Photosynthesis, the conversion of light energy into chemical energy in cells that contain chlorophyll, a green pigment. Photosynthesis occurs in most plants and algae and in some bacteria and protozoans. The process is also called carbon fixation, because it includes the production of carbon compounds that store the chemical energy for use in cell growth. These compounds—mainly sugars and starches, collectively called carbohydrates—also serve as building blocks for more complex foods, such as fats and proteins. For photosynthesis to occur, water, carbon dioxide, chlorophyll, and light are necessary.

The Reactions of Photosynthesis

Two main chemical reactions occur in photosynthesis. One takes place only in the presence of light and is called the light reaction; the other can occur with or without light and is called the dark reaction.

The Light Reaction has the following steps:

  • Light enters a cell and is absorbed by chlorophyll. The light's energy raises the energy level of some chlorophyll electrons, freeing them from the chlorophyll molecules.
  • Molecules of water (H2O) from the environment take part in chemical reactions in the cell. Electrons from the hydrogen atoms in each of these water molecules are attracted to the chlorophyll molecules lacking the electrons freed in step 1. This attraction causes the water molecules to break apart into oxygen atoms, protons, and electrons. The oxygen atoms join together in pairs, forming molecules of oxygen. Oxygen molecules, called free oxygen, are released into the environment.
  • The electrons freed from the chlorophyll molecules and the protons freed from the water molecules take part in chemical reactions in the cell. These reactions result in the production of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide diphosphate (NADPH2).
The Dark Reaction

The chemical energy possessed by ATP and NADPH2 is used in making carbohydrates from hydrogen and carbon dioxide. (The carbon dioxide is obtained from the environment.) The carbohydrates then possess the chemical energy given up by ATP and NADPH2.

The generalized, overall chemical equation for photosynthesis is:

6CO2+12H2O + light → C6H12O6 + 6O2 + 6H2O

The carbohydrate in this equation (C6H12O6) is glucose, a simple sugar. Glucose is only one of several compounds that can be formed by photosynthesis.

Importance of Photosynthesis

Photosynthesis is the most important chemical process for life. Through photosynthesis, the sun's energy is made available to all organisms. For example, when an animal eats a plant, it obtains chemical energy that the plant acquired through photosynthesis; when a second animal eats a plant-eating animal, it obtains some of the chemical energy that the first animal obtained by eating plants.

As organisms respire, they take in free oxygen and give off carbon dioxide. Respiration is dependent on photosynthesis because photosynthesis is the source of virtually all the free oxygen in the atmosphere and in bodies of water. In addition, photosynthesis removes carbon dioxide from the atmosphere and from bodies of water. If this carbon dioxide were not removed, it would eventually smother the organisms that produce it.

Coal and petroleum, composed of the remains of various kinds of organisms, contain energy that was captured from the sun's rays by photosynthesis millions of years ago.