The Two Parts of Photosynthesis
Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light-independent reactions (Figure 1). In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll; that energy is converted into stored chemical energy in the form of NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate). Protein complexes and pigment molecules work together to produce both NADPH and ATP.
In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. The light-dependent reactions utilize certain molecules to temporarily store the energy: These are referred to as energy carriers. The energy carriers that move energy from light-dependent reactions to light-independent reactions can be thought of as “full” because they are rich in energy. After the energy is released, the “empty” energy carriers return to the light-dependent reaction to obtain more energy.
Moreover, the actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem, two types of which are found embedded in the thylakoid membrane: photosystem II (PSII) and photosystem I (PSI) (Figure 2). The two complexes differ on the basis of what they oxidize (i.e., the source of the low-energy electron supply) and what they reduce (i.e., the place to which they deliver their energized electrons).