Aerobic RespirationCambridge IGCSE Biology: Revision notes
Section 1
What is aerobic respiration and why is it essential?
Aerobic respiration is the process by which cells break down glucose in the presence of oxygen to release energy. The overall equation is:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
This process occurs in mitochondria and is the main way organisms release usable energy for life processes. Aerobic respiration is far more efficient than anaerobic respiration, releasing approximately 30-32 ATP molecules per glucose molecule compared to only 2 ATP from anaerobic respiration.
The energy released is stored in the form of ATP (adenosine triphosphate), which is the universal energy currency of cells. Living organisms use this energy continuously to carry out essential life functions.
Examiners expect you to state that energy is released and stored in ATP, not just 'released'. Use 'aerobic respiration' not 'respiration' alone to avoid confusion with gas exchange.
Think of ATP as a rechargeable battery: glucose is 'charged up' during respiration, and cells 'discharge' ATP to power their activities, then recharge it again.
Section 2
How is energy used in living organisms?
Cells use the energy from ATP for seven main purposes:
| Use of Energy | Example/Explanation |
|---|---|
| Muscle contraction | Sliding of actin and myosin filaments requires ATP; enables movement and locomotion |
| Protein synthesis | Assembly of amino acids into proteins on ribosomes requires ATP energy |
| Cell division | Separation of chromosomes and formation of spindle fibres during mitosis/meiosis requires ATP |
| Active transport | Movement of molecules against concentration gradient across cell membranes requires ATP |
| Growth | Cell division, protein synthesis and tissue formation all require ATP energy |
| Nerve impulses | Maintaining ion gradients across neurone membranes (especially sodium-potassium pump) requires ATP |
| Maintaining constant body temperature | Heat production through increased respiration in mammals; shivering generates heat via muscle contraction |
These processes are constantly occurring, so cells must continuously respire to meet their energy demands. The rate of respiration increases when these activities are more intense.
Examiners test this extensively—you must list specific uses, not just 'energy for life'. Link each use to ATP: state that ATP 'provides' or 'releases' energy, not just that the process 'needs energy'.
If asked how a sprinter uses energy: muscles contract (ATP breaks down to release energy for actin-myosin sliding), nerve impulses send signals (ATP maintains Na⁺/K⁺ pumps), body temperature increases (increased respiration produces heat).
Section 3
What are the stages of aerobic respiration?
Aerobic respiration occurs in three main stages:
1. Glycolysis (in cytoplasm)
- Glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each)
- Net gain of 2 ATP and 2 NADH
- Does not require oxygen (anaerobic stage)
2. The Link Reaction (in mitochondrial matrix)
- Each pyruvate is converted to Acetyl-CoA (2 carbons)
- Releases CO₂ and produces NADH
- Occurs twice per glucose molecule
3. The Electron Transport Chain/Krebs Cycle (in inner mitochondrial membrane)
- Acetyl-CoA enters the Krebs cycle
- Produces large amounts of NADH and FADH₂
- These electron carriers donate electrons to the electron transport chain
- As electrons move through the chain, energy is used to pump protons, creating a gradient
- ATP synthase uses this gradient to produce approximately 28-30 ATP per glucose
- Oxygen is the final electron acceptor, combining with electrons and protons to form water
The total yield is approximately 30-32 ATP per glucose molecule. The presence of oxygen is essential because it acts as the final electron acceptor; without it, the electron transport chain stops and aerobic respiration cannot continue.
At IGCSE, you need to know the three stages and where they occur, plus that most ATP comes from the electron transport chain. You do not need to memorise all Krebs cycle intermediates.
Section 4
How does temperature affect respiration in yeast?
Yeast is a single-celled fungus that respires aerobically and anaerobically. When investigating the effect of temperature on respiration, scientists typically measure the rate of carbon dioxide production or oxygen consumption as a measure of respiration rate.
