Photosynthesis is the process by which green plants and certain types of bacteria make carbohydrates, beginning only with carbon dioxide (CO 2 ) and water (H 2 O). Carbohydrates are complex chemical compounds that occur widely in plants and that serve as an important food source for animals. Sugar, starch, and cellulose are among the most common carbohydrates. The energy needed to make photosynthesis possible comes from sunlight, which explains the term photo ("light") synthesis ("to make"). The absorption of sunlight in plants takes place in specific molecules known as chlorophyll (KLOR-uh-fill) that give plants their green color.
Photosynthesis can be represented by means of a simple chemical equation:
In this equation, C 6 H 12 O 6 represents a simple sugar known as glucose. Molecules of glucose later combine with each other to form more complex carbohydrates, such as starch and cellulose. The oxygen formed during photosynthesis is released to the air. It is because of this oxygen that animal life on Earth is possible.
Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.
Chlorophyll: A compound in plants that makes possible the conversion of light energy to chemical energy.
Dark reactions: Those reactions in the photosynthesis process that can occur in the absence of sunlight.
Glucose: A sugar, or simple carbohydrate, that serves as an energy source for cells.
Light reactions: Those reactions in the photosynthesis process that can occur only in the presence of sunlight.
The equation for photosynthesis shown above is very misleading. It suggests that changing carbon dioxide and water into carbohydrates is a simple, one-step process. Nothing could be further from the truth. Scientists have been working for well over 200 years trying to find out exactly what happens during photosynthesis. Although the major steps of the process are understood, researchers are still unable to duplicate the process in the laboratory.
The equation above seems to say that six carbon dioxide molecules (6 CO 2 ) and six water molecules (6 H 2 O) somehow get joined to each other to form one carbohydrate molecule (C 6 H 12 O 6 ). Instead, the process occurs one small step at a time. During each of the many stages of photosynthesis, a single atom or an electron is transferred from one compound to another. Only after dozens of steps have taken place has the overall reaction shown above been completed.
What scientists have learned is that two general kinds of reactions are involved in photosynthesis: the light reactions and the dark reactions. Light reactions, as their name suggests, can take place only in the presence of sunlight. In those reactions, light energy is used to generate certain kinds of energy-rich compounds. These compounds do not themselves become part of the final carbohydrate product. Instead, they are used to "carry" energy from one compound to another in the process of photosynthesis.
The dark reactions are able to take place in the absence of sunlight, although they often occur during the daylight hours. During the dark reactions, the energy-rich compounds produced in the light reactions generate the compounds from which carbohydrates are eventually produced.
[ See also Plant ]