C₃ pathways

C₄ 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 3phosphoglycerate

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 photo-respiratory losses are high.

4. It is a faster process of carbon fixation and photo-respiratory 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 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.

C₃ leaves

C₄ leaves

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 bundlesheath 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