Unit 7 Autotrophic Nutrition
Key Unit Competence
To be able to describe the process of photosynthesis and explain the various environmentalfactors that influence the rate of photosynthesis.
LEARNING OBJECTIVES
At the end of this unit, the learner will be able to:
• State and explain the types of autotrophic nutrition.
• Explain the role of light in autotrophic nutrition.
• State the pigments involved in light absorption.
• Appreciate the importance of photosynthesis as an energy transfer process that produces
complex organic compounds using light energy absorbed by chloroplast pigments.
• Recall the structure of the leaf in relation to photosynthesis.
• Use their knowledge of plant cells and leaf structure from the section on cell structure while
studying photosynthesis.
• State the sites and stages of photosynthesis in chloroplasts.
• Describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and
xanthophylls) in light absorption in the grana.
• Describe the relationship between the structure and function in the chloroplast, using
diagrams and electron micrographs.
• Interpret absorption and action spectra of chloroplast pigments.
• Carry out an investigation of limiting factors.
• outline the three main stages of the Calvin cycle.
• Describe and outline the conversion of the Calvin cycle intermediates to carbohydrates,
lipids and amino acids and their uses in the plant cell.
• Relate the anatomy and physiology of the leaves of C4 and CAM plants to high rates of
carbon fixation and low rates of transpiration.
• Explain the term limiting factor in relation to photosynthesis and the effects of the changes
in the limiting factors on the rate of photosynthesis.
• Apply knowledge and understanding of limiting factors to increase crop yields in protected
environments, such as glasshouses.
• Investigate the effect of light intensity or light wavelength on the rate of photosynthesis.
• Acknowledge that environmental factors influence the rate of photosynthesis and investigation
shows how they can be managed in protected environments used in crop production.• Differentiate between C4, CAM and C3 plants during carbon dioxide fixation.
INTRODUCTORY ACTIVITY
Rwanda has pledged to plant two million hectares of trees by 2020. Adopting a forest landscape
restoration (FLR) approach, the country has committed to the Bonn Challenge: a global
aspiration to restore 150 millioan hectares of the world’s deforested and degraded lands by
2020 and 350 million hectares by 2030.Can you advocate for planting more trees in Rwanda? Why?
ACTIVITY 1
The term ‘autotroph’ consists of two words ‘auto’ and ‘troph’. ‘Auto’ means ‘self ’ and ‘troph’
means ‘nutrition’. Search from the internet and using books about autotrophic nutrition. Makereport on the same and present it to the class.
All organisms require macromolecules like carbohydrates, proteins and fats for their growth and
development. Some organisms produce these organic compounds from inorganic sources on
their own. Such organisms are called autotrophs or producers and the process of synthesizing
complex compounds from simple inorganic sources is called autotrophic nutrition. While others
including humans are heterotrophs or consumers, which depend on autotrophs for source of
chemical energy. Green plants are autotrophs and require chlorophyll, sunlight, oxygen andminerals for preparing their own food.
7.1 TYPES OF AUTOTROPHIC NUTRITION
Chemoautotrophic: An autotrophic nutrition where organisms get energy from oxidation ofchemicals, mainly inorganic substances like hydrogen sulphide and ammonia.
Photoautotrophic: An autotrophic nutrition where organisms get energy from sunlight
and convert it into usable form like sugars. Green plants and some bacteria like, green
sulphur bacteria can make their own food from simple inorganic substances by a processcalled photosynthesis.
7.2 TESTS FOR STARCH AND FOR OXYGEN
ACTIVITY 2
Aim: To show that starch is important for photosynthesis in terrestrial plants.
Materials Required: Two potted plants, ethyl alcohol, iodine solution, sauce pan and burner.
Procedure: Take two potted plants. Keep one in dark and other in well illuminated condition. After
24 hrs, take leaves from each plant. Boil ethyl alcohol in a sauce pan and dip leaves for 30 s. Place
them in a beaker of ethyl alcohol until they turn white. Take leaves out and cover with iodine
solution.
Observation: Leaf taken from well illuminated condition plant turned bluish-black while other
tested negative.
Discussion: This shows that starch is present in terrestrial plants that carry out photosynthesis.
After performing the experiment, try to answer the following questions:
• What is the role of ethyl alcohol in experiment?• What made leaves turn blue-black?
