Photosynthesis is the process by which plants and other organisms convert light energy into chemical energy through several stages (Bassham). It is one process which does not only benefit the organism that synthesizes it, but the environment and other species as well.
Photosynthesis is a two-step process. In the first series of reactions or the Light Dependent Process light hits chlorophyll and excites electrons to a higher state. Along an electron transport process, light energy is transformed into ATP and NADPH. These are used to make covalent carbon to carbon bonds in the next step. Water is also split and oxygen is released as a by-product. This occurs in the grana. The Light Independent Process or Dark Reactions on the other hand, are the second set of reactions. These reactions take place in the stroma of chloroplasts. Atmospheric carbon dioxide is modified by adding hydrogen to form carbohydrates. This process by which carbon is incorporated with other substances to produce organic compounds is called carbon fixation. (Farabee, 2010).
Photosynthetic electron transfers
Charge separation reactions or the initial electron transfer in the photosynthetic reaction center initiates a long series of reduction-oxidation reactions. The electron passes along a number of cofactors and fills up the “electron hole” on the chlorophyll. Organisms that are capable of photosynthesis have two types of reaction centers: photosystem II and photosystem I (PS II and PS I). Both of these photosystems are pigment/protein complexes located in thylakoids. Thylakoids are specialized membranes inside chloroplasts and are found in the grana. Since prokaryotes or single celled organisms do not have chloroplasts, pigment/protein complexes can be found in the cytoplasmic membrane, in its invaginations or in thylakoid membranes that are part of more complex structures within the cell (Vermaas, 2007).
In organisms that produce oxygen, all chlorophyll is in thylakoids and is associated with both photosystems or with antenna proteins that provide energy for the photosystems. PS II is where water is split and oxygen is generated. When the reaction center chlorophyll in PS II is oxidized, an electron is pulled from tyrosine, which also gets an additional electron from the water-splitting complex. After this reaction is completed, electrons go into molecules called plastoquinone in the thylakoid membrane and into the cytochrome b6f complex. Meanwhile, PS I hastens light-induced charge separation in a similar way with PS II. The difference is that PS I electrons are transferred to NADP, which can be converted into NADPH and used for carbon fixation. The end result of the two photosystems is water oxidation, oxygen evolution and NADPH production, with light providing the energy for the said process (Vermaas, 2007).
Factors affecting photosynthesis
Several factors can affect the rate at which the process is completed. Some of these factors include light, temperature, carbon dioxide and water. At moderate temperatures and low to medium light conditions, the rate of photosynthesis increases as light intensity increases, but is not affected by temperature. However, as the intensity of light increases, the rate tends to be dependent on temperature. An increase in carbon dioxide also results to an increase in the rate of photosynthesis, but the degree varies depending on the species and condition of the plants. For plants that grow on land, the availability of water also contributes as a limiting factor. Not only is it directly required by the photosynthetic process, but is also transpired form the leaves through the stomates. These openings allow the entry of carbon dioxide, but also seves as an exit of water vapour (Bassham).
Basically, photosynthesis is an oxidation-reduction reaction. Light is used to oxidize water to produce oxygen, hydrogen ions and electrons. The hydrogen ions and electrons are then transferred to carbon dioxide, which is converted into other organic products. The unused electrons and hydrogen ions are used to reduce nitrate and sulphate to amino and sulfhydryl groups. Starch and sucrose are some of the major organic products of this process (Bassham).
Bassham, J. A. (n.d.). Photosynthesis. Retrieved November 1, 2011, from Encyclopedia Britannica: http://www.britannica.com/EBchecked/topic/458172/photosynthesis
Farabee, M. (2010, May 18). Photosynthesis. Retrieved November 1, 2011, from http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BiobookPS.html
Vermaas, W. (2007, June 12). An Introduction to Photosynthesis and Its Applications. Retrieved November 1, 2011, from Arizona State University: http://photoscience.la.asu.edu/photosyn/education/photointro.html