Photosynthesis is necessary because it forms the basis of the food chain. Photosynthesis, which occurs in green plants and algae, is the process by which sunlight combines with carbon dioxide, water and a chemical called chlorophyll to produce simple sugars. These sugars provide the plants with an energy source, and, when they are eaten by a predator, these sugars provide energy for the animal.
The chemical energy found in the sugars produced by photosynthesis permeates the entire food chain. When the actual molecules produced by the plant are broken apart by predators, the energy inside them is used and stored as necessary. When these animals, called primary consumers, are eaten by other predators, known as secondary consumers, the energy transfers from one animal to another. This process continues throughout the food chain, as even scavengers and decomposing organisms rely on the initial production of food by photosynthetic organisms. However, at each step in the food chain, some energy is lost as heat.
A few types of organisms produce their own food by harnessing chemical energy. This is most common in very unique habitats, such as near the hot-water vents of the ocean floor. This is necessary at the ocean floor because sunlight never reaches this depth, which means that photosynthesis never occurs there.