ACTIVITY 3
Aim: To show that oxygen is required by aquatic plants.
Materials Required: Beaker, funnel, aquatic plant Elodea/Hydrilla and boiling tube.
Procedure: Take a few fresh twigs of aquatic plant Elodea/Hydrilla with one
end intact and put in a beaker full of water. Keep an inverted funnel on the
plant such that all plants are inside the funnel and their cut ends facing up. Now
take a boiling tube full of water and invert it on the funnel. Put the wholeapparatus in sunlight
Observation: Observe the air bubbles
accumulating in the boiling tube.
Discussion: The air shows positive test for oxygen gas.
After performing the experiment, answer the following questions:
• What is the positive test for oxygen?• How is oxygen available for aquatic plants?
7.2.1 Importance of Autotrophic Nutrition
Photosynthesis is a process by which plants and other organisms, such as algae and bacteria
synthesize their own food using the energy of light for their growth and development. The food
produced by plants is in the form of carbohydrates. In preliminary studies, Julius von Sachs
proposed that glucose is the first product of photosynthesis. It is stored in chloroplasts within
plant cells. It provides energy in the form of food to organisms that feed on plants. It has been
rightly said “ALL FLESH IS GRASS”, as all organisms (herbivores, carnivores and omnivores)
are directly or indirectly dependent on plants as a source of energy. It is the means by which
solar energy is captured by plants for use by all organisms. In 1782, Jean Senebier proved that
green plants can produce oxygen in presence of light and carbon dioxide. It is the single most
important biological process that can replenish oxygen which is required for existence of all
other organisms. Have you ever thought about what will happen if there is no photosynthesis? This
unit focuses on the photosynthetic machinery, the reactions in this physiochemical process andthe factors affecting photosynthesis.
7.3 ADAPTATION OF PLANTS FOR PHOTOSYNTHESIS
Photosynthesis occurs not only in eukaryotic organisms such as green plants but also in
prokaryotic organisms like blue green algae and green sulphur bacteria.
In higher plants, photosynthesis occurs in the green part of the plant (Figure 7.1). Leaves are
adapted to carry photosynthesis efficiently. Most leaves are broad and flat to capture maximum
light. Also the bifacial nature of the leaf allows it to collect incident light on the upper surface
and diffuse light on the lower surface. The photosynthetic tissue is located between the upper
and lower epidermis. It consists of one to three layers of compactly arranged, elongated and
cylindrical palisade mesophyll cells, and loosely arranged, irregular and isodiametric spongy
mesophyll cells. In monocotyledonous leaf, there is no distinction of palisade and spongyparenchyma.
Figure 7.1: (A) Structure of leaf showing photosynthetic cells; (B) EM of chloroplast cells;
(C) Sectional view of chloroplast.
The mesophyll cells in leaves contain large number of chloroplasts that transform light energyinto ATP and NADPH which are then used to convert CO2 into sugars
7.4 STRUCTURE OF CHLOROPLAST–THE ORGANELLE FORPHOTOSYNTHESIS.
Chloroplast is the photosynthetic machinery. It is a double membrane organelle that contains
series of parallel membranes called thylakoids or lamellae, suspended in fluid like matrix called
stroma. The thylakoids are flattened discs arranged in stacks called grana. In a typical chloroplast
as many as 40-60 grana may be present and each granum may contain 2-100 thylakoids. The
stroma contains DNA, ribosomes, soluble proteins and enzymes, while pigments are confined
to thylakoids. Thylakoids have large surface for absorption of light and the space within them‘lumen’ allows rapid accumulation of protons.
7.4.1 Pigments of Chlorophyll
A pigment is a substance that absorbs light of different wavelengths. Pigments are involved
in absorption of light of certain wavelength. While some wavelengths are absorbed, other arereflected or scattered, which imparts them colour. The absorbed wavelength of light has the
correct energy to excite specific transitions of electrons in the pigments. Photosynthesis depends
on light absorption by pigments in leaves. However, it can be carried out in isolated chloroplastbut not in isolated pigments.
ACTIVITY 4
Aim: To perform chromatography to separate and identify chloroplast.
Materials Required: Whatmann’s filter paper No.1, mortar and pestle and
solvent.
Procedure: Cut a vertical strip (10 cm) × (2.5 cm) of Whatmann’s filter
paper No.1. Make it V-shaped at one end. Draw a horizontal line with a
pencil (not pen) about half an inch from the bottom.
