Edexcel International GCSE Biology 4BI1: Respiration – Study Guide

This study guide teaches sub-topic 2(f), Respiration (specification points 2.34–2.39), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered, and none of these six points carries a bold B reference, so all of them can be examined on Paper 1 and Paper 2. It applies to the November and June series examined under Issue 3.

Use it with the revision notes and the practice questions. Mitochondria, where aerobic respiration happens, are covered in the cell structure revision notes and the cell structure and biological molecules guide. The full course is on the Edexcel IGCSE Biology hub, the printable checklist lists every point, and the free diagnostics show where to focus.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.34	Explain how respiration produces ATP in living organisms	1 and 2
2.35	Know that ATP provides energy for cells	1 and 2
2.36	Describe the differences between aerobic and anaerobic respiration	1 and 2
2.37	Word equation and balanced symbol equation for aerobic respiration	1 and 2
2.38	Word equation for anaerobic respiration in plants and in animals	1 and 2
2.39	Practical: carbon dioxide and heat released by respiring seeds or other organisms	1 and 2

Respiration in plants during day and night, and net gas exchange in leaves, belong to sub-topic 2(g) and are Paper 2 only (2.44B). They are not taught here.

What respiration is (2.34)

Respiration is a set of chemical reactions in every living cell that break down nutrient molecules, mainly glucose, to release energy. The energy released is used to make ATP.

Three points are tested again and again:

• Respiration happens in all living cells, all the time: in plants, animals, fungi and bacteria, day and night.
• Respiration is controlled by enzymes, so its rate depends on temperature and pH.
• Respiration releases energy. It does not “make” or “produce” energy. Energy cannot be created; it is transferred from glucose to ATP. Some is lost as heat.

Do not confuse respiration with breathing. Breathing (ventilation) moves air in and out of the lungs. Respiration is the chemical process inside cells.

ATP: the energy carrier (2.34–2.35)

ATP stands for adenosine triphosphate. When glucose is broken down in respiration, the energy released is used to make ATP. The cell then breaks ATP down whenever it needs energy for a process. ATP is the direct source of energy for cells; glucose is not used directly.

Cells use the energy from ATP for:

• muscle contraction
• active transport of ions and molecules against a concentration gradient (for example, mineral ions into root hair cells)
• building large molecules from small ones, such as proteins from amino acids
• cell division and growth
• sending nerve impulses.

In mammals and birds, heat released by respiration keeps body temperature constant.

Cells that use a lot of energy, such as muscle cells and sperm cells, contain many mitochondria. More mitochondria means more aerobic respiration and more ATP.

Aerobic respiration (2.36–2.37)

Aerobic respiration uses oxygen. It takes place mainly in the mitochondria, and it breaks glucose down completely to carbon dioxide and water.

Word equation:

glucose + oxygen → carbon dioxide + water

Balanced symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Learn the symbol equation exactly. Note that it is the photosynthesis equation reversed, but the two processes are not the same: they happen in different organelles and are controlled by different enzymes.

Worked example 1: checking the symbol equation is balanced

Question. Show that the balanced equation for aerobic respiration has the same number of each type of atom on both sides.

Answer. Count each element on each side.

Atom      Left side                  Right side
C         6 (in C₆H₁₂O₆)            6 × 1 = 6 (in 6CO₂)
H         12 (in C₆H₁₂O₆)           6 × 2 = 12 (in 6H₂O)
O         6 + (6 × 2) = 18           (6 × 2) + (6 × 1) = 18

Carbon 6 = 6, hydrogen 12 = 12, oxygen 18 = 18. The equation is balanced.

The common slip is forgetting the oxygen inside glucose, which gives 12 oxygen atoms on the left instead of 18.

Anaerobic respiration (2.36, 2.38)

Anaerobic respiration happens without oxygen. It takes place in the cytoplasm. Glucose is only partly broken down, so much less energy is released and much less ATP is made from each glucose molecule.

In animals (for example, human muscle cells during hard exercise):

glucose → lactic acid

No carbon dioxide is made in animal anaerobic respiration.

In plants (for example, root cells in waterlogged soil) and in yeast:

glucose → ethanol + carbon dioxide

Yeast is a fungus, not a plant, but it uses the same reaction. Anaerobic respiration in yeast is used in bread making and brewing; that application is covered in Topic 5.

Why release less energy? In anaerobic respiration much of the chemical energy stays locked up in the product. Lactic acid and ethanol still contain energy; carbon dioxide and water from aerobic respiration contain very little.

