Understanding Respiration: Edexcel IGCSE Human Biology Explained

Respiration is the process that releases energy from glucose inside every living cell. It is not breathing. That single distinction trips up more students than almost any other point in the edexcel igcse human biology specification.

The specification covers aerobic respiration, anaerobic respiration, the differences between them, and the role of ATP in energy transfer. This is a compact section, but it connects to nearly everything else in the course: muscles need energy to contract, active transport requires energy, nerve impulses depend on energy, and temperature regulation relies on the heat released by respiration. A clear understanding of edexcel igcse human biology respiration underpins your performance across multiple sections of the 4HB1 specification. These edexcel igcse human biology revision notes walk you through every testable point, with worked examples and self-check questions to lock in your understanding.

Aerobic respiration

Aerobic respiration occurs in the mitochondria and requires oxygen. It is the main source of energy for cells at rest and during moderate activity.

Word equation:

glucose + oxygen → carbon dioxide + water (+ energy released)

Balanced chemical symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy released)

Exam precision: You must know both the word equation and the balanced symbol equation. The edexcel mark scheme awards separate marks for each. Make sure the symbol equation balances: six O₂ on the left, six CO₂ and six H₂O on the right. Count the atoms: 6 carbon, 12 hydrogen, and 18 oxygen on each side.

Worked example: balancing the equation

A student writes: C₆H₁₂O₆ + O₂ → CO₂ + H₂O. Is this balanced?

Answer: No. Count the atoms on each side.

Left: C = 6, H = 12, O = 6 + 2 = 8
Right: C = 1, H = 2, O = 2 + 1 = 3

The equation is not balanced. The correct balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Now: Left: C = 6, H = 12, O = 6 + 12 = 18. Right: C = 6, H = 12, O = 12 + 6 = 18. Both sides match.

Anaerobic respiration

Anaerobic respiration occurs without oxygen. In humans, it takes place in the cytoplasm of cells when oxygen supply is insufficient, such as during intense exercise.

Word equation:

glucose → lactic acid (+ small amount of energy released)

Anaerobic respiration releases much less energy per molecule of glucose than aerobic respiration. This is because the glucose molecule is only partially broken down: lactic acid still contains chemical energy that has not been released. The build-up of lactic acid in muscles causes fatigue and a burning sensation during intense exercise.

Differences between aerobic and anaerobic respiration

Feature	Aerobic respiration	Anaerobic respiration
Oxygen required?	Yes	No
Where it occurs	Mitochondria	Cytoplasm
Products	Carbon dioxide and water	Lactic acid (in humans)
Energy released	Large amount	Small amount
Glucose breakdown	Complete	Incomplete

Worked example: explaining why anaerobic respiration releases less energy

Explain why anaerobic respiration releases less energy than aerobic respiration.

Answer: In anaerobic respiration, glucose is only partially broken down. The product, lactic acid, still contains chemical energy that has not been released. In aerobic respiration, glucose is completely broken down into carbon dioxide and water, releasing all the available chemical energy. Therefore, aerobic respiration releases more energy per molecule of glucose.

The role of ATP

ATP (adenosine triphosphate) is the molecule that transfers energy within cells. The specification requires you to understand the addition and removal of a phosphate group:

Energy storage: When energy is released from glucose during respiration, it is used to add a phosphate group to ADP (adenosine diphosphate), forming ATP. This reaction requires energy.
Energy release: When a cell needs energy for a process (such as muscle contraction, active transport, or protein synthesis), ATP is broken down by removing a phosphate group, releasing energy and forming ADP + phosphate.

Think of ATP as a rechargeable battery. Respiration charges it (ADP + P_i → ATP, using energy). Cellular processes discharge it (ATP → ADP + P_i, releasing energy). The cycle repeats continuously. Cells that are highly active, such as muscle cells and liver cells, contain large numbers of mitochondria precisely because they need to regenerate ATP at a high rate. A resting muscle cell uses ATP more slowly than one mid-contraction, but even at rest, every cell in the body is continuously cycling ATP to power basic maintenance: pumping ions, synthesising proteins, and repairing membranes.

Why ATP matters for the exam: The edexcel specification explicitly names ATP and requires you to understand both directions of the reaction. A common mistake is describing ATP as "energy." ATP is not energy itself; it is a molecule that stores and transfers energy. The energy is released when the phosphate group is removed.

