AQA GCSE Biology (8461)

4.4.2 Respiration

Respiration is the chemical reaction that releases energy from glucose in every living cell — it is NOT breathing. This page works through the three GCSE pathways (aerobic, anaerobic in muscles, anaerobic in yeast and plants), what energy is used for, the body's response to exercise, and the oxygen debt. The exact AQA marking phrases for 'lactic acid', 'oxygen debt' and 'fermentation' are pulled out so you can drop them straight into 4-mark answers.

Why this matters

Every living cell needs energy continuously. That energy comes from breaking chemical bonds in glucose — a process called cellular respiration. Aerobic respiration uses oxygen and happens in the mitochondria; it releases a lot of energy and produces CO₂ and water. When demand outstrips oxygen supply (for example during a sprint) cells switch to anaerobic respiration, which breaks glucose down incompletely, releases much less energy, and produces lactic acid in animals (or ethanol + CO₂ in yeast and plants — fermentation). The body responds to exercise by raising heart rate, breathing rate and breath volume to bring more glucose and oxygen to the muscles; the liver releases glucose from glycogen stores. When the exercise is intense enough, anaerobic respiration kicks in alongside aerobic, building up an oxygen debt that must be repaid afterwards. The waste lactic acid is carried in the blood to the liver, where it is broken down — which is why you keep breathing fast for several minutes after stopping.

How to learn this topic

Build on what you already know

GCSE 4.1.1: cells contain mitochondria — the site of aerobic respiration.
GCSE 4.1.3: active transport moves substances against a gradient and needs energy.
GCSE 4.4.1: photosynthesis produces glucose; the reverse of respiration in some ways.
KS3: breathing brings oxygen into the lungs and removes carbon dioxide.
Chemistry 5.4: exothermic reactions release energy to the surroundings.

Define respiration as an exothermic reaction that happens continuously in living cells.
Aerobic respiration: word equation, symbol equation, location (mitochondrion).
Five uses of the energy released: muscle contraction, building molecules, keeping warm, active transport, nervous impulses.
Anaerobic respiration in muscles: glucose → lactic acid, much less energy, incomplete breakdown.
Anaerobic respiration in yeast + plants: glucose → ethanol + CO₂ = fermentation; economic uses (bread, brewing).
Response of the body to exercise: heart rate, breathing rate, breath volume, glycogen → glucose.
Oxygen debt: anaerobic respiration during exercise → lactic acid; extra O₂ needed afterwards to react with it; liver does the breakdown.

Key terms

cellular respiration: An exothermic chemical reaction that releases energy from glucose. Occurs continuously in every living cell. (Examiners reward 'releases energy from glucose'. Don't say 'makes energy' — energy is not made, it is transferred.)
exothermic reaction: A reaction that releases energy to the surroundings. Aerobic respiration is exothermic. (AQA marking phrase: 'exothermic reactions release energy'. Worth stating explicitly in extended answers.)
aerobic respiration: Respiration that uses oxygen. Takes place mainly in the mitochondria. Breaks glucose down completely to CO₂ and water, releasing a lot of energy. Word equation: glucose + oxygen → carbon dioxide + water. (Aerobic = uses oxygen. State the equation in words. CO₂ AND water (both needed for full marks).)
anaerobic respiration: Respiration that does NOT use oxygen. Glucose is broken down incompletely, releasing much less energy per glucose. In muscles: glucose → lactic acid. In yeast/plants: glucose → ethanol + CO₂. (Mark 'incomplete breakdown of glucose' and 'releases much less energy than aerobic respiration'.)
lactic acid: The waste product of anaerobic respiration in animal (muscle) cells. Builds up in muscles during vigorous exercise; transported in the blood to the liver to be broken down. (Lactic acid → in MUSCLES. Ethanol + CO₂ → in YEAST and PLANTS. Don't mix them up.)
oxygen debt: The extra oxygen needed by the body after vigorous exercise to react with the lactic acid that built up during anaerobic respiration, removing it from the cells. (Two marking-phrase parts: (1) 'extra oxygen needed to react with lactic acid', (2) 'this is the oxygen debt'. Both wanted.)
fermentation: Anaerobic respiration in yeast cells. Produces ethanol and carbon dioxide. Used in brewing (the ethanol becomes alcohol) and in bread-making (CO₂ makes the bread rise). (Term 'fermentation' is specifically YEAST anaerobic respiration. Marking phrase: 'glucose → ethanol + carbon dioxide'.)
mitochondrion: A subcellular organelle in eukaryotic cells where aerobic respiration takes place. Cells with high energy demands (muscle, liver) have many mitochondria. (Aerobic respiration → mitochondrion. Anaerobic respiration → cytoplasm. Location is examinable.)
glycogen: A storage carbohydrate made of joined glucose molecules. Stored in muscles and the liver. Broken down to glucose during exercise to fuel respiration. (Animals store GLYCOGEN. Plants store STARCH. Don't confuse the two.)
breath volume: The volume of air moved into the lungs in a single breath. Increases during exercise so more oxygen reaches the alveoli per breath. (Three things increase during exercise: heart rate, breathing rate, AND breath volume. State all three for full marks.)
active transport: The movement of substances against a concentration gradient. Requires energy from respiration. (One of the five named uses of energy in 4.4.2. Examiners often ask for examples — root hair cells absorbing mineral ions; gut lining absorbing glucose against the gradient.)
vigorous exercise: Exercise intense enough that the lungs and heart cannot supply oxygen to muscle cells fast enough — triggering anaerobic respiration in muscles. (Use this exact phrase in answers about oxygen debt — 'during vigorous exercise, anaerobic respiration occurs'.)

