom
hormone that inhibits germination and water is the ood stores. All parts o the embryo need
needed to wash it out o the seed. Germination glucose or aerobic cell respiration.
involves growth o the embryo root and shoot and
this also requires water. Most vegetable crop varieties have been bred to
germinate quickly they do not usually have long
The metabolic rate o a dry and dormant seed periods o seed dormancy. Nevertheless, growers
is close to zero, but ater absorption o water, o vegetable crops sometimes have difculty in
metabolic processes begin again, including energy getting crops to germinate ater sowing.
release by aerobic cell respiration. Another
requirement or germination is thereore a supply Choose one o the possible causes o crop ailure
o oxygen. Because germination involves enzyme- shown in the mind-map, to investigate.
catalysed metabolic reactions, warmth is required
and germination oten ails at low temperatures. Design an experiment and see whether you obtain
evidence or or against your cause.
Another metabolic process occurring at the start You will need to decide:
o germination is synthesis o gibberellin, a plant
hormone. Several genes have to be expressed to which seed type to use
produce the various enzymes o the metabolic
pathway leading to gibberellin. This hormone how to vary the actor that you are
stimulates mitosis and cell division in the embryo. investigating
In starchy seeds it also stimulates the production
o amylase. This enzyme is needed to break how to keep other actors constant
how to collect your results, including how to
assess whether germination has occurred.
Seed too old not viable
any more.
Soil temperature too high Seed needed darkness for
or too low. germination but was sown
on the soil surface.
Soil was too dry and Slugs, snails or other pests
the seeds remained ate the seedlings or mice
dehydrated. ate the seeds.
Seeds needed light for Seed kept in unsuitable
germination but were sown conditions, e.g. too hot.
below the soil surface. Seeds sown too deeply,
so ran out of food before
Soil waterlogged and shoot reached the light.
anaerobic, so seedlings
died of ethanol poisoning.
435
9 PLANT BIOLOGY (AHL)
Data-based questins: Fire and seed dormancy in a plant
of the chaparral
Emmenanthe pendulifora grows in chaparral b) Suggest a hypothesis or the germination
(shrubland) in Caliornia. It is rarely seen in o plants o Emmenanthe pendulifora ater
unburnt chaparral, but appears ater fres, fres, based on the dierences
growing to about 250 mm, owering, orming in staining that you have described. [2]
seed and dying in a ew months. The electron
micrographs below show the results o an 4 S uggest two advantages to Emmenanthe
experiment in which seeds o the plant were
treated with smoke or 3 minutes and then pendulifora o dormancy ending ater fres in
soaked in a solution o lanthanum nitrate
hexahydrate. the chaparral. [2]
A = control A
seed
1 The scale bars in the electron micrographs testa
represent 1 m. Calculate the thickness
o waxy cuticle between the testa and the waxy
cuticle
embryo and ood stores inside the control
seed. [2]
2 The lanthanum solution appears as dark B = smoke- B
treated testa
staining in the electron micrographs and seed
shows how ar water was able to penetrate.
Deduce how ar water could penetrate into waxy
cuticle
the control seeds. [2] embryo
3 a) Compare the staining o the waxy cuticle Figure 12 Two electron micrographs of Emmenanthe
in the smoke-treated seeds with the pendulifora. (A) control seed (above) ; ( B) smoke-
staining o the cuticle in the control seeds. treated seed (below)
[2]
ToK
what are the limitatins f the telelgical viepint?
The teleological viewpoint argues that nature tends toward denite ends; i.e., that
nature has intention and that natural selection is a directed process.
Within the tissues supporting the seeds othe jalapeo pepper, Capsicum annuum,
is a chemical known as capsaicin. Chewing by human and mammal molars
destroy the seed tissue and releases the chemical. This irritates the mucous
membrane and causes a pain sensation. The pain production rom consuming the
seeds suggests that it is an adaptation by the plant to protect itselrom mammal
consumption. Despite this, the jalapeo pepper is part othe cuisine oseveral
cultures. Bird digestive tracts do not damage the seeds and are unafected by
the capsaicin. Further, the bird distributes the seeds aiding seed distribution and
providing ertilizer to assist germination. A teleological statement in this case is
that the pepper wants to be eaten by the bird and that humans are not meant to
eat the seed. Critics othe teleological viewpoint argue that evolution by natural
selection is not a directed process and that mutations arise by chance, with those
mutations that impart an advantage being more likely to persist in the population.
