Respiration is not breathing. It is a chemical process happening in every cell you have, right now, taking glucose apart in careful stages so the energy inside it can be packed into ATP.
📚 What you need to know
Cell respiration is the controlled release of energy from organic compounds to produce ATP.
It is a catabolic process, happening in every living cell, as a pathway of many small enzyme-controlled steps.
Glucose is the main respiratory substrate. Lipids and proteins can be used, but need converting first.
Aerobic respiration uses oxygen, oxidises glucose completely, and yields about 36 ATP per glucose.
Anaerobic respiration happens without oxygen, oxidises glucose partially, and yields only 2 ATP.
Anaerobic products differ by organism: lactate in animals, ethanol and carbon dioxide in yeast and plants.
Aerobic reactions happen in the cytoplasm and mitochondria; anaerobic ones only in the cytoplasm.
What respiration actually is
Food molecules hold a lot of chemical energy. Respiration is the process that gets it out in a form the cell can use, and the key word in the definition is controlled.
Definition
cell respiration = the controlled release of energy from organic compounds to produce ATP
If a cell released all the energy in a glucose molecule in one step, the temperature would spike and proteins would be damaged. So instead the molecule is dismantled bit by bit, in a pathway of many small reactions, each one catalysed by its own enzyme. A little energy comes out at each stage and some of it is captured as ATP.
The total energy released is identical in both panels. Splitting it into stages is what lets the cell capture some of it instead of losing the lot as heat.
Respiration is catabolic and exergonic — a big molecule broken into smaller ones, with energy given out. If those words feel unfamiliar, they are from the metabolism page, and examiners like seeing them here.
What gets respired
Glucose is the main respiratory substrate. It can enter the pathway directly, which makes it the easiest fuel to oxidise.
Lipids can be respired, but they have to be converted into intermediates first before they can join the pathway.
Proteins are mostly structural, so they are only used as fuel when glucose and lipids have run short.
Aerobic respiration
When oxygen is available, glucose is broken down completely into carbon dioxide and water. Complete oxidation means almost all the chemical energy is released, which is why the ATP yield is so high.
Learn this as a word equation. You are expected to be able to write it out, and the ATP is worth adding even though it is not usually shown as a product.
Glucose is completely oxidised, so a large amount of energy is released.
Yield is roughly 36 ATP per glucose molecule.
Most of the reactions happen in the mitochondria of eukaryotic cells.
Carbon dioxide is a waste product and must be excreted — except in plants, which can use it for photosynthesis.
Water is a useful by-product that contributes to the organism’s water supply.
Anaerobic respiration
Without oxygen, only the first stage of respiration can run. Glucose is only partially oxidised, so most of its energy stays locked inside the products — and the ATP yield collapses to just 2 per glucose.
Two ATP is not much, but it is far better than none — it keeps muscles working for the short burst before oxygen catches up.
When does a cell go anaerobic?
When demand outruns supply — during vigorous exercise, muscle cells need ATP faster than oxygen can be delivered.
When oxygen simply cannot reach the organism, for example plant roots in waterlogged soil.
Why the yield is so low: lactate and ethanol are still organic molecules holding plenty of chemical energy. Because oxygen is missing, the cell cannot break them down any further, so roughly 36 ATP that could have been made never are.
Comparing the two
Compare
Aerobic
Anaerobic
Oxygen needed
Yes
No
Oxidation of glucose
Complete
Partial
ATP yield per glucose
High, about 36
Low, about 2
Products
Carbon dioxide and water
Lactate, or ethanol and carbon dioxide
Where it happens
Cytoplasm and mitochondria
Cytoplasm only
Energy released
Large amount
Small amount
Both types make ATP. Students often write that anaerobic respiration “does not produce ATP” — it does, just far less of it. That single word “less” is what the mark scheme wants.
Respiration is not breathing
This costs marks every year, so be precise:
Cell respiration is a chemical process inside cells that releases energy and makes ATP.
Gas exchange is the movement of oxygen and carbon dioxide across a surface, such as the alveoli.
Breathing (ventilation) is the physical movement of air in and out of the lungs.
They are linked — breathing supplies the oxygen that respiration uses — but they are three different things with three different answers.
Worked examples
WORKED EXAMPLE
A cell needs 180 ATP molecules. Calculate how many glucose molecules it must respire (a) aerobically and (b) anaerobically. Comment on the difference.
Part (a): divide by the aerobic yield180 ÷ 36 = 5 glucose moleculesPart (b): divide by the anaerobic yield180 ÷ 2 = 90 glucose moleculesCompare90 ÷ 5 = 18 times more glucose needed5 aerobically, 90 anaerobically — 18 times as much fuelthis is why glucose stores drain so quickly during a sprint
WORKED EXAMPLE
Explain why anaerobic respiration releases far less energy from glucose than aerobic respiration. [3]
Step 1: what oxygen allows
With oxygen, glucose is completely oxidised to carbon dioxide and water.
Step 2: what happens without it
Only the first stage runs, so glucose is only partially oxidised.
Step 3: where the energy ends up
Lactate or ethanol still contain chemical energy that cannot be released.
Partial oxidation leaves energy locked in the products, so only about 2 ATP is made“complete” versus “partial” oxidation is the phrase that earns the mark
WORKED EXAMPLE
A student says: “Yeast in a sealed bottle of grape juice will respire aerobically the whole time.” Explain why this is wrong and state what the yeast produces. [3]
Step 1: what happens to the oxygen
The bottle is sealed, so the dissolved oxygen is soon used up and cannot be replaced.
Step 2: what the yeast switches to
With no oxygen it respires anaerobically in the cytoplasm.
Step 3: name the products
Yeast is not an animal, so it makes ethanol, not lactate.
Oxygen runs out, so the yeast respires anaerobically, making ethanol and carbon dioxidecheck the organism before you name the product — animals give lactate, yeast gives ethanol
💡 Exam tip
Learn both word equations. You are expected to be able to write them from memory with glucose as the substrate.
Put the word controlled in your definition of respiration. It is the difference between respiration and burning.
Quote the yields: about 36 ATP aerobically, 2 ATP anaerobically. Numbers make an answer specific.
Say complete or partial oxidation when comparing the two. It is the cleanest way to explain the difference.
Check whether the organism is an animal or a yeast or plant before naming the anaerobic product.
If asked where it happens, give both locations for aerobic: cytoplasm first, then mitochondria.
⚠ Common mix-up
Confusing respiration with breathing or gas exchange. Respiration is a chemical process happening inside cells.
Saying anaerobic respiration makes no ATP. It makes about 2 per glucose — less, not none.
Giving lactate as the product for yeast. Yeast and plants make ethanol and carbon dioxide.
Saying all aerobic respiration happens in the mitochondria. The first stage is in the cytoplasm.
Writing “energy is produced”. Energy is released or transferred, never created.
Thinking plants do not respire. Every living cell respires, all the time, including plant cells in daylight.
Calling carbon dioxide the only product of anaerobic respiration in animals. Animals make lactate and no carbon dioxide by this route.
Up next: Cell Respiration (Skills) — what changes the rate, how a respirometer measures oxygen uptake, and the calculation that turns a moving drop of liquid into a rate.
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