The substances formed as a result of digestion like glucose, amino acids, free fatty acids, glycerol, phospholipids, nitrogenous bases, minerals, vitamins, cholesterol, water etc., finally reach blood circulation after their absorption. They are used in oxidation to produce energy or in the synthesis of complex molecules in cells. In this way, these substances becomes the part of protoplasm. This utilization of substances by cells is known as assimilation and the assimilated substances are called metabolites. In some process of assimilation energy is either required or releases. The assimilation of main components of food takes place as follows: 1. Assimilation of carbohydrates: After reaching liver, monosaccharides are sent to heart by blood and from heart they are sent to different parts of the body. In tissues they (glucose) are oxidise to produce energy. Excess of glucose is converted into glycogen. This process is called glycogenesis. It is stored in muscles and liver Whenever required, liver cells start converting amino acid and glycerol into glucose. 2. Assimilation of Proteins: Amino acids reach in each cell of body and forms cell and its components. They also take part in the formation of enzymes and different hormones. Surplus amino acids produce ammonia by deamination in liver cells. Ammonia forms urea which is excreted as urine. 3. Assimilation of fat: Fat are used as concentrated source of energy in our body. Apart from this, fat and phospholipids are utilized in the formation or repair of bio membranes. Steroid hormones are also synthesized from fats. Surplus fat is stored as adipose layer beneath skin and functions as insulating layer. Complex fatty acid, triglycerides, phospholipids and cholesterol enclose protein capsule to form chylomicron fat globule and release to lymph. From where central lymph vessel releases them into blood.