Respiratory Substrates

A respiratory substrate is any biological molecule that can be broken down in respiration to release energy and make ATP. The three main groups are carbohydrates, lipids and proteins.Carbohydrates are the main substrate for most cells. Glucose is respired first. Stores of glycogen (in animals) and starch (in plants) are hydrolysed to glucose when needed. Brain cells and red blood cells depend on glucose. Other monosaccharides such as fructose and galactose can also enter glycolysis.Lipids are used when carbohydrate is in short supply, for example during long periods of exercise or fasting. Triglycerides are hydrolysed into glycerol and fatty acids, which enter the respiration pathway at different points. Lipids are an important long-term energy store.Proteins are only respired in large amounts during starvation, when carbohydrate and lipid stores are used up. Proteins are hydrolysed to amino acids, which are deaminated before they can be respired. Using proteins means breaking down useful tissue such as muscle.So there is an order of use: carbohydrates first, then lipids, then proteins. All three are ultimately fed into the link reaction or Krebs cycle, and most of their ATP is made by oxidative phosphorylation.Different substrates release different amounts of energy per gram and use different amounts of oxygen, which is why the respiratory quotient (RQ) can be used to work out which substrate an organism is respiring.