Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
CHAPTER 2
Leaf Structure
and Function
" Dan Dialah yang menurunkan air hujan dari langit, lalu
kami tumbuhkan dengan air itu segala macam tumbuh-
tumbuhan, maka Kami keluarkan dari tumbuh-tumbuhan
itu tanaman yang menghijau, Kami keluarkan dari
tanaman yang menghijau itu butir yang banyak; dan dari
mayang kurma mengurai tangkai-tangkai yang menjulai
dan kebun-kebun anggur"
Surah Al-An'am]: Ayat 99
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
2.1 STRUCTURE OF A LEAF
1 Leaf is the main organ of a plant which carries out photosynthesis
2 The structure of a leaf can be divided into two parts
External structure
Internal structure.
The External Structure of a Leaf
Structure Characteristic Explanation
1. Lamina Lamina is flat-shaped Provide a wide surface
To expose the cells containing
Lamina is thin chloroplasts to the maximum
amount of sunlight.
To allow gases involved in
photosynthesis to diffuse
efficiently in the leaf.
2. Petiole Petiole is the leaf stalk Provide support the lamina
Petiole that connects the lamina
to the stem of the plant
Stretches out into the
lamina
Producing a network of
middle veins
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Internal Structure of a Leaf Lamina
1. Leaves grow without overlapping in most plant
2. Such arrangement is called leaf mosaic
3. To enable them to receive optimum light to carry out
photosynthesis
Upper epidermis B A Cuticle
Palisades C
mesophyll
Spongy D G Xylem
H Phloem
mesophyll
Lower epidermis E
F I
Stoma Guard cell
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Structure Characteristic Function
1. Cuticle
Waxy and waterproof layer Reduces excessive loss of water
2. Upper epidermis Covers the upper and lower through evaporation
parts of the leaf epidermis. (transpiration).
3. Lower epidermis To allows sunlight to pass
Transparent layer through it.
4. Palisade mesophyll
5. Spongy mesophyll Located on the upper surface Protect the leaf from mechanical
of the leaf under the cuticle injuries
cells layer.
Transparent and does not Light can pass through it.
contain any chloroplasts
Located at the lower surface Protect the leaf from mechanical
of the leaf. injuries
Consists of stomata. To allow exchange of gaseous
Have a pair of guard cells To regulate the opening and
closing of stoma
Arranged vertically and To receive maximum light
closely-packed. exposure
Contain many chloroplasts. Sites for photosynthesis
Has irregular-shaped Increases the internal surface
area for gaseous exchange
Loosely arranged and have Ease carbon dioxide and water
many intercellular air spaces. absorption through the leaves to
palisade mesophyll cells during
Has a smaller number of photosynthesis
chloroplasts than palisade
mesophyll Sites for photosynthesis
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Structure Characteristic Function
6. Xylem Transports water and mineral salts
Vessels that are elongated, absorbed from the roots to the leaf
Xylem hollow and connected to each
other from its roots to the To provide mechanical support and
7. Phloem leaves strength to the plants
Has thick and lignified walls Transports organic substances
are and thickened produced during photosynthesis
from the leaves to other parts of the
Has elongated and plants
continuous tube structures.
2.2 MAIN ORGAN FOR GASEOUS EXCHANGE
The Necessity of Gaseous Exchange in Plants
1. Plant synthesises their own food through photosynthesis
2. Plants need to exchange gases and absorb light in order to carry out photosynthesis
efficiently
3. The exchange of oxygen and carbon dioxide between plants and the surroundings occurs
through stomata.
4. Stomata are the pores located on the lower epidermis of the leaf
5. Each stoma is guarded by a pair of guard cells
6. To controls the opening and closing of the stoma by changing their shapes.
7. Guard cells contain chloroplasts to carry out photosynthesis.
The Mechanism of Stomata Opening and Closing
1. Depends whether guard cells turgid or flaccid
2. The condition of the guard cells depends on;
Potassium ion (K+) uptake by the cells
Sucrose concentration in the sap of the guard cells
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Opening of Stoma
K+ H2O
H2O K+
Stoma close K+ and H2O diffuse into Stoma open
guard cell
Uptake of potassium ions by guard cell Sucrose concentration in the guard cell
sap
1. The potassium ions enter the guard cells 1. Photosynthesis occurs in the presence
by active transport of light
2. The solute potential in the guard cells 2. The concentration of sucrose in the
increases guard cell becomes high.