Notes

What is respiration?

Cellular respiration is the chemical reaction that releases energy from glucose in every living cell. It is exothermic — it transfers energy to the surroundings — and it happens continuously, day and night, in every cell of every living organism.

Do not muddle respiration with breathing. Breathing (gas exchange) moves air in and out of the lungs. Respiration is the chemical reaction that happens inside cells. They are linked — breathing supplies the oxygen for aerobic respiration — but they are not the same thing.

Aerobic respiration

Aerobic respiration uses oxygen and takes place mainly in the mitochondria.

Word equation:

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

Symbol equation:

> C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Key features:

Exothermic — energy is released as the glucose is broken down.
Complete breakdown of glucose — releases a large amount of energy per glucose molecule.
Happens in mitochondria (more active cells like muscle and liver have many more mitochondria).

What is the energy used for?

The energy released by respiration powers everything a living organism does. AQA wants you to know five named uses:

Muscle contraction — movement of the body, heartbeat, breathing muscles.
Building larger molecules from smaller ones — e.g. amino acids → proteins, glucose → starch in plants, glucose → glycogen in animals.
Keeping warm — mammals and birds maintain a constant body temperature using heat released from respiration.
Active transport — moving substances against a concentration gradient (e.g. root hair cells absorbing mineral ions).
Nervous impulses — transmitting signals along neurons.

Anaerobic respiration in muscles

During vigorous exercise, muscles need energy faster than the heart and lungs can deliver oxygen. They switch to anaerobic respiration, which does NOT need oxygen.

Word equation (muscles):

> glucose → lactic acid (+ a little energy released)

Key features:

Takes place in the cytoplasm of muscle cells (not in mitochondria).
Incomplete breakdown of glucose — releases much less energy per glucose molecule than aerobic respiration.
Produces lactic acid, which builds up in the muscles and causes fatigue and cramp.
Allows muscles to keep working when oxygen supply runs out.

Anaerobic respiration in plants and yeast — fermentation

In plant cells (and yeast cells, which are single-celled fungi), anaerobic respiration produces different waste products.

Word equation (yeast and plant cells):

> glucose → ethanol + carbon dioxide (+ a little energy)

In yeast this is called fermentation, and it has huge economic importance:

Brewing alcoholic drinks — yeast respires anaerobically using sugars from grain (beer) or grapes (wine). The ethanol is the alcohol; the CO₂ bubbles give beer its fizz.
Bread-making — yeast respires anaerobically in dough. The CO₂ bubbles get trapped, making the bread rise; the ethanol evaporates in the oven.

Plants respire anaerobically in roots that get waterlogged (lack of oxygen) — the ethanol can damage the cells over time.

Response of the body to exercise

When you start exercising, your muscles need more energy. The body responds in several ways to bring more glucose and oxygen to the muscles and to remove waste CO₂ faster:

Heart rate increases — heart beats faster and harder, pumping more oxygenated blood per minute to the muscles.
Breathing rate increases — you breathe more times per minute.
Breath volume (depth) increases — each breath moves a larger volume of air, so more oxygen reaches the alveoli per breath.
Glycogen stored in muscles is broken down to glucose — to fuel the increased respiration in muscle cells.

If the exercise is intense enough that the lungs and heart cannot supply oxygen fast enough, muscle cells start respiring anaerobically in addition to aerobically. Lactic acid begins to build up.

Oxygen debt

The oxygen debt is the extra oxygen the body needs after exercise to react with the lactic acid that built up during anaerobic respiration. It is why you keep breathing fast and deep for several minutes after you stop running.

What happens:

During vigorous exercise, anaerobic respiration occurs in muscles.
Lactic acid accumulates in the muscle cells and the blood.
When exercise stops, breathing and heart rate stay raised — bringing in extra oxygen.
Lactic acid is transported in the blood to the liver.
In the liver, lactic acid is converted back to glucose (or fully broken down) using the extra oxygen — this repays the oxygen debt.
Once the lactic acid is cleared, breathing and heart rate return to normal.

The definition of oxygen debt for an exam answer: the amount of extra oxygen the body needs after exercise to react with the lactic acid produced by anaerobic respiration and remove it from cells.

Quick comparison table

|---|---|---|---|

| Oxygen used? | Yes | No | No |

Memorising this table is enough to answer most short-answer respiration questions. Add the oxygen-debt marking-phrase chain for the longer 4-mark questions, and the five named uses of energy for the 'why do organisms respire' questions, and you have the full topic covered.