436
QuesTions
Questions
1 The graphs in fgure 1 3 show the results o 2 In order to prevent transer o pollen rom
investigations into the rate at which water is an anther o one plant to the stigma o the
able to diuse though the waxy cuticle o plants, same plant (sel-pollination) , the sunower
which is called the water permeance o the ( Helianthus spp.) anther sheds its pollen beore
cuticle. Figure 1 3 shows the relationship between the stigma is mature enough to receive it.
temperature and water permeance o our Early in the morning the anther is exposed by
species o plant. Figure 1 3 shows the relationship elongation o the flaments. The anthers open
between the thickness o cuticular wax and at this time to release their pollen (anthesis) .
water permeance. The results o the experiment The stigma appears above the anthers by late
show how important it is to test hypotheses, aternoon, and by the ollowing morning it is
even when it may seem that this is not necessary. ully receptive.
permeance 106 /ms1 20 Hedera To see how the flament ( F) and the style
Camellia (S) are aected by light, their lengths were
measured at time intervals starting 1 2 hours
15 Pyrus beore anthesis ( 1 2) . Some plants were grown
Liriodendron in continuous white light (L24) and some
plants grown under cycles o 1 6 hours white
10 light ollowed by 8 hours dark (L1 6/D8) . The
results are shown in the graph.
5
0 40 45 50 55 light
35 temperature /C L24
dark light dark light
(a) light 5.5
L1 6 /D 8 S (L16/D8) 5.0
6 S (L24) 4.5
11 F (L16/D8) 4.0
permeance for water /ms1 5 F (L24)
10
4 lament length/mm
9
3 style length/mm
2 8 3.5
1 7 3.0
2.5
0
0123456 6 2.0
(b) thickness of cuticular wax /m 5 1.5
-12-10-8 - 6- 4-2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
Figure 13 Factors afecting water
permeance o waxy cuticle time after start of anthesis/hours
Source: Lobello et al, Journal ofExperimental Botany, (2000) , 51,
pages 14031412
a) Using the data in fgure 1 3, describe the
relationship between temperature and a) Filaments o the plants grown in continuous
water permeance. [2] white light increased in length by 0.25 mm
b) Discuss the consequences or plants o the in the 28 hours ater anthesis. Calculate
eect o temperature on cuticular water how much the flaments o the plants
permeance. [3] grown in alternating white light and dark
c) Using the data in fgure 1 3, state the increased during the same period. [1 ]
thickness o cuticular wax with:
b) Compare the increase in the length o the
(i) the highest water permeance style in the plants grown in continuous
(ii) the lowest water permeance. [2] white light with those grown in alternating
d) Evaluate the hypothesis that the water white light and dark. [2]
permeance o the cuticle is positively The table compares the percentage o ovules
that have been ertilized and developed
correlated with its thickness, using the data into seeds in sunower plants grown under
in fgure 1 3. [3]
437
9 PLANT BIOLOGY (AHL)
continuous white light with those grown The graph below shows the sodium ion
under alternating light and dark. The numbers concentration in plant parts o sweet
represent the mean one standard deviation. pepper grown in 1 5 mM sodium chloride or
three weeks.
