Question

Give comparison between the following :
(a) $C_3$ and $C_4$ pathways,
(b) Cyclic and noncyclic photophosphorylation.
(c) Anatomy of leaf in $C_3$ and $C_4$ plants.

Answer 1

(a)

C₃ pathway	C₄ pathway
1. Known as Calvin Cycle ( due to Malvin Calvin.)	Known as Hatch and Slack (due to Marshall Davidson Hatch and C.R. Slack.)
2. This cycle is present in all the plants.	This cycle is present only in C₄ plants.
3. The first stable product is a 3 Carbon (3C) compound, called Phosphoglyceric acid (PGA).	The first stable product is a 4 Carbon (4C) compound, called Oxaloacetic acid (OOA).
4. The carbon dioxide fixation takes place at only one place.	The carbon dioxide fixation takes place twice (first in mesophyll cells; second in bundle-sheath cells).
5. These plants can only perform photosynthesis when the stomata is open.	These plants can perform photosynthesis even if the stomata is closed.
6. Their leaves do not have Kranz Anatomy.	Their leaves do have Kranz Anatomy.
7. Example: Cotton, sunflower, rye, oats, et cetera.	Example: Sugarcane, maize, amaranthus, et cetera.

(b)

Cyclic photophosphorylation.	Noncyclic photophosphorylation.
1. Predominant in bacteria.	Predominant in green plants.
2. Only photosystem I (PS I) is involved.	Both photosystem I and II (PS I and II) are involved.
3. Since PS I is involved, the active reaction center is P₇₀₀.	The active reaction center is P₆₈₀.
4. The movement of electrons is cyclic.	The movement of electrons is non-cyclic.
5. This process produces no NADPH and no O₂, but it does make ATP.	This process produces both NADPH and ATP, and O₂ is evolved as a by-product.
6. Photolysis (water-splitting) is absent.	Photolysis (water-splitting) is present.
7. The electrons travel back to PS I and completes the cycle.	The electrons from PS I accepted by NADPH, making the cycle incomplete.

(c) Leaf anatomy in

1. C₃ plants: Leaves of C₃ plants display only one type of cell, called mesophyll cells. It contains mesophyll chloroplast due to which, C₃ pathway occur.

These plants show normal leaf anatomy.

2. C₄ plants: Leaves of C₄ plants display two type of cells: [outer] mesophyll cells and [inner] spongy bundle-sheath cells. So, the leaves of C₄ plants contain two types of chloroplast: mesophyll chloroplast and bundle sheath chloroplast i.e., dimorphic chloroplast. Thus, C₄ pathway occurs.

This anatomy of leaves in C₄ plants is called Kranz Anatomy.