Respiratory Substrates
Different Respiratory Substrates
- A respiratory substrate is any organic molecule broken down to release energy for the synthesis of ATP.
- Triglycerides are hydrolysed into fatty acids, which enter the Krebs cycle via Acetyl CoA and glycerol.
- Glycerol is converted to pyruvate before undergoing oxidative decarboxylation, producing an acetyl group which then goes on to form Acetyl CoA.
- Gram for gram, in terms of net energy release biological macromolecules are arranged in this order.
- Lipids > Alcohols > Proteins > Carbohydrates
- Proteins have to be hydrolysed into amino acids, then deaminated before they can enter the respiratory pathway. This requires ATP, so the net production of ATP from protein is reduced.
- Triglycerides are hydrolysed into fatty acids, which enter the Krebs cycle via Acetyl CoA and glycerol.
- The respiratory quotient of a substrate is the ratio of CO2 to O2 produced/consumed to break down that molecule.
- E.g. 6 Oxygen molecules are required to completely respire one glucose molecule, and this results in the production of six carbon dioxide molecules, giving an RQ of 1.0.
- More carbon-hydrogen bonds in lipids mean more ATP is produced from them in respiration. This also means more oxygen is required to break them down, so has an RQ of less than one.
- Carbohydrates: RQ = 1.0
- Proteins: RQ = 0.9
- Lipids: RQ = 0.7
- By measuring the oxygen consumed and carbon dioxide released at each stage of respiration, the type of substrate can be determined.
Low Carbohydrate Diets
- By removing carbohydrates, weight can be lost quickly, but there are problems cutting out molecules commonly used for food...
- Triglycerides are hydrolysed into fatty acids and glycerol. The fatty acids are broken down in the mitochondria to produce acetyl groups which combine with CoA, required in the Krebs cycle.
- Carbohydrates are needed to keep the Krebs cycle going, if they are in short supply the body will make them from pyruvate in gluconeogenesis.
- The same can happen with Oxaloacetate, reducing the number of oxaloacetate molecules in the krebs cycle reduces the rate at which ATP can be produced.
- Oxaloacetate can be replaced by the conversion of pyruvate from the breakdown of glycerol from lipids. However this only produces a small amount of pyruvate meaning ideally carbohydrates are required.
- Proteins can be hydrolysed into amino acids, then deaminated in the liver, with the remaining keto acids converted into glucose molecules, leading to the breakdown of muscle tissue and increased stress on liver and kidneys to reduce nitrogenous waste.
- If Acetyl CoA is not used and allowed to build up, it is converted into ketone bodies by the liver, which are used as a respiratory substrate by the brain instead of glucose.
- However, an excess in ketone production is ketosis, leading to a dangerous condition called ketoacidosis, resulting in the accumulation of keto acids, reducing the pH of the blood to dangerous levels causing denaturation of proteins.
- Triglycerides are hydrolysed into fatty acids and glycerol. The fatty acids are broken down in the mitochondria to produce acetyl groups which combine with CoA, required in the Krebs cycle.
Respirometers and Practical Investigations
- A respirometer is a piece of equipment used to measure the respiratory quotient.
- They can be used to investigate the effect of variables of respiration rate, such as temperature
- For example, a respirometer containing peas could be set up in different temperature water baths, with potassium hydroxide to absorb carbon dioxide, and therefore any change in height of the coloured fluid is due to oxygen production and so the rates of respiration can be compared.