light treatments percentage of 30
fertiized ovues 28
Continuous white light (L24) 26
Alternating light and dark 11.40 7.76 24
( L1 6 /D 8 ) 22
58.26 4.06 20
lament length/mm 18
sodium ion concentration/mM kg-1c) Explain the dierences in the percentages o16
ovules ertilized using the data in the graph 14
about the growth o flaments and styles. [3] 12
10
d) Explain how standard deviation (SD) 8
6
shown in this table can be used to help in 4
2
comparing the eect o light treatments on 0
the ertilization o ovules. [3] root
To analyse the eect o growth regulators on leaves fruit stem
flament elongation, urther experiments were
perormed in the dark, white light and red light. plant part
The owers were treated with auxin or with
gibberellic acid and compared to a control with Source: M Blom-Zandstra et. al. , Sodium fuxes in sweet pepper
no growth regulator. The results are shown in exposed to varying sodium concentrations, Journal ofExperimental
the bar chart below. Botany (1 November 1998) , vol. 49, issue 328, pp. 18631868, by
permission o Oxord University Press
6 a) (i) State the concentration o sodium ions
control auxin gibberellic acid
in ruits. [1 ]
5
(ii) Calculate the percentage increase in
4
sodium ion concentration between
3
root and stem. [1 ]
2
b) Suggest why a high sodium ion
1
concentration in the cells o the stem
0
continuous dark continuous white continuous red is important in providing support to this
light light
type o plant. [1 ]
c) State one possible use o sodium in plants. [1 ]
e) Identiy, with reasons, which actors [3] d) Scientists also ound that the concentrations
promote and which actors inhibit the [2] o sodium ion in cells o the stem and in
elongation o flaments. xylem sap were the same. Explain why this
led the scientists to believe there was no
) Explain the disadvantages to a plant o active transport between xylem and stem. [2]
s e l- p o llinatio n.
e) Suggest one possible method o
transport o sodium ions between xylem
3 S weet pepper ( Capsicum annuum) is an and stem. [1 ]
important widespread agricultural crop.
Scientists studied the transport and distribution
o sodium in sweet pepper by growing plants in
sodium chloride solutions.
438
10 GeN etI cS aN d evo lU tI o N
(aHl)
Inruin independent assortment of chromosomes and
the unique composition of alleles in daughter
Inheritance follows principles that have been cells. Gene pools change over time.
discovered by research from the 1 9th century
onwards. Genes may be linked or unlinked
and are inherited accordingly. Meiosis leads to
10.1 Meiosis
Unrsning Skis
Chromosomes replicate in interphase before Drawing diagrams to show chiasmata formed
meiosis. by crossing over.
Crossing over is the exchange of DNA material Nur f sin
between non-sister homologous chromatids.
Making careful observations: careful
Chiasmata formation between non-sister observation and record keeping turned
chromatids in a bivalent can result in an up anomalous data that Mendels law of
exchange of alleles. independent assortment could not account for.
Thomas Hunt Morgan developed the notion of
Crossing over produces new combinations of linked genes to account for the anomalies.
alleles on the chromosomes of the haploid cells.
Homologous chromosomes separate in meiosis.
Independent assortment of genes is due to
the random orientation of pairs of homologous
chromosomes in meiosis I.
Sister chromatids separate in meiosis II.
439
10 Genetics and evolution (aHl)
tetrad/ chromosom rpliation
bivalent
Chromosomes replicate in interphase beore meiosis.
non-sister sister
chromatids chromatids Like mitosis, meiosis follows a period of interphase with the cell
cycle phases of G , S and G . In the S phase, DNA is replicated so
prophase I of meiosis
12
Figure 1
that each chromosome consists of two chromatids. At the start of
TOK meiosis, the chromosomes condense and are visible as two chromatids,
called sister chromatids. Unlike mitosis, pairing, or synapsis, occurs
What is the role of chance in where homologous chromosomes come to align beside each other.
scientific discovery? The combination is referred to as a tetrad as it is composed of four
chromatids. It is also referred to as a bivalent as it is composed
The theory that chromosome behavior of a homologous pair. In many eukaryotic cells, a protein-based
accounts or Mendel's principles structure forms between the homologous chromosomes called the
o segregation and independent synaptonemal complex.
assortment is known as the Sutton-
Boveri chromosome theory o exhang of gnti matrial
inheritance. Sutton and Boveri
were two scientists who worked Crossing over is the exchange o DNA material between
independently, but it was Sutton who non-sister homologous chromatids.
was the frst to publish his research.
Boveri studied Parascaris equorum, a D uring prophase I of meiosis breaks in the D NA occur. Following these
roundworm with large cells, containing chromosome breaks, non-sister chromatids invade a homologous
only two pairs o chromosomes. sequence on a non-sister chromatid and bind in the region of the break.
