Photosynthesis in Higher Plants
Summary***Green plants make their own food by photosynthesis. During this process carbon dioxide from the atmosphere is taken in by leaves through stomata and used for making carbohydrates, principally glucose and starch. Photosynthesis takes place only in the green parts of the plants, mainly the leaves. Within the leaves, the mesophyll cells have a large number of chloroplasts that are responsible for CO2 fixation. Within the chloroplasts, the membranes are sites for the light reaction, while the chemosynthetic pathway occurs in the stroma. Photosynthesis has two stages: the light reaction and the carbon fixing reactions. In the light reaction the light energy is absorbed by the pigments present in the antenna, and funnelled to special chlorophyll a molecules called reaction centre chlorophylls. There are two photosystems, PS I and PS II. PS I has a 700 nm absorbing chlorophyll a P700 molecule at its reaction centre, while PS II has a P680 reaction centre that absorbs red light at 680 nm. After absorbing light, electrons are excited and transferred through PS II and PS I and finally to NAD forming NADH. During this process a proton gradient is created across the membrane of the thylakoid. The breakdown of the protons gradient due to movement through the F0 part of the ATPase enzyme releases enough energy for synthesis of ATP. Splitting of water molecules is associated with PS II resulting in the release of O2 , protons and transfer of electrons to PS II.
In the carbon fixation cycle, CO2 is added by the enzyme, RuBisCO, to a 5- carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.
Some tropical plants show a special type of photosynthesis called C4 pathway. In these plants the first product of CO2 fixation that takes place in the mesophyll, is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates.
In the carbon fixation cycle, CO2 is added by the enzyme, RuBisCO, to a 5- carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO also catalyses a wasteful oxygenation reaction in C3 plants: photorespiration.
Some tropical plants show a special type of photosynthesis called C4 pathway. In these plants the first product of CO2 fixation that takes place in the mesophyll, is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried out for the synthesis of carbohydrates.
EXERCISE
Q1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how? Q2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain. Q3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why? Q4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants? Q5. Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments? Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable? Q7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why? Q8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve is light a limiting factor? (b) What could be the limiting factor/s in region A? (c) What do C and D represent on the curve? Q9. Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants
Q1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how? Q2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain. Q3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why? Q4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants? Q5. Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments? Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable? Q7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why? Q8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve is light a limiting factor? (b) What could be the limiting factor/s in region A? (c) What do C and D represent on the curve? Q9. Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants
Stroma lamellae connect thylakoids of two different grana.They are also known as stroma thylakoids. They ensure that maximum energy from sunlight is captured in photosynthesis. The stroma lamellae also contain Photosystem I and chlorophyll.
Calvin cycle = carboxylation + reduction + regeneration
EXERCISE******
Q1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how? Answer1: One cannot distinguish whether a plant is C3or C4 by observing its leaves and other morphological features externally. Unlike C3plants, the leaves of C4plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C3plant, while maize is a C4plant.
Q2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain.Answer2:The leaves of C4plants have a special anatomy called Kranz anatomy. This makes them different from C3plants. Special cells, known as bundle-sheath cells, surround the vascular bundles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features help prevent photorespiration in C4plants, thereby increasing their ability to photosynthesise.
Q3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why? Answer3:The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C4plants have a mechanism for increasing the concentration of carbon dioxide. In C4plants, the Calvin cycle occurs in the bundle-sheath cells. The C4compound (malic acid) from the mesophyll cells is broken down in the bundles heath cells. As a result, CO2 is released. The increase in CO2 ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, C4plants are highly productive.
Q4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants? Ans. The enzyme RuBisCo is absent from the mesophyll cells of C4plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In C4plants, the Calvin cycle occurs in the bundle-sheath cells. The primary CO2acceptor in the mesophyll cells is phosphoenol pyruvate –a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and CO2fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase.
Q5. Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments? Ans. Chlorophyll-amolecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction centres for both photosystems I and II. Chlorophyll-band other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation. Therefore, chlorophyll-a is essential for photosynthesis. If any plant were to lack chlorophyll-aand contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis. Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable? Ans.Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of chlorophyll-a molecules stops and they get broken slowly. This changes the colour of the leaf gradually to light green. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. They are always present in plants.Q7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why? Ans. Light is a limiting factor for photosynthesis. Leaves get lesser lightfor photosynthesis when they are in shade. Therefore, the leaves or plants in shade perform lesser photosynthesis as compared to the leaves or plants kept in sunlight. In order to increase the rate of photosynthesis, the leaves present in shade have more chlorophyll pigments. This increase in chlorophyll content increases the amount of light absorbed by the leaves, which in turn increases the rate of photosynthesis. Therefore, the leaves or plants in shade are greener than the leaves or plants kept in the sun.
Q8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve is light a limiting factor? (b) What could be the limiting factor/s in region A? (c) What do C and D represent on the curve? Ans.
Q9. Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants Ans. a. C3 and C4 pathwaysC3 pathway: i. The primary acceptor of CO2 is RUBP - a six carbon compoundii. The first stable product is 3phosphoglycerateiii. It occurs only in the mesophyll cells of the leavesiv. It is a slower process of carbon fixation and photo-respiratory losses are highC4 pathways:i. The primary acceptor of CO2 is phosphoenol pyruvate - a three carbon compoundii. The first stable product is oxaloacetic acidiii. It occurs in the mesophyll and bundle-sheath cells of the leaves.iv. It is a faster process of carbon fixation and photo-respiratory losses are low.
b. Cyclic and non-cyclic photophosphorylations.Cyclic photophosphorylation:i. It occurs only in photosystem I ii. It involves only the synthesis of ATPiii. In this process, photolysis of water does not occur. Therefore, oxygen is not producediv. In this process, electrons move in a closed circle.
