9.1: What is Photosynthesis?
Photosynthesis is a multipart, biochemical process that occurs in plants as well as in some bacteria. It captures carbon dioxide and solar energy to produce glucose. Glucose stores chemical energy in the form of carbohydrates. The overall biochemical formula of photosynthesis is 6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2. Photosynthesis releases oxygen into the atmosphere and is largely responsible for maintaining the Earth’s atmospheric oxygen content.
Photosynthetic reactions occur in chloroplasts, specialized membrane-enclosed compartments in the plant cell. Chloroplasts consist of coin-like stacks of thylakoids. One such stack is called a granum. The thylakoid membranes are enriched with chlorophyll, a green pigment that gives plants and especially their leaves their green color. The chlorophyll molecule absorbs light energy in the form of photons from violet-blue, and orange and red wavelengths. The photons initiate a cascade that powers the reactions of Photosystem II and Photosystem I that produce ATP and NADPH. These two molecules are then used to power the light-independent reactions of the Calvin Cycle that take place in the stroma of the chloroplast to produce complex carbohydrates.
Some plants, like corn and cacti that grow in dry, hot climates, use modified carbon-fixing processes to produce sugars, such as the C4 and CAM pathways. These climate adaptations reduce photorespiration—a process where the enzyme rubisco binds O2 and burns sugar rather than producing it by binding CO2. CAM and C4 plants separate CO2 fixation and glucose synthesis in time, or by using specialized cell compartments for both processes.