Q.Distinguish between intracellular and extracellular digestion?

Intracellular and extracellular digestion differ in where the food is broken down:

• Intracellular digestion takes place inside individual cells rather than in a gut cavity. Food particles are taken into cells and digested within them. Sponges digest this way, and choanocytes help trap the food particles.
• Extracellular digestion takes place outside the cells, in a cavity or gut, before the products are absorbed. It occurs, for example, in the gastro-vascular cavity of coelenterates. …

Digestion is the breaking down of food, and animals differ in where this breakdown occurs relative to their cells. This single difference in location is what separates intracellular from extracellular digestion.

• Intracellular digestion. Here food is digested within individual cells rather than in any gut cavity. The cell takes in food particles and breaks them down internally. Sponges are the clear example: they have no gut, and digestion takes place within their cells, with the flagellated choanocytes helping to trap the food particles that are then digested.
• Extracellular digestion. Here food is broken down outside the cells, in a body cavity or gut, and the digested products are then absorbed. In coelenterates a central gastro-vascular cavity handles this outside-the-cell breakdown. …

Method: Trace a single food particle

Instead of defining the two terms abstractly first, follow one food particle's journey through two different animals step by step and let the distinction emerge from the trace.

Trace a food particle entering a sponge: it is captured by a flagellated collar cell (choanocyte), taken INTO that individual cell by phagocytosis, and broken down entirely inside that one cell's own food vacuole. The particle never enters any shared body cavity — its whole digestive journey happens within a single cell. By this trace, digestion here is purely intracellular.

Now trace a food particle entering a coelenterate such as Hydra: it passes through the mouth into a shared central cavity, the gastrovascular cavity, where digestive cells lining that cavity release enzymes into the space itself, beginning to break the food down OUTSIDE any individual cell. Only afterward do gastrodermal cells take up the partly-digested fragments and finish the job inside themselves. This trace shows digestion beginning outside the cells (extracellular) and finishing inside them (intracellular) — the two-stage process noted for coelenterates and ctenophores. …