WAEC Biology 2013 Theory — Question 2

Model answer

(a) Photosynthesis refers to the process by which green plants manufacture their food using CO₂ and H₂O in the presence of sunlight and with the release of oxygen as a by-product. (b)(i) External factors affecting the rate of photosynthesis include: light intensity, carbon dioxide, water and temperature. (ii) Internal factors affecting the rate of photosynthesis include chlorophyll and enzymes. (c)(i) The major products of the light-dependent stage of photosynthesis include hydrogen ion (H⁺), hydroxyl ion (OH⁻), nicotinamide adenine dinucleotide phosphate hydrogen (NADPH₂), and oxygen gas. (ii) H⁺ generates H₂O and oxygen gas: 4OH⁻ → 2H₂O + O₂ + 4e⁻. NADPH₂ releases hydrogen which is eventually fixed on to form carbohydrate. Oxygen gas is harnessed by living things for respiration. (d)(i) There are no green plants at the lower depths of some lakes due to the fact that the light intensity decreases as we go down an aquatic body. At a level where the amount of light is minimal, the rate of photosynthesis will be so low that the plant can barely survive. (ii) Decomposers break down dead organic matter to release nutrients back to the soil for plant uptake.