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The rate of cellular respiration is influenced by several factors, including temperature, substrate availability, and oxygen concentration. This lesson also covers the practical techniques used to measure respiration rate, particularly the use of respirometers. These topics are required by the Edexcel A-Level Biology (9BI0) specification.
Respiration is a series of enzyme-controlled reactions, so temperature affects the rate in the same way as any enzyme-catalysed process.
| Temperature range | Effect on respiration rate |
|---|---|
| Below optimum | Rate increases with temperature. Molecules have more kinetic energy, so enzyme-substrate collisions occur more frequently and with more energy. The Q₁₀ value is typically around 2 (rate doubles per 10°C rise). |
| At optimum (~30–40°C for most organisms) | Maximum rate of respiration. |
| Above optimum | Rate decreases rapidly. Enzymes begin to denature — their tertiary structure unfolds, active sites change shape, and substrate binding is reduced. |
| Extreme temperatures (>50°C) | Enzymes are fully denatured; respiration rate drops to zero. |
Exam Tip: When asked about temperature and respiration, always mention kinetic energy, frequency of enzyme-substrate collisions, and (if above optimum) denaturation with reference to changes in tertiary structure and active site shape.
The rate of respiration depends on the availability of respiratory substrates:
| Substrate level | Effect |
|---|---|
| Low | Rate is limited by substrate availability (glycolysis has less glucose to process) |
| Increasing | Rate increases as more substrate molecules are available to enter glycolysis |
| Saturating | Rate plateaus — enzymes (e.g. hexokinase, PFK) become saturated and work at Vmax |
Different substrates produce different amounts of ATP per gram:
Oxygen is the final electron acceptor in the electron transport chain. Its concentration determines whether respiration is aerobic or anaerobic.
| O₂ level | Effect |
|---|---|
| High (aerobic) | Full aerobic respiration occurs: glycolysis + link reaction + Krebs cycle + oxidative phosphorylation. Maximum ATP yield (~32 per glucose). |
| Low (partially anaerobic) | Oxidative phosphorylation is reduced; anaerobic pathways supplement ATP production. Overall ATP yield falls. |
| Zero (anaerobic) | Only glycolysis + fermentation. ATP yield is just 2 per glucose. |
Louis Pasteur observed that yeast consumes glucose much faster under anaerobic conditions than aerobic conditions. This is because:
Exam Tip: If asked to explain why glucose consumption increases in anaerobic conditions, reference the lower ATP yield per glucose molecule and the need to increase glycolysis rate to compensate.
A respirometer (also called a manometer in some designs) measures the rate of oxygen consumption by an organism. This is the most common method for measuring aerobic respiration rate in the required practicals.
| Component | Purpose |
|---|---|
| Sealed chamber | Contains the living organism (e.g. germinating seeds, woodlice, maggots) |
| Soda lime / KOH | Absorbs CO₂ produced by respiration |
| Capillary tube with fluid | Measures volume change as fluid moves |
| Control tube | Contains dead organisms or glass beads of equal volume; accounts for changes in temperature or pressure |
| Water bath | Maintains constant temperature |
| Syringe | Resets the apparatus by pushing air back to the starting position |
| Consideration | Why it matters |
|---|---|
| Equilibration time | Allow 5–10 minutes for the apparatus to equilibrate to the water bath temperature before recording |
| Control tube | Essential to account for volume changes due to temperature or atmospheric pressure fluctuations |
| CO₂ absorber | Without it, the CO₂ produced would replace the O₂ consumed, and no volume change would be detected (if RQ = 1) |
| Constant temperature | Use a water bath to prevent thermal expansion/contraction of gases affecting results |
| Organism activity | Use organisms of similar size and mass for reproducible results |
Exam Tip: Without soda lime, a respirometer would show no movement of the fluid when the organism is respiring carbohydrates (RQ = 1, meaning the volume of O₂ consumed equals the volume of CO₂ produced). The soda lime removes CO₂, ensuring that only O₂ consumption causes a volume change.
To investigate the effect of temperature on respiration rate:
To investigate different respiratory substrates:
If the coloured fluid moves 3.2 cm in 10 minutes and the capillary tube has a cross-sectional area of 0.8 mm²:
Volume of O₂ consumed = distance × cross-sectional area = 3.2 cm × 0.8 mm² = 3.2 × 10 mm × 0.8 mm² = 32 × 0.8 = 25.6 mm³
Rate = 25.6 mm³ / 10 min = 2.56 mm³ min⁻¹
To express per gram of organism, divide by the mass: If the seeds weigh 5 g: Rate = 2.56 / 5 = 0.512 mm³ min⁻¹ g⁻¹
Exam Tip: Always show your working clearly and include units. Examiners award marks for correct units and for dividing by mass to give a rate per gram.
