Photosynthesis occurs in two major stages, involving several chemical reactions: the first is the clear phase (also called photochemistry) and the second is dark phase (also known as phase chemistry).
Generally speaking, the main events of photosynthesis are the absorption of light energy by chlorophyll; the reduction of one electron acceptor called NADP, which becomes NADPH2; The ATP formation and the glucose synthesis
The dark phase of photosynthesis need not occur in the dark. What the name means is that it occurs even in the absence of light - it only needs ATP and NADH2 to occur.
Clear or photochemical phase: Water breakdown and oxygen release
This phase occurs in the membrane of the tilacoids and a complex of pigments in the money, electron acceptors, water molecules and light participate. As a result of this phase we have the production of oxygen, ATP (from ADP + Pi) and also the formation of a substance called NADPH2;. Both ATP and NADPH2; will be used in the dark phase.
In the clear phase, light enters chloroplasts and reaches the pigment complex, while causing changes in water molecules. How does this light action result in products that can be used in the second phase of photosynthesis?
One of the remarkable events of the clear phase are the so-called cyclic photophosphorylations and acyclic.
In cyclic photophosphorylation, when hit by sunlight, the chlorophyll molecule releases electrons. These electrons are collected by certain organic molecules called electron acceptors, which send them to a chain of cytochromes (substances associated with the photosynthesizing system and which are so called because they have color). Hence, the electrons return to chlorophyll.
You may ask: what is the advantage of this electron transport cycle?
The answer is that by returning to the chlorophyll molecule from the cytochromes, the electrons release energy because they return to their original energy levels. And this energy is harnessed for the synthesis of ATP molecules that will be used in the dark phase of photosynthesis.
Note that the path taken by the electrons is cyclic. For this reason, this pathway is commonly referred to as cyclic photophosphorylation, due to the occurrence of synthesis of numerous ATP molecules in a cyclic process, with the participation of light and chlorophyll molecules.
At the same time, water molecules - when struck by sunlight - are "broken" (the term is used “Water photolysis” to designate the breakdown of water molecules) and release protons (H+), electrons (and-) and oxygen molecules. Protons are captured by NADP molecules, which convert to NADPH2; oxygen molecules are released into the medium; and the electrons return to chlorophyll, replacing those it lost at the beginning of the process. See more details about this step of photosynthesis below.