Anaerobic RespirationCambridge IGCSE Biology: Revision notes
Section 1
What is anaerobic respiration and how does it differ from aerobic respiration?
Anaerobic respiration is the release of energy from glucose without using oxygen. It occurs in the cytoplasm and does not require a continuous oxygen supply, making it useful when oxygen availability is limited.
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen required | Yes | No |
| Location | Mitochondria (mainly) | Cytoplasm |
| Glucose broken down | Completely | Partially |
| ATP produced | Large amount (about 38 ATP per glucose) | Small amount (2 ATP per glucose) |
| End products | Carbon dioxide and water | Lactate (animals) or ethanol and CO₂ (yeast/plants) |
| Speed | Slower | Faster |
Why is this important? During intense exercise, muscles may not receive oxygen quickly enough, so they rely on anaerobic respiration to produce ATP rapidly, even though it yields less energy.
Examiners expect you to compare ATP yields and identify the different end products in different organisms. State clearly that animals produce lactate whilst yeast produces ethanol and carbon dioxide.
Think of anaerobic respiration as a 'quick sprint' that uses stored energy reserves rapidly but inefficiently, versus aerobic respiration as a 'steady jog' that uses fuel efficiently.
Section 2
What are the equations for anaerobic respiration in different organisms?
In animals (including humans):
Glucose → 2 Lactate + Energy (2 ATP)
C₆H₁₂O₆ → 2 C₃H₆O₃ + Energy
In yeast and plants:
Glucose → 2 Ethanol + 2 Carbon dioxide + Energy (2 ATP)
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + Energy
Key differences:
- Animal anaerobic respiration produces lactate, which accumulates in muscles and causes muscle fatigue
- Yeast and plant anaerobic respiration produces ethanol (used in brewing and fermentation) and carbon dioxide
- Both pathways produce only 2 ATP per glucose molecule, compared to approximately 38 ATP in aerobic respiration
Students often forget that the equations must be balanced and show both ATP production and the correct end products. Don't write 'lactate is produced' without specifying the correct molecular products and energy yield.
In an exam, if asked why yeast is used in bread-making, explain: yeast respires anaerobically, producing CO₂ gas which makes bread rise, and ethanol which evaporates during baking. This is a practical application that examiners reward.
Section 3
How does temperature affect respiration in yeast?
Temperature has a significant effect on the rate of respiration in yeast, which can be investigated by measuring the volume of gas produced (CO₂) or the rate of fermentation.
Effect of increasing temperature:
- 0–10°C: Very slow respiration; enzyme activity is minimal
- 10–40°C: Respiration rate increases steadily; enzymes work faster as molecular collisions increase and enzyme-substrate complexes form more frequently
- 40–50°C: Respiration rate begins to decline; enzymes start to denature (lose their 3D shape)
- Above 50°C: Respiration rate drops significantly; most enzymes are permanently denatured and non-functional
Optimal temperature: Yeast respires fastest at approximately 37–40°C (body temperature or slightly above).
Why this happens: Respiration depends on enzyme activity. Temperature affects how often enzyme and substrate molecules collide. However, excessive heat breaks the hydrogen bonds holding enzyme structure together, destroying their active site.
When describing experimental results, examiners want to see: the rate increases then decreases (bell-curve pattern), linking this to enzyme activity and denaturation. Use 'enzyme activity' rather than vague terms like 'the respiration is faster'.
If experimenting with yeast, measuring CO₂ production at 20°C, 30°C, 40°C, and 50°C shows a clear pattern: slow at 20°C, fastest at 40°C, slower again at 50°C. Explain this using enzyme kinetics and denaturation.
Section 4
What are the main uses of energy from respiration in living organisms?
ATP (adenosine triphosphate) produced during respiration is used for many essential processes:
- Muscle contraction: ATP provides energy for myosin filaments to pull actin filaments, enabling movement
- Protein synthesis: ATP powers ribosomes to link amino acids together in building new proteins
- Cell division: ATP is required for spindle fibre formation, chromosome movement, and cytokinesis
- Active transport: ATP provides energy for carrier proteins to move substances against concentration gradients (e.g., glucose uptake, ion pumps)
- Growth: ATP supplies energy for cell division and synthesis of new cellular structures
- Nerve impulses: ATP powers sodium-potassium pumps that maintain the resting potential and allow action potentials
- Maintenance of constant body temperature: ATP is used for metabolic heat production in mitochondria, particularly through non-shivering thermogenesis in brown adipose tissue
Why this matters: Different organisms and tissues have different energy demands. Muscle tissue requires lots of ATP for contraction; rapidly dividing cells (like bone marrow) require ATP for cell division; endocrine glands require ATP for protein synthesis.
Examiners test whether you can link specific ATP uses to specific cellular structures and processes. For example: say 'ATP is hydrolysed to provide energy for myosin-actin interaction' rather than just 'muscle contraction needs energy'.
ATP is like a rechargeable battery in cells—it's constantly being used (spent) and regenerated (recharged) through respiration. Different 'appliances' (cell processes) consume ATP at different rates depending on the cell type.
Section 5
Why is anaerobic respiration important despite producing less ATP?
Although anaerobic respiration produces far less ATP than aerobic respiration (only 2 ATP per glucose compared to ~38), it serves critical functions:
In muscles during exercise:
- Provides immediate ATP when oxygen delivery cannot meet demand
- Allows brief periods of intense activity (e.g., sprinting) before oxygen becomes available
- Produces energy rapidly because fewer enzymatic steps are required
- The lactate produced is transported to the liver and converted back to glucose (Cori cycle)
In yeast and microorganisms:
- Allows survival in oxygen-poor environments
- Used commercially for fermentation (brewing, bread-making, yogurt production)
- Enables anaerobic decomposition in sewage treatment
Why lactate matters:
- Lactate accumulation in muscles causes muscle fatigue and the 'burning' sensation during intense exercise
- Once exercise stops and oxygen becomes available again, lactate is oxidised back to pyruvate and used in aerobic respiration
- This is why oxygen debt (or EPOC—excess post-exercise oxygen consumption) occurs: extra oxygen is needed after exercise to clear lactate
The trade-off: Anaerobic respiration is energy-inefficient but time-efficient—useful for short bursts but unsustainable for long periods.
When explaining why anaerobic respiration exists, don't just say 'muscles need energy quickly.' Explain: aerobic respiration is too slow to meet ATP demand during intense exercise, so anaerobic respiration provides rapid ATP production while lactate is cleared during recovery.
Must Know
- Anaerobic respiration is the release of energy from glucose without oxygen, occurring in the cytoplasm and producing only 2 ATP per glucose (compared to ~38 in aerobic respiration)
- In animals: Glucose → 2 Lactate + Energy; in yeast/plants: Glucose → 2 Ethanol + 2 CO₂ + Energy
- Temperature dramatically affects respiration rate in yeast: enzymes work fastest at 37–40°C; respiration rate increases with temperature up to the optimum, then declines as enzymes denature
- ATP from respiration is used for: muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses, and maintaining constant body temperature
- Despite low ATP yield, anaerobic respiration is crucial because it provides rapid energy during intense exercise when oxygen is limited, allowing muscles to function briefly at maximum intensity; lactate is later cleared during recovery using oxygen (oxygen debt/EPOC)
- Examiners expect you to: compare equations, explain enzyme kinetics with temperature, link ATP use to specific cellular processes, and justify why anaerobic respiration exists despite inefficiency
That's the notes covered.
Carry on to the next subtopic.