20.6: Photosystems

Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.

Functioning of Photosystems

Photosystems contain many pigment molecules, such as chlorophylls and carotenoids, arranged in a particular organization across two domains — the antenna complex and the reaction center. The main aim of the pigment molecules distributed in the antenna complex is to absorb light in the form of photons and funnel them to the special chlorophyll pair of the reaction center.

There are two types of photosystems — photosystem II (PSII) and photosystem I (PSI) that are structurally similar but differ on the basis of the source of the low-energy electron supplier and the acceptor to which they deliver their energized electrons. Both these photosystems work in concert.

The PSII reaction center, also known as P680, absorbs a photon that excites an electron in the chlorophyll. The high-energy electron breaks free and is passed on to the primary electron acceptor, and ultimately to PSI through the electron transport chain. P680's missing electron is replaced by extracting a low-energy electron from water; thus, water is "split" during this stage of photosynthesis, and PSII is re-reduced after every photoact. Splitting one H₂O molecule releases two electrons, two hydrogen atoms, and one atom of oxygen. The oxygen molecules are released into the environment while the hydrogen ions play a critical role in establishing a proton gradient across the thylakoid membrane that is essential for the synthesis of ATP in the chloroplast.

As electrons move through the proteins that reside between PSII and PSI, they lose energy and must be re-energized by PSI; hence, another photon is absorbed by the PSI antenna. This energy is relayed to the PSI reaction center called P700. P700 is oxidized and sends a high-energy electron to NADP⁺ to form NADPH. Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the energy to reduce NADP⁺ into NADPH.

After the energy from the sun is converted into chemical energy in the form of ATP and NADPH molecules, the cell has the fuel needed to build carbohydrate molecules for long-term energy storage. This is achieved in the second phase of photosynthesis, also known as the light-independent or dark phase of photosynthesis, which occurs in the chloroplast stroma.

This text is adapted from Openstax, Biology 2e, Chapter 8, Section 8.2:The Light-dependent Reactions of Photosynthesis.

Photosynthetic organisms capture sunlight through the pigment-protein complexes called photosystems, embedded within the chloroplast's thylakoid membrane.

These complexes are categorized into photosystem I or PSI and photosystem II or PSII.

Inside the chloroplast, PSI complexes are predominantly located in the unstacked regions, called the stromal lamellae, while PSII complexes are present within the stacked granal lamellae.

Each photosystem is a collection of about 200 chlorophyll and 50 carotenoid pigment molecules, distributed across two different domains of the photosystem─the core domain called the reaction center and a peripheral domain called the antenna complex.

Although all pigment molecules absorb photons, only a few chlorophyll molecules associated with the reaction center can convert absorbed light energy to chemical energy.

The pigments in the antenna complex only funnel the absorbed energy to the reaction center.

The photosystems also have associated cofactors essential for their functioning.

For instance, PSI has a ferredoxin cofactor, a key junction in the electron transport chain, while PSII contains an oxygen-evolution complex that catalyzes water oxidation, a step crucial for photosynthesis.