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Photosynthesis

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Leaf

In plants, photosynthesis occurs mainly within the leaves.

Image: Dan / FreeDigitalPhotos.net
Organisms need energy to survive. Some organisms are capable of absorbing energy from sunlight and using it to produce sugar and other organic compounds such as lipids and proteins. The sugars are then used to provide energy for the organism. This process, called photosynthesis, is used by plants and some protists, bacteria, and blue-green algae.

Photosynthesis Equation

In photosynthesis, solar energy is converted to chemical energy. The chemical energy is stored in the form of glucose (sugar). Carbon dioxide, water, and sunlight are used to produce glucose, oxygen, and water. The chemical equation for this process is:

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

6 molecules of carbon dioxide (6CO2) and 12 molecules of water (12H2O) are consumed in the process, while glucose (C6H12O6), six molecules of oxygen (6O2), and six molecules of water (6H2O) are produced.

This equation may be simplified as: 6CO2 + 6H2O + light → C6H12O6 + 6O2.

Photosynthesis in Plants

In plants, photosynthesis occurs mainly within the leaves. Since photosynthesis requires carbon dioxide, water, and sunlight, all of these substances must be obtained by or transported to the leaves. Carbon dioxide is obtained through tiny pores in plant leaves called stomata. Oxygen is also released through the stomata. Water is obtained by the plant through the roots and delivered to the leaves through vascular plant tissue systems. Sunlight is absorbed by chlorophyll, a green pigment located in plant cell structures called chloroplasts. Chloroplasts are the sites of photosynthesis. Chloroplasts contain several structures, each having specific functions:

    Outer and inner membranes: protective coverings that keep chloroplast structures enclosed.

    Stroma: dense fluid within the chloroplast. Site of conversion of carbon dioxide to sugar.

    Thylakoid: flattened sac-like membrane structures. Site of conversion of light energy to chemical energy.

    Grana: dense layered stacks of thylakoid sacs. Sites of conversion of light energy to chemical energy.

    Chlorophyll: a green pigment within the chloroplast. Absorbs light energy.

Stages of Photosynthesis

Photosynthesis occurs in two stages. These stages are called the light reactions and the dark reactions. The light reactions take place in the presence of light. The dark reactions do not require direct light, however dark reactions in most plants occur during the day.

Light reactions occur mostly in the thylakoid stacks of the grana. Here, sunlight is converted to chemical energy in the form of ATP (free energy containing molecule) and NADPH (high energy electron carrying molecule). Chlorophyll absorbs light energy and starts a chain of steps that result in the production of ATP, NADPH, and oxygen (through the splitting of water). Oxygen is released through the stomata. Both ATP and NADPH are used in the dark reactions to produce sugar.

Dark reactions occur in the stroma. Carbon dioxide is converted to sugar using ATP and NADPH. This process is known as carbon fixation or the Calvin cycle. The Calvin cycle has three main stages: carbon fixation, reduction, and regeneration. In carbon fixation, carbon dioxide is combined with a 5-carbon sugar [ribulose1,5-biphosphate (RuBP)] creating a 6-carbon sugar. In the reduction stage, ATP and NADPH produced in the light reaction stage are used to convert the 6-carbon sugar into two molecules of a 3-carbon carbohydrate, glyceraldehyde 3-phosphate. Glyceraldehyde 3-phosphate is used to make glucose and fructose. These two molecules (glucose and fructose) combine to make sucrose or sugar. In the regeneration stage, some molecules of glyceraldehyde 3-phosphate are combined with ATP and are converted back into the 5-carbon sugar RuBP. With the cycle complete, RuBP is available to be combined with carbon dioxide to begin the cycle over again.

Photosynthesis Summary

In summary, photosynthesis is a process in which light energy is converted to chemical energy and used to produce organic compounds. In plants, photosynthesis typically occurs within the chloroplasts located in plant leaves. Photosynthesis consists of two stages, the light reactions and the dark reactions. The light reactions convert light into energy (ATP and NADHP) and the dark reactions use the energy and carbon dioxide to produce sugar. For a review of photosynthesis, take the Photosynthesis Quiz.

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