The relationship between temperature and respiration follows a predictable pattern:
| Temperature Range | Effect on Respiration Rate | Reason |
|---|---|---|
| 0–10°C | Very slow increase in rate | Enzyme (respiration enzyme) activity is low; substrate and enzyme molecules have little kinetic energy |
| 10–40°C | Rapid increase in rate | Enzyme activity increases; more enzyme-substrate collisions occur; molecular kinetic energy increases |
| Optimum temperature (~37–40°C for yeast) | Maximum respiration rate | Enzymes are maximally active; rate is at peak |
| Above 40°C | Rapid decrease in rate | Enzymes denature (lose 3D structure); active sites no longer fit substrate molecules |
| Above 80°C | Rate approaches zero | Enzymes are permanently denatured; respiration ceases |
Key experimental observations:
- As temperature increases from 10 to 40°C, respiration rate increases exponentially (approximately doubles for every 10°C increase)
- Beyond the optimum temperature, the rate decreases sharply and irreversibly
- This pattern is true for all enzyme-catalysed reactions, not just respiration
Why this matters: Living organisms must maintain temperatures within a narrow range. Mammals maintain constant body temperature partly because most enzymes have optimal activity at 37°C.
Examiners want to see you explain the mechanism: below optimum, temperature ↑ → kinetic energy ↑ → more enzyme-substrate collisions → rate ↑. Above optimum, bonds break → enzyme denatures → active site changes → rate ↓.
Students often say 'enzymes die' or 'enzymes are killed' above optimum temperature. Enzymes are not alive, they denature (their 3D structure breaks down irreversibly).
In a yeast respiration experiment, at 20°C the rate is 10 units; at 30°C it might be 20 units; at 40°C it might be 40 units (doubling each 10°C). But at 50°C it drops to 5 units because enzymes denature.
Section 5
What experimental methods are used to measure respiration in yeast?
To investigate the effect of temperature on respiration, scientists use several measurable indicators of respiration rate:
1. Carbon Dioxide Production
- Yeast cells produce CO₂ as a waste product of respiration
- Gas can be collected and measured using gas collection tubes or measuring cylinders over water
- Volume of gas produced per unit time = respiration rate
- Advantage: simple, visible product; easily quantifiable
2. Oxygen Consumption
- Aerobic respiration requires O₂ uptake
- Measured using respirometers (apparatus that measures change in gas volume in a sealed system)
- As yeast uses O₂, pressure decreases; change measured with manometer
- Advantage: directly measures aerobic respiration
3. Temperature Control
- Water bath maintained at constant temperatures (e.g. 10°C, 20°C, 30°C, 40°C, 50°C, 60°C)
- Yeast suspension placed in water bath to reach thermal equilibrium
- Essential to keep all other variables constant (glucose concentration, yeast concentration, pH, time)
Key experimental considerations:
- Independent variable: temperature
- Dependent variable: rate of respiration (CO₂ produced or O₂ consumed per unit time)
- Control variables: glucose concentration, yeast concentration, pH, initial mass of yeast, time
- Repeat measurements at each temperature to obtain mean values and assess reliability
Data presentation: Results are typically plotted as a line graph with temperature on the x-axis and respiration rate on the y-axis, showing the characteristic rise-to-optimum-then-fall curve.
Examiners expect you to identify which variable is independent (temperature) and dependent (respiration rate), plus list at least three control variables. State why controls matter: to prove temperature alone causes the change in respiration rate.
Must Know
- Aerobic respiration releases energy from glucose in the presence of oxygen, producing CO₂, water and ATP (~30–32 ATP per glucose); it occurs in mitochondria
- Energy from ATP is used for: muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses and maintaining body temperature
- Aerobic respiration has three stages: glycolysis (cytoplasm, 2 ATP), link reaction (mitochondrial matrix, releases CO₂), and electron transport chain (inner mitochondrial membrane, ~28–30 ATP); oxygen is the final electron acceptor
- Temperature affects yeast respiration rate: rate increases with temperature until ~37–40°C (optimum), then decreases rapidly because enzymes denature above this temperature; this follows enzyme kinetics (low temp = low kinetic energy/collisions; high temp = denatured active sites)
- In yeast respiration experiments: independent variable = temperature, dependent variable = respiration rate (measured by CO₂ production or O₂ consumption), control variables = glucose concentration, yeast mass, pH, time; results show an exponential rise then sharp fall, proving the inverse relationship between denaturation and enzyme activity
That's the notes covered.
Carry on to the next subtopic.