PHOTOSYSTEMS: COMPONENTS, OPERATION AND TYPES - BIOLOGY - 2024

The photosystems are functional units of the photosynthetic process. They are defined by their forms of association and particular organization of photosynthetic pigments and protein complexes capable of absorbing and transforming light energy, in a process that involves the transfer of electrons.

Two types of photosystems are known, called photosystems I and II because of the order in which they were discovered. Photosystem I has very high amounts of chlorophyll a compared to the amount of chlorophyll b, while photosystem II has very similar amounts of both photosynthetic pigments.

Photosystem I diagram. Taken and edited from: Pisum.

Photosystems are located in the thylakoid membranes of photosynthetic organisms such as plants and algae. They can also be found in cyanobacteria.

Chloroplasts

Chloroplasts are spherical or elongated organelles about 5 µm in diameter that contain photosynthetic pigments. Inside it, photosynthesis occurs in plant cells.

They are surrounded by two outer membranes and inside they contain sac-like structures, also surrounded by two membranes, called thylakoids.

The thylakoids are stacked forming a group that receives the name of grana, while the fluid that surrounds the thylakoids is called the stroma. Additionally, the thylakoids are surrounded by a membrane called the lumen that delimits the intrathylakoid space.

The conversion of light energy into chemical energy during photosynthesis occurs within the membranes of thylakoids. On the other hand, the production and storage of carbohydrates as a result of photosynthesis occurs in the stromas.

Photosynthetic pigments

They are proteins capable of absorbing light energy to use it during the photosynthetic process, they are totally or partially bound to the thylakoid membrane. The pigment directly involved in the light reactions of photosynthesis is chlorophyll.

There are two main types of chlorophyll in plants, called chlorophylls a and b. However, in some algae other types of chlorophyll such as c and d may be present, the latter present only in some red algae.

There are other photosynthetic pigments such as carotenes and xanthophylls that together make up carotenoids. These pigments are isoprenoids generally composed of forty carbon atoms. Carotenes are non-oxygenated caroteinoids, while xanthophylls are oxygenated pigments.

In plants only chlorophyll a is directly involved in light reactions. The remaining pigments do not directly absorb light energy, but act as accessory pigments by transmitting the energy captured from light to chlorophyll a. In this way, more energy is captured than the chlorophyll alone could capture.

Photosynthesis

Photosynthesis is a biological process that allows plants, algae, and some bacteria to take advantage of the energy that comes from sunlight. Through this process, plants use light energy to transform atmospheric carbon dioxide and water obtained from the soil, into glucose and oxygen.

Light causes a complex series of oxidation and reduction reactions that allow the transformation of light energy into chemical energy necessary to complete the photosynthesis process. Photosystems are the functional units of this process.

Components of photosystems

Antenna complex

It is made up of a large number of pigments, including hundreds of chlorophyll molecules a and even greater amounts of accessory pigments, as well as phycobilins. The complex antenna allows a large amount of energy to be absorbed.

It works like a funnel or like an antenna (hence its name) that captures the energy from the sun and transforms it into chemical energy, which is transferred to the reaction center.

Thanks to the transfer of energy, the chlorophyll a molecule in the reaction center receives much more light energy than it would have acquired on its own. Also, if the chlorophyll molecule receives too much light, it could photooxidize and the plant would die.

Reaction center

It is a complex made up of chlorophyll a molecules, a molecule known as a primary electron receptor, and numerous protein subunits surrounding them.

Functioning

Generally, the chlorophyll molecule a present in the reaction center, and which initiates the light reactions of photosynthesis, does not receive photons directly. The accessory pigments, as well as some chlorophyll a molecules present in the antenna complex, receive the light energy, but do not use it directly.

This energy absorbed by the antenna complex is transferred to the chlorophyll a of the reaction center. Each time a chlorophyll a molecule is activated, it releases an energized electron that is then absorbed by the primary electron receptor.

As a consequence, the primary acceptor is reduced, while chlorophyll a recovers its electron thanks to water, which acts as the final electron liberator and oxygen is obtained as a by-product.

Types

Photosystem I

It is found on the outer surface of the thylakoid membrane and has a low amount of chlorophyll b, in addition to chlorophyll a and carotenoids.

Chlorophyll a in the reaction center better absorbs wavelengths of 700 nanometers (nm), which is why it is called P700 (pigment 700).

In photosystem I, a group of proteins from the ferrodoxin group - iron sulfide - act as final electron acceptors.

Photosystem II

It acts first in the process of transforming light into photosynthesis, but it was discovered after the first photosystem. It is found on the inner surface of the thylakoid membrane and has a higher amount of chlorophyll b than photosystem I. It also contains chlorophyll a, phycobilins and xanthophylls.

In this case, the chlorophyll a in the reaction center absorbs better the 680 nm wavelength (P680) and not the 700 nm wavelength as in the previous case. The final electron acceptor in this photosystem is a quinone.

Photosystem II diagram. Taken and edited from: Original work was by Kaidor..

Relationship between photosystems I and II

The photosynthetic process requires both photosystems. The first photosystem to act is the II, which absorbs light and so the electrons in the chlorophyll of the reaction center are excited and the primary electron acceptors capture them.

Electrons excited by light travel to photosystem I through an electron transport chain located in the thylakoid membrane. This displacement causes an energy drop that allows the transport of hydrogen ions (H +) through the membrane, towards the lumen of the thylakoids.

The transport of hydrogen ions provides an energy differential between the lumen space of the thylakoids and the chloroplast stroma, which serves to generate ATP.

The chlorophyll in the reaction center of photosystem I receives the electron coming from photosystem II. The electron can continue in cyclical electron transport around photosystem I, or be used to form NADPH, which is then transported to the Calvin cycle.

References

1. MW Nabors (2004). Introduction to Botany. Pearson Education, Inc.
2. Photosystem. On Wikipedia. Recovered from en.wikipedia.org.
3. Photosystem I, In Wikipedia. Recovered from en.wikipedia.org.
4. Photosynthesis - Photosystems I and II. Recovered from britannica.com.
5. B. Andersson & LG Franzen (1992). The photosystems of oxygenic photosynthesis. In: L. Ernster (Ed.). Molecular mechanisms in bioenergetics. Elvieser Science Publishers.
6. EM Yahia, A. Carrillo-López, GM Barrera, H. Suzán-Azpiri & MQ Bolaños (2019). Chapter 3 - Photosynthesis. Postharvest physiology and biochemistry of fruits and vegetables.