Concepts: The respiratory balance sheet, RQ and the amphibolic pathway

4.1 The balance sheet and its assumptions

A respiratory balance sheet is the theoretical calculation of ATP produced when one glucose molecule is oxidised step by step. The calculation is possible only if certain assumptions hold. The pathway must proceed in an orderly sequence from glycolysis through the link reaction to the citric acid cycle and the electron transport system. The NADH made in glycolysis must be transported into the mitochondrion and its electrons must pass through the chain. The intermediates of the pathway must not be pulled away for other cellular work. The ATP yield of each carrier is also fixed as an assumption. With these assumptions the net gain comes to 38 ATP per glucose.

In a living plant none of these assumptions is fully true. Intermediates are continuously withdrawn for biosynthesis, some carriers are used in other reactions, and some protons leak. The real yield therefore varies with the cell type and its needs. The value of the exercise is the accounting: it shows where the energy goes and why the earlier steps matter.

4.1.1 Respiratory quotient

The respiratory quotient, RQ, is the ratio of the volume of CO2 released to the volume of O2 consumed during respiration. When carbohydrate is the substrate and is completely oxidised, the RQ is 1 because glucose has six oxygens of its own and the volume of CO2 leaving equals the volume of O2 taken in. Fats have less oxygen in their molecules, so they need extra oxygen from outside and their RQ is less than 1, usually around 0.7. Proteins have an RQ of about 0.9. For proteins the ratio is about 0.9, and in a living tissue the substrate is usually a mixture, so the measured RQ falls between these values.

4.1.2 The amphibolic pathway

The respiratory pathway is mainly catabolic because it breaks down fuels and releases energy. It is also anabolic because several of its intermediates are used as building blocks for biosynthesis. Fats are broken into glycerol and fatty acids, fatty acids are degraded to acetyl CoA and glycerol enters as PGAL, while proteins are broken into amino acids that enter the cycle after deamination. Since the same pathway serves both breakdown and building, it is called amphibolic, and all three major fuel groups meet in the respiratory pathway.

This also explains why respiration cannot be switched off completely when a plant stops growing. The intermediates are needed for repair, for the synthesis of new compounds and for the maintenance of the cell, which is why a basal rate of respiration continues in every living cell. The respiratory quotient is measured by tracking gases in a sealed chamber around plant material: a respirometer records the oxygen taken up and the carbon dioxide released, and the ratio of the two is the quotient. Values close to one suggest carbohydrate, values near 0.7 suggest fat, and in a living tissue the substrate is usually a mixture, so the measured value falls between these cases. The figure is a guide rather than a fingerprint, since a respiring tissue rarely uses one pure substrate and since air leaks or gas solubility can shift a measurement. Because respiration supplies intermediates as well as ATP, a growing plant keeps respiration high even when its ATP demand is modest, and ripening fruits send carbon through respiration to build pigments, flavours and wall materials. This is also why a fruit that is detached from the plant keeps respiring and softening, using its own stored sugars as the substrate.

4.1.3 Exercise practice

Exercise 2. Respiratory substrates are the compounds oxidised during respiration to release energy. The most common substrate is glucose, a carbohydrate.

Exercise 8. The assumptions are an orderly pathway, transport of glycolytic NADH into the mitochondrion, no use of intermediates for other purposes and fixed ATP yields for NADH and FADH2. They are not fully valid in a living system.

Exercise 9. The pathway is amphibolic because intermediates leave it for biosynthesis while fuels are being broken down in the same pathway.

Exercise 10. RQ is the volume of CO2 released divided by the volume of O2 consumed. For fats the value is less than one, close to 0.7.

Exercise 12. The step-wise release of energy allows the cell to capture energy in ATP in small packages, to regulate the process through enzymes and to avoid damage from a sudden large release of energy.