Photosynthesis Explained Step by Step

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

The overall equation is:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• $CO_2$ is carbon dioxide from the air.
• $H_2O$ is water, usually absorbed by roots in plants.
• $C_6H_{12}O_6$ is glucose, a sugar that stores energy.
• $O_2$ is oxygen released into the environment.

In plant cells, photosynthesis occurs in chloroplasts. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy, especially red and blue wavelengths.

Leaves are well suited for photosynthesis because they have many chloroplast-containing cells. Small openings called stomata allow carbon dioxide to enter and oxygen to leave.

Photosynthesis has two connected stages:

1. Light-dependent reactions occur in thylakoid membranes. Chlorophyll captures light energy, water is split, and oxygen is released. The captured energy is transferred into energy-carrying molecules.
2. The Calvin cycle occurs in the stroma of the chloroplast. It uses carbon dioxide and the energy from the first stage to build sugar molecules.

Although the Calvin cycle does not directly require light, it depends on energy produced by the light-dependent reactions.

Suppose a leaf receives sunlight, takes in $6$ molecules of carbon dioxide, and has access to $6$ molecules of water.

$6CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2$

Step by step:

1. Light is absorbed by chlorophyll.
2. Water supplies electrons and hydrogen; splitting water produces oxygen.
3. Carbon dioxide provides carbon atoms.
4. The carbon atoms are assembled into one glucose molecule.
5. Six oxygen molecules are released.

The glucose can be used immediately in cellular respiration, converted to starch for storage, or used to build plant tissues.