Name the CO 2 acceptor in the mesophyll cells during HSK pathway.

The Hatch-Slack pathway (HSK) is also called the C 4 cycle in which the first stable product is a 4 carbon compound, oxaloacetic acid. This cycle is present in most of the monocot plants in which two types of photosynthetic cells found in their leaves, i.e, mesophyll cells and bundle sheath cells. In the C 4 cycle, carbon dioxide from the environment reaches to mesophyll cells of the leaf through stomata. This carbon dioxide absorbed by phosphoenolpyruvic acid (PEP) and in the presence of the PEP Carboxylase enzyme, converts it into oxaloacetic acid. Hence, the CO 2 acceptor in the mesophyll cells during HSK pathway is phosphoenolpyruvic acid (PEP).

Give any two examples of C 4 plants.

The C 4 cycle is also called Hatch-Slack pathway in which the first stable product is a 4 carbon compound, oxaloacetic acid. This cycle is present in most of the monocot plants in tropical region in which two types of photosynthetic cells found in their leaves, i.e, mesophyll cells and bundle sheath cells. Examples: Maize, sugarcane, millet, sorghum, etc.

Why is phorespiration avoided in C 4 plants?

Photorespiration is the process of oxidation that is initiated in the chloroplast and takes place only during the day. RuBisCO, the enzyme required for CO 2 fixation in the Calvin cycle, is responsible for the photorespiration. The active site of the enzyme RuBisCO can bind to both CO 2 and O 2 . In C 4 plants, a special anatomical feature is seen, called Kranz anatomy. The large bundle sheath cells are arranged in two or three layers around the vascular bundles. Two types of chloroplast are found in C 4 plants, the chloroplast present in mesophyll cell is small and has well-developed grana while the chloroplast found in bundle sheath cell is large and without grana. Thylakoids are found in the form of stroma lamellae. Hence, in C 4 plants, the light reaction takes place in mesophyll cells. In C 4 plants, PEP carboxylase (PEPCase) enzyme catalyses the carboxylation and fixes CO 2 at low concentration in the mesophyll cells. In the bundle sheath cells, CO 2 concentration is more hence, RuBisCO functions as carboxylase, and photorespiration is avoided.

Where does photolysis of H 2 O occur? What is its significance?

The photolysis of H 2 O occurs in the water-splitting complex of photosystem l that takes place in thylakoid membranes of chloroplasts where light is utilised to break water molecules, as discussed in the equation below: 2H 2 O → 4H + + 4e - + O 2 Significance of photolysis: It helps to liberate the oxygen in the photosynthesis process. It also provides the hydrogen required for the reaction.