IB Biology respiration is the enzyme-controlled release of energy from carbon compounds so that cells can produce ATP. For exams, you must connect the substrates, products, locations and energy transfers rather than treating respiration as a single equation.
Under the current course, C1.2 Cell respiration contains material for both SL and HL, followed by additional HL detail. SL students focus on ATP, aerobic and anaerobic respiration, and factors affecting respiration rate. HL students must also explain glycolysis, NAD, the link reaction, the Krebs cycle, electron transport, chemiosmosis, yeast respiration and respiratory substrates.
What cell respiration actually means
The official IB definition presents cell respiration as a system for producing ATP within cells using energy released from carbon compounds. Glucose and fatty acids are principal respiratory substrates, although cells can use other organic compounds.
Respiration is not the same as breathing or gas exchange. Gas exchange moves oxygen and carbon dioxide between an organism and its environment, while cell respiration consists of metabolic reactions inside cells. Aerobic respiration depends on oxygen supplied through gas exchange, but the two processes remain distinct.
The overall aerobic word equation is:
glucose + oxygen → carbon dioxide + water
Energy released from glucose is not simply released as heat. Some is transferred to ATP, allowing the cell to use it for active transport, macromolecule synthesis and movement of cells or cell components.
ATP and energy transfer
ATP, or adenosine triphosphate, is a nucleotide that distributes energy within cells. Hydrolysis removes its terminal phosphate:
ATP + water → ADP + inorganic phosphate + released energy
The reverse process requires energy:
ADP + inorganic phosphate + energy → ATP + water
ATP is useful because it releases a manageable quantity of energy rapidly and can be regenerated continuously. It also transfers phosphate groups to other molecules, making them more reactive in a process called phosphorylation.
An exam answer should not say that ATP “stores all the cell’s energy” or that respiration “creates energy.” Energy is conserved and transferred. Respiration releases energy from carbon compounds, and some of that energy is used to synthesize ATP.
Aerobic and anaerobic respiration compared
The IB expects students to compare these processes using specific features, not merely state that one uses oxygen.
| Feature | Aerobic respiration | Anaerobic respiration in humans |
|---|---|---|
| Oxygen | Required | Not required |
| Substrate | Carbohydrates and lipids can be used | Glucose is used |
| Location | Cytoplasm and mitochondria | Cytoplasm only |
| ATP yield | Much higher | Low, with a net yield of 2 ATP per glucose from glycolysis |
| Human waste products | Carbon dioxide and water | Lactate |
| Extent of substrate breakdown | More complete oxidation | Incomplete breakdown |
The human anaerobic word equation is:
glucose → lactate
Anaerobic respiration allows ATP production to continue when oxygen delivery cannot meet demand, such as during intense muscular activity. Its ATP yield is low because glucose is not fully oxidized.
At HL, students should explain that pyruvate is converted to lactate to regenerate NAD. This enables glycolysis to continue by providing oxidized NAD to accept more hydrogen.
The stages of aerobic respiration at HL
HL questions frequently test the sequence of events and the relationship between mitochondrial structure and function. Learn each stage as part of a connected pathway.
| Stage | Location in a eukaryotic cell | Main events |
|---|---|---|
| Glycolysis | Cytoplasm | Glucose is converted into two pyruvate molecules; ATP and reduced NAD are produced |
| Link reaction | Mitochondrial matrix | Pyruvate is oxidized and decarboxylated; an acetyl group is formed and reduced NAD is produced |
| Krebs cycle | Mitochondrial matrix | Acetyl groups are oxidized and decarboxylated; ATP and reduced NAD are produced |
| Electron transport and chemiosmosis | Inner mitochondrial membrane | Reduced NAD supplies electrons; a proton gradient drives ATP synthesis |
Glycolysis
In glycolysis, one six-carbon glucose molecule is converted through a series of stepwise reactions into two three-carbon pyruvate molecules. ATP is initially used, but more ATP is subsequently produced, giving a net gain of ATP. NAD accepts hydrogen and becomes reduced NAD.
Glycolysis occurs in the cytoplasm and does not itself require oxygen. It is therefore the common initial pathway for aerobic and anaerobic respiration.
Link reaction and Krebs cycle
When oxygen is available, pyruvate enters the mitochondrial pathway. During the link reaction, pyruvate is oxidized and decarboxylated, meaning hydrogen and carbon dioxide are removed. The resulting acetyl group proceeds into the Krebs cycle.
The Krebs cycle involves further oxidation and decarboxylation of acetyl groups. It produces carbon dioxide, a small amount of ATP and reduced electron carriers, particularly reduced NAD. Its central purpose is therefore not direct production of most ATP, but the transfer of energy to reduced carriers.
Electron transport and chemiosmosis
Reduced NAD supplies high-energy electrons to an electron transport chain in the inner mitochondrial membrane. As electrons pass along the chain, released energy is used to pump protons from the matrix into the intermembrane space.
This creates an electrochemical proton gradient. Protons then move back into the matrix through ATP synthase, and the enzyme uses this flow to synthesize ATP from ADP and inorganic phosphate. This coupling of proton movement to ATP production is chemiosmosis.
Oxygen acts as the terminal electron acceptor. It accepts electrons and combines with protons to form water, allowing electron transport to continue. Avoid writing that oxygen directly produces ATP or is needed for glycolysis.