The rate of respiration can be studied using metabolic inhibitors:
| Inhibitor | Target | Observation in respirometer |
|---|---|---|
| Cyanide | Complex IV of ETC | O₂ consumption drops to near zero |
| DNP | Uncouples ETC from ATP synthase | O₂ consumption increases (ETC runs faster without back-pressure from proton gradient) |
| Oligomycin | ATP synthase | O₂ consumption decreases (ETC slows as proton gradient builds up) |
| Malonate | Competitive inhibitor of succinate dehydrogenase (Krebs cycle) | O₂ consumption decreases |
| Term | Definition |
|---|---|
| Respirometer | An apparatus that measures the rate of O₂ consumption by a living organism |
| Soda lime | A chemical that absorbs CO₂, used in respirometers to ensure only O₂ changes are measured |
| Q₁₀ | The factor by which the rate of a reaction increases for a 10°C rise in temperature |
| Pasteur effect | The observation that glucose consumption by yeast increases dramatically in anaerobic conditions |
| Equilibration | Allowing the apparatus to stabilise at the experimental temperature before taking readings |
This material sits in Edexcel 9BI0 Topic 5 (On the Wild Side — Photosynthesis, Energy and Ecosystems) and integrates the rate-controlling factors of cellular respiration: temperature, substrate concentration, oxygen availability and metabolic inhibitors. Synoptic links run across all four respiratory stages: lesson 4 (glycolysis) — where PFK is the major regulatory enzyme and a frequent rate-limiting step in fast-glycolytic tissues; lesson 5 (link reaction and Krebs cycle) — where matrix enzymes set flux through the cycle; lesson 6 (oxidative phosphorylation) — where Complex IV is the most common rate-limiting step under physiological conditions; and lesson 7 (anaerobic respiration) — the fallback when O2 supply fails. Further connections: Topic 1 (enzymes) — every step in respiration obeys Michaelis-Menten kinetics and the standard rate-vs-temperature curve, so Q10 behaviour follows from enzyme theory; Topic 7 (exercise physiology) — VO2max integrates all these factors at the whole-organism scale, with respiration matching ATP demand during increasing workload. Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks): A respirometer measures O2 uptake by germinating peas at 20 °C and 30 °C. The rate at 20 °C is 1.20 mm3 min−1 g−1; the rate at 30 °C is 2.76 mm3 min−1 g−1.
(a) Calculate the Q10 for respiration in this temperature range. (2)
(b) Use your Q10 value to predict the rate at 40 °C, stating the assumption. (2)
(c) Explain why the actual rate at 50 °C is likely to be lower than your prediction, referring to enzyme structure. (4)
Solution with mark scheme:
(a) M1 (AO2) — Q10 = (rate at T+10) / (rate at T) = 2.76 / 1.20.
A1 (AO2) — Q10 = 2.30 (accept 2.3 to 2 s.f.).
(b) M1 (AO2) — Predicted rate at 40 °C = rate at 30 °C × Q10 = 2.76 × 2.30 = 6.35 mm3 min−1 g−1.
A1 (AO3) — Assumption: Q10 remains constant across this range — i.e. enzymes are still operating below their thermal denaturation threshold and the rate-limiting step has not changed.
(c) M1 (AO1) — At 50 °C, kinetic energy is higher, so collision frequency continues to increase.
M1 (AO1) — However, the increased thermal motion disrupts hydrogen bonds and ionic interactions that maintain enzyme tertiary structure.
M1 (AO2) — The active site distorts, substrate can no longer bind efficiently (or at all), and enzymes progressively denature — the change is partial at first, complete at high temperatures.
A1 (AO3) — Above the optimum, denaturation of any enzyme along the respiratory pathway (e.g. cytochrome oxidase / Complex IV in the inner mitochondrial membrane, where membrane-protein folding is also temperature-sensitive) becomes the new rate-limiting step, dropping flux below the Q10 extrapolation. (Total: 8 marks; M5 A3.)
