AQA GCSE Biology 8461: Bioenergetics – Study Guide

This guide teaches section 4.4 Bioenergetics of the AQA GCSE Biology (8461) specification, for teaching from September 2016 and exams from 2018 onwards. It covers every point from 4.4.1.1 to 4.4.2.3: photosynthesis, its rate, the uses of glucose, respiration, the response to exercise and metabolism. Bioenergetics is assessed on Paper 1 (topics 1 to 4), which is set at Foundation and Higher Tier. Content the specification marks (HT only) is labelled Higher tier only below. Required practical activity 6 belongs to this topic.

For quick recall, use the Bioenergetics revision notes. To test yourself, use the Bioenergetics practice questions. The course hub is AQA GCSE Biology, and the printable checklist lists every specification point.

What this unit covers

Spec point	What you must be able to do	Tier
4.4.1.1 Photosynthetic reaction	Give the word equation; recognise CO₂, H₂O, O₂, C₆H₁₂O₆; describe photosynthesis as endothermic	Both
4.4.1.2 Rate of photosynthesis	Explain the effects of temperature, light intensity, CO₂ concentration and amount of chlorophyll; measure and calculate rates; plot and interpret graphs with one limiting factor	Both
4.4.1.2 Rate of photosynthesis	Explain interacting limiting factors and graphs with two or three factors; use the inverse square law; relate limiting factors to greenhouse economics	Higher tier only
Required practical 6	Investigate the effect of light intensity on the rate of photosynthesis using pondweed	Both
4.4.1.3 Uses of glucose	List the five uses; explain why plants need nitrate ions	Both
4.4.2.1 Aerobic and anaerobic respiration	Describe respiration as exothermic; compare aerobic and anaerobic; give the three word equations	Both
4.4.2.2 Response to exercise	Describe changes in heart rate, breathing rate and breath volume; explain lactic acid build-up, oxygen debt and fatigue	Both
4.4.2.2 Response to exercise	Explain how lactic acid is removed by the liver; define oxygen debt	Higher tier only
4.4.2.3 Metabolism	Define metabolism; explain how sugars, amino acids, fatty acids and glycerol are used to build and break down molecules	Both

4.4.1.1 The photosynthetic reaction

Photosynthesis happens in the chloroplasts of plant cells and algae. The word equation is:

                        light
carbon dioxide + water  ------>  glucose + oxygen

You must recognise the formulae: carbon dioxide CO₂, water H₂O, glucose C₆H₁₂O₆ and oxygen O₂.

Photosynthesis is an endothermic reaction: energy is transferred from the environment to the chloroplasts by light. The chlorophyll in the chloroplasts absorbs the light. This links to the structure of the leaf, which you met in the Organisation topic.

4.4.1.2 Rate of photosynthesis

The four factors

• Light intensity. Light supplies the energy. More light means a faster rate, until another factor limits it.
• Carbon dioxide concentration. CO₂ is a reactant. More CO₂ means a faster rate, until another factor limits it.
• Temperature. Photosynthesis is controlled by enzymes. Warming speeds it up; above the optimum the enzymes are denatured and the rate falls sharply.
• Amount of chlorophyll. Less chlorophyll (for example, from magnesium deficiency or leaf disease) means less light is absorbed, so the rate falls.

A limiting factor is the factor in shortest supply, which stops the rate increasing. On a graph with one factor on the x-axis, the rising part of the line shows that factor is limiting. Where the line levels off, something else has become limiting.

Measuring the rate – required practical 6

The specification requires you to investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.

1. Put a piece of pondweed in a boiling tube of water (often with sodium hydrogencarbonate added to supply CO₂).
2. Place a lamp at a measured distance from the tube.
3. Stand a large beaker of water between the lamp and tube as a heat shield, and check the temperature with a thermometer, so temperature stays constant.
4. Leave the weed to adjust, then count the oxygen bubbles, or collect the gas in a syringe or measuring cylinder, for a fixed time.
5. Repeat at several distances, taking multiple readings at each distance.
6. Plot a graph of light intensity (or distance) against rate.

Worked example 1 – calculating a rate

A student collects 6.0 cm³ of oxygen from pondweed in 5 minutes. Calculate the rate of photosynthesis.

rate = volume of oxygen ÷ time
     = 6.0 ÷ 5
     = 1.2 cm³ per minute

State the unit. A rate is always “something per unit time”.

Interacting factors (Higher tier only)

The factors interact, and any one of them may be the limiting factor. Suppose rate is measured against light intensity at two CO₂ concentrations:

• At low light intensity, both lines rise together. Light is limiting.
• The low-CO₂ line levels off first. There, CO₂ is limiting.
• The high-CO₂ line rises further, then levels off. Now temperature (or chlorophyll) may be limiting.

To decide which factor is limiting at a point: if raising that factor would still raise the rate, it is the limiting factor. If two lines at different CO₂ levels are separate at a point, CO₂ is limiting there.

The inverse square law (Higher tier only)

Light intensity is inversely proportional to the square of the distance from the light source:

light intensity ∝ 1 / distance²

So doubling the distance cuts the light intensity to one quarter. In practical write-ups, 1/d² is often used as a measure of relative light intensity.

Worked example 2

A light meter reads 400 units when a lamp is 10 cm from the pondweed. Find the reading at 20 cm and at 25 cm.

