| $C_{3}$ pathway | $C_{4}$ pathway |
|---|---|
| $(1)$ The primary acceptor of $CO_{2}$ is $RuBP$ - a five-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 $NADPH$. |
| $(3)$ 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. |
| $C_{3}$ leaves | $C_{4}$ leaves |
|---|---|
| $(1)$ Bundle-sheath cells are absent or poorly developed. | $(1)$ Bundle-sheath cells are present (Kranz anatomy). |
| $(2)$ $RuBisCo$ 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 is negligible or absent. |
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