Make leaf extract by crushing 20 g leaves in 20 mL acetone. With the help of capillary
tube load
a small drop of leaf extract at the
centre of pencil
mark and air dry. Repeat the previous
step
for 5-6 times. Insert paper strip in chromatographic
chamber pre-saturated with solvent
(1:9:: benzene:petroleum ether) such that only tip of paper is dipped into solvent. Do not dip the loaded
pigment into solvent. Allow it to run for few (1-2) hours. Take out strip and mark solvent front and
different coloured separated pigments.
Observation: Different bands of different colours are formed.
Discussion: Discuss and label the different bands formed on the filter paper. Also answer whythere are different bands formed.
Chlorophyll a is the major pigment involved in trapping light energy. It is the principal pigment
involved in photosynthesis. It is of universal occurrence. It is a large molecule composed of four
pyrrole rings with Mg at centre, and a long hydrocarbon phytol chain. It absorbs maximum
wavelengths of 430 nm and 660 nm.
Chlorophyll b constitutes one-fourth of the total chlorophyll content. It has a similar structure
as that of Chlorophyll a, except that the –CH3 group in chlorophyll a is replaced by –CHO
group in chlorophyll b. It absorbs maximum wavelengths of 460 nm and 680 nm.
Carotenes are tetraterpenes or polyunsaturated hydrocarbons containing 40 carbon atoms and
variable number of hydrogen atoms and no other elements. b-carotene is the common form
found abundantly in orange, yellow and green fruits and vegetables. Carotenes protect plant
against photo-oxidation.
Xanthophylls are yellow coloured pigments. They are structurally similar to carotenes, butcontain oxygen atoms. These are more common in young and etiolated leaves.
7.4.2 Absorption and Action Spectra
A plot showing absorption of light of different wavelengths of a pigment is called absorptionspectrum.
Figure 7.2: (a) Absorption spectrum of chloroplast pigments;
(b) Action spectrum of green plants.
Each pigment absorbs a specific wavelength. We can plot an absorption spectrum showing
the ability of pigments to absorb lights of different wavelengths. From Figure 7.2(a), it can be
concluded that chlorophyll a and b show absorption peaks at blue and red light. On the other
hand, action spectrum is the plot of graph depicting the rate of a light sensitive process at
different wavelength of light. The action spectrum of photosynthesis shows that most of the
photosynthesis also takes place in blue and red light. The absorption spectrum of a pigment
when compared with action spectrum of photosynthesis, gives the function of the pigment.
Therefore, it can be concluded that chlorophyll a is the chief photosynthetic pigment. The other
pigments like chlorophyll b, carotenes and xanthophylls are called accessory pigments and
form the antenna complex. They collect light of different wavelength and transfer it to reaction
centre (basic model of energy transfer). This is called Light Harvesting Complex (Figure 7.3).LHC is made up of hundreds of pigment molecules bound to proteins.
Figure 7.3: Light harvesting complex
In 1985, R Huber, H Michael and J Dissenhofer crystallized and worked on of light harvesting
complex of Rhodobacter, and got Nobel Prize in 1988. In green plants, pigments are organized
into two discrete photochemical complexes: Photosystem I (PSI) and Photosystem II (PSII),
named after their sequence of discovery. Each photosystem has all the pigments which help
in more efficient absorption of light. The chlorophyll a molecule forms the reaction centre.
In PSI, the reaction centre chlorophyll a molecule has absorption maxima at 700 nm, hence
called P700 and in PSII it has absorption maxima at 680 nm, called P680.
7.4.3 Role of Light
When a light photon is absorbed, an electron is excited from pigment molecule to a higher
energy level (triplet state). It remains there for 10–9 s and then fall to ground state. Sometimes,
it can emit the energy in the form of light and heat as it reaches the ground state. This process
is called fluorescence. When electron remains at triplet state for more than 10–9 s and then
comes back to ground state, the energy is lost in the form of heat and light. This happens evenafter the source is put off. Such a process is called phosphorescence.
1. Complete the sentences by correct terms:
(a) Chloroplast is a double membrane organelle that contains a parallel membrane called
................. .
(b) ................. is the yellow coloured pigment found in young and etiolated leaves.
(c) Most of the photosynthesis takes place at ................. and ................. light.