Anaerobic respiration in yeast
At HL, students must also know anaerobic respiration in yeast:
glucose → ethanol + carbon dioxide
This pathway is economically useful. Carbon dioxide expands dough during baking, while ethanol production is used in brewing. In both yeast and humans, the pathway regenerates NAD so glycolysis can continue, but the final products differ.
A common mistake is to use the human lactate equation when a question refers to yeast. Identify the organism before giving the products.
Respiratory substrates and respiration rate
The syllabus requires HL students to compare lipids and carbohydrates as respiratory substrates. Lipids contain proportionally more hydrogen and less oxygen than carbohydrates, so their oxidation transfers more electrons and can yield more energy per unit mass. However, anaerobic respiration uses carbohydrates rather than lipids.
Respiration rate may be investigated by measuring:
- oxygen consumption
- carbon dioxide production
- loss of respiratory substrate
- heat production
A simple rate calculation is:
rate = change in measured quantity ÷ time
Temperature, substrate availability, oxygen concentration, organism mass and activity can affect measured rates. Because respiration is enzyme controlled, increasing temperature generally raises rate up to an optimum, after which enzyme denaturation and membrane disruption can reduce it.
In a respirometer, carbon dioxide may be absorbed so that a decrease in gas volume represents oxygen uptake. Suitable controls, equal organism masses, constant temperature and time for equilibration improve validity. When interpreting results, distinguish random uncertainty, such as reading a scale, from a systematic problem, such as an unnoticed gas leak.
How respiration appears in IB examinations
The current Biology assessment includes multiple-choice, data-based, short-answer and extended-response questions. Respiration can therefore appear as factual recall, an unfamiliar experiment, a calculation or a longer mechanistic explanation.
Command terms determine the required response:
- State: give a brief answer without explanation, such as naming oxygen as the terminal electron acceptor.
- Compare: give corresponding similarities and differences, ideally feature by feature.
- Explain: give linked biological reasons, not an isolated list of facts.
- Calculate: show substitution, working and units.
- Evaluate: discuss strengths, limitations and their effects on the conclusion.
For an “explain chemiosmosis” question, a high-quality sequence is: electrons pass through carriers, released energy pumps protons, a gradient forms, protons flow through ATP synthase, and ATP is synthesized. Each statement causes or enables the next.
For comparison questions, use paired language: “Aerobic respiration requires oxygen, whereas anaerobic respiration does not.” A two-column plan helps ensure that every feature addresses both processes.
Common respiration mistakes
Students commonly lose marks by:
- defining respiration as breathing
- saying that energy is created rather than transferred
- placing glycolysis in the mitochondrion instead of the cytoplasm
- claiming that oxygen is directly used in the Krebs cycle
- omitting NAD when explaining HL pathways
- confusing the electron transport chain with ATP synthase
- giving ethanol as the human anaerobic product
- describing a graph without explaining the biological cause
- quoting an exact total ATP yield when the question only requires a relative comparison
Exact ATP totals vary according to the accounting model and cell conditions. For current IB preparation, prioritize the syllabus language: anaerobic respiration has a low yield, while aerobic respiration has a much higher yield.
A practical exam-focused revision method
Start by drawing the pathway from memory with locations, inputs and outputs. Then practise explaining it aloud as a causal sequence rather than memorizing disconnected labels.
Use the IB Biology resource hub to review syllabus-aligned notes and videos, then test recall with IB Biology flashcard strategies. Flashcards are most useful for definitions and locations; they cannot replace data analysis or extended-response practice.
Next, complete respiration questions in the IB Biology Questionbank. Open the worked solution after making a complete attempt, and use available per-question video solutions to observe how the method converts biological knowledge into marks. The broader Questionbank guide explains how instant feedback can support targeted correction.
Record each error under one of four headings: missing knowledge, incorrect terminology, weak command-term response or data-handling error. Jojo AI can help explain an error, but you should then close the explanation and rewrite the answer independently. The RevisionDojo exam-preparation workflow provides a useful notes-to-practice-to-feedback structure.
Conclusion
Respiration in IB Biology centres on a manageable set of testable ideas: ATP transfers energy, glycolysis begins glucose breakdown, aerobic pathways produce a much higher ATP yield, anaerobic pathways regenerate NAD, and chemiosmosis links a proton gradient to ATP synthesis. SL students need secure comparisons and experimental interpretation, while HL students must explain the pathway mechanistically.
The strongest preparation combines accurate terminology with repeated application. Use RevisionDojo Study Notes and Flashcards to establish the content, then move to the Questionbank and per-question worked or video solutions to see how that knowledge should be expressed under exam conditions.
Sources and referenced URLs
- Official IB Biology subject brief, first assessment 2025
- Official IB Biology curriculum updates
- Official IB Biology programme page
- IB Biology guide for the first assessment 2025
- NCBI overview of the mitochondrial electron transport chain
- OpenStax explanation of oxidative phosphorylation
- RevisionDojo IB Biology resources
- RevisionDojo IB Biology Questionbank
- RevisionDojo Questionbank guide
- RevisionDojo guide to IB Biology flashcards
- RevisionDojo exam-preparation workflow