3. The water potential in the guard cells 3. The water potential in the guard cells
decreases. decreases
4. The water molecules from the epidermal 4. The water molecules from the epidermal
cells diffuse into the guard cells by cells diffuse into the guard cells by
osmosis osmosis
5. The guard cells become turgid and curve 5. The guard cells become turgid and curve
outwards outwards
6. The stoma opens 6. The stoma opens
Bio Concept: Biology Vs Chemistry
Water potential refer to Solute Water
free water molecule in the
solution Low solute High solute
The higher free water potential potential
molecule, the higher water
potential High water Low water
The higher the solute, the potential potential
lower free water molecule
Low water potential AB
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Closing of Stoma
K+ H2O
H2O K+
Stoma open K+ and H2O diffuse out into Stoma close
epidermal cell
Uptake of potassium ions by guard cell Sucrose concentration in the guard cell
sap
1. The potassium ions move out from the
guard cells 1. Photosynthesis does not occur in the
absence of light
2. The solute potential in the guard cells 2. The concentration of sucrose in the
decreases guard cell becomes low
3. The water potential in the guard cells 3. The water potential in the guard cells
increases. increases
4. The water molecules from the guard 4. The water molecules from the guard
cells diffuse out to epidermal cells by cells diffuse out to epidermal cells by
osmosis osmosis
5. The guard cells become flaccid and 5. The guard cells become flaccid and
curve inwards curve inwards
6. The stoma close 6. The stoma close
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Distribution of Stomata on the Upper and Lower Epidermis of Monocot and Eudicot
Leaves
1. Each plant has abundant of stoma on the lower epidermis compare on upper epidermis
2. Lower epidermis not exposed directly to sunlight
3. Reduce excessive water lost through evaporation when stoma open for absorption of
carbon dioxide
4. Guard cell in monocot is shape like dumbbell while in eudicot is shape like a bean
The Effect of Water Deficiency in Plants on Stomatal Opening and Closing
1. Water from plants is lost in the form of water vapour to the surroundings through the
stomata.
2. The rate of water loss from the plants is high when stoma widely opens
3. The opening and closing of the stoma are dependent on the turgor pressure of the guard
cells.
Fresh Plant Wilted Plant
1. Guard cells become turgid when the 1. Guard cells become flaccid when the
plant obtains enough water, the. plant lacks water
2. The inner cell wall of the guard cells is 2. The thin and more elastic outer cell wall
thick and less elastic as compared to 3. Causes the guard cells to lose turgidity
the outer cell wall. 4. The stoma to close
3. The thin and more elastic outer cell wall
4. Causes the guard cells to curve
outwards
5. The stoma to open
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
2.3 MAIN ORGAN FOR TRANSPIRATION
The Necessity of Transpiration in Plants
1. Transpiration is a process of water loss in the form of water vapour
2. Through evaporation from the plants to the atmosphere
3. 90% of water evaporate out through stomata pores in the leaves
4. Water diffuses into the root system by osmosis continuously to replace the loses of water
5. Plant roots absorb water and mineral salts from soil
6. The importance of transpiration are;
To give cooling effect. : Water absorbs heat energy from leaves and evaporates as
water vapour
Produces a pulling force : To moves the water and mineral salts continuously in the
xylem vessels from the roots to all plant cells
3
Water evaporite to the
atmosphere through
stoma
1 2
The xylem transport
Root absorb water water upward in the
from the soil plant stem
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Environmental Factors that Affect the Rate of Transpiration
Factors Explanation
a) Light intensity
1. The higher the light intensity, the higher the rate of
Rate of transpiration transpiration until it becomes constant
2. Because the relative air humidity, temperature and
air movement become the limiting factors
Light intensity
b) Relative air humidity
Rate of transpiration 1. The lower relative air humidity of the surrounding
atmosphere, the higher rate of transpiration
2. Faster water vapour escapes from the stomata.
Relative air humidity
c) Temperature
Rate of transpiration 1. An increase in temperature, increases the rate of
transpiration
2. Increasing kinetic energy of water molecules.
Temperature (°C)
d) Air movement
Rate of transpiration 1. The faster the air movement, the faster the rate of
transpiration.
