Bioenergetics

1. Plants capture energy from light

Think of a growing tomato plant. It makes new leaves, roots and fruit, but it does not eat food. Instead, it uses photosynthesis to make glucose, a sugar, from carbon dioxide and water.

Carbon dioxide enters leaves from the air. Roots take up water, which travels to the leaves. Inside green plant cells, chloroplasts contain chlorophyll, a green pigment that absorbs light.

Photosynthesis is endothermic: energy is taken in from the surroundings. Light transfers that energy to the chloroplasts. Some of it becomes stored in the glucose made.

Recognise these formulae: carbon dioxide CO₂, water H₂O, oxygen O₂ and glucose C₆H₁₂O₆.

2. What happens to the glucose?

A plant can use its glucose straight away or turn it into other useful substances.

To make amino acids, plants need nitrate ions as well as material from glucose. Roots absorb nitrate ions from the soil. A plant with too little nitrate cannot make enough protein for healthy growth.

3. What changes the rate of photosynthesis?

Rate means how quickly something happens. One way to estimate photosynthesis rate is to measure how much oxygen a plant produces in a set time.

A limiting factor is something in short supply that prevents the rate increasing further. On a graph, check the axes first. A steep rising line means the rate is increasing quickly. A horizontal section means the rate has stopped increasing, even though the x-axis value is still rising.

When drawing a graph, put the factor you changed on the horizontal axis and the rate on the vertical axis. Use sensible, evenly spaced scales, add units, plot carefully and draw a suitable line or curve through the trend. Do not force the line through the origin unless the evidence supports it.

4. Required practical 5: light and pondweed

Aim: investigate how light intensity affects the rate of photosynthesis in an aquatic plant, such as pondweed. Complete practical work in school with your teacher’s guidance.

1. Place a healthy, freshly cut piece of pondweed in a clear container of sodium hydrogencarbonate solution. This provides a source of dissolved carbon dioxide. Put the cut end upwards so bubbles can escape.
2. Position a lamp at a measured distance from the pondweed. Measure to the plant, not just to the edge of the container. Use the same lamp throughout.
3. Check the temperature. A water bath and a low-heat LED lamp help reduce temperature changes. Keep background light steady.
4. Allow the plant time to adjust at each distance before recording. Use the same settling time each time.
5. Count the bubbles released in a fixed time, for example one minute. Repeat at least three times and calculate a mean. Record unusual results and repeat suspicious readings rather than silently removing them.
6. Move the lamp to several other measured distances. Repeat the same process, keeping the same piece of pondweed, solution concentration and volume, temperature and recording time.

Independent variable: light intensity, changed by moving the lamp. Distance is the measurement used to change it. Dependent variable: oxygen production per unit time, estimated using bubbles per minute.

Safety: keep electrical equipment and plugs away from water, avoid touching hot lamps, handle glass carefully, and let the teacher manage any sharp blade used to cut pondweed. Do not look directly into a bright lamp.

Evaluate the method: bubbles can have different sizes, making counts an imperfect measure of oxygen volume. Some oxygen dissolves in the water. Collecting oxygen and measuring its volume over time with suitable gas-collection equipment gives a better estimate than bubble counts. Repeat readings improve confidence, but they do not remove a consistent temperature error.

5. Turn results into rates

Worked example: a plant releases 72 bubbles in 2 minutes. Rate = 72 ÷ 2 = 36 bubbles per minute. Alternatively, 2 minutes = 120 seconds, so the rate is 72 ÷ 120 = 0.60 bubbles per second.

Three one-minute counts are 34, 36 and 38. Mean = (34 + 36 + 38) ÷ 3 = 36 bubbles per minute. Use the same unit when comparing results.

These results suggest that the rate increases as the lamp moves closer. They do not show that light is the only factor that ever affects photosynthesis.

6. Higher only: factors can work together

Higher only. Light, carbon dioxide and temperature interact. Increasing one factor only helps while that factor is limiting. For example, a well-lit plant may still grow slowly if it has too little carbon dioxide.

At low light, the two curves are close: adding carbon dioxide makes little difference because light is limiting. At high light, raising carbon dioxide increases the rate. Carbon dioxide was limiting the lower curve. The upper curve’s plateau tells us another factor prevents a further rise; we need more evidence before naming it. A temperature experiment could test whether temperature is limiting.

Higher only — inverse-square relationship: for a suitable lamp setup, relative light intensity is proportional to 1 ÷ distance². It is a useful model; background light and reflections can make real results differ.

