During aerobic respiration, cells respire substrates such as glucose to produce ATP.
Some events that occur during aerobic respiration are:
- The respiratory substrate breaks down into smaller and smaller molecules. These series of reactions are described as catabolism.
- Coenzymes take part in various reactions. In some reactions, coenzymes are reduced or oxidised.
- Carbon dioxide is released.
(a) Aerobic respiration occurs in four successive stages: glycolysis (G), link reaction (LR), Krebs cycle (KC) and oxidative phosphorylation (OP).
Complete Table 3.1 to show which events occur in each stage of aerobic respiration. Use a tick (✓) to show that the event does occur or a cross (✗) to show that the event does not occur.
(b) A new hand-held technological device shows the main type of respiratory substrate being used in the cells of a person.
The device consists of a carbon dioxide sensor and air-flow meter. The person inhales through the device for a fixed time and then exhales into it.
The device calculates the respiratory quotient (RQ) value to show whether the cells are mainly respiring carbohydrates or lipids.
(i) Explain how the device calculates the RQ value and how this shows whether the cells are mainly respiring carbohydrates or lipids.
(ii) State the difference in the relative energy values of carbohydrates and lipids as respiratory substrates, and explain the reasons for the difference.
▶️ Answer/Explanation
(a)
Explanation:
1. Catabolism occurs in all stages as glucose is progressively broken down into smaller molecules throughout aerobic respiration.
2. Coenzyme reduction/oxidation happens in all stages: NAD is reduced in glycolysis, link reaction and Krebs cycle, while FAD is reduced in Krebs cycle. Oxidative phosphorylation involves oxidation of reduced coenzymes.
3. Coenzyme covalent bonding only occurs in the link reaction when coenzyme A binds with acetate to form acetyl CoA.
4. Carbon dioxide release occurs in the link reaction (1 CO₂ per pyruvate) and Krebs cycle (2 CO₂ per cycle), but not in glycolysis or oxidative phosphorylation.
(b)(i)
The device calculates RQ by measuring the ratio of carbon dioxide exhaled to oxygen inhaled (RQ = CO₂ produced/O₂ consumed).
For carbohydrates, the RQ is 1 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). For lipids, the RQ is about 0.7 (e.g., palmitic acid: C₁₆H₃₂O₂ + 23O₂ → 16CO₂ + 16H₂O).
The air-flow meter measures oxygen inhaled, and the CO₂ sensor measures carbon dioxide exhaled. The device then calculates the ratio to determine if the person is mainly respiring carbohydrates (RQ ≈ 1) or lipids (RQ ≈ 0.7).
(b)(ii)
Lipids have higher energy value (37-40 kJ g⁻¹) than carbohydrates (15-17 kJ g⁻¹).
This is because:
1. Lipids contain more hydrogen atoms per gram, which yield more reduced NAD/FAD during oxidation.
2. The greater number of C-H bonds in lipids releases more energy when broken compared to C-O bonds in carbohydrates.
3. More protons are pumped across the mitochondrial membrane during lipid oxidation, creating a larger proton gradient for ATP synthesis.