Oxidative phosphorylation and respiratory quotientEdexcel International A Level Biology: Revision notes

The reduced coenzymes made in glycolysis, the link reaction and the Krebs cycle (reduced NAD and reduced FAD) carry hydrogen to the electron transport chain. This is a series of electron carriers embedded in the inner mitochondrial membrane, which is folded into cristae to give a large surface area.

1. Reduced NAD and FAD donate hydrogen atoms, which split into protons (H⁺) and electrons; the coenzymes are reoxidised and can be reused.
2. The electrons pass from carrier to carrier by redox reactions, releasing energy at each step.
3. At the end of the chain oxygen is the final electron acceptor. It combines with electrons and H⁺ to form water.

The energy released as electrons pass along the chain is used to pump H⁺ ions from the matrix into the intermembrane space. This builds up a proton (electrochemical) gradient across the inner membrane.

The inner membrane is impermeable to H⁺. The ions can return to the matrix only through channels in the enzyme ATP synthase. As they flow down their gradient, the energy is used to join ADP and inorganic phosphate to form ATP. This is chemiosmosis.

The whole process is oxidative phosphorylation: the phosphorylation of ADP using energy released by oxidation of reduced coenzymes. It makes most of the ATP in aerobic respiration.

Oxygen is the final electron acceptor. Without it electrons cannot leave the chain, so the carriers stay reduced and the chain stops. Protons are no longer pumped, so ATP is no longer made by oxidative phosphorylation.

Reduced NAD and FAD are then not reoxidised, so the Krebs cycle and link reaction also stop.

Substances that uncouple respiration, such as molecules that carry H⁺ across the inner membrane, remove the proton gradient. Electron transport and oxygen uptake continue (or increase), but little ATP is made and the energy is released as heat.

The respiratory quotient (RQ) shows which substrate is being respired:

$RQ = \dfrac{\text{volume of CO}_2\text{ produced}}{\text{volume of O}_2\text{ consumed}}$ in the same time.

• Carbohydrate: about 1.0
• Protein: about 0.9
• Lipid: about 0.7, because lipids contain proportionally more hydrogen and less oxygen, so more oxygen is needed to oxidise them
• Anaerobic respiration: RQ above 1, because carbon dioxide is produced with little or no oxygen used

Worked example: 190 cm³ CO₂ and 250 cm³ O₂ per minute gives RQ = 190 ÷ 250 = 0.76, so mainly lipid is being respired.

A simple respirometer measures oxygen uptake by germinating seeds or small invertebrates.

• The organisms are in a sealed tube connected to a capillary tube containing a drop of coloured liquid.
• Potassium hydroxide solution (or soda lime) absorbs the carbon dioxide, so the volume of gas falls only because oxygen is used, and the liquid moves towards the organisms.
• Volume of oxygen = distance moved × cross-sectional area of the capillary ($\pi r^2$). Rate = volume ÷ time.
• A control tube with glass beads or boiled seeds of the same volume allows for changes in temperature and pressure; use a water bath to keep the temperature constant.

To find the RQ, run a second tube without potassium hydroxide. Its movement shows oxygen taken up minus carbon dioxide produced, so carbon dioxide produced = O₂ uptake (with KOH) − movement without KOH.

Worked example: with KOH 24 mm³; without KOH 4.8 mm³. CO₂ = 24 − 4.8 = 19.2 mm³. RQ = 19.2 ÷ 24 = 0.80.