The photosystems are protein complexes located in thylakoid membranes of chloroplasts where grouped photosynthetic pigments such as chlorophyll, able to capture the light energy from the sun and transform it into chemical energy.
The chloroplasts contain molecules over three hundred chlorophyll required for photosynthesis. This means that these molecules act together as a photosynthetic unit or photosystem, in which only one member of the group, the chlorophyll in the reaction center, acts by transferring electrons to an acceptor.
All chlorophylls collaborate in forming a kind of antenna to be able to capture the light that reaches them with different wavelengths. When one is excited by capturing a photon, it transfers that energy to the adjacent molecule, and this to another, until it reaches the chlorophyll located in the reaction center.
The reaction center has two special chlorophyll a molecules (a1 and a2), the released electrons of which are sent towards the electron transport chain of the thylakoid membrane.
The photosystems are protein complexes associated with photosynthetic pigments. They are distinguished:
- The antenna complex, consisting mainly of photosynthetic pigments that capture light energy, transform it into chemical energy and transmit it to other pigments to the photochemical reaction center. It works like a funnel collecting the light energy directing it towards the reaction center. The antenna presents about 300 molecules of photosynthetic pigments, mainly chlorophyll a, chlorophyll b and carotenes , associated with lipids and proteins.
- The photochemical reaction center, where the target pigments are located, which receive the energy captured by the antenna pigments and transfer it to the first electron acceptor, initiating the chain of chemical reactions. The reaction center is made up of three molecules:
- A target chlorophyll molecule, which captures the excited electrons coming from the antenna, and yields to the primary electron acceptor.
- The primary electron acceptor that is reduced with the electron that comes from chlorophyll.
- The final electron donor, a molecule that gives electrons to the target molecule so that it can recover the lost electron. This electron donor is water, which oxidizes and gives oxygen as a by-product.
When the antenna complex picks up a photon of light, an electron jumps into a higher-energy orbital. This energy is transmitted from one chlorophyll molecule to another nearby one and thus, through a chain reaction, this energy reaches one of the chlorophyll molecules in the photochemical reaction center, which responds by releasing a high-energy electron that is captured by the primary acceptor.
The electron leaves a vacuum in the chlorophyll of the reaction center, which is occupied by a low-energy electron from an electron donor, water, or an electron-carrier molecule depending on which photosystem it is that has been activated.
Types of photosystems
In the thylakoid membranes of chloroplasts there are two types of photosystems, photosystem I (PS I) and photosystem II (PS II).
- Photosystem II (PS II). It is located in the thylakoid membranes that pile up to form the grana, and appear embedded in the side of the membrane close to the intrathylakoid space. Its reaction center contains two molecules of chlorophyll a2, called P680, which have their maximum absorption at a wavelength of 680 nm. This photosystem is activated with wavelengths shorter than PS I, 680 nm and less, which are the most efficient for the evolution of O2. Plants, algae, and cyanobacteria have photosystem I and photosystem II that work together, releasing O2 to the atmosphere, and therefore carry out oxygenic photosynthesis.
- Photosystem I (PS I). It is located in the membranes of the non-stacked thylakoids, in contact with the stroma. In the reaction center there are two molecules of chlorophyll a1 called P700, since they have their point of maximum absorption at a wavelength of 700 nm, but they are not responsible for the evolution of O2. Chlorophyll P700 gives a pair of electrons to a primary acceptor which, in turn, gives them to a transport chain whose last acceptor is NADP+, which is reduced to NADPH
The electron hole that has become free in the chlorophyll P700 molecule is filled with a pair of electrons from plastocyanin.
Only photosynthetic bacteria have something similar to this complex. They do not give off O2, and their photosynthesis is called anoxygenic.
When sunlight falls on the antenna pigments of the two photosystems, they absorb energy, and excite the pigments in the reaction centers, transferring the electrons from these centers to a primary electron acceptor. These pigments of the reaction centers are left with one electron less, going from being excited to oxidized (with a positive charge), being called P+680 and P+700.
This electron is picked up by an acceptor which, in turn, is reduced. The chlorophyll molecule that was oxidized, replenishes its electron from the photolysis of water, so that the oxygen is free in the reaction and is released into the atmosphere.
H2O + light → 2H+ + 2e- + ½ O2