Photosynthesis Defined

A great variety of living things on earth, including all green plants, synthesize their foods from simple molecules, such as carbon dioxide and water. For this process, the organisms require energy, and that energy is derived from sunlight.

Figure 1 shows the energy relationships in living cells. Light energy is captured in the chloroplast of plant cells and used to synthesize glucose molecules, shown as C 6H 12O 6. In the process, oxygen (O 2) is released as a waste product. The glucose and oxygen are then used in the mitochondrion of the plant and animal cell, and the energy is released and used to fuel the synthesis of ATP from ADP and P. In the reaction, C0 2 and water are released in the mitochondrion to be reused in photosynthesis in the chloroplast.

Figure 1. Energy relationships in living cells.


The process of utilizing energy to synthesize carbohydrate molecules is referred to as photosynthesis. Photosynthesis is actually two separate processes. In the first process, energy‐rich electrons flow through a series of coenzymes and other molecules. This electron energy is trapped. During the trapping process, adenosine triphosphate (ATP) molecules and molecules of nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) are formed. Both ATP and NADPH are rich in energy. These molecules are used in the second half of the process, where carbon dioxide molecules are bound into carbohydrates to form organic substances such as glucose.