Why does photosynthesis occur?
(Lansing State Journal, September 15,1993)
Plants use photosynthesis as a means of obtaining energy from sunlight. Plants do not use light energy directly but rather they convert and store it as chemical energy- packets called ATP and NADPH. These energy packets are, in turn, used with carbon dioxide to build a variety of chemicals in the plant, including carbohydrates, fats and proteins - the common compounds we call food.
When we eat plants as food, we break down, or metabolize, food compounds to small molecules. Some of these molecules are used to build more complex structures, such as bones and muscles in every part of our bodies. Other metabolites are used to produce the energy packets, such as ATP, needed to put together those complex structures.
The creation of ATP in animals requires oxygen and produces carbon dioxide, just the opposite of photosynthesis, which uses carbon dioxide and produces oxygen. This molecular recycling of gases makes animals and plants dependent on each other. So photosynthesis provides food for most forms of life on Earth, including you.