Aerobic and anaerobic respiration

What you'll learn

Respiration is one of the most fundamental processes in biology, occurring continuously in every living cell to release energy. This revision guide covers both aerobic and anaerobic respiration as specified in the AQA GCSE Biology specification. You'll learn the key differences between these processes, their chemical equations, where they occur in cells, and how organisms use them in different situations.

Key terms and definitions

Respiration — an exothermic reaction that occurs continuously in living cells to release energy from glucose.

Aerobic respiration — respiration that requires oxygen and produces carbon dioxide, water, and large amounts of ATP.

Anaerobic respiration — respiration without oxygen that produces lactate (in animals) or ethanol and carbon dioxide (in plants and yeast), releasing much less energy than aerobic respiration.

Mitochondria — organelles in the cytoplasm where aerobic respiration takes place.

Metabolism — the sum of all chemical reactions in a cell or organism, many of which are controlled by enzymes.

Oxygen debt — the amount of extra oxygen the body needs after exercise to react with accumulated lactate and remove it from cells.

Fermentation — another term for anaerobic respiration in yeast and plant cells that produces ethanol and carbon dioxide.

Core concepts

What is respiration and why is it important?

Respiration is not breathing. While breathing supplies oxygen to cells, respiration is the chemical process that releases energy from glucose inside cells.

All living organisms respire continuously. The energy released is used for:

• Building larger molecules from smaller ones (e.g. proteins from amino acids)
• Muscle contraction in animals
• Maintaining constant body temperature in mammals and birds
• Active transport of molecules across cell membranes

Respiration is an exothermic reaction, meaning it transfers energy to the environment. This energy is used to make ATP (adenosine triphosphate), the molecule that powers most cellular processes.

The rate of respiration varies depending on activity level. During exercise, muscle cells respire more rapidly to meet increased energy demands. Plant cells respire more quickly when they are growing or germinating.

Aerobic respiration — the complete process

Aerobic respiration requires oxygen and is the most efficient way to release energy from glucose. It occurs in the mitochondria of cells.

The word equation is:

glucose + oxygen → carbon dioxide + water

The symbol equation (required for Higher tier) is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Aerobic respiration releases approximately 2,800 kJ of energy per mole of glucose — far more than anaerobic respiration.

The process involves multiple enzyme-controlled steps that gradually break down glucose molecules. This controlled release of energy prevents damage to cells and allows the energy to be captured efficiently as ATP.

Evidence that aerobic respiration is occurring includes:

• Oxygen consumption increases
• Carbon dioxide production increases
• Temperature rises (due to energy release)
• ATP levels increase

Anaerobic respiration in animals

When insufficient oxygen is available, cells switch to anaerobic respiration. This commonly occurs during vigorous exercise when oxygen cannot be supplied to muscle cells quickly enough.

The word equation in animal cells is:

glucose → lactate

The symbol equation is:

C₆H₁₂O₆ → 2C₃H₆O₃

Key characteristics of anaerobic respiration in animals:

• Occurs in the cytoplasm (not mitochondria)
• Does not require oxygen
• Releases much less energy than aerobic respiration (approximately 120 kJ per mole of glucose)
• Is reversible — lactate can be broken down once oxygen is available
• Produces lactate, which builds up in muscles causing fatigue and pain

After vigorous exercise, you continue breathing heavily. This extra oxygen is needed to break down the accumulated lactate in a process called oxidation. The volume of oxygen needed to do this is the oxygen debt.

Lactate is transported in the blood to the liver where it is either:

• Oxidised to carbon dioxide and water
• Converted back to glucose

Anaerobic respiration in plants and yeast

Plant cells and yeast cells can also respire anaerobically, but they produce different products from animal cells.

The word equation is:

glucose → ethanol + carbon dioxide

The symbol equation is:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

This process is called fermentation and has important economic applications:

Brewing and winemaking: Yeast ferments the sugars in grapes or grain to produce ethanol. The carbon dioxide produced is either released (wine) or captured to carbonate the drink (beer, champagne). Brewers control oxygen levels carefully — too much oxygen and yeast will respire aerobically, producing no alcohol.

Bread making: Yeast is mixed into dough. As it ferments the sugars, carbon dioxide gas is produced which causes the dough to rise. The small amount of ethanol produced evaporates during baking.

Bioethanol production: In countries like Brazil, yeast ferments sugar from sugar cane to produce ethanol fuel. This is used as a renewable alternative to petrol.

Unlike lactate from animal anaerobic respiration, ethanol is toxic to yeast. When ethanol concentration reaches approximately 14%, the yeast dies, which is why wines naturally stop fermenting at this alcohol level.

Comparing aerobic and anaerobic respiration

Feature	Aerobic	Anaerobic
Oxygen required?	Yes	No
Location in cell	Mitochondria	Cytoplasm
Energy released per glucose	Large amount (~2,800 kJ/mol)	Small amount (~120 kJ/mol)
Products in animals	CO₂ + water	Lactate
Products in plants/yeast	CO₂ + water	Ethanol + CO₂
Glucose breakdown	Complete	Incomplete

The efficiency difference is significant: aerobic respiration releases approximately 23 times more energy per glucose molecule than anaerobic respiration.

Both processes start with glucose and occur in all living organisms, but aerobic respiration is preferred when oxygen is available due to its greater energy yield.

