Photosynthesis

Photosynthesis is a biological process by which plants, algae, and certain bacteria use sunlight to convert carbon dioxide and water into sugars that store energy. The process releases oxygen, which is returned to the atmosphere; this occurs in most plant species.

This process takes place in cellular structures called chloroplasts, found in leaf cells and other green parts of plants. The green color comes from a pigment called chlorophyll, which absorbs light in the blue and red wavelengths, while green light is mostly reflected.

Two stages

The first stage is called the light-dependent reaction. It occurs in structures within the chloroplast called thylakoids. In this stage, light is absorbed, water molecules are split, oxygen is released, and energy-rich compounds are created for use in the second stage.

The second stage is the Calvin cycle, which does not directly require light. It takes place in the stroma of the chloroplast. Carbon dioxide from the atmosphere is incorporated into existing compounds, and sugar is produced. Plants use this sugar for growth and energy storage.

Limiting factors

Light intensity, atmospheric carbon dioxide concentration, temperature, and water availability determine the rate of photosynthesis. When any one of these decreases, it becomes the limiting factor. Excessively high temperatures can damage the enzymes that drive the process.

Ecological importance

Photosynthesis forms the basis of food chains on Earth: it converts solar energy into the biosphere. It also maintains atmospheric oxygen levels and removes carbon dioxide from the air. Fossil fuels—petroleum and coal—originated from organisms that performed photosynthesis in ancient times.