Free Factors Affecting the Rate of Respiration revision notes for OCR A Level Biology – covering specification point 5.2.1 (l).
Factors Affecting the Rate of Respiration
The main factors that affect the rate of respiration are:
- Temperature: affects enzyme activity across all stages of respiration.
- Substrate concentration: determines the availability of glucose for glycolysis.
- Oxygen availability: determines if aerobic respiration can occur.
Temperature
The effect of temperature on respiration depends on if it is below, above, or at the optimum:
- Below: Molecules have lower kinetic energy, which reduces the frequency of successful collisions between enzymes and their substrates.
- Above: Molecules have excessive kinetic energy, which disrupts the hydrogen and ionic bonds that maintain the tertiary structure of proteins.
- Optimal: Molecules have the right level of kinetic energy to maximise the frequency of enzyme-substrate collisions without damaging the structure of the protein.
It is useful to know that respiration rates can be manipulated for commercial purposes. For example, fruits and vegetables are stored at low temperatures to prevent them from ripening, sprouting, or otherwise spoiling. This maximises businesses’ profit margins.
The table below outlines a set of example respiration rates for fruits and vegetables stored at different temperatures:
| Organism | Respiration Rate (mg CO2 kg-1 h-1) | ||||
|---|---|---|---|---|---|
| 0 °C | 5 °C | 10 °C | 15 °C | 20 °C | |
| Apple | 3.0 | 5.0 | 9.0 | 15.0 | 30.0 |
| Asparagus | 60.0 | 95.0 | 197.0 | 244.0 | 388.0 |
| Cauliflower | 18.0 | 21.0 | 34.0 | 46.0 | 81.0 |
| Onion | 3.0 | 3.5 | 7.5 | 10.5 | 16.5 |
| Potato | 2.0 | 6.0 | 9.0 | 12.0 | 28.0 |
Substrate Concentration
The concentration of respiratory substrate (primarily glucose) affects the rate of glycolysis, which in turn determines the production of pyruvate, reduced NAD, and acetyl-CoA, which is required for later stages.
The effect of substrate concentration on the rate of respiration depends on whether it is low, high, or saturated:
- Low substrate concentrations: The rate of glycolysis is limited because enzymes have fewer substrate molecules available, producing less pyruvate and reduced NAD.
- High substrate concentration: The rate of respiration increases, as more enzyme-substrate complexes can form, producing more pyruvate and reduced NAD.
- Saturating concentrations: The rate of respiration plateaus at its Vmax, because all enzyme active sites are occupied; adding more substrate has no further effect.
Oxygen Availability
Oxygen availability determines whether aerobic or anaerobic respiration occurs.
The effects of reduced or absent oxygen are due to its effect on the ETC, and the subsequent effects of its limitation:
- The ETC stops as electrons cannot be transferred to oxygen as the final electron acceptor.
- Chemiosmosis and oxidative phosphorylation reduce or stop as the electrochemical gradient decreases.
- The link reaction and the Krebs cycle stop as their inputs of NAD and FAD are no longer regenerated at the ETC.
