All the substances or materials that are used as raw materials in the process of respiration are called respiratory substances. Carbohydrates are the main respiratory substances. However, proteins, amino acids, fats, fatty acids, glycerol and various types of organic acids are used as respiratory substances. Glucose is the main translocated sugar in the animal body. It is used as a direct inhalation agent. Sucrose is the main translocated sugar in plants. It is broken down to produce glucose and fructose and is used in digestion.