Photosynthesis is the process by which plants, algae and some bacteria use light energy to drive the synthesis of organic compounds. In plants, algae and certain types of bacteria, photosynthesis releases O2 and removes CO2 from the atmosphere while synthesizing carbohydrates (oxygenic photosynthesis). Other types of bacteria use light energy to create organic compounds but do not produce oxygen (anoxygenic photosynthesis). Photosynthesis provides the energy and reduced carbon required for the survival of virtually all life on Earth, as well as the molecular oxygen necessary for the survival of oxygen consuming organisms.

The overall equation for photosynthesis is simple, but the mechanism is a complex set of physical and chemical reactions that must occur in a coordinated manner for the synthesis of carbohydrates. To produce a sugar molecule such as sucrose, plants require nearly 30 distinct proteins that work within a complicated membrane structure. Only the green parts of plants release oxygen. Both water and CO2 are required for photosynthetic growth.

Oxygenic Photosynthetic Organisms

The photosynthetic process in all plants and algae as well as in certain types of photosynthetic bacteria involves the reduction of CO2 to carbohydrate and removal of electrons from H20, which results in the release of O2. In this process, known as oxygenic photosynthesis, water is oxidized by the photosystem II reaction center, a multisubunit protein located in the photosynthetic membrane. Years of research have shown that the structure and function of photosystem II is similar in plants, algae and certain bacteria, so that knowledge gained in one species can be applied to others. This is a common feature of proteins that perform the same reaction in different species. This homology at the molecular level is important because there are estimated to be 300,000-500,000 species of plants.

The synthesis of carbohydrate from carbon and water requires a large input of light energy. The standard free energy for the reduction of one mole of CO2 to the level of glucose is +478 kJ/mol. Because glucose, a six carbon sugar, is often an intermediate product of photosynthesis, the net equation of photosynthesis is frequently written as :

  • 6CO2 + 12H2O + Light Energy -----> C6H12O6 + 6O2 + 6H2O.

where CH2O represents a carbohydrate (e.g., glucose, a six-carbon sugar). The standard free energy for the synthesis of glucose is +2,870 kJ/mol.

The biochemical conversion of CO2 to carbohydrate is a reduction reaction that involves the rearrangement of covalent bonds between carbon, hydrogen and oxygen. The energy for the reduction of carbon is provided by energy rich molecules that are produced by the light driven electron transfer reactions. Carbon reduction can occur in the dark and involves a series of biochemical reactions.

Photosynthetic bacteria use light energy to produce ATP, an organic molecule that serves as an energy source for many biochemical reactions. Plants, algae and cyanobacteria require two reaction centers, photosystem II and photosystem I, operating in series.

Most of the proteins required for the conversion of light energy and electron transfer reactions of photosynthesis are located in membranes.

A key element in photosynthetic energy conversion is electron transfer within and between protein complexes and simple organic molecules. The electron transfer reactions are rapid (as fast as a few picoseconds) and highly specific.

Photosystem I vs. II

Two families of reaction centers in photosystems exist: type I reaction centers (chloroplasts and in green-sulphur bacteria) and type II reaction centers (chloroplasts and in non-sulphur purple bacteria). Oxygenic photosynthesis requires both photosystems I and II, and can be performed by cyanobacteria, which are believed to be the progenitors of the photosystem-containing chloroplasts of eukaryotes (i.e., plants), which also perform oxygenic photosynthesis.

The photosystem I was named "I" since it was discovered before photosystem II, but this does not represent the order of the electron flow. Each photosystem can be identified by the wavelength of light to which it is most reactive (700 and 680 nanometers, respectively for PSI and PSII in chloroplasts), the amount and type of light-harvesting complexes present and the type of terminal electron acceptor used.

Photosystem II uses light energy to drive two chemical reactions - the oxidation of water and the reduction of plastoquinone. Photosystem II is the only known protein complex that can oxidize water, resulting in the release of O2 into the atmosphere. Despite years of research, little is known about the molecular events that lead to water oxidation. Energetically, water is a poor electron donor. Water oxidation requires two molecules of water and involves four sequential turnovers of the reaction center. Each photochemical reaction creates an oxidant that removes one electron. The net reaction results in the release of one O2 molecule, the deposition of four protons into the inner water phase, and produces two reduced plastoquinone molecules). Photosystem II is an energy transforming enzyme that must switch between various high energy states that involve the creation of the powerful oxidants required for removing electrons from water and the complex chemistry of plastoquinone reduction which is strongly influenced by protons.

The photosystem I complex catalyzes the oxidation of plastocyanin, a small soluble Cu- protein, and the reduction of ferredoxin, a small FeS protein. Photosystem I is composed of a heterodimer of proteins that act as ligands for most of the electron carriers. The reaction center is served by an antenna system that consists of about two hundred chlorophyll molecules (mainly chlorophyll a) and primary photochemistry is initiated by a chlorophyll a dimer, P700. In contrast to photosystem II, many of the antenna chlorophyll molecules in photosystem I are bound to the reaction center proteins. Also, FeS centers serve as electron carriers in photosystem I and, so far as is known, photosystem I electron transfer is not coupled to proton translocation. Primary charge separation occurs between a primary donor, P700, a chlorophyll dimer, and a chlorophyll monomer (Ao).

Anoxygenic Photosynthetic Organisms

Some photosynthetic bacteria can use light energy to extract electrons from molecules other than water. Some photosynthetic bacteria use hydrogen sulfide (H2S) instead of water for photosynthesis and these organisms release sulfur instead of oxygen. These organisms are of ancient origin, presumed to have evolved before oxygenic photosynthetic organisms. Anoxygenic photosynthetic organisms occur in the domain Bacteria and include Purple Bacteria, Green Sulfur Bacteria, Green Gliding Bacteria, and Gram Positive Bacteria.

Created by admin. Last Modification: Wednesday 16 of September, 2015 00:08:05 CDT by admin.