Carbohydrates are the main substrates for respiration. Different types of carbohydrates guch as disaccharides (such as sucrose) and polysaccharides (such as starch in plants and glycogen in animals) are enzymatically hydrolyzed to form the simplest form of carbohydrate, the monosaccharides.
The monosaccharides produced by hydrolysis of starch (plant) or glycogen (animal) are glucose. Glucose-i-phosphate is also produced by the phosphorolytic degradation of starch and maltose. Hydrolysis of sucrose produces the mixture of glucose and fructose. Glucose, fructose and glucose-i-phosphate enter the first common phase respiration known as glycolysis.
Fats are generally used as respiratory substrate after the exhaustion of carbohydrate reserve. Fatty acids are oxidized to give acetyl CoA, which enters the respiratory metabolism through the citric acid cycle. Proteins are used as respiratory substrates only when carbohydrate and fat reserves are exhausted.
Proteins are hydrolyzed to form amino acids which are then further modified into ketoacids before entering into the citric acid cycle. The entry points of different substrates of respiration into the respiratory metabolic pathways.