Carbohydrate, mainly glucose is the main substrate inside cells. Lipids i.e. fatty acids and glycerol are also used. Fatty acids are used when the carbohydrates are exhausted.
A molecule of lipid yields much more energy than a molecule of glucose. Proteins are not normally used for respiration.
However during starvation they are hydrolysed to amino acids, dearnination follows and the products enter Kreb’s cycle as urea is formed. Use of body protein in respiration result to body wasting, as observed during prolonged sickness or starvation.
The ratio of the amount of carbon (IV) oxide produced to the amount of oxygen used for each substrate is referred to as Respiratory Quotient (RQ) and is calculated as follows:
Q.= Amount of carbon (IV) oxide produced/ Amount of oxygen used
Carbohydrates have a respiratory quotient of 1.0 lipids 0.7 and proteins 0.8. Respiratory quotient value can thus give an indication of types of substrate used. Besides values higher than one indicate that some anaerobic respiration is taking place.