Give comparison between the following:
(a) C $_{3}$ and C $_{4}$ pathways
(b) Cyclic and non-cyclic photophosphorylation
(c) Anatomy of leaf in C $_{3}$ and C $_{4}$ plants.

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Solution

(a)
$C_{3}$ pathways $C_{4}$ pathways
1. The primary acceptor of CO $_{2}$ is RUBP – a six-carbon compound. 1. The primary acceptor of CO $_{2}$ 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 $N A D P H_{2}$ .
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 $C_{3}$ plants Anatomy of leaf in $C_{4}$ 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.

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$C_{3}$ pathways	$C_{4}$ pathways
1. The primary acceptor of CO $_{2}$ is RUBP – a six-carbon compound.	1. The primary acceptor of CO $_{2}$ 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.

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 $N A D P H_{2}$ .
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.

Anatomy of leaf in $C_{3}$ plants	Anatomy of leaf in $C_{4}$ 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.