(a) What is photosynthesis? (b) List: (i) four external factors (ii) two internal factors; that affect the rate of photosynthesis. (c)(i)List the major prod...

(a) Photosynthesis

Photosynthesis is the process by which green plants manufacture their own food (glucose) from carbon dioxide and water, using light energy trapped by chlorophyll, and releasing oxygen as a by-product. It is summarised as:

(b) Factors affecting the rate of photosynthesis

• (i) Four external factors: light intensity, concentration of carbon dioxide, temperature, and availability of water.
• (ii) Two internal factors: amount of chlorophyll present, and the number/distribution of stomata (also acceptable: accumulation of end products, leaf structure).

(c)(i) Major products of the light-dependent stage

• Oxygen.
• ATP (energy).
• Reduced hydrogen carrier (NADPH₂), formed from the splitting of water (photolysis).

(c)(ii) Importance of each product

• Oxygen: released to the atmosphere and used by living organisms for respiration.
• ATP: supplies the energy used in the light-independent (dark) stage to fix carbon dioxide into glucose.
• Reduced NADP (NADPH₂): supplies the hydrogen used to reduce carbon dioxide to carbohydrate in the dark stage.

(d)(i) Why there are no green plants at the lower depths of some lakes

Light does not penetrate to the great depths of the water; without light the green plants cannot carry out photosynthesis to make food, so they cannot survive there.

(d)(ii) Why decomposers are important to flowering plants

Decomposers break down dead organisms and wastes, releasing mineral nutrients (for example nitrates, phosphates, carbon dioxide) back into the soil and air, which the flowering plants absorb and use for growth and photosynthesis.