2. The movement of air carries away water molecules
that have been diffused out from the leaves.
Wind speed
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
2.4 MAIN ORGAN FOR PHOTOSYNTHESIS
The Necessity of Photosynthesis in Plants
1. Plants are autotrophic organisms that produce their own food through photosynthesis.
2. Glucose is the product of photosynthesis
3. Glucose is used by other organisms to generate energy through oxidation of food
4. Energy is needed to carry out living processes such as growth and reproduction
The Adaptation of the Internal Structure of a Leaf to Photosynthesis
Structure Explanation
1. Cuticle
Transparent waxy cuticle on the upper and lower epidermis
2. Upper epidermis o allows sunlight to penetrate the upper and lower
epidermis into the palisade mesophyll.
Transparent because does not chloroplast
o allows sunlight to penetrate into the palisade
mesophyll.
3. Lower epidermis
Stomata are present in the lower epidermis:
Stoma open with the presence of light
o allow gaseous exchange to occur
4. Palisade mesophyll Arranged vertically to near the surface of the leaf
o To maximize absorption of sunlight
Closely-packed to increase surface are
o To maximize absorption of sunlight
Contain many chloroplasts.
o Chloroplast contain chlorophyll to absorb light energy
for photosynthesis
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
5. Spongy mesophyll Have many intercellular air spaces.
cells o Allow efficient gaseous exchange during
photosynthesis
8. Xylem
Have a smaller number of chloroplasts than palisade
mesophyll
o Chloroplast contain chlorophyll to absorb light energy
for photosynthesis
Transports water and mineral salts absorbed from the roots to
the leaf
9. Phloem
Transports organic substances produced during
photosynthesis from the leaves to other parts of the plants
Chloroplast Structure
Inner P
membrane
Outer Q
membrane
R Stroma
Granum S T Thylakoid
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U
Lamellae
Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Structure Characteristic Function
a) Thylakoid
b) Granum Disc-shaped containing Chlorophyll trap sunlight
c) Stroma chlorophyll. Site for light-dependent reaction
A disc-shape stack of Increase surface are for optimal
thylakoid photosynthesis
Colourless fluid Site for light-independent reaction
surrounding granum To produce glucose
Why leaves change colour during autumn
1. Chlorophylls absorb red and blue light and reflect the green part of the spectrum
2. Cause leaves appear green. (Form 1: Adding and subtraction of light)
3. Daylength decreasing in autumn causes trees to stop producing several plant hormones
4. Causing ageing chlorophylls cannot replaced
5. Chlorophylls are broken down into colourless molecules and lose green colour
characteristic
6. Causing other pigment become visible
Carotenoids – orange
Xanthophylls – yellow
Anthocyanins - red
Stage in Photosynthesis Tree
map
STAGE IN PHOTOSYNTHESIS
Light-dependent reaction Light-independent reaction
Occur in Occur in
Thylakoid Stroma
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
GRANA
Hydrogen Photolysis of Hydroxide
ion H+ water ion OH-
NADP+ H2O
NADPH Loose
electron Become
e-
e- Chlorophyll OH + OH Hydroxyl
group
Electron Combine To produce
exited
Water Oxygen
H2O O2
ATP
CO2
STROMA
ATP
ADP Reduction 5-carbon organic
NADPH compound
NADP+
Glucose
Condensation
Starch
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Light-Dependent Reaction
1. Chlorophyll at thylakoids absorb light energy
2. Light energy used to break down water molecule form hydrogen ions (H+) and hydroxide
ions (OH-)
