Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed by the cells of an organism. In humans, digestion begins in the mouth with the help of teeth, tongue, and salivary glands. It ends in the small intestine.

Assimilation is the movement of digested food molecules into the cells of the body where they are used. For example, glucose is used in respiration to provide energy. Amino acids are used to build new proteins. Proteins are required for the growth and development of the body.

The grass-eating animals quickly swallow the fodder and store it in the separate part of the stomach called rumen. In the rumen, food is partially digested. The partially digested food is called the cud. The cud is then brought to the mouth in small lumps and chewed. The process of chewing the cud is called rumination and the animals are called ruminants.

Amoeba is a microscopic, single-celled organism found in water. It is a simple organism having an irregular shape. It is capable of constantly changing its shape and position. Amoeba engulfs its food by wrapping finger like projections called false feet or pseudopodia. These projections help in movement and capturing food. Amoeba does not have a mouth or a digestive system. The food particle inside the Amoeba forms a food vacuole.