Q: By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.

C4 plants live in hot moist or arid and nonsaline or saline habitats. Internally the leaves show kranz anatomy. In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by large sized bundle sheath cells which are arranged in a wreath-like manner (kranz - wreath). The mesophyll and bundle sheath cells are connected by plasmodesmata or cytoplasmic bridges. The chloroplasts of the mesophyll cells are smaller. They have well developed grana and a peripheral reticulum but no starch. Mesophyll cells are specialised to perform light reaction, evolve 02 and produce assimilatory power (ATP and NADPH). They also possess enzyme PEPcase for initial fixation of CO2 The chloroplasts of the bundle sheath cells are agranal.

Q: 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.

(a) C3 pathwaysC4 pathways 1. The primary acceptor of CO2 is RUBP – a six-carbon compound. 1. The primary acceptor of CO2 is phosphoenol pyruvate – a three-carbon compound.2. The first stable product is 3-phosphoglycerate. 2. The first stable product is oxaloacetic acid.3. It occurs only in the mesophyll cells of the leaves. 3. It occurs in the mesophyll and bundle-sheath cells of the leaves.4. It is a slower process of carbon fixation and photorespiratory losses are high. 4. It is a faster process of carbon fixation and photorespiratory losses are low.(b) Cyclic photophosphorylation Non-cyclic photophosphorylation 1. It occurs only in photosystem I. 1. It occurs in photosystems I and II. 2. It involves only the synthesis of ATP. 2. It involves the synthesis of ATP and NADPH2. 3. In this process, photolysis of water does not occur. Therefore, oxygen is not produced. 3. In this process, photolysis of water takes place and oxygen is liberated. 4. In this process, electrons move in a closed circle. 4. In this process, electrons do not move in a closed circle.(c) Anatomy of leaf in C3 plants Anatomy of leaf in C4 plants 1. Bundle-sheath cells are absent. 1. Bundle-sheath cells are present. 2. RuBisCo is present in the mesophyll cells. 2. RuBisCo is present in the bundle-sheath cells.3. The first stable compound produced is 3-phosphoglycerate –a three-carbon compound. 3. The first stable compound produced is oxaloacetic acid – a four-carbon compound. 4. Photorespiration occurs. 4. Photorespiration does not occur.

Question 1

By looking at a plant externally can you tell whether a plant is C3 or C4? Why and how?

Accepted Answer

It is not possible to distinguish externally between a C3 and C4 plant, but generally tropical plants are adapted for C4  cycle.

Question 2

By looking at which internal structure of a plant can you tell whether a plant is C3 or C4? Explain.

Accepted Answer

C4​   plants live in hot moist or arid and nonsaline or saline habitats. Internally the leaves show kranz anatomy. In kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles. Vascular bundles are surrounded by large sized bundle sheath cells which are arranged in a wreath-like manner (kranz - wreath). The mesophyll and bundle sheath cells are connected by plasmodesmata or cytoplasmic bridges. The chloroplasts of the mesophyll cells are smaller. They have well developed grana and a peripheral reticulum but no starch. Mesophyll cells are specialised to perform light reaction, evolve 02​ and produce assimilatory power (ATP and NADPH). They also possess enzyme PEPcase for initial fixation of CO2​  The chloroplasts of the bundle sheath cells are agranal.

Question 3

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?

Accepted Answer

The productivity of a plant is measured by the rate at which it photosynthesizes. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. C4​ plants have a mechanism for increasing the concentration of carbon dioxide. In C4​ plants, the Calvin cycle occurs in the bundle-sheath cells. The C4​ compound (malic acid) from the mesophyll cells is broken down in the bundle-sheath 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, C4​ plants are highly productive.

Question 4

RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?

Accepted Answer

RuBisCO is an enzyme which acts both as carboxylase (carboxylation during photosynthesis) and oxygenase (during photorespiration). But RuBisCO carries out more carboxylation in C4​ plants. In C4​ plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of CO2​ is phosphoenol pyruvate or PER It combines with carbon dioxide in the presence of PEP carboxylase or PEPcase to form oxaloacetic acid or oxaloacetate. Malic acid or aspartic acid is translocated to bundle sheath cells through plasmodesmata. Inside the bundle sheath cells they are decarboxylated (and deaminated in case of aspartic acid) to form pyruvate and CO2​. CO2​ is again fixed inside the bundle sheath cells through Calvin cycle. RuBP of Calvin cycle is called secondary or final acceptor of CO2​ in C4​ plants. Pyruvate is sent back to mesophyll cells.

Question 5

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?

Accepted Answer

Chlorophyll-a molecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylation. They form the reaction centres for both photosystems I and II. Chlorophyll-b and 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-a and contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis.

Question 6

Why the colour of a leaf kept in the dark frequently is yellow or pale green? Which pigment do you think is more stable?

Accepted Answer

Leaf has a green colour pigment known as chlorophyll. The production of the chlorophyll pigment is essential for photosynthesis is directly proportional to the amount of light available. In the absence of light, the production of chlorophyll a molecule stops and they degenerate. 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.

Question 7

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?

Accepted Answer

Light is a limiting factor for photosynthesis. Leaves get lesser light for 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.

Question 8

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?

Accepted Answer

(a) At regions A and B light is the limiting factor.(b) In the region A, light can be a limiting factor.(c) C is the region where the rate of photosynthesis is not increased when light intensity is increased. D is the point where some other factors become limiting.

Question 9

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.

Accepted Answer

(a) C3​ pathwaysC4​ pathways 1. The primary acceptor of CO2​ is RUBP – a six-carbon compound. 1. The primary acceptor of CO2​ is phosphoenol pyruvate – a three-carbon compound.2. The first stable product is 3-phosphoglycerate.  2. The first stable product is oxaloacetic acid.3. It occurs only in the mesophyll cells of the leaves. 3. It occurs in the mesophyll and bundle-sheath cells of the leaves.4. It is a slower process of carbon fixation and photorespiratory losses are high. 4. It is a faster process of carbon fixation and photorespiratory losses are low.(b)  Cyclic photophosphorylation Non-cyclic photophosphorylation 1. It occurs only in photosystem I. 1. It occurs in photosystems I and II. 2. It involves only the synthesis of ATP. 2. It involves the synthesis of ATP and NADPH2​. 3. In this process, photolysis of water does not occur. Therefore, oxygen is not produced. 3. In this process, photolysis of water takes place and oxygen is liberated. 4. In this process, electrons move in a closed circle. 4. In this process, electrons do not move in a closed circle.(c)  Anatomy of leaf in C3​ plants  Anatomy of leaf in C4​ plants 1. Bundle-sheath cells are absent. 1. Bundle-sheath cells are present. 2. RuBisCo is present in the mesophyll cells. 2. RuBisCo is present in the bundle-sheath cells.3. The first stable compound produced is 3-phosphoglycerate –a three-carbon compound.  3. The first stable compound produced is oxaloacetic acid – a four-carbon compound. 4. Photorespiration occurs. 4. Photorespiration does not occur.

Chapter 13 : Photosysnthesis in Higher Plants