(d) PSII has absorption maxima at .................2. The graph below shows the strictum of two pigments in the chlorophyll
a.Is that absorption or action spectrum?
b. Identify the pigment X and Y.
7.5 MECHANISM OF PHOTOSYNTHESIS
ACTIVITY 5
Aim: Investigation to determine the effect of light intensity or light wavelength on the rate of
photosynthesis using a redox indicator.
Materials Required: Pestle and mortar, centrifuge, filter paper, test tube, sucrose buffer, DCPIP dye.
Procedure: Crush the interveinal portion of leaves in chilled sucrose buffer using pestle and
mortar. Filter it and centrifuge the filtrate at 200-300 rpm for 2-3 minutes. Collect the supernatant
and again centrifuge at higher speed above (5000 rpm). Collect pellet and dissolve in small amount
of buffer. Take a test tube and to each test tube add sucrose buffer, chloroplast extract, DCPIP dye
according to following table and keep in different light conditions for 10-20 minutes.Observation:
Discussion: Discuss the change in colour obtained. Also state the role of DCPIP in the experiment.
The process of photosynthesis takes place in two major phases: Light reaction and Dark
reaction. Both these phases are interdependent. Green plants convert light energy to chemical
energy. The light energy is used to transfer electron from water to reduce NADP, which in
reduced form, can further be used to fix carbon dioxide. Thus, carbon fixation does not
need light directly and is called dark reaction. Both light and dark reactions take place inchloroplasts.
Table 7.1
Light Reaction
Photochemical phase or Light reaction in which solar energy is trapped by chlorophyll and
stored in the form of chemical energy of ATP and as reducing power in NADPH2. The ATP
and NADPH2 together constitute the assimilatory power of the plant. Oxygen is evolved in
the light reaction by splitting of water (photolysis of water).
Mechanism of Light Reaction
The two photosystems (PS I and PS II) absorb different wavelengths of light. The light energy
absorbed anywhere in the harvesting zone of a photosystem is passed to its photocentre. When
the photocentre acquires a sufficient quantum of energy, it emits an electron. This electron with
high potential energy moves down to an electron transport chain, and results in the formation
of ATP. Thus, the primary function of the two photosystems which interact each other, is to
trap light energy and converts it to the chemical energy (ATP). This chemical energy stored inthe form of ATP is used by the living cells.
7.6 DARK REACTION/CARBON ASSIMILATION/CARBON FIXATION
7.6.1 Calvin Cycle
It is carbon assimilation process which utilizes assimilatory power generated from light reaction
to produce sugars. It occurs in stroma of chloroplasts. Melvin Calvin got Nobel Prize for his
outstanding work on carbon assimilation. Melvin Calvin, Andrew Benson and James Bassham
gave the Calvin cycle of dark reaction. They used autoradiography to detect path of cycle,
and chromatography to separate constituents. The first product that showed radioactivity
was a three carbon (3-C) compound Phosphoglyceric acid (PGA) and hence the cycle is alsocalled C-3 cycle.
Figure 7.4: Calvin cycle
The process of carbon assimilation can be described under three stages: carboxylation, reduction
and regeneration (Figure 7.4).
Carboxylation: It is the process of fixation of carbon in stable organic intermediate,
phosphoglyceric acid. This reaction is catalyzed by an enzyme called RuBPcarboxylase-
oxygenase (RUBISCO). Rubisco-bis-phospahte (RuBP) is the initial acceptor or substratefor dark reaction.
Reduction or Glycolytic Reversal: It is the process involving reduction of carbon. It is
a multistep process that utilizes 12 ATP molecules and 12 NADPH for release of one
molecule of glucose. The glucose can further be converted into starch for storage or sucrosefor transport.
Regeneration: This process requires 6 ATP molecules to regenerate 6 molecules of RuBP,
which is crucial for continuity of Calvin cycle.
Another important requirement is high concentration of CO2. The efficiency of photosynthesis
declines at low concentration of CO2. This is because the enzyme RUBISO has low affinity
with carbon dioxide as compared to oxygen. At low CO2 concentration, RUBISCO catalyzes
the reaction between RuBP and oxygen. The oxygenation of RuBP in presence of light and
oxygen is called Photorespiration. It occurs in chloroplast, peroxisome and mitochondria.