Exam tips

Always state aerobic respiration USES OXYGEN and happens in the MITOCHONDRIA. These two phrases get marks.
Write the word equation: 'glucose + oxygen → carbon dioxide + water'. Both products (CO₂ AND water) needed for the equation mark.
List ALL FIVE uses of energy when asked: muscle contraction, building larger molecules, keeping warm, active transport, nervous impulses.
For anaerobic respiration in MUSCLES: 'glucose → lactic acid'. For YEAST/PLANTS: 'glucose → ethanol + carbon dioxide'. Different products — don't swap them.
When describing oxygen debt, use the full marking-phrase chain: anaerobic respiration occurs → lactic acid is produced → extra oxygen needed to react with the lactic acid → this is the oxygen debt → lactic acid taken to the liver.
Anaerobic respiration releases MUCH LESS ENERGY because the breakdown of glucose is INCOMPLETE. Both halves of that sentence are marked separately.
During exercise, name all THREE changes: heart rate UP, breathing rate UP, breath volume UP. Add 'glycogen is broken down to glucose'.
Fermentation is YEAST anaerobic respiration. Examiners love this term — use it. Add the economic uses: bread-making and brewing.
Aerobic respiration is EXOTHERMIC — it releases energy. Stating 'exothermic reactions release energy' is often a mark in itself.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What is the difference between respiration and breathing?

Respiration is the chemical reaction inside every living cell that releases energy from glucose. Breathing (ventilation) is the physical movement of air in and out of the lungs. They are connected — breathing brings in the oxygen used by aerobic respiration, and removes the CO₂ that respiration produces — but they are not the same process. If a GCSE question asks about respiration, they want the chemical reaction, not the breathing-in-and-out part.

Why does anaerobic respiration release less energy than aerobic respiration?

Because the breakdown of glucose is INCOMPLETE. In aerobic respiration, glucose is broken down all the way to CO₂ and water, releasing all of the energy stored in the glucose. In anaerobic respiration there is no oxygen to complete the breakdown — so the glucose is only partially broken down (to lactic acid in muscles, or to ethanol + CO₂ in yeast). Most of the energy stays locked up in those products. That's why anaerobic respiration is fast (no oxygen needed, no waiting for the lungs) but inefficient (releases only a small fraction of the energy).

What exactly is the oxygen debt?

The oxygen debt is the EXTRA oxygen the body needs AFTER exercise — over and above what would normally be needed at rest — to react with the LACTIC ACID that built up during anaerobic respiration. During hard exercise, muscles can't get enough oxygen, so they respire anaerobically and make lactic acid. After the exercise, you keep breathing fast and deep so that the extra oxygen can be carried in the blood to the liver, where lactic acid is broken down. Once all the lactic acid is cleared, the debt is 'repaid' and breathing returns to normal.

Why is fermentation economically important?

Fermentation is anaerobic respiration in yeast cells: glucose → ethanol + carbon dioxide. Two big industries depend on it. (1) BREWING — yeast ferments sugar in grain (beer) or grapes (wine); the ethanol IS the alcohol; the CO₂ gives the drink its fizz. (2) BREAD-MAKING — yeast in dough ferments sugars and produces CO₂; the bubbles get trapped in the gluten and make the bread rise; the ethanol evaporates during baking. So fermentation is what makes both alcoholic drinks and risen bread possible — that's billions of pounds of food and drink industry every year.

What is the energy from respiration used for?

AQA wants you to know FIVE named uses. (1) MUSCLE CONTRACTION — movement, heartbeat, breathing. (2) BUILDING LARGER MOLECULES — amino acids → proteins, glucose → starch (plants) or glycogen (animals). (3) KEEPING WARM — mammals and birds use heat released by respiration to maintain a constant body temperature. (4) ACTIVE TRANSPORT — moving substances against a concentration gradient, e.g. roots absorbing mineral ions. (5) NERVOUS IMPULSES — transmitting signals along neurons. List all five in extended answers — examiners give a mark per named use.

What changes in the body during exercise?

Three things increase straight away. (1) HEART RATE — the heart beats faster and harder, pumping more oxygenated blood to the muscles per minute and removing CO₂ faster. (2) BREATHING RATE — you take more breaths per minute. (3) BREATH VOLUME — each breath is deeper, so more oxygen reaches the alveoli per breath. On top of that, GLYCOGEN stored in muscles and the liver is broken down to glucose to fuel respiration. If the exercise is intense enough that the lungs and heart still can't keep up, the muscles switch to anaerobic respiration, building up lactic acid and creating an oxygen debt.

Why are mitochondria important for respiration?

Mitochondria are the subcellular organelles where AEROBIC respiration takes place. Inside the mitochondria, the enzymes that break glucose down using oxygen are arranged on folded inner membranes — giving a large surface area for the reactions. Cells with high energy demand — muscle cells, liver cells, sperm cells — have many more mitochondria than cells with low demand. If a cell has few mitochondria, it can't release much energy aerobically. Anaerobic respiration happens in the cytoplasm instead, but releases much less energy per glucose.