Photosystem
Photosystems are functional and structural units of
PSII will absorb red light, and PSI will absorb far-red light. Although photosynthetic activity will be detected when the photosystems are exposed to either red or far-red light, the photosynthetic activity will be the greatest when plants are exposed to both wavelengths of light. Studies have actually demonstrated that the two wavelengths together have a synergistic effect on the photosynthetic activity, rather than an additive one.[1]
Each photosystem has two parts: a reaction center, where the photochemistry occurs, and an antenna complex, which surrounds the reaction center. The antenna complex contains hundreds of chlorophyll molecules which funnel the excitation energy to the center of the photosystem. At the reaction center, the energy will be trapped and transferred to produce a high energy molecule.[2]
The main function of PSII is to efficiently split water into oxygen molecules and protons. PSII will provide a steady stream of electrons to PSI, which will boost these in energy and transfer them to NADP+ and H+ to make NADPH. The hydrogen from this NADPH can then be used in a number of different processes within the plant.[2]
Reaction centers
Reaction centers are multi-protein complexes found within the thylakoid membrane.
At the heart of a photosystem lies the reaction center, which is an enzyme that uses light to reduce and oxidize molecules (give off and take up electrons). This reaction center is surrounded by light-harvesting complexes that enhance the absorption of light.
In addition, surrounding the reaction center are pigments which will absorb light. The pigments which absorb light at the highest energy level are found furthest from the reaction center. On the other hand, the pigments with the lowest energy level are more closely associated with the reaction center. Energy will be efficiently transferred from the outer part of the antenna complex to the inner part. This funneling of energy is performed via resonance transfer, which occurs when energy from an excited molecule is transferred to a molecule in the ground state. This ground state molecule will be excited, and the process will continue between molecules all the way to the reaction center. At the reaction center, the electrons on the special chlorophyll molecule will be excited and ultimately transferred away by electron carriers. (If the electrons were not transferred away after excitation to a high energy state, they would lose energy by fluorescence back to the ground state, which would not allow plants to drive photosynthesis.) The reaction center will drive photosynthesis by taking light and turning it into chemical energy[3] that can then be used by the chloroplast.[2]
Two families of reaction centers in photosystems can be distinguished: type I reaction centers (such as
Each of the photosystem can be identified by the
Type I photosystems use ferredoxin-like iron-sulfur cluster proteins as terminal electron acceptors, while type II photosystems ultimately shuttle electrons to a quinone terminal electron acceptor. Both reaction center types are present in chloroplasts and cyanobacteria, and work together to form a unique photosynthetic chain able to extract electrons from water, creating oxygen as a byproduct.
Structure of PSI and PSII
A reaction center comprises several (about 25-30)
Each photosystem has two main subunits: an antenna complex (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is transformed into chemical energy. At the reaction center, there are many polypeptides that are surrounded by pigment proteins. At the center of the reaction center is a special pair of chlorophyll molecules.
Each PSII has about 8 LHCII. These contain about 14 chlorophyll a and chlorophyll b molecules, as well as about four carotenoids. In the reaction center of PSII of plants and cyanobacteria, the light energy is used to split water into oxygen, protons, and electrons. The protons will be used in proton pumping to fuel the ATP synthase at the end of an electron transport chain. A majority of the reactions occur at the D1 and D2 subunits of PSII.
In oxygenic photosynthesis
Both
At the core of photosystem II is P680, a special chlorophyll to which incoming excitation energy from the antenna complex is funneled. One of the electrons of excited P680* will be transferred to a non-fluorescent molecule, which ionizes the chlorophyll and boosts its energy further, enough that it can split water in the oxygen evolving complex (OEC) of PSII and recover its electron.[citation needed] At the heart of the OEC are 4 Mn atoms, each of which can trap one electron. The electrons harvested from the splitting of two waters fill the OEC complex in its highest-energy state, which holds 4 excess electrons.[2]
Electrons travel through the
When the electron reaches photosystem I, it fills the electron deficit of light-excited reaction-center chlorophyll P700+ of PSI. The electron may either continue to go through cyclic electron transport around PSI or pass, via ferredoxin, to the enzyme NADP+ reductase. Electrons and protons are added to NADP+ to form NADPH. This reducing (hydrogenation) agent is transported to the Calvin cycle to react with
Photosystem repair
In intense light, plants use various mechanisms to prevent damage to their photosystems. They are able to release some light energy as heat, but the excess light can also produce
See also
References
External links
- Photosystems I + II: Imperial College, Barber Group
- Photosystem I: Molecule of the Month in the Protein Data Bank
- Photosystem II: Molecule of the Month in the Protein Data Bank
- Photosystem II: ANU
- UMich Orientation of Proteins in Membranes Superfamily » 1.1.002. Photosystems (7 families) - Orientations of Proteins in Membranes (OPM) database at the Wayback Machine (archived October 13, 2017) – Calculated spatial positions of photosynthetic reaction centers and photosystems in membrane
- Rutherford AW, Faller P (January 2003). "Photosystem II: evolutionary perspectives". Philos. Trans. R. Soc. Lond. B Biol. Sci. 358 (1429): 245–53. PMID 12594932.