It is a wasteful process as during this process carbon dioxide is released and efficiency of
photosynthesis decreases (Figure 7.5).
Figure 7.5: Photorespiration and C-3 cycle
7.6.2 Hatch and Slack Cycle
In some temperate plants such as wheat, rice, potato and bean only Calvin cycle occurs. Such plants
are called C-3 plants. While in some other plants dual carboxylation takes place: (1) carboxylation
of phosphoenol pyruvate (PEP) and (2) carboxylation of RuBP. Such plants are called C-4 plants
e.g. maize, sugar cane and sorghum. In these, the first product formed during carbon dioxide
fixation is a four carbon compound oxalo acetic acid (OAA). C-4 plants have special type of leaf
anatomy called Kranz Anatomy. They have special large cells around vascular bundles called
bundle sheath cells. These are characterized by having large number of chloroplasts, thick walls
and no intercellular spaces. The shape, size and arrangement of thylakoids in chloroplasts arealso different in bundle sheath cell as compared to mesophyll cell chloroplasts.
Table 7.2
Figure 7.6: Hatch and slack pathway
The pathway followed by C-4 plants is called C-4 cycle or Hatch and Slack pathway. This was
discovered by Hatch and Slack in sugar cane. The primary CO2 acceptor is a 3-carbon molecule
phosphoenol pyruvate (PEP). The reaction is catalyzed by PEPcarboxylase or PEPcase in
mesophyll cell chloroplast. It forms 4-carbon compounds like OAA, malic acid or aspartate,
which are transported to the bundle sheath cells. In bundle sheath cells, these acids are broken
down to release CO2 and 3-carbon molecule. The 3-carbon molecule is transported back to
mesophyll cells and converted to PEP again, while CO2 enters into C-3 cycle to form sugars
(Figure 7.6). C-4 plants are more efficient than C-3 plants as in C-4 plants, photosynthesis canoccur at low concentration CO2 and photorespiration is negligible or absent.
Table 7.3
APPLICATION
1. Complete with appropriate terms:
(a) Photorespiration occurs in ........................, ..................... and ........................ .
(b) C4 plants have special type of leaf anatomy called ........................ .
(c) In C3 plants, initial acceptor of CO2 is ........................ .
(d) Hatch and Slack pathway occurs in ........................ and ........................ of chloroplast.2. Giving examples, write short note on C4 plants.
7.7 FACTORS AFFECTING PHOTOSYNTHESIS
Aim: To investigate the effect of carbon assimilation on the rate of photosynthesis.
Materials Required: Elodea plant, glass rod, sodium bicarbonate.
Procedure: Take a fresh, healthy twig of Elodea plant with one end intact and tie it gently to a
glass rod. Put the glass rod with plant in a boiling tube containing
water and add 1mg/mL sodium bicarbonate and keep it in
moderate light condition. Note the numbers of bubbles escaping
from cut end per minute. Again add same amount of sodium
bicarbonate and note the number of bubbles escaping from cut
end per minute. Do you find the number of bubbles increasing?
Repeat this step until bubbles escaping per minute do not
increase. Then take set up under high light intensity and note the
numbers of bubbles.
Observation: The bubbles evolved gradually.Discussion: Discuss what the bubbles are and why they evolved.
Carbon assimilation is directly related to the productivity of plants. It carries implications for the
sustainability of the human population. The total carbon assimilation is known as Gross primary
productivity and the one available for increase in biomass is known as net primary productivity.
Net primary productivity is determined by deleting loss due to respiration by plants. It is the
biomass available for animals. By studying factors affecting photosynthesis, one can learn to
manage world resources in time. The rate of photosynthesis can be influenced by many factors
like number, size, orientation and age of leaf, sunlight, temperature, carbon dioxide and water.
However, when several factors can affect a process, the rate of reaction is governed by the factor
which is limiting. This is called Blackman’s (1905) law of limiting factor. For example, despite
the presence of green leaf, optimal carbon dioxide, the plant may not photosynthesize if light
intensity is very low. Thus, light behaves as limiting factor and controls the rate of photosynthesis(Figure 7.7). Hence, rate will be determined by the factor available at sub-optimal level.
Figure 7.7: Graph depicting Blackmann’s law of limiting factor.
7.7.1 External Factors
ACTIVITY 7
Aim: To show effect of carbon dioxide on the rate of photosynthesis.
