(a) (i) State the role of ATP in cellular respiration. (ii) What ensures sufficient exchange of gases in plants? (iii) State the conditions on which the direction of diffusion of gases in plant depend upon. OR (b) (i) What is the internal energy reserve in plants and animals? (ii) How desert plants perform photosynthesis if their stomata remain closed during the day?

Part (a)

(i) Role of ATP in cellular respiration:

• ATP (Adenosine Triphosphate) acts as the energy currency of the cell.
• The energy released during cellular respiration is immediately used to synthesise ATP from ADP and inorganic phosphate ($P_i$):

$$\text{ADP} + \text{P}_i \xrightarrow{\text{Energy}} \text{ADP}\sim\text{P} = \text{ATP}$$

• When the cell needs energy for endothermic reactions, the terminal phosphate linkage of ATP is broken using water, releasing a fixed amount of energy ($30.5\text{ kJ/mol}$) to drive these cellular activities.

(ii) What ensures sufficient exchange of gases in plants?:

• Plants have large intercellular spaces in their tissues (especially leaves) which ensure that all cells are in direct contact with air.
• Gaseous exchange occurs via diffusion through stomata on leaves, lenticels on stems, and the general surface of roots.

(iii) Conditions on which the direction of diffusion of gases in plants depends:

• The direction of diffusion depends on:

1. Environmental conditions (e.g., day or night).
2. The requirements of the plant (photosynthesis vs. respiration).

Example*: During the day, $CO_2$ generated during respiration is immediately used up in photosynthesis; hence, the net event is $O_2$ release. At night, when photosynthesis does not occur, $CO_2$ elimination is the major diffusion event.

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OR

Part (b)

(i) Internal energy reserve:

• In Plants: Starch serves as the internal energy reserve, stored when carbohydrates produced during photosynthesis are not immediately consumed.
• In Animals: Glycogen serves as the internal energy reserve, stored in liver and muscle tissues.

(ii) Photosynthesis in desert plants:

• Desert plants keep their stomata closed during the day to prevent excessive water loss through transpiration.
• To perform photosynthesis, they take up carbon dioxide at night (when stomata are open) and convert it into an intermediate compound (malic acid).
• During the day, when stomata are closed, the chlorophyll absorbs solar energy and uses it to act upon this intermediate to complete the process of photosynthesis and produce carbohydrates.