What Are the Two Phases of Photosynthesis?

Photosynthesis occurs in the leaves.

Plants use photosynthesis to make and store their food. The trigger for this process is light, preferably natural sunlight. There are two phases: light dependent reaction and light independent reaction.

Photosynthesis

Plants have chlorophyll in their leaves and stems. This green chemical absorbs light and uses this energy to convert carbon dioxide and water into glucose, a sugar.

Light Dependent Reaction

This phase of photosynthesis takes place when there is a light source. In the natural world, this is in daylight. It takes the hydrogen from water, releases oxygen, changes the light energy into chemical energy and makes glucose.

Light Independent Reaction

The second phase of photosynthesis occurs after the sun goes down. It uses oxygen and glucose already made within the plant to create and release water and carbon dioxide. Depending on how much energy the plant has stored during the daytime, this process can continue long into the night.

Aerobic Respiration

Plants use aerobic respiration to release energy from the food they produce. In the daytime the process of photosynthesis makes more energy than the plant can use. At night, when photosynthesis changes from light-dependent to light-independent, the aerobic respiration process takes precedence.

The Calvin Cycle

The light independent reaction, or dark cycle, is also called the Calvin cycle after Melvin Calvin, who charted the path of carbon dioxide during photosynthesis. In 1961 he received the Nobel Prize in chemistry.

References

• Photosynthesis
• Biology of Animals and Plants
• Biology GCSE: Respiration

Resources

• Read this Article in Spanish
• Newton Ask a Scientist, Department of Energy