3. This process known as photolysis of water
4. Light energy excites electrons in chlorophyll pigments to a higher level.
5. The excited electrons from the chlorophyll go through a series of electron carriers.
6. Energy from the electrons is used to generate energy in the form of ATP.
7. Electrons are accepted by the NADP+
8. NADP+ combines with H+ from photolysis of water
9. Forms NADPH which is a reducing agent.
10. Chlorophyll pigment attracts electrons from hydroxide ion OH- to become stable.
11. Hydroxide ions lose electrons and form oxygen
Light-Independent Reaction
1. 5-carbon organic compounds fix carbon dioxide gases to form 6-carbon organic
compounds.
2. NADPH and ATP from the light-dependent reaction reduces the organic compounds
3. To form glucose monomers.
4. Glucose monomers condensate to form starch molecules.
5. Starch granules are stored in the stroma of chloroplasts.
The overall reaction for photosynthesis can be represented by the following chemical
reaction
12H20 + 6C02 Light energy C6H12O6 + 602 + 6H20
Chlorophyll Glucose Oxygen Water
Wate Carbon
dioxide
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Comparison Between Light-Dependent and Light-Independent Reactions In
Photosynthesis
Similarities
Both reactions are catalysed by enzyme
Both reactions occur in chloroplast
Differences
Aspect Light-Dependent Light-Independent
Reaction Water Carbon dioxide
substance Oxygen and water molecule Glucose
Reaction product
Process involves Photolysis of water Reduction of carbon dioxide
ATP molecule Produced Used
Site of reaction Thylakoid Stroma
Environmental Factors that Affect the Rate of Photosynthesis
Rate of photosynthesisFactors Explanation
(cm/min-1)a) Temperature
The reaction of photosynthesis is catalysed by
Temperature (OC) enzymes.
Changes of surrounding temperature will affect
enzyme activity and also the rate of photosynthesis.
The optimum temperature in general is between
25°C to 30°C.
A very high temperature denatures the enzymes
The process of photosynthesis is stopped
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
FactorsRate of photosynthesis (cm/min-1) Explanation
b) Carbon dioxide concentration
The increase in carbon dioxide concentration,
30°C at high light increases the rate of photosynthesis
intensity Because more carbon dioxide can fix with 5 -carbon
organic compound to form 6-carbon organic
30°C at low light compound
intensity More glucose is produced
At P, the rate of photosynthesis is constant
P After P, increases carbon dioxide no longer
Carbon dioxide concentration (%) increasing the rate of photosynthesis
Absent of free NADPH and ATP to reduce 6-carbon
organic compound into glucose
Light intensity become limiting factor
Increasing light intensity will increase the rate of
photosynthesis
c) Light intensity Rate of photosynthesis (cm/min-1) The increase in light intensity, increases the rate of
photosynthesis
0.13% CO2 High light intensity will produce more NADPH and
0.03% CO2 ATP
More free NADPH and ATP to reduce 6-carbon
P organic compound into glucose
Light intensity (Lux) At P, the rate of photosynthesis is constant
After P, increases light intensity no longer
increasing the rate of photosynthesis
Absent of free 6-carbon organic compound to
reduced by NADPH and ATP into glucose
Carbon dioxide concentration become limiting
factor
Increasing carbon dioxide concentration will
increase the rate of photosynthesis
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Effect of Different Light Intensities and Light Colour on The Rate of Photosynthesis
1. The rate of photosynthesis in plants is different throughout the day
2. Rate of photosynthesis is also affected by;
Light intensity
Colour of light.
