Photosynthesis is a vital biochemical process in which plants, algae, and some bacteria harness the energy of sunlight to produce food. Ultimately, nearly all living things depend on energy produced from photosynthesis for their nourishment, making it vital to life on Earth. It is also responsible for producing the oxygen that makes up a large portion of the Earth's atmosphere. Organisms that produce energy through photosynthesis are called phototrophs.
Plants are autotrophs, which means they are able to synthesize food directly from inorganic compounds, instead of eating other organisms or relying on material derived from them. Most notably, they use carbon dioxide gas and water to produce sugars and oxygen gas. The energy for these processes comes from photosynthesis. Glucose, for example, is variously used to form other organic compounds, such as the building material cellulose, or it may be used as a fuel. This takes place through processes of respiration that are found in both animals and plants.
Plants capture light using the pigment chlorophyll, which gives them their green color. This is contained in organelles (compartments within the cells) called chloroplasts. Although all green parts of a plant have chloroplasts, most of the energy is produced in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, contain about half a million chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant, waxy cuticle, that protects the leaf from excessive absorption of light and evaporation of water. The transparent, colourless epidermal layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place.