Introduction
When we walk or ride a bicycle, we are using up energy. Even when we are not doing any apparent activity, energy is needed to maintain a state of order in our body. We also need materials from outside in order to grow, develop, synthesise protein, and other substances needed in the body. This source of energy and materials is the food we eat.
How do living things get their food?
The general requirement for energy and materials is common in all organisms, but it is fulfilled in different ways. Some organisms use simple food material obtained from inorganic sources in the form of carbon dioxide and water. These organisms, the autotrophs, include green plants and some bacteria. Other organisms utilize complex substances. These complex substances have to be broken down into simpler ones before they can be used for the upkeep and growth of the body. To achieve this, organisms use bio-catalysts called enzymes. Thus, heterotrophs survival depends directly or indirectly on autotrophs. Heterotrophic organisms include animals and fungi.
How do Organisms Obtain Their Food?
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, Nutrition in Amoeba - Amoeba takes in food using temporary finger-like extensions of the cell surface which fuse over the food particle forming a food-vacuole (Fig. 6.5).
Figure 6.5 Nutrition in Amoeba
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 covers the entire surface of the cell.