Types of Digestion

Two types of digestion are seen in heterotrophs:

1. Intracellular
2. Extracellular

Intracellular Digestion

All the five steps of nutrition occur inside the cell itself, as in Amoeba, Paramecium and other unicellular animals.

Food particles such as minute bacteria are enclosed (caught) by pseudopodia (pseudo means false, podia means feet) to form a food vacuole (Ingestion). Enzymes from cytoplasm are secreted into the food vacuole to break down complex food (Digestion). Digested food is absorbed into the cytoplasm (Absorption). The absorbed food is used up wherever required in the cell (Assimilation). The undigested unabsorbed food is expelled, when the food vacuole comes near the cell surface and bursts open (Egestion).

Food vacuoles are temporary structures and every time the Amoeba feeds, a new food vacuole is produced. All free-living unicellular animals carry out intracellular digestion.

Extracellular Digestion

Digestion occurs outside the cell. All animals (excluding sponges) carry out extracellular digestion. They have either a cavity, a tube, or a food canal which receives the ingested food. Digestive enzymes are poured over the food, and the products of digestion are absorbed back into the cells. The undigested, unabsorbed food is thrown out of the digestive cavity.