Materials Required: Elodea, beaker, NaHCO3, lamp.
Procedure: Place a pond weed Elodea upside in a test tube containing water at 25°C. Place the
tube in a beaker of fresh water. Place excess sodium bicarbonate (NaHCO3) in the water to
give a constant saturated solution of CO2. Place the lamp at a fixed distance from the plant.
Maintain the room temperature at 20°C. Count the number of oxygen bubbles given off by the
plant in a one minute period.
Observation: The bubbles are formed of oxygen.Discussion: Discuss why was NaHCO3 added to water.
CO2 concentration: Carbon dioxide is the inorganic substrate for photosynthesis. Increase inconcentration up to 0.05% in atmosphere can cause an increase in CO2 fixation. Carbon dioxide
is the major limiting factor, especially in C-3 plants; C-4 plants are more productive even at low
concentration of CO2. Nevertheless, both C-3 and C-4 plants show increase in rate of photosynthesis
at high CO2 concentration and high light intensities. The fact that C-3 plants respond to higher
CO2 concentration by showing increased rates of photosynthesis leading to higher productivity
has been used for some green house crops such as tomatoes and bell pepper. They are allowed togrow in carbon dioxide enriched atmosphere as in glasshouses leading to higher yields.
ACTIVITY 8
Aim: To show the effect of light intensity on photosynthesis in terrestrial plants.
Materials Required: Spinach, sodium bicarbonate solution, detergent, lamp.
Procedure: Take out uniform size discs from fresh leaves of spinach. Place discs in 0.2%
sodium bicarbonate solution and a drop of liquid detergent in a syringe. Plunge out air present
in between tissue, such that intercellular spaces are occupied by sodium bicarbonate and all leaf
discs sink to bottom. Then put leaf discs in the beaker containing water exposed to on lamp.
Count the number of leaves that then float on surface at regular interval of time. Similarly,
repeat the experiment with increasing light condition.
Observation: Count and note the number of leaves floating.Discussion: Discuss why the leaves floated.
ACTIVITY 9
Aim: To show the effect of light intensity on aquatic plants.
Materials Required: Elodea, glass rod, sodium bicarbonate.
Procedure: Take a fresh, healthy twig of Elodea plant with one end intact and tie it gently to a
glass rod. Put the glass rod with plant in a tube or jar containing water and a pinch of sodium
bicarbonate. Keep it in under a light source at a distance of 50 cm/ low light condition. Note
the numbers of bubbles escaping from cut end per minute. Place the apparatus at distance of
30 cm from the light source and count the number of bubbles evolving per minute. Similarly,
place the apparatus at variable distances from light source.
Observation: Count the number of bubbles evolving per minute.Discussion: Discuss any change in the number of bubbles.
Light: Light is an important factor to carry out photosynthesis. It is rarely a limiting factor
in nature as photosynthesis can occur even at low light intensities. There is a direct relation
between light and CO2 fixation. With increase in light intensity the rate of photosynthesisincreases. However, at higher light intensities, rate does not increase linearly but light
saturation occurs. At very high light intensity, there is breakdown of chlorophyll molecules
called photo-oxidation and the rate of photosynthesis decreases. The quality of light and
time of exposure also governs photosynthesis. Green plants show high rate of photosynthesisat red and blue light.
ACTIVITY 10
Aim: To show effect of temperature on photosynthesis.
Materials Required: Elodea plant, boiling tube, sodium bicarbonate.
Procedure: Take a fresh, healthy twig of Elodea plant with one end intact and tie it gently to a
glass rod. Put the glass rod with plant in a boiling tube containing water and a pinch of sodium
bicarbonate. Keep it in under moderate light condition. Note the temperature and numbers of
bubbles escaping from cut end per minute. Heat or cool the water in a boiling tube.
Observation: Count the number of bubbles at different temperatures.Discussion: Discuss the change in number of bubbles.
Temperature: The dark reactions are dependent on temperature as they are enzymatic. Rate
of photosynthesis is best at optimum temperature. Different plants have different temperature
optima that also depend on their habitats.
Water: Only about 1% of water absorbed by plants is used in photosynthesis. It is an
important factor for various metabolic processes in plant. Water may not have direct affect on
photosynthesis even though it is one of the reactants in light reaction. In water stress plants
wilt and their stomata close. Thus reducing availability of carbon dioxide and decreasing
the rate of photosynthesis. Water stress will also alter the hydration of enzymatic proteins,
affecting their activities.
