AQA GCSE Combined Science: Biology: Bioenergetics

A grower keeps tomato plants in a greenhouse. The rate of photosynthesis of the plants — and so how well they grow — can be limited by light intensity, carbon dioxide concentration and temperature.

Explain how each of these three factors can act as a limiting factor for the rate of photosynthesis, and how the grower could use this knowledge to increase the yield of tomatoes. (6 marks)

A student measured the rate of photosynthesis of pondweed (counting bubbles of gas per minute) at different distances of a lamp from the pondweed. Light intensity follows the inverse square law: it is proportional to $\dfrac{1}{d^{2}}$d21​, where $d$d is the distance from the lamp.

Distance of lamp, $d$d / cm	Relative light intensity ($\propto 1/d^{2}$∝1/d2)	Bubbles per minute
10	1.00	48
20	0.25	13
30	?	6

(a) The relative light intensity at 10 cm is taken as $1.00$1.00. Using the inverse square law, calculate the relative light intensity at 30 cm (relative to the value at 10 cm). Show your working. (3 marks)

(b) Suggest one variable, other than light intensity, that the student should keep constant to make this a fair test. (1 mark)

Plants make glucose during photosynthesis. The word equation for photosynthesis is:

carbon dioxide + water → glucose + oxygen (using light energy transferred by chlorophyll)

(a) State two uses the plant makes of the glucose produced in photosynthesis. (2 marks)

(b) Glucose can be converted into a storage carbohydrate inside the plant. Name this storage carbohydrate. (1 mark)

During hard exercise, the muscles may not receive enough oxygen for aerobic respiration, so they begin to respire anaerobically as well.

(a) Name the substance produced when muscle cells respire anaerobically. (1 mark)

(b) Compare anaerobic respiration with aerobic respiration in terms of the amount of energy released per glucose molecule, and explain why this difference occurs. (2 marks)

After a sprint, an athlete continues to breathe deeply and quickly for several minutes, even though they have stopped running. This is because they have built up an oxygen debt.

Explain why the athlete must take in extra oxygen after exercise has finished. (2 marks)

Respiration takes place in the cells of all living organisms.

State whether respiration is an exothermic or an endothermic reaction. (1 mark)