Oxygenic photosynthesis Anoxygenic photosynthesis This type of photosynthesis involves oxygen release. Photolysis of water occurs in this process which results in release of oxygen as a by-product. This type of photosynthesis does not involve release of oxygen. Water is used as a source of reducing agent in the green plants and cyanobacteria.
Therefore, photosynthesis by these bacteria does not involve oxygen. In the domain Bacteria, oxygenic photosynthesis is limited to only one phylum, i. Because O2 is produced, photosynthesis in these organisms is called oxygenic photosynthesis.
Some photosynthetic bacteria can use light energy to extract electrons from molecules other than water.
Photosynthesis is a process which synthesizes carbohydrates glucose from water and carbon dioxide, utilizing the energy from sunlight by green plants, algae , and cyanobacteria. As a result of photosynthesis, gaseous oxygen is released to the environment. It is an extremely important process for the existence of life on earth. Photosynthesis can be divided into two categories such as oxygenic and anoxygenic photosynthesis based on the generation of oxygen. The key difference between oxygenic and anoxygenic photosynthesis is that oxygenic photosynthesis generates molecular oxygen during the synthesis of sugar from carbon dioxide and water while anoxygenic photosynthesis does not generate oxygen.
Overview and Key Difference 2. What is Oxygenic Photosynthesis 3. What is Anoxygenic Photosynthesis 4. The energy of sunlight is converted into chemical energy by photosynthesis. The light is captured by the green pigments called chlorophylls possessed by photosynthetic organisms. Using this absorbed energy, chlorophyll reaction centers of the photosystems are excited and release electrons which contain high energy. These high energy electrons flow via several electron carriers and convert water and carbon dioxide into glucose and molecular oxygen.
Due to the generation of molecular oxygen, this process is known as oxygenic photosynthesis and also called noncyclic photophosphorylation. These two photosynthetic apparatus contain two reaction centers P and P Upon absorption of light, the reaction center P become excited and releases high energy electrons.
While in the "excited" state, the pigment now has a much lower reduction potential and can donate the "excited" electrons to other carriers with greater reduction potentials. These electron acceptors may, in turn, become donors to other molecules with greater reduction potentials and, in doing so, form an electron transport chain.
This electrochemical gradient generates a proton motive force whose exergonic drive to reach equilibrium can be coupled to the endergonic production of ATP, via ATP synthase. If the electrons are deposited back on the original pigment in a cyclic process, the whole process can start over.
This electron must come from a source with a smaller reduction potential than the oxidized pigment and depending on the system there are different possible sources, including H 2 O, reduced sulfur compounds such as SH 2 and even elemental S 0.
When a compound absorbs a photon of light, the compound is said to leave its ground state and become "excited". Figure 1. A diagram depicting what happens to a molecule that absorbs a photon of light. Attribution: Marc T. Facciotti original work. What are the fates of the "excited" electron?
There are four possible outcomes, which are schematically diagrammed in the figure below. These options are:. As the excited electron decays back to its lower energy state, the energy can be transferred in a variety of ways. While many so called antenna or auxiliary pigments absorb light energy and transfer it to something known as a reaction center by mechanisms depicted in option III in Figure 2 , it is what happens at the reaction center that we are most concerned with option IV in the figure above.
Here a chlorophyll or bacteriochlorophyll molecule absorbs a photon's energy and an electron is excited. This initiates the electron transport reactions. The result is an oxidized reaction center that must now be reduced in order to start the process again. How this happens is the basis of electron flow in photophosphorylation and will be described in detail below. Early in the evolution of photophosphorylation, these reactions evolved in anaerobic environments where there was very little molecular oxygen available.
Two sets of reactions evolved under these conditions, both directly from anaerobic respiratory chains as described previously. These are known as the light reactions because they require the activation of an electron an "excited" electron from the absorption of a photon of light by a reaction center pigment, such as bacteriochlorophyll.
The light reactions are categorized either as cyclic or as noncyclic photophosphorylation, depending upon the final state of the electron s removed from the reaction center pigments. If the electron s returns to the original pigment reaction center, such as bacteriochlorophyll, this is cyclic photophosphorylation; the electrons make a complete circuit and is diagramed in Figure 4.
In this case the reaction center must be re-reduced before the process can happen again.
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