Once crossing over is complete, the non-sister chromatids continue
Historians o science have pointed to adhere at the site where crossing over occurred. These connection
out that Sutton was aided by the points are called chiasmata (plural) or chiasma (singular) . Evidence
serendipitous use o the research suggests that connections via chiasmata are essential for the successful
organism, Brachystola magna, completion of meiosis.
a grasshopper. Sutton began his
research in Kansas and the great (a) (b) (c) (d)
abundance o grasshoppers in that
state contributed to its use as a Figure 2 The process of crossing over
research organism. Brachystola
magna had eleven pairs o chiasmata formation
chromosomes. This made it much
easier to distinguish individual Chiasmata ormation between non-sister chromatids
chromosomes by their size and shape. in a bivalent can result in an exchange o alleles.
Using similar techniques to Boveri,
he documented the confguration o A further consequence of crossing over besides stabilizing bivalents
chromosomes undergoing meiosis, at chiasmata is to increase genetic variability. This occurs because the
and made the observation that each process of crossing over results in the exchange of DNA between the
chromosome has a well-defned maternal and paternal chromosomes. Crossing over can decouple linked
shape that is conserved in each cell combinations of alleles and therefore lead to independent assortment.
generation. This prompted Sutton to Further, crossing over can occur multiple times and between different
proclaim that chromosomes may chromatids within the same homologous pair.
constitute the physical basis ofthe
Mendelian law ofheredity.
440
10.1 Meiosis
explaining discrpancis in
Mndlian ratios
Making careful observations: careful observation and
record keeping turned up anomalous data that Mendels
law of independent assortment could not account for.
Thomas Hunt Morgan developed the notion of linked
genes to account for anomalies.
Mendels paper was published in 1 866. Initially it had little impact
and was cited about three times over the next thirty-ve years.
S tarting with the turn o the century, there was increasing recognition
o his work. At the same time, some discrepancies arose between
observations and Mendels principle o independent assortment.
William Bateson and Reginald Punnett conducted crosses with sweet
peas. One o the parent plants had long pollen (LL) and purple
fowers (PP) . The other had round pollen (ll) and red fowers (pp) .
As expected all o the F had long pollen and purple fowers (LlPp) .
1
The surprising result came in the F2 generation o a dihybrid cross.
Instead o the expected ratio o 9:3:3:1 , there were ar more o the
individuals with the parental phenotypes seen in the P generation and
much smaller numbers o the non-parental phenotypes, known as
recombinants.
Even though Bateson and Punnett realized their results did not
conorm to Mendels principle o independent assortment, they
did not develop a clear explanation or the discrepancy. Thomas
Hunt Morgan observed similar discrepancies in ruit fies. His
discovery o sex linkage led him to develop a theory o gene linkage
that accounted or the higher than expected number o parental
phenotypes and the notion o crossing over to explain the presence o
the recombinants.
Nw combinations of allls
Crossing over produces new combinations of alleles
on the chromosomes of the haploid cells.
Figure 3 summarizes how crossing over can produce new
combinations o alleles on the chromosomes o the haploid cells. The
red and the blue diagrams represent homologous chromosomes. As
such they have the same length, the same centromere position and
the same gene content, but they will have dierent combinations o
alleles. In the diagram, the chromosomes are the locus or genes A,
B and D. The individual is heterozygous or all three alleles; i.e, the
individual has the genotype AaBbDd. Because the genes are linked, the
individual can produce gametes with the combinations AbD and aBd.
441
10 Genetics and evolution (aHl)
The diagram illustrates how crossing over can produce additional
combinations of alleles.
Ab Da B d
DNA replication
Ab Da B d
Ab Da B d
synapsis and recombination
ab meiosis I B D
Ab DA B d
ab da D
meiosis II
DA B
Ab da B d
Figure 3 Crossing over occurs multiple times and between diferent chromatids
within the same homologous pair
diagrams of crossing over
Drawing diagrams to show chiasmata formed by crossing over.
A chiasma is an X-shaped knot-like structure The position at which the crossing over is going to
that forms where crossing over has occurred. To occur can be shown with breaks at the same point
draw a bivalent with one or more chiasmata it in two chromatids, one in each chromosome. As
is necessary to use two coloured pens or pencils, the position of the crossover is random you can
so that the two homologous chromosomes, i.e., show this anywhere along the bivalent. You can
the maternal and paternal chromosomes can be have more than one crossover if you want.
distinguished. A series of drawings can be used to
show different stages in the process. Remember to It is hard to show the crossover while the
start with the chromosomes very elongated. chromosomes are still tightly paired as part
of it will be hidden, but one of the new
442
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