Worked example: ATP in muscle contraction

Explain the role of ATP in muscle contraction during a sprint.

Answer: During a sprint, muscle cells need a rapid supply of energy. Glucose is broken down by respiration (initially aerobic, then increasingly anaerobic as oxygen supply cannot keep up with demand). The energy released is used to convert ADP and a phosphate group into ATP. When the muscle fibres contract, ATP is broken down back into ADP and phosphate, releasing the energy needed for the contraction. This cycle continues throughout the sprint.

Investigating inspired and expired air

The specification requires you to investigate the difference between inspired (inhaled) and expired (exhaled) air for carbon dioxide concentration. The standard method is:

Breathe air through limewater using two connected test tubes (one for inspired air, one for expired air).
Inspired air passes through limewater on the way in; expired air passes through a separate limewater sample on the way out.
The limewater connected to expired air turns milky (cloudy) faster and more completely, showing that expired air contains more CO₂ than inspired air.

The reason is that cells produce CO₂ as a waste product of aerobic respiration. This CO₂ is transported in the blood to the lungs and exhaled. The difference in CO₂ concentration between inspired and expired air is substantial: inspired air contains roughly 0.04% CO₂, while expired air contains approximately 4%, a hundred-fold increase. This difference directly reflects the rate at which cells throughout the body are respiring aerobically.

Component	Inspired air (approximate)	Expired air (approximate)
Oxygen	21%	16%
Carbon dioxide	0.04%	4%
Nitrogen	79%	79%
Water vapour	Variable	Saturated (higher)

Connecting respiration to other topics

Respiration does not sit in isolation. Understanding how it links to the rest of the specification strengthens your answers across the board:

Gas exchange: Oxygen for aerobic respiration enters the blood at the alveoli; CO₂ produced by respiration leaves via the lungs.
Active transport: Requires ATP, which is produced by respiration. Without respiration, active transport stops.
Exercise: During exercise, muscles respire faster. The heart rate and breathing rate increase to deliver more oxygen and remove more CO₂.
Temperature regulation: Respiration is exothermic; it releases heat. This heat maintains body temperature. When the body is cold, shivering (rapid muscle contraction) increases the rate of respiration and heat production.
Digestion: The glucose that fuels respiration comes from the digestion of carbohydrates in the alimentary canal.

Self-check questions

Write the balanced symbol equation for aerobic respiration.
Write the word equation for anaerobic respiration in humans.
State two differences between aerobic and anaerobic respiration.
Explain why muscles produce lactic acid during a sprint.
Describe the role of ATP in energy transfer within a cell.
A student says "respiration is breathing." Explain why this statement is incorrect.
Describe an experiment to show that expired air contains more CO₂ than inspired air.
Explain why aerobic respiration releases more energy than anaerobic respiration.

Answers: (1) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. (2) glucose → lactic acid. (3) Any two: aerobic uses oxygen, anaerobic does not; aerobic occurs in mitochondria, anaerobic in cytoplasm; aerobic releases more energy; aerobic produces CO₂ and water, anaerobic produces lactic acid; aerobic completely breaks down glucose, anaerobic only partially. (4) During a sprint, muscles contract rapidly and need energy faster than oxygen can be supplied. Anaerobic respiration occurs without oxygen, breaking down glucose incompletely and producing lactic acid as a by-product. (5) Energy from respiration is used to add a phosphate group to ADP to form ATP. When the cell needs energy, ATP is broken down to ADP and phosphate, releasing energy for processes like muscle contraction or active transport. (6) Respiration is a chemical reaction that releases energy from glucose in cells. Breathing (ventilation) is the physical movement of air in and out of the lungs. They are linked but are different processes. (7) Pass inspired air through one sample of limewater and expired air through another. The limewater exposed to expired air turns milky faster and more completely, showing higher CO₂ concentration. (8) In aerobic respiration, glucose is completely broken down, releasing all the chemical energy stored in the molecule. In anaerobic respiration, glucose is only partially broken down to lactic acid, which still contains unreleased chemical energy.

These edexcel igcse human biology notes on respiration cover every specification point the exam tests. On the Green Bridge CBT platform, you can work through respiration edexcel igcse practice questions and igcse 4hb1 respiration revision sets. For the complete course, explore the edexcel igcse human biology explained series and the full bank of edexcel igcse human biology practice questions.