The type of digestive system is different in various organisms.
In single-celled organisms, the food is taken in by the entire surface and in multi cellular organisms different parts perform different specific functions.
For example, Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole . Inside the food vacuole, complex substances are broken down into simpler ones which then diffuse into the cytoplasm. The remaining undigested material is moved to the surface of the cell and thrown out.