Response to exercise

During exercise, your body shows clear responses related to respiration:

Immediate responses:

• Heart rate increases to pump more oxygenated blood to muscles
• Breathing rate increases to take in more oxygen and remove carbon dioxide
• Breath volume increases (deeper breathing)
• Blood flow to muscles increases

Why these responses occur:

• Muscle cells respire more rapidly during exercise to release energy for contraction
• They require more oxygen and glucose
• They produce more carbon dioxide which must be removed

During vigorous exercise:

• Oxygen demand exceeds supply
• Muscles begin anaerobic respiration
• Lactate accumulates in muscle tissue
• Muscles become fatigued and painful
• An oxygen debt builds up

After exercise:

• Breathing and heart rate remain elevated
• Extra oxygen is used to break down lactate
• Oxygen debt is repaid
• Lactate is removed from muscles

You can measure these changes practically using simple equipment:

• Count pulse rate per minute before, during, and after exercise
• Count breathing rate per minute
• Use a spirometer or similar device to measure breathing volume
• Take blood samples to measure lactate concentration (in research settings)

Worked examples

Example 1: Calculation question

Question: A student measured their breathing rate before and after exercise. Before exercise, they took 15 breaths per minute. After exercise, they took 32 breaths per minute. Calculate the percentage increase in breathing rate. [2 marks]

Answer:

• Increase = 32 – 15 = 17 breaths per minute [1 mark]
• Percentage increase = (17 ÷ 15) × 100 = 113.3% or 113% [1 mark]

Mark scheme notes: The first mark is for calculating the difference. The second mark requires the correct formula and calculation. Answers should be given to 3 significant figures or rounded to the nearest whole number.

Example 2: Explaining observations

Question: A student investigated anaerobic respiration in yeast. They set up two boiling tubes of yeast and glucose solution. Tube A was sealed with a bung and tube B was left open to the air. After 30 minutes, only tube A produced bubbles. Explain this observation. [3 marks]

Answer:

• Tube A has no/limited oxygen available [1 mark]
• Yeast respires anaerobically/ferments [1 mark]
• Producing carbon dioxide which forms bubbles [1 mark]
• Tube B has oxygen available so yeast respires aerobically (not producing CO₂ gas that forms bubbles) [1 mark — any 3 marks from the 4 available]

Mark scheme notes: This question tests understanding of conditions needed for fermentation. You need to link the absence of oxygen to anaerobic respiration and identify the gas produced.

Example 3: Extended response

Question: Compare aerobic respiration and anaerobic respiration in animal cells. [6 marks]

Answer:

• Both break down glucose [1 mark]
• Aerobic respiration requires oxygen but anaerobic respiration does not [1 mark]
• Aerobic respiration occurs in mitochondria, anaerobic occurs in cytoplasm [1 mark]
• Aerobic respiration produces carbon dioxide and water [1 mark]
• Anaerobic respiration (in animals) produces lactate [1 mark]
• Aerobic respiration releases much more energy than anaerobic respiration [1 mark]

Mark scheme notes: "Compare" means you must give similarities AND differences. Structure your answer clearly with comparative statements. This is a 6-mark question so aim for 6 developed points with correct scientific terminology.

Common mistakes and how to avoid them

• Confusing respiration with breathing. Remember: breathing is the physical movement of air in and out of lungs; respiration is the chemical reaction in cells that releases energy. Never say "cells breathe" — they respire.

• Stating that respiration "produces energy". Energy cannot be created or destroyed. Respiration releases or transfers energy from glucose. Use precise terminology: "respiration releases energy" or "energy is transferred from glucose".

• Forgetting that plants respire all the time. Students often think plants only photosynthesise. Plants respire 24 hours a day in all living cells. They photosynthesise only in cells containing chloroplasts when light is available.

• Writing incomplete equations. In the symbol equation for aerobic respiration, ensure you balance it correctly: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. The numbers matter for full marks in exams.

• Mixing up products of anaerobic respiration. In animals: lactate only. In plants and yeast: ethanol AND carbon dioxide. Don't mix these up or say animals produce ethanol.

• Not explaining why anaerobic respiration is "less efficient". Don't just state it releases less energy — explain that glucose is only partially broken down in anaerobic respiration, so less energy can be released from each glucose molecule.

Exam technique for "Aerobic and anaerobic respiration"

• Command words matter. "State" or "Name" needs just a simple answer (1 mark). "Explain" requires a reason or mechanism (usually 2+ marks). "Compare" requires both similarities and differences. "Suggest" means apply your knowledge to an unfamiliar situation.

• Learn both word and symbol equations. Higher tier students must know symbol equations. Foundation students need word equations but may be asked to balance or complete a given symbol equation. Practice writing them from memory.

• Show your working in calculations. Even if your final answer is wrong, you can gain marks for correct method. Write out the formula you're using, substitute the numbers, then calculate.

• Use data from tables and graphs. Questions often provide experimental data. Quote specific values from the data when explaining patterns: "At 2 minutes, the breathing rate was 18 breaths per minute but at 4 minutes it had increased to 26 breaths per minute, showing..."

Quick revision summary

Respiration releases energy from glucose in all living cells. Aerobic respiration requires oxygen, occurs in mitochondria, and produces carbon dioxide and water, releasing large amounts of energy. Anaerobic respiration occurs without oxygen in the cytoplasm. In animals it produces lactate; in plants and yeast it produces ethanol and carbon dioxide. Anaerobic respiration releases much less energy because glucose is incompletely broken down. During vigorous exercise, muscles respire anaerobically when oxygen supply is insufficient, creating an oxygen debt that must be repaid afterwards.