Comparing aerobic and anaerobic respiration (2.36)

Feature	Aerobic	Anaerobic (animals)	Anaerobic (plants and yeast)
Oxygen needed?	Yes	No	No
Breakdown of glucose	Complete	Incomplete	Incomplete
Products	Carbon dioxide and water	Lactic acid	Ethanol and carbon dioxide
Energy (ATP) per glucose	Large amount	Small amount	Small amount
Main site in cell	Mitochondria	Cytoplasm	Cytoplasm

When a “describe the differences” question is worth 3 marks, give three separate differences, each as a comparison (“aerobic needs oxygen but anaerobic does not”). A list about only one type of respiration does not show a difference.

Practical: carbon dioxide and heat from respiring organisms (2.39)

You need to know how to show that living organisms release carbon dioxide and heat when they respire. Germinating seeds (such as peas or beans) are used because they respire quickly while they grow. Small animals such as woodlice or maggots also work for the carbon dioxide test.

Showing that carbon dioxide is released

Two indicators are used:

Indicator	Normal air	More carbon dioxide	Less carbon dioxide
Hydrogen-carbonate indicator	Red / orange	Yellow	Purple
Limewater	Clear	Turns milky (cloudy)	–

Method outline.

1. Put the same volume of hydrogen-carbonate indicator in two boiling tubes.
2. Place germinating seeds on a gauze platform above the indicator in one tube, so they do not touch it.
3. Set up a control tube with the same mass of seeds that have been boiled (killed) and cooled, or with glass beads of the same volume.
4. Seal both tubes with a bung and leave them at the same temperature, in the dark if you are using seeds, so photosynthesis cannot take up carbon dioxide.
5. Record the colour of the indicator at intervals.

The indicator in the tube with living seeds turns from red to yellow. The control stays red. This shows the change was caused by respiration of living cells.

Worked example 2: interpreting indicator colours

Question. A student sets up three sealed tubes in the dark, each containing red hydrogen-carbonate indicator: tube A has live maggots, tube B has glass beads, tube C has live maggots and is kept 10 °C warmer than A. After 20 minutes, A is orange-yellow, B is red and C is yellow. Explain the results.

Answer.

• Tube A: maggots respire and release carbon dioxide, which dissolves in the indicator and lowers its pH, so it changes towards yellow.
• Tube B: glass beads do not respire, so no carbon dioxide is added and the indicator stays red. This is the control; it shows the change in A is caused by the living maggots.
• Tube C: at the higher temperature, respiration enzymes work faster, so carbon dioxide is released faster and the indicator turns fully yellow in the same time.

Showing that heat is released

Method outline.

1. Soak peas in water for a day so they start to germinate.
2. Boil a second batch to kill them, then cool.
3. Rinse both batches in dilute disinfectant. This kills microorganisms on the surface, which would otherwise respire and release heat themselves.
4. Put each batch in a vacuum flask. The flask insulates the seeds, so heat released is not lost to the surroundings.
5. Plug each flask loosely with cotton wool, with a thermometer passing into the seeds. The cotton wool lets oxygen in for aerobic respiration but keeps heat in.
6. Record the temperature in each flask every day for several days.

The flask of germinating peas warms up; the flask of boiled peas stays at about room temperature. The boiled peas are the control: their cells are dead, so they do not respire.

Worked example 3: processing heat data

Question. Germinating peas start at 21.0 °C and reach 27.0 °C after 4 days. Boiled peas start and finish at 21.0 °C. Calculate the mean rate of temperature rise of the germinating peas, and explain the difference between the flasks.

Answer.

Temperature rise = 27.0 − 21.0 = 6.0 °C
Mean rate = 6.0 ÷ 4 = 1.5 °C per day

The germinating peas respire and release energy as heat. The vacuum flask stops the heat escaping, so the temperature rises. The boiled peas are dead and cannot respire, so their temperature does not change.

Always include the unit (°C per day) and use the same time unit given in the question.

Variables and reliability

• Independent variable: living or dead seeds (or, in an extension, temperature or type of seed).
• Dependent variable: temperature rise, or the time taken for the indicator to change colour.
• Control variables: mass or number of seeds, type of seed, volume of indicator, starting temperature, size of flask.
• Repeat each set-up and calculate a mean to improve reliability.

Common errors

• Writing “respiration produces energy”. Write “releases energy” or “produces ATP”.
• Giving carbon dioxide as a product of anaerobic respiration in human muscle.
• Leaving out carbon dioxide from the plant/yeast anaerobic equation.
• Writing O instead of O₂, or 6H₂O as H₁₂O₆, in the symbol equation.
• Saying anaerobic respiration releases “no energy”. It releases less.
• Forgetting to explain why the seeds are disinfected, or why boiled seeds are used.
• Describing a colour change with no starting colour: write “from red to yellow”.

Next steps

Condense this with the revision notes, then test yourself with the practice questions. For more on the cell structures involved, use the cell structure practice questions.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3, first examination June 2019, Pearson Education Limited – Section 2, Structure and functions in living organisms, sub-topic (f) Respiration, points 2.34–2.39.