Question (6 marks): A student investigates the effect of cyanide on the respiration rate of yeast using a respirometer. With no inhibitor, O2 uptake is 4.20 mm3 min−1. After adding 0.1 mmol dm−3 cyanide, O2 uptake falls to 0.18 mm3 min−1. Explain the result with reference to the site of action of cyanide and the consequence for upstream respiratory stages.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Identifying cyanide as an inhibitor of Complex IV (cytochrome c oxidase) in the inner mitochondrial membrane |
| 2 | AO1.2 | Stating that Complex IV catalyses the transfer of electrons from reduced cytochrome c onto O2, the final electron acceptor of the ETC |
| 3 | AO2.1 | Reasoning that without electron flow through Complex IV, proton pumping at Complexes I, III and IV ceases, the proton gradient collapses, and chemiosmotic ATP synthesis stops |
| 4 | AO2.2 | Recognising that O2 uptake falls to near zero because O2 is no longer being reduced to H2O at Complex IV |
| 5 | AO3.1 | Predicting that reduced NAD and reduced FAD accumulate (cannot be re-oxidised), so the link reaction and Krebs cycle stall for lack of oxidised coenzymes |
| 6 | AO3.2 | Concluding that the cell switches to anaerobic glycolysis with lactate fermentation to keep regenerating NAD+, which sustains a tiny residual ATP supply but cannot match aerobic demand — explaining cyanide's lethality at the whole-organism level |
Total: 6 marks (UMS-band-anchored at A; AO1 = 2, AO2 = 2, AO3 = 2). Specimen question modelled on the Edexcel 9BI0 paper format. The structure mirrors Edexcel's preference for combining numerical observation with mechanistic reasoning that traces an effect from a specific molecular target back through the integrated respiratory pathway.
Lesson 4 (glycolysis) — substrate-level phosphorylation and PFK as a regulatory step. Glycolysis is cytoplasmic and O2-independent. Its absolute requirement is NAD+ at the G3PDH step, plus a continual supply of glucose. Phosphofructokinase (PFK) is allosterically inhibited by high ATP and citrate (signal that downstream stages are saturated) and activated by AMP and ADP (signal of energy demand). In fast-glycolytic muscle fibres at peak demand, PFK can become the rate-limiting step.
Lesson 5 (link reaction and Krebs cycle) — matrix-enzyme flux. Pyruvate dehydrogenase, citrate synthase and isocitrate dehydrogenase are key regulated enzymes; high reduced NAD : NAD+ ratios inhibit them, providing feedback when oxidative phosphorylation is saturated. Krebs cycle flux therefore matches ETC throughput.
Lesson 6 (oxidative phosphorylation) — usually THE rate-limiting step. Under most physiological conditions, Complex IV sets the pace of the entire pathway. Its Km for O2 is very low (~1 μM), so O2 supply only becomes limiting in severe hypoxia — not at the partial pressures found in normally-perfused tissue. ATP synthase activity (set by ADP and Pi availability) feedback-controls proton flux and therefore overall flux through the chain — high ATP : ADP slows everything; low ATP : ADP accelerates it.
Lesson 7 (anaerobic respiration) — the fallback. When O2 supply genuinely fails (e.g. cyanide poisoning, severe ischaemia, sprinting muscle), lactate dehydrogenase regenerates NAD+ from reduced NAD by reducing pyruvate, allowing glycolysis to continue at 2 ATP per glucose. This ties directly to the Pasteur effect — yeast switching to fermentation on O2 removal consumes 16× more glucose to meet the same ATP demand.
Topic 1 (enzymes and Michaelis-Menten kinetics) — substrate availability follows hyperbolic curves. Each respiratory enzyme has a characteristic Km for its substrate. Below Km, rate rises roughly linearly with substrate; above Km, the enzyme approaches Vextmax and rate plateaus. Q10 derives from the Arrhenius equation — a 10 °C rise approximately doubles or triples the rate constant for most biochemical reactions, until denaturation reverses the trend.
Topic 7 (exercise physiology) — whole-body integration. VO2max is the maximum sustainable rate of O2 consumption by the body and integrates every rate-affecting factor: cardiac output, capillary density, mitochondrial density, enzyme abundance and substrate availability. Trained endurance athletes increase VO2max by adapting all of these — not by changing the biochemistry of any single step.
| AO | Typical share on rate-of-respiration questions | Earned by |
|---|---|---|
| AO1 (knowledge) | 25–35% | Defining Q10; naming respirometer components; identifying soda lime's role; naming Complex IV as cyanide's target; stating that respiration is enzyme-catalysed |
| AO2 (application) | 35–45% | Calculating Q10 from rate data; calculating rate per gram from respirometer fluid displacement; predicting rate at a new temperature; deducing substrate from RQ |
| AO3 (analysis / evaluation) | 25–35% | Explaining why O2 is not normally rate-limiting at physiological concentrations; identifying the rate-limiting step and how it shifts; evaluating respirometer assumptions (constant T, control tube role); predicting effects of inhibitors on upstream and downstream stages |
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