IB Biology HLCellular RespirationPaper 1 & 2~12 min read
Cell Respiration
Glucose holds a lot of energy. Release it all at once and you get a fire. Cells release it in a long series of small steps instead, and capture as much of it as they can in ATP.
📚 What you need to know
Cell respiration is the controlled release of energy from organic compounds to produce ATP. It happens in every cell.
It is a catabolic process, controlled by enzymes in a pathway of small steps.
Releasing the energy in one uncontrolled step would damage cells and kill tissue.
Glucose is the main respiratory substrate because it can enter glycolysis directly. Lipids and proteins need converting first.
Aerobic respiration uses oxygen, oxidises glucose completely to carbon dioxide and water, and yields about 36 ATP per glucose.
Anaerobic respiration happens without oxygen, oxidises glucose only partially, and yields about 2 ATP per glucose.
Aerobic respiration takes place in the cytoplasm and mitochondria; anaerobic respiration takes place in the cytoplasm only.
Anaerobic products differ: animals make lactate; yeast and plants make ethanol and carbon dioxide.
What respiration is for
Organic food molecules contain a great deal of chemical energy. The cell’s problem is not getting at it — burning glucose would do that — but getting at it safely and in useful amounts.
So enzymes control the release through a pathway of small reactions, and the energy is used to attach a phosphate group to ADP, making ATP. The energy in that ATP then goes to:
fuelling anabolic processes such as protein synthesis
muscle contraction
active transport across membranes
moving molecules and components around inside the cell
generating heat to maintain body temperature in warm-blooded animals
DefinitionCell respiration is the controlled release of energy from organic compounds in cells to form ATP.
Respiration and breathing are not the same thing and examiners love the difference. Respiration is a chemical process inside cells. Gas exchange is the physical movement of oxygen and carbon dioxide at the alveoli. You can hold your breath; you cannot hold your respiration.
Choosing a fuel
Glucose is the main respiratory substrate, but it is not the only one.
Glucose enters glycolysis directly, which makes it easier to oxidise than the alternatives.
Lipids can be used but must be broken down first.
Proteins are mainly structural, so they are only used as fuel when glucose and lipids are not available — a last resort.
The four stages, and where they happen
Aerobic respiration is one pathway split across two compartments. It helps enormously to hold this map in your head before you meet the details.
Keep coming back to this map. Every later page in this topic is a zoomed-in view of one of these four boxes.
Aerobic respiration
Requires oxygen and gives a large yield of ATP from glucose.
Oxygen allows glucose to be completely oxidised to carbon dioxide and water.
Yields roughly 36 ATP per glucose, compared with 2 from anaerobic respiration.
Carbon dioxide is a waste product that must be excreted — except in plants, which use it for photosynthesis.
Water is a useful by-product. Some desert animals drink very little and survive largely on this metabolic water.
In eukaryotes, most of the reactions happen in the mitochondria.
Aerobic respiration
glucose + oxygen → carbon dioxide + water (+ energy transferred to ATP)
Anaerobic respiration
Sometimes oxygen is not available. That happens in two quite different situations:
The oxygen supply cannot keep up with demand in heavily respiring cells — vigorous exercise is the classic case, where muscles still need a quick supply of ATP.
Oxygen simply cannot reach the organism, for example in waterlogged soil.
Without oxygen, glucose is only partially oxidised, so only a small part of its chemical energy is transferred to ATP. The only ATP-producing stage that continues is the first one, giving about 2 ATP per glucose. The roughly 36 ATP that oxygen would have allowed are simply not made.
Two ATP is not much, but it is far better than none, and it gives a short burst of energy when oxygen runs out. Different organisms handle it differently: plants and yeast produce ethanol and carbon dioxide, while animals produce lactate.
Look at where the energy goes: lactate and ethanol still contain most of the original energy of the glucose. That is why the yield is so poor.
Comparing the two
Feature
Aerobic respiration
Anaerobic respiration
Oxidation of glucose
Complete
Incomplete
Oxygen required
Yes
No
Relative ATP yield
High, about 36 molecules
Low, 2 molecules
Products
Carbon dioxide and water
Lactate, or ethanol and carbon dioxide
Where it happens
Cytoplasm and mitochondria
Cytoplasm only
Is ATP produced?
Yes
Yes — just far less of it
Where the 36 comes from. Different textbooks quote anything from 30 to 38 ATP per glucose, because the exact figure depends on assumptions about how efficiently the cell shuttles and uses its proton gradient. Use the figure your course gives, and always show your working so the examiner can follow the reasoning rather than just the number.
Worked examples
WE 1
Explain the need for a pathway
Explain why the energy in glucose is released in a series of small steps rather than all at once. (3 marks)
Point 1: the danger
Organic molecules hold a lot of chemical energy, and releasing it in one uncontrolled step would damage cells and cause tissue death.
Point 2: the mechanism
Enzymes control the release through a pathway, and each small step is catalysed by its own enzyme.
Point 3: the benefit
Small releases can be captured efficiently as ATP rather than being lost as heat, and the cell can regulate the pathway step by step.
Controlled steps mean usable energy instead of destructive heat“it would burn the cell” is the right idea but say damage cells and tissues for the mark
WE 2
Compare the ATP yields
A muscle cell respires 10 molecules of glucose aerobically and 10 anaerobically. Calculate the difference in ATP produced and explain the cause. (3 marks)
Step 1: aerobic
10 × 36 = 360 ATPStep 2: anaerobic
10 × 2 = 20 ATP, so the difference is 360 − 20 = 340 ATP.
Step 3: the explanation
Without oxygen glucose is only partially oxidised, so only glycolysis produces ATP; the link reaction, Krebs cycle and oxidative phosphorylation cannot run.
340 ATP fewer, because the glucose is never fully oxidisedname the stages that stop — it turns a calculation mark into an explanation mark
💡 Exam tips
Be able to write simple word equations for both types of respiration with glucose as the substrate.
Remember ATP is produced in both aerobic and anaerobic respiration.
Never confuse respiration with gas exchange or breathing.
Say completely oxidised and partially oxidised — these are the phrases mark schemes look for.
Learn the locations: cytoplasm for glycolysis, mitochondria for everything else.
Water is a useful by-product of aerobic respiration, not just waste.
⚠ Common mistakes
Saying anaerobic respiration makes no ATP. It makes about 2 per glucose.
Saying respiration creates energy. Energy is transferred from glucose to ATP.
Claiming anaerobic respiration happens in mitochondria. It is cytoplasm only.
Giving lactate as the anaerobic product in yeast. Yeast produces ethanol and carbon dioxide.
Writing that plants do not respire. Every living cell respires, all the time.
Treating proteins as a normal fuel. They are used only when glucose and lipids are unavailable.
Up next: Cell Respiration (Skills). Before pulling the pathway apart, the next page covers how respiration rate is actually measured in a lab, including the respirometer calculation.
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