Non-Cyclic photophosphorylationi. It occurs in photosystems I and IIii. It involves the synthesis of ATP and NADPH2.iii. In this process, photolysis of water takes place and oxygen is liberated.iv. In this process, electrons do not move in a closed circle
c. Anatomy of the leaves in C3 and C4 plantsC3 leaves:i. Bundle - sheath cells are absentii. RuBisCo is present in the mesophyll cells.iii. The first stable compound produced is 3-phosphoglycerate - a three - carbon compound.iv. Photorespiration occursC4 leave: i. Bundle-sheath cells are presentii. RuBisCo is present in the bundlesheath cellsiii. The first stable compound produced is oxaloacetic acid - a four carbon compoundiv. Photorespiration does not occur.
Q1. By looking at a plant externally can you tell whether a plant is C3 or C4 ? Why and how? Answer1: One cannot distinguish whether a plant is C3or C4 by observing its leaves and other morphological features externally. Unlike C3plants, the leaves of C4plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a C3plant, while maize is a C4plant.
Q2. By looking at which internal structure of a plant can you tell whether a plant is C3 or C4 ? Explain.Answer2:The leaves of C4plants have a special anatomy called Kranz anatomy. This makes them different from C3plants. Special cells, known as bundle-sheath cells, surround the vascular bundles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features help prevent photorespiration in C4plants, thereby increasing their ability to photosynthesise.
Q3. Even though a very few cells in a C4 plant carry out the biosynthetic – Calvin pathway, yet they are highly productive. Can you discuss why? Answer3:The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C4plants have a mechanism for increasing the concentration of carbon dioxide. In C4plants, the Calvin cycle occurs in the bundle-sheath cells. The C4compound (malic acid) from the mesophyll cells is broken down in the bundles heath cells. As a result, CO2 is released. The increase in CO2 ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, C4plants are highly productive.
Q4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants? Ans. The enzyme RuBisCo is absent from the mesophyll cells of C4plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In C4plants, the Calvin cycle occurs in the bundle-sheath cells. The primary CO2acceptor in the mesophyll cells is phosphoenol pyruvate –a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and CO2fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase.
Q5. Suppose there were plants that had a high concentration of Chlorophyll b, but lacked chlorophyll a, would it carry out photosynthesis? Then why do plants have chlorophyll b and other accessory pigments? Ans. Chlorophyll-amolecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction centres for both photosystems I and II. Chlorophyll-band other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation. Therefore, chlorophyll-a is essential for photosynthesis. If any plant were to lack chlorophyll-aand contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis. Q6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green? Which pigment do you think is more stable? Ans.Since leaves require light to perform photosynthesis, the colour of a leaf kept in the dark changes from a darker to a lighter shade of green. Sometimes, it also turns yellow. The production of the chlorophyll pigment essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of chlorophyll-a molecules stops and they get broken slowly. This changes the colour of the leaf gradually to light green. During this process, the xanthophyll and carotenoid pigments become predominant, causing the leaf to become yellow. These pigments are more stable as light is not essential for their production. They are always present in plants.Q7. Look at leaves of the same plant on the shady side and compare it with the leaves on the sunny side. Or, compare the potted plants kept in the sunlight with those in the shade. Which of them has leaves that are darker green ? Why? Ans. Light is a limiting factor for photosynthesis. Leaves get lesser lightfor photosynthesis when they are in shade. Therefore, the leaves or plants in shade perform lesser photosynthesis as compared to the leaves or plants kept in sunlight. In order to increase the rate of photosynthesis, the leaves present in shade have more chlorophyll pigments. This increase in chlorophyll content increases the amount of light absorbed by the leaves, which in turn increases the rate of photosynthesis. Therefore, the leaves or plants in shade are greener than the leaves or plants kept in the sun.
Q8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions: (a) At which point/s (A, B or C) in the curve is light a limiting factor? (b) What could be the limiting factor/s in region A? (c) What do C and D represent on the curve? Ans.
Q9. Give comparison between the following: (a) C3 and C4 pathways (b) Cyclic and non-cyclic photophosphorylation (c) Anatomy of leaf in C3 and C4 plants Ans. a. C3 and C4 pathwaysC3 pathway: i. The primary acceptor of CO2 is RUBP - a six carbon compoundii. The first stable product is 3phosphoglycerateiii. It occurs only in the mesophyll cells of the leavesiv. It is a slower process of carbon fixation and photo-respiratory losses are highC4 pathways:i. The primary acceptor of CO2 is phosphoenol pyruvate - a three carbon compoundii. The first stable product is oxaloacetic acidiii. It occurs in the mesophyll and bundle-sheath cells of the leaves.iv. It is a faster process of carbon fixation and photo-respiratory losses are low.
b. Cyclic and non-cyclic photophosphorylations.Cyclic photophosphorylation:i. It occurs only in photosystem I ii. It involves only the synthesis of ATPiii. In this process, photolysis of water does not occur. Therefore, oxygen is not producediv. In this process, electrons move in a closed circle.
Non-Cyclic photophosphorylationi. It occurs in photosystems I and IIii. It involves the synthesis of ATP and NADPH2.iii. In this process, photolysis of water takes place and oxygen is liberated.iv. In this process, electrons do not move in a closed circle
c. Anatomy of the leaves in C3 and C4 plantsC3 leaves:i. Bundle - sheath cells are absentii. RuBisCo is present in the mesophyll cells.iii. The first stable compound produced is 3-phosphoglycerate - a three - carbon compound.iv. Photorespiration occursC4 leave: i. Bundle-sheath cells are presentii. RuBisCo is present in the bundlesheath cellsiii. The first stable compound produced is oxaloacetic acid - a four carbon compoundiv. Photorespiration does not occur.