. . How do Organisms obtain their Nutrition? Since the food and the way it is obtained differ, the digestive system is different in various organisms.
In single-celled organisms, the food may be taken in by the entire surface. But as the complexity of the organism increases, different parts become specialised to perform different 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 (Fig. .
). 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. In Paramoecium , which is also a unicellular organism, the cell has a definite shape and food is taken in at a specific spot.
Food is moved to this spot by the movement of cilia which cover the entire surface of the cell.