Cell Respiration
IB Biology SL· C.2.1 - C.2.5 (2025 IB Biology Syllabus)· 12 min read
1. Core Definition and Purpose of Cell Respiration★★☆☆☆⏱ 3 min
Cell respiration is not the same as physical breathing or gas exchange: it is a continuous, enzyme-controlled metabolic process that breaks down energy-rich organic compounds (glucose, lipids, amino acids) to synthesise ATP. Unlike uncontrolled burning of glucose, which releases almost all energy as heat, the stepwise reactions of respiration capture ~40% of the stored chemical energy to form ATP, with the rest released as heat to help maintain body temperature in endotherms.
Cell Respiration
The enzyme-controlled release of energy from organic compounds to produce ATP, the immediate energy source for all cellular processes including active transport, muscle contraction, and protein synthesis.
Example:
A yeast cell breaking down glucose to power growth in an anaerobic bread dough environment.
State two reasons why all living eukaryotic cells must carry out cell respiration continuously.
- 1
First reason: ATP cannot be stored in large quantities in cells, so a constant supply of new ATP is required to power essential ongoing processes like active transport across membranes.
- 2
Second reason: No other usable energy source exists for most cellular work, so even non-photosynthetic cells cannot generate ATP via any other pathway.
Test your understanding of the core definition
Which of the following is the most accurate description of cell respiration?
A) Gas exchange between lung alveoli and blood
B) Controlled breakdown of organic molecules to produce ATP
C) Release of heat energy to raise body temperature
D) Synthesis of glucose using light energy
Reveal answer
B —A describes physical breathing, C is only a secondary byproduct, D describes photosynthesis.
2. Aerobic vs Anaerobic Respiration Pathways★★★☆☆⏱ 4 min
Glycolysis is the shared first step for all respiration, occurring in the cytoplasm with no oxygen required, producing a small net yield of 2 ATP per glucose molecule. If no oxygen is available, pyruvate from glycolysis enters fermentation pathways: human cells produce lactic acid as waste, while yeast and plant cells produce ethanol and carbon dioxide. If oxygen is available, pyruvate moves into the mitochondrion to enter the full aerobic pathway, which generates a much larger total ATP yield.
Feature | Anaerobic Respiration | Aerobic Respiration |
|---|---|---|
Oxygen Requirement | Absent or very low | Mandatory |
Location | Cytoplasm only | Cytoplasm + Mitochondrion |
Net ATP per Glucose | 2 molecules | ~30 molecules |
Human Waste Products | Lactic acid | CO₂ + Water |
Relative ATP Production Speed | Very fast | Slower |
Calculate the total net ATP produced from 6 glucose molecules via anaerobic respiration in human muscle cells, and state the waste product generated.
- 1
Recall that anaerobic respiration in human cells produces a net yield of 2 ATP per glucose molecule.
- 2
Multiply the per-glucose yield by 6: 6 * 2 = 12 total net ATP molecules.
- 3
The only waste product generated in this pathway is lactic acid, no carbon dioxide is released during human anaerobic respiration.
3. Mitochondrial Structure and Function Link★★★☆☆⏱ 3 min
Aerobic respiration after glycolysis occurs entirely inside the mitochondrion, whose specialised structure is adapted to maximise ATP production. The organelle has a smooth outer membrane, and a highly folded inner membrane that forms projections called cristae, enclosing a gel-like fluid called the mitochondrial matrix.
Explain how the structure of mitochondrial cristae adapts it for high rates of aerobic respiration.
- 1
Cristae are the tightly folded projections of the inner mitochondrial membrane.
- 2
The high degree of folding creates an extremely large surface area to embed thousands of protein carrier molecules required for the electron transport chain.
- 3
This maximises the total number of aerobic ATP synthesis reactions that can run simultaneously in a single mitochondrion.
4. Respirometer Experiments and Rate Calculations★★★★☆⏱ 2 min
A respirometer measures the rate of aerobic respiration by tracking pressure changes in a sealed chamber. Potassium hydroxide (KOH) placed inside the chamber absorbs all carbon dioxide released by respiring tissue, so the only volume change comes from oxygen being consumed by the organism, which pulls a drop of coloured fluid along a calibrated capillary tube.
A respirometer containing 4g of germinating wheat seeds shows a 18 mm movement of the coloured fluid in 45 minutes. Calculate the relative rate of oxygen consumption in mm per hour.
- 1
First convert the measured time from minutes to hours: 45 minutes = 0.75 hours.
- 2
Divide the total distance moved by the time in hours: 18 mm / 0.75 hours = 24 mm per hour.
- 3
This value represents the relative aerobic respiration rate of the 4g sample of germinating wheat tissue.
5. Common Pitfalls
Wrong move:
Stating that human anaerobic respiration produces ethanol and carbon dioxide
Why:
Confuses human fermentation pathways with yeast/plant pathways, one of the most common 1-mark deductions on SL papers
Correct move:
Explicitly note that lactic acid is the only waste product of human anaerobic respiration, no CO₂ is generated in this pathway.
Wrong move:
Describing cell respiration as 'breathing'
Why:
Fails to distinguish the cellular metabolic process from organ-level gas exchange, loses the definition mark entirely
Correct move:
Always qualify cell respiration as an enzyme-controlled metabolic process that releases energy from organic compounds.
Wrong move:
Citing outdated 36/38 ATP per glucose values for aerobic respiration
Why:
The 2025 IB SL syllabus specifies an approximate yield of ~30 ATP, older values are not accepted in modern mark schemes
Correct move:
Use the syllabus standard value of ~30 net ATP per glucose molecule for full marks.
Wrong move:
Forgetting to mention KOH absorbs CO₂ when explaining respirometer function
Why:
This is a mandatory mark scheme point for all respirometer questions, missing it drops 1-2 marks automatically
Correct move:
Explicitly state that potassium hydroxide removes all released CO₂, so only O₂ uptake causes measurable pressure change.
Wrong move:
Claiming glycolysis only occurs in the presence of oxygen
Why:
Glycolysis is the universal first step for all respiration, and has no oxygen requirement at all
Correct move:
Note that glycolysis runs in the cytoplasm for both aerobic and anaerobic respiration, regardless of oxygen availability.
6. Quick Reference Cheatsheet
Process | Location | Oxygen Required? | Net ATP per Glucose |
|---|---|---|---|
Glycolysis | Cytoplasm | No | 2 |
Human Anaerobic Respiration | Cytoplasm | No | 2 |
Yeast Anaerobic Respiration | Cytoplasm | No | 2 |
Full Aerobic Respiration | Cytoplasm + Mitochondrion | Yes | ~30 |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2023 · Paper 2
Compare aerobic and anaerobic respiration pathways
- 2022 · Paper 1
Identify location of glycolysis in eukaryotic cells
- 2021 · Paper 3
Analyse respirometer data for germinating seeds
What's Next
Mastering cell respiration gives you a core foundation for understanding all biological energy transfer, which is tested across ~30% of IB Biology SL exam papers. This topic directly links to photosynthesis, the complementary process that generates the organic compounds used as respiration substrates, as well as ecosystem energy flow in Unit 4. You will also build on this knowledge when exploring muscle contraction and athletic performance in the human physiology optional topic. Make sure to practice drawing the fully annotated mitochondrion diagram, as this 3-mark question appears almost every exam cycle, and work through past respirometer data analysis questions to avoid calculation errors.