The nutritional requirements and the various ways used by heterotrophic organisms to obtain nutrition. Whether food is used to give energy or to build the body, the large molecules of food have to be broken down into simpler constituents before they can be used by the body.
The process by which the food is broken down into simpler molecules is known as digestion. This breakdown is achieved with the aid of enzymes and can take place inside the cell - intracellular digestion or outside the cell - extracellular digestion often in a specialised digestive tract. Let us first consider intracellular digestion and see how it is different from extracellular digestion.