3. Light spectrum consists of seven colours in a certain sequence (violet, indigo, blue, green,
yellow, orange and red)
4. Each colour has a different wavelength
5. The rate of photosynthesis is the highest in red and blue light
6. Red light is absorbed by chlorophyll pigment
7. Blue light is absorbed by carotenoid pigments before being transferred to the chlorophyll
8. These two lights have enough amount of energy to excite electrons in the light-dependent
reaction
Chlorophyll b
Light absorption Carotenoid Chlorophyll a
400 500 600 700 Wavelength (nm)
Violet Yellow Orange Red
Blue Green
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
The Characteristics of Greenhouse Which Can Help the Growth of Plant
Artificial lighting system
Heating Plant Carbon dioxide
system generator
Cooling
system Water sprinkler
system
Characteristic Explanation
1. Carbon dioxide generator
To generate carbon dioxide
2. Heating system and Cooling At least 0.03% of CO2 is supplied all the time.
system Increase the rate of photosynthesis
To regulate the temperature
Kept up at the optimum level / between 15°C to
30°C all the time
For maximum enzyme activities
Increase the rate of photosynthesis
To minimise the rate of transpiration.
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
3. Artificial lighting system
To regulate light intensity.
4. Water sprinkler system Maximum amount and correct wave length of light
5. Glass building (red and blue) are supply all time
Increase the rate of photosynthesis
Photosynthesis can be carried out by the plants all
the times
To regulate nutrients/ fertiliser
o Sufficient nutrients are supplied all the time.
To regulate water
o Sufficient water is supplied all the time
To prevent the attack of pest
Reduce the cost for pesticide
2.5 COMPENSATION POINT
Compensation point
Uptake CO2 P
increases Q
R
Uptake CO2
decreases
At Point R
1. Light intensity is low during early in the morning or at shade area
2. Causing stoma closed and decreasing uptake of carbon dioxide
3. Low light intensity causing rate of photosynthesis become low
4. Rate of respiration is higher than rate of photosynthesis
5. Glucose used in respiration is faster than glucose produced in photosynthesis
6. Net loss in glucose
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
At Point Q
1. Light intensity increasing until it reaches a level when the rate of respiration equals to the
rate of photosynthesis
2. Known as compensation point
3. Glucose produced in photosynthesis is used in the respiration of plants
4. No net gain in glucose
At Point P
1. Light intensity continues to increase beyond the compensation point
2. The rate of photosynthesis becomes faster compared to the rate of respiration.
3. Carbon dioxide needs to be absorbed from the atmosphere at this time
4. To compensate the rate of its usage in photosynthesis
5. Excessive oxygen is released into the atmosphere
6. Rate of glucose production exceeds the rate of glucose usage
7. Excess glucose is stored in the form of starch by the plants
The Difference Between Compensation Point in Plants Exposed to Light with Plants
Under a Shady Place
Uptake CO2 Compensation Exposed plant
increases point Shade plant
Uptake CO2 Compensation
decreases point
Light intensity (Lux)
1. Shade plants tend to have lower compensation point than plant exposed to light
2. Because shade plant has;
Lower maximum photosynthesis rate
Lower light saturation range
3. Leaves of shade plant are;
Larger and thinner than plant exposed to light
Chloroplasts are evenly distributed between the palisade and spongy
mesophyll layers
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Comparison Between Photosynthesis and Respiration in Plants
Similarities
Both processes take place in living organisms
Both processes involve the uptake and release of gases
Differences
Aspect Photosynthesis Respiration
Organisms
involved Green plants and All living organisms
photosynthetic bacteria
Type of cells
Happens in the cells containing
Type of Happens in all cells
metabolism
chlorophyll
Anabolism process Catabolism process
happens happens
Synthesis of glucose using Breakdown of glucose to
carbon dioxide and water produce energy
Site Chloroplast Mitochondria
Reaction Carbon dioxide and water Oxygen and glucose
substances
Products Glucose Energy
By-products Oxygen and water Carbon dioxide and water
Energy Light energy is absorbed and Chemical energy is converted to
involvement converted into chemical energy ATP and heat energy is
released
Light requirement Needs light Does not need light
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Jisrun Najaah Fi Ilmi Al Insan Chapter 2: Leaf Structure and Function
Effects if the rate of respiration and the rate of photosynthesis remain the same at the
compensation point
Cause Effects
1. Stunted growth and Plant can’t produce flowers, seed and fruits
development in plant Less food sources to organism
2. Reduce production of Reduce cellular respiration
oxygen Produced less energy (ATP)
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