Oxygen concentration: Atmospheric oxygen content affects photosynthesis directly or indirectly.
The decrease in rate of respiration at high oxygen concentration was first observed by O. Warburg
in 1920 in Chlorella. The phenomenon is called the Warburg effect.
Chemical pollutants: Plant growth has been adversely affected by accumulation of various
undesirable chemicals. Heavy metals such as lead, mercury, cadmium seem to be affecting
photosynthesis through stomata closure. Air pollutants like SO2, NO2 and O3 are also known
to affect photosynthesis at higher concentrations.
7.7.2 Internal Factors
Adaptation of leaf: Leaves are arranged on plants to minimize overlapping. The shape, size,age and orientation of leaf influences the absorption of light and thus effects photosynthesis.
Most leaves are broad for more absorption of light. The anatomy of the leaf is also highly
specialized for absorption of light. The epidermis is transparent and also acts as convex lens
to focus and intensify light reaching mesophyll cells for maximum absorption. The palisade
layer also helps in absorption of more light. Presence of hairs, salt glands and epicuticular waxincrease the reflection of light and thereby reducing the absorption.
Absorption of carbon dioxide is also dependent on leaf surface area and number of stomata.
Spongy parenchyma has large intercellular space so that carbon dioxide can easily diffuse.
Opening and closing of stomata is yet another factor that governs photosynthesis as the
exchange of gases is affected when stomata close. In some succulent plants such as Bryophyllum,
Kalanchoe, stomata open during night and close during day to reduce the rate of transpiration.
Such plants have special mechanism for photosynthesis called Crassulacean Acid Metabolism
(CAM), where CO2 fixation takes place in different time (day and night) as per availability
of carbon dioxide and light (Figure 7.8). CO2 is taken up by the plant during night time as
stomata open during night. It is fixed and converted to malic acid, which is stored in vacuole.
During day time when assimilatory power is available, malic acid is released from vacuole
and reverted back to pyruvate by releasing carbon dioxide. This CO2 then enters C-3 cycle
and assimilated. Pyruvate is stored in chloroplast in the form of starch and released duringnight time through glycolysis.
Figure 7.8: Crassulacean acid metabolism in CAM plants
Fill in the columns 2 and 3 in this table to highlight the differences between C3 and C4plants.
7.8 SUMMARY
• Organisms that are autotrophic can make their own food from inorganic substances with
help of energy.
• Photosynthesis is the process where the source of energy is light. It is carried out by green
plants, algae and some bacteria.
• Photosynthesis takes place in green parts of a plant, mainly leaves. Within leaves,
chloroplasts in mesophyll cells are the site of photosynthesis.
• Photosynthesis has two stages: light reaction and dark reaction. Light reaction is a
photochemical reaction, in which light energy is absorbed by the pigments present in
antenna molecules of light harvesting complex. While, in dark reaction carbon is reduced
in the stroma of chloroplast.
• Chlorophyll, a molecule is the reaction centre which has two special forms PSI and PSII
with absorbance maxima at 700 nm and 680 nm, respectively.
• In temperate plants, C-3 cycle takes place with the help of enzyme RUBISCO. The C-3
cycle includes: carboxylation, reduction and regeneration. In some tropical plants, C-4
cycle takes place.
• C-4 cycle includes dual carboxylation that takes place in mesophyll cells chloroplast and
bundle sheath cell chloroplasts.
• Various environmental factors such as light, temperature, carbon dioxide concentration,
oxygen concentration and air pollutants are responsible for the plant productivity onaccount of photosynthesis.
7.9 GLOSSARY
• Absorption spectrum: Absorption spectrum is the graph plot that shows the measure
of absorption of radiation of different wavelengths.
• Action spectrum: Action spectrum is the graph plot that shows rate of physiological
activity at different wavelengths of light.
• ATP synthase: It is an important lipid binding protein. It is used to generate ATP from
ADP. It is present on thylakoid membrane.
• Autotrophs: An autotroph is an organism that produces complex organic compounds
such as carbohydrates, fats and proteins from simple inorganic substances.
• Chemoautotrophs: Chemoautotrophs are organisms that obtain energy by the oxidation
of electrons donors in their environment.