Worked example: at 10 cm, 1/d² = 1/100 = 0.010 cm⁻². At 20 cm it is 1/400 = 0.0025 cm⁻². The second value is one-quarter of the first. These are relative values, not measured light intensity in lux.

In a greenhouse, lamps, heaters and extra carbon dioxide cost money. A grower should compare the extra income from a bigger crop with the extra running cost. Raising a factor that is not limiting can waste money. The fastest growth does not always give the greatest profit.

7. Respiration: energy for living cells

You need energy to move, make new molecules and keep warm. Your cells transfer this energy through respiration. Plants need respiration too: their roots, leaves and other living cells carry it out continuously.

Respiration is exothermic: energy is transferred out of the reaction. Some is used for the cell’s activities and some warms the surroundings. Breathing moves air into and out of the lungs; respiration is a set of chemical reactions in cells.

Aerobic respiration uses oxygen and breaks down glucose fully.

Recognise glucose (C₆H₁₂O₆), oxygen (O₂), carbon dioxide (CO₂) and water (H₂O). Oxygen reaches muscle cells from the blood. Carbon dioxide leaves the cells and is carried to the lungs to be breathed out.

Respiration provides energy for reactions that build larger molecules from smaller ones, muscle movement and keeping body temperature suitable.

8. When oxygen supply cannot keep up

During a sprint, muscles may need energy faster than oxygen can be delivered. They can also carry out anaerobic respiration, which does not use oxygen.

Glucose is not broken down fully, so much less energy is transferred per glucose molecule than in aerobic respiration. Lactic acid builds up. During prolonged vigorous activity, muscles become fatigued and do not contract as effectively.

Anaerobic respiration also happens in yeast and some plant cells. The products are different:

In yeast, this process is called fermentation. In bread-making, carbon dioxide makes the dough rise. In brewing, ethanol is the useful alcohol product. Baking removes much of the ethanol from bread.

Do not write that anaerobic respiration produces no energy. It transfers less energy, but that energy still helps cells work.

9. Why you breathe harder during exercise

Working muscles respire faster. Your heart rate increases, so blood delivers oxygen and glucose faster. Your breathing rate increases and each breath becomes deeper: breath volume is the amount of air moved in one breath.

Together, these changes help supply the muscles with more oxygenated blood. If the supply is still too small for the demand, anaerobic respiration contributes more, lactic acid builds up and an oxygen debt develops.

Breathing and heart rate stay above resting levels for a while after exercise. The body still needs extra oxygen during recovery.

Worked example: at rest, a person takes 12 breaths each minute, with 0.5 dm³ per breath. Air moved per minute = 12 × 0.5 = 6 dm³ min⁻¹. During exercise, 25 breaths of 1.6 dm³ move 40 dm³ min⁻¹. This measures ventilation, not the volume of oxygen actually absorbed.

Exercise investigations are useful supporting activities, not a separate Biology required practical for this topic. Take part only in teacher-approved activity suitable for your health. Do not use exhaustion as a target.

10. Metabolism: making and breaking molecules

Metabolism means all the chemical reactions happening in a cell or organism. It includes both building molecules and breaking them down. Enzymes control these reactions. Energy transferred by respiration helps keep many of the building reactions going.

Digestion breaks large food molecules into smaller ones. Cells can then use these smaller molecules to build the carbohydrates, proteins and lipids they need. For example, absorbed amino acids can be joined in a different order to make a human protein.

11. Build a clear scientific answer

For a calculation, write the relationship, substitute numbers, calculate and give a unit. For a graph comparison, use values and units rather than only saying “higher”.

For a six-mark method, organise your answer around what you change, what you measure, what you keep the same, repeats and safety. For an explanation, link cause and effect: “The muscles need more energy, so their respiration rate increases…”

All questions here are original exam-style practice, not copied from AQA papers. Higher-only work is labelled separately from optional challenges.

Required practical 5: analyse your evidence

Use the full method in Learn, then analyse these invented results. At 30 cm, one-minute bubble counts were 18,20 and 22. At 15 cm, they were 35,37 and 36. Water temperature stayed at 20°C.

Higher extension: halving distance gives approximately four times the light intensity in the inverse-square model. Here the rate rises by only 1.8 times. Another factor may be limiting; bubble counting is also only an estimate.

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One final thought

Use a scientific idea and a specific example in your reflection.