AQA GCSE Biology 4.4 Bioenergetics: a complete overview of photosynthesis and respiration

What module 4.4 actually demands

Bioenergetics is about how living things capture and release energy. The two big processes are photosynthesis (taking in energy) and respiration (releasing energy). The examiners reward correct equations, the endothermic versus exothermic distinction, confident graph and limiting-factor reasoning, and clear explanation of the body's response to exercise.

This guide walks through the module and ties together the matching dot-point pages, each with its own practice questions.

Photosynthesis

Photosynthesis is an endothermic reaction: plants and algae use light energy, captured by chlorophyll in the chloroplasts, to make glucose. Carbon dioxide and water become glucose and oxygen. The glucose is used for respiration, stored as starch, or turned into cellulose, proteins (with nitrate ions) and fats and oils.

The rate of photosynthesis

The rate is controlled by four limiting factors: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. On a graph the rate rises then plateaus when another factor becomes limiting; too high a temperature denatures the enzymes. Light follows the inverse square law, so doubling the distance from a lamp quarters the intensity. Greenhouse growers control these factors to raise yield, balancing the extra cost against the profit. The required practical uses pondweed to measure how light intensity affects the rate.

Respiration

Respiration is an exothermic reaction happening continuously in all living cells. Aerobic respiration uses oxygen and releases the most energy, producing carbon dioxide and water. Anaerobic respiration releases much less energy: in muscles it makes lactic acid, and in yeast and plants it makes ethanol and carbon dioxide (fermentation). The energy released powers movement, keeps mammals warm and builds larger molecules.

Response to exercise

During exercise, heart rate, breathing rate and breath volume rise to deliver more oxygen and glucose and remove carbon dioxide. In vigorous exercise the muscles also respire anaerobically, building up lactic acid that causes fatigue. The oxygen debt is the extra oxygen needed afterwards to break down the lactic acid (carried to the liver), which is why you keep breathing hard after stopping.

How module 4.4 is examined

• Equations. Word and (higher tier) symbol equations for photosynthesis and respiration.
• Graphs. Limiting-factor and rate-against-distance graphs, including the inverse square law.
• Practical. The pondweed investigation, with variables and conclusions.
• Application. Greenhouse economics and the body's response to exercise.

Check your knowledge

A mix of recall and application questions covering module 4.4. Attempt them under timed conditions, then check against the solutions.

1. Write the word equation for photosynthesis. (2 marks)
2. State whether photosynthesis is endothermic or exothermic and explain why. (2 marks)
3. Name the four limiting factors of photosynthesis. (2 marks)
4. Explain why the rate of photosynthesis levels off as light intensity increases. (2 marks)
5. Write the word equation for aerobic respiration. (2 marks)
6. Name the products of anaerobic respiration in yeast. (1 mark)
7. State two ways the body responds to exercise. (2 marks)
8. Define oxygen debt. (2 marks)