• Emerson enhancement effect: Increase in rate of photosynthesis when two wavelengths
of light: 680 nm and 700 nm are given simultaneously.
• Fluorescence: Emission of light of higher wavelength (usually red) by a substance or
system when exposed to light.
• Kranz anatomy: Occurrence of wreath of bundle sheath cells around vascular bundles
in leaves is called Kranz anatomy. The mesophyll cells and bundle sheath cells show
chloroplast dimorphism.
• Law of limiting factor: If a chemical process is affected by more than one factor, the
rate of process will be determined by the factor which is nearest to minimal value.
• Light Harvesting Complex: It is a complex of proteins subunits and photosynthetic
pigments. It collects light energy and transfers it to the reaction centre.
• Phosphorescence: Emission of light of higher wavelength (usually red) by a substance
or system even after light is put off.
• Photoautotrophs: Photoautotrophs are the organisms that carry out metabolism by using
light energy. They can carry out photosynthesis.
• Photon: The elementary particle of light.
• Photorespiration: The oxygenation of Ribulose bisphosphate in presence of light and
oxygen in green plants is called photorespiration.
• Photosynthesis: A process of synthesizing organic compounds from inorganic substances
in presence of light in green plants.
• Photosystems: Photosystems are functional and structural units of protein complexes
involved in photosynthesis. There are two types of photosystems : PSI and PSII.
• Pigment: A pigment is a material that changes the colour of reflected or transmitted
light as a result of wavelength selective absorption.
• Quantum: Quantum is the minimum amount of any physical entity involved in interaction.
A photon is a single quantum of visible light and referred as light quantum.
• Red drop phenomenon: Decrease in rate of photosynthesis beyond 680 nm of light.
• Redox potential: It is the measure of tendency of a chemical species to acquire electronsand thereby reduced. It is also called reduction potential.
END UNIT ASSESSMENT 7
I. Choose whether the given statements are True (T) or False (F)
1. Organisms that are heterotrophic can make their own food.
2. Photosynthesis has two stages—light reaction and dark reaction.
3. CAM cycle includes triple carboxylation.
4. Environmental factors improve crop yield.
5. Pigment is a material that changes colour of reflected or transmitted light.
6. Within leaves, chloroplasts are responsible for respiration.
II. Multiple Choice Questions
1. Green plants require which of the following for photosynthesis?
(a) Sunlight (b) CO2
(c) O2 (d) Water
2. C-4 cycle occurs in
(a) Wheat (b) Rice
(c) Sugar cane (d) All of the above
3. What is true about action spectrum?
(a) It can be carried out in isolated pigments
(b) It gives the function of pigments
(c) It is used to identify pigments
(d) It does not involve light
4. By looking at which internal structure, you can tell whether a plant is C-3 or C-4?
(a) Mesophyll cell (b) Bundle sheath cells
(c) Vascular bundles (d) Epidermal cells
5. How many ATP are required to produce 2 molecules of glucose?
(a) 12 (b) 24
(c) 18 (d) 36
6 Autotrophs are commonly called producers because they
(a) produce young plants
(b) produce CO2 from light energy
(c) produce sugars from chemical energy(d) produce water from light energy
III. Long Answer Type Questions
1. State and explain the types of autotrophic nutrition. Also explain the role of light in
autotrophic nutrition.
2. Analyse and appreciate the importance of photosynthesis as an energy transfer process.
3. State the role of chloroplast and structure of leaf in photosynthesis. Giving illustrative
diagrams, explain your answer.
4. State the pigments involved in light absorption. Throw light on absorption and action
spectra of chloroplast pigments.
5. Outline the three main stages of the Calvin cycle. State the uses of Calvin cycle
intermediaries in plant cell.
6. Summarize the limiting factors affecting photosynthesis. Also state how this can help
yield crop production.
7. Compare anatomy of C4 and CAM plants.
8. Differentiate between C4, CAM and C3 plants during carbon dioxide fixation.
9. Investigate the effect of light intensity or light wavelength on the rate of photosynthesis.
10. Describe the relationship between the structure and function in the chloroplast, using
diagrams and electron micrographs.
11. Acknowledge the importance of autotrophic nutrition in sustaining the balance of
life on Earth. Also state the ways to keep the environment sustained. Predict various
facts related to photosynthesis that state the importance of nutrition for all livingbeings.