7.1: Introduction
The early earths’ atmosphere consisted of hydrogen gas, nitrogen gas, water vapor, methane, ammonia, carbon monoxide and carbon dioxide. Oxygen was not present until the evolution of the first photosynthetic organisms, the cyanobacteria, about 3.5 billion years ago.
Photosynthesis is the process that transforms light energy from the sun into chemical energy. Photosynthetic organisms include plants, algae, cyanobacteria and Euglena, a photosynthetic protozoa. Photosynthesis occurs inside the specialized organelles known as chloroplasts. Photosynthesis can be summarized by the following equation:
\[\ce{6 CO2 + 6 H2O -> C6H12O6 + 6O2}\]
Photosynthesis is not one reaction but rather a metabolic pathway, a series of reactions that can be divided into 2 separate phases. In the first phase, the light dependent reaction, the pigment chlorophyll absorbs light energy. This energy is used to split water molecules. The breaking of the water molecules releases energy that is stored as ATP and NADPH and oxygen is released as a bi-product. The second phase of photosynthesis is the Calvin Cycle where the energy harvested in the light dependent reaction is used to fixate carbon atoms from carbon dioxide and glucose is formed.