Photosynthesis is the process by which green plants (and some bacteria) change light energy and carbon dioxide gas into usable chemical energy in the form of a sugar called glucose. The overall biochemical reaction for photosynthesis is:
Carbon dioxide plus water yields glucose and oxygen gas. Light energy (usually from the sun) and a special green pigment molecule called chlorophyll are needed to complete this reaction.
The process of photosynthesis has two parts: the light cycle, and the “Calvin” cycle.
When light strikes a chlorophyll molecule, it excites some of its electrons, and causes them to leave the molecule. Light also splits a water molecule into hydrogen and oxygen. The oxygen is a waste product, and leaves the plant as oxygen gas. But the electrons and hydrogen create two energy-carrying molecules, ATP and NADPH. Therefore, the light reaction changes light energy into chemical energy, which is necessary for the next part of photosynthesis.
In the Calvin cycle, the ATP and NADPH produced in the light reaction are used to “fix” carbon dioxide into sugar. “Fixing” means plants take the carbon dioxide gas, which can’t be used by animals, and change it into glucose, which they can use.
Why is photosynthesis so important? The ability to capture the energy of sunlight to make glucose puts plants at the bottom of the food chain, making them the primary producers of the energy needed for much of life on earth. Even the waste product of photosynthesis, oxygen, is necessary for animals to survive. Hundreds of millions of years ago, the earth’s atmosphere was mostly carbon dioxide. Thanks to photosynthesis, there’s now enough oxygen to sustain animal life.