Photosynthesis in plants and algae takes place in chloroplasts and entails two steps:
Energy transferring (energy‐transduction) reactions (commonly called the light‐dependent or light reactions)
Carbon fixation reactions (sometimes inappropriately called the dark reactions)
The energy transferring reactions are photochemical processes that take place in two physically separate but chemically linked photosystems: Photosystem I (PsI) and Photosystem II (PsII). Photosystems are pigment molecules that capture energy from the sun and are arranged in the thylakoid membranes of the chloroplasts. The chlorophyll and other pigments of both photosystems absorb light energy, most of which is stored temporarily in energy‐rich chemical bonds of ATP (adenosine triphosphate) and the electron carrier NADPH (reduced nicotinamide adenine dinucleotide phosphate). ATP and NADPH supply the energy for the resultant carbon fixation reactions of step two. Oxygen (O 2) is a by‐product of water molecules splitting in the initial energy exchanges of step one. The three products of the energy transfer phase are ATP, NADPH, and O 2.
The carbon fixation reactions of the second step of photosynthesis are biochemical and use the energy of ATP and the reducing power of NADPH to repackage the energy in a form that can be transported and stored, as the carbohydrates sugar and starch. Carbon fixation reactions do not require light; if cellular energy is available, the reactions occur.
Plants have developed three different pathways for photosynthetic carbon fixation, one basic procedure and two modifications of it.
- C3 Pathway (also called the Calvin cycle after its 1961 Nobel Prize−winning discoverer). This method is used by most common temperate zone species.
- C4 or Hatch‐Slack Pathway. An additional step is added to the Calvin cycle, making it more efficient for plants structurally modified to do so. Many common grasses and tropical plants use this pathway; it is a necessary adaptation in areas of high light intensity, high temperatures or semi‐aridity.
- CAM (crassulacean acid metabolism) Pathway. Another Calvin cycle modification is made by succulents and other plants growing in areas of high temperatures, high light, and low moisture (deserts especially). In this modification, carbon fixation takes place at night in a pathway similar to C 4 photosynthesis and, in addition, during the day carbon is fixed in the same cells using the C 3 pathway. This pathway is named for the family of plants, Crassulaceae, in which it was first discovered.
The final products of carbon fixation are a disaccharide sugar, sucrose, and a polysaccharide, starch. The sucrose is formed from two monosaccharides (6‐carbon or hexose sugars), glucose and fructose, joined together by an extra oxygen atom. Stored energy is transported from cell to cell in plants by the water‐soluble sucrose. (In vertebrates, glucose is the transported sugar.)
Starch molecules are strings of glucose molecules too large to move through membranes, and, therefore, useful for storing energy. As energy is needed, the starch is converted to sucrose and transported. Plants build and fuel their bodies from these carbohydrates.
Two intermediate carbohydrates (manufactured before sucrose or starch) are the first detectable products in the C 3 and C 4 Pathways. In the C 3 Pathway the product is PGA (3‐phosphoglycerate) (3 carbons), and in C 4 photosynthesis the first detectable product is oxaloacetate (4‐carbons).