The main source of energy for all living things on the earth is the sun. The energy received from the sun travels 150 million kilometres to reach the earth. This energy comes in two forms: light and heat. Heat energy cannot be captured directly by the plants or animals. But the heat energy does warm up the surroundings of plants and animals.

Many animals also directly use heat energy to control their body temperature. Reptiles, such as snakes, lie out in the sun to warm up their bodies. While this heat helps to warm the bodies of animals, there is not enough energy to meet all the needs of an animal. To obtain this energy, the animal requires food, which can be broken down in the animal's body to provide the necessary energy for the animal's life processes.

Only plants can capture light energy directly. Through the process of photosynthesis, plants convert the light energy into stored energy. Because green plants can manufacture their own food, plants are called autotrophs or self-nourishing.

Photosynthesis is possible because green plants contain an energy-capturing substance called chlorophyll. The plant gets its green colour because chlorophyll is green. Many seaweeds and other plants that do not appear to be green also have chlorophyll and therefore can convert the sun's energy into food. In these plants the greenness is hidden by other pigments.

The chlorophyll captures the light energy and uses this energy to build carbohydrates from simple raw materials (water, carbon dioxide and minerals). Carbohydrates are complex energy-storing materials that the plant can use to sustain its life processes.

The raw materials that are needed for photosynthesis are the same raw materials that make up carbohydrates-carbon, hydrogen and oxygen. The carbon dioxide (C02) breathed out by animals is the source of carbon (C) and oxygen (02). Hydrogen (H2) is taken from water (H20). These raw materials enter the plant through its roots and leaves. Carbon dioxide is taken in through pores, called stomata, in the leaf's surfaces. Water enters the plants through the roots and is channelled up the stem and into the veins of the leaves. In the case of water plants, there is water all around the plant. The raw materials are dissolved in the water and taken into the plant simply through any surface.