In details

The Photochemical Stage of Photosynthesis


See in more detail the photochemical phase.

It is also called the "clear phase" of photosynthesis, since its occurrence is entirely dependent on light. As this is a step that counts with the participation of chlorophyll molecules, it happens inside the tilacoids, in whose inner faces of their membranes the molecules of this photosynthesizing pigment are "anchored".

At this stage, chlorophyll, when illuminated, loses electrons, which causes "voids" in the molecule. The fate of the lost electrons and the reoccupation of these voids can follow two distinct mechanisms, called cyclic photophosphorylation and acyclic photophosphorylation.

In the so-called photosystem I, chlorophyll predominates The. This, when illuminated, loses a pair of excited (energy-rich) electrons. In the chlorophyll molecule, an electron "void" is established. The electron pair is collected by a series of cytochromes, substances that accept additional electrons, becoming unstable and transferring these electrons to other molecules.
As they pass through the cytochrome chain, the electrons gradually lose energy, which is used in phosphorylation (ATP production by joining one more phosphate group to an ADP molecule). How this phosphorylation is made possible by the light energy captured by the chlorophyll electrons is called photophosphorylation.

After passing through the cytochrome chain, the electrons return to the chlorophyll molecule, occupying the "void" they had left. As electrons return to chlorophyll, the process is cyclic.

This mechanism employs two photosynthesis systems: the photosystem I it's the photosystem II. In photosystem I, chlorophyll predominates Thewhile photosystem II predominates chlorophyll B.
Chlorophyll The, illuminated, loses a pair of activated electrons, collected by a special acceptor, the ferridoxine. At the same time, chlorophyll B, excited by light, loses a pair of electrons that, after crossing a chain of cytochrome, occupy the "void" left in the chlorophyll molecule The. During the passage of these electrons through the cytochrome chain, there is energy release and ATP (phosphorylation) production. Like the "electron void" of chlorophyll The is not filled by the same electrons that came out of this molecule, the mechanism is called acyclic photophosphorylation.

Inside chloroplasts, water is decomposed in the presence of light. This reaction is the photolysis of water. (or Hill's reaction).

Of the water photolysis products, the electrons will occupy the "voids" left by chlorophyll's loss of electrons. B. H + protons, along with electrons lost by chlorophyll The, will transform NADP (nicotinamide adenine dinucleotide phosphate) into NADPH. At the same time, oxygen is released. This is an important aspect of photosynthesis: All oxygen generated in the process comes from photolysis of water.

Photosynthetic beings use water as a source of hydrogen atoms to reduce NADP. These hydrogen atoms are later employed in reducing CO2 even carbohydrate. The general process equation is as follows:

The value no generally corresponds to six, which leads to glucose formation (C6H12O6). However, since all released oxygen comes from water, the equation must be corrected to:

Thus, one can explain the origin of a quantity 2n of oxygen atoms from an amount of 2n water molecules (H2O).