20 cm:  distance × 2   ->  intensity × (1/2)² = × 1/4
        400 × 1/4 = 100 units

25 cm:  intensity = 400 × (10/25)²
                  = 400 × 0.16
                  = 64 units

Greenhouse economics (Higher tier only)

Growers can add heat, light or CO₂ to a greenhouse to raise the rate of photosynthesis and yield. Each costs money, so the grower only gains if the extra crop is worth more than the cost. Adding a factor that is not limiting adds cost without raising yield.

Worked example 3

A grower pays £120 a week to burn paraffin, which raises the CO₂ concentration and temperature. Yield rises by 90 kg a week. The crop sells for £2.40 per kg. Extra lamps would cost £150 a week and raise yield by a further 20 kg. Should the grower use each?

Paraffin: extra income = 90 × 2.40 = £216
          gain = 216 − 120 = £96 a week   -> worth doing

Lamps:    extra income = 20 × 2.40 = £48
          gain = 48 − 150 = −£102 a week  -> a loss

The lamps add little yield because light is no longer the main limiting factor, so they are not cost effective.

4.4.1.3 Uses of glucose from photosynthesis

The glucose made in photosynthesis may be:

1. used for respiration;
2. converted into insoluble starch for storage;
3. used to produce fat or oil for storage;
4. used to produce cellulose, which strengthens the cell wall;
5. used to produce amino acids for protein synthesis.

To make proteins, plants also need nitrate ions, absorbed from the soil. Starch, glucose and protein can be identified with simple qualitative reagents (iodine for starch, Benedict’s test for sugars, Biuret reagent for protein), the same tests used in the Organisation topic.

4.4.2.1 Aerobic and anaerobic respiration

Respiration is an exothermic reaction that is continuously occurring in living cells. The energy it transfers supplies all the energy needed for living processes. Organisms need energy for:

• chemical reactions to build larger molecules;
• movement;
• keeping warm.

Aerobic respiration (uses oxygen):

glucose + oxygen  ->  carbon dioxide + water

Recognise the formulae C₆H₁₂O₆, O₂, CO₂ and H₂O.

Anaerobic respiration in muscles (no oxygen):

glucose  ->  lactic acid

Anaerobic respiration in plant and yeast cells:

glucose  ->  ethanol + carbon dioxide

In yeast this is called fermentation. It matters economically in making bread (the CO₂ makes dough rise) and alcoholic drinks (the ethanol).

	Aerobic	Anaerobic
Oxygen needed?	Yes	No
Products	Carbon dioxide and water	Lactic acid (muscles); ethanol and carbon dioxide (plants, yeast)
Oxidation of glucose	Complete	Incomplete
Energy transferred	Much more	Much less

Anaerobic respiration transfers much less energy because the oxidation of glucose is incomplete.

4.4.2.2 Response to exercise

During exercise, muscles need more energy. The body responds:

• heart rate increases;
• breathing rate increases;
• breath volume increases.

Together these supply the muscles with more oxygenated blood (and remove more CO₂).

If not enough oxygen reaches the muscles, they respire anaerobically. The incomplete oxidation of glucose causes a build-up of lactic acid and creates an oxygen debt. During long periods of vigorous activity, muscles become fatigued and stop contracting efficiently.

(Higher tier only) Blood flowing through the muscles transports the lactic acid to the liver, where it is converted back into glucose. Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells. This is why you keep breathing hard after you stop.

Worked example 4

A student’s heart rate is 68 beats per minute at rest and 153 beats per minute after a sprint. Calculate the percentage increase.

increase = 153 − 68 = 85
% increase = 85 ÷ 68 × 100 = 125%

Divide by the starting value (68), not the final one.

4.4.2.3 Metabolism

Metabolism is the sum of all the reactions in a cell or the body. The energy transferred by respiration is used for the continual, enzyme-controlled processes of metabolism that synthesise new molecules.

Metabolism includes:

• converting glucose to starch (plants), glycogen (animals) and cellulose (plant cell walls);
• forming a lipid molecule from one glycerol and three fatty acids;
• using glucose and nitrate ions to form amino acids, which are used to make proteins;
• respiration;
• breaking down excess proteins to form urea for excretion.

The small units are the key: sugars build carbohydrates, amino acids build proteins, and fatty acids plus glycerol build lipids. Breaking the large molecules down releases the small units again.

Common errors

• Writing that photosynthesis “gives out energy”. It is endothermic; respiration is exothermic.
• Saying plants photosynthesise instead of respiring. Plants respire all the time; they photosynthesise only in light.
• Saying higher temperature “kills” the enzymes. Enzymes are denatured, not killed.
• Giving “lactic acid and carbon dioxide” for anaerobic respiration in muscles. Muscles make lactic acid only.
• Describing anaerobic respiration as producing “no energy”. It transfers less energy, not none.
• Using distance instead of 1/distance² when a question asks for relative light intensity (Higher tier only).
• Forgetting to control temperature in required practical 6. The lamp heats the water unless you use a heat shield.
• Missing the unit on a rate, such as bubbles per minute or cm³ per minute.

Where to go next

Condense this guide with the Bioenergetics revision notes, then try the Bioenergetics practice questions. Check your wider Paper 1 knowledge with the free 10-minute diagnostics.

Official syllabus

AQA GCSE Biology (8461) specification, Version 1.0, for teaching from September 2016 and exams from 2018 onwards (AQA), section 4.4 Bioenergetics.