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Water is key to almost every organism existence, as humans, we need water to keep our blood flow stable, our muscles healthy and so much more. Humans also rely on oxygen for aerobic respiration, which in turn provides energy for everything we do. The single biggest producer of oxygen are plants, they photosynthesise and produce the oxygen we need to survive, a process which is dependent on water being available. This discussion will detail how plants use water in photosynthesis, how this affects later stages of photosynthesis and other more niche aspects how plants make use of water.   Given its importance water is a surprisingly simple molecule consisting of 2 hydrogen atoms covalently bonded to s single oxygen atom. This does however lead to some important characteristics, an example of this is the hydrogen bonding network. Water forms weak H-bonds, primarily electrostatic attraction of the oxygen atom to a hydrogen atom of nearby a water molecule. Many water molecules form H-bonds resulting in a relatively strong network of constantly rearranging H-bond networki. This forms an important part of the delivery of water to parts of the plant where photosynthesis occurs for instance in the xylem, the cohesion-tension theory suggests the transpiration of water out of a leaf causes a pull on water molecules further down the xylem, creating a chain effect where the H-bond cohesion aids the transport of water up the stem of a plant.   Osmotic potential results in net movement of water in or out of a cell, if the inside of a cell has a more negative water potential than on the outside, water will travel through the cell wall and cell membrane into the cell via osmosis. This is usually due to higher concentrations of solute in the cell compared to that outside the cell.     Once water molecules reach chloroplasts they undergo multiple changes leading to the splitting of the moleculesii. This process of splittingwater is part of Photosystem II, a complex protein structure found embedded in the thylakoid membrane, as a part of four redox reactionsiii. These so-called S-states describe the oxidation state of the water oxidising complex and can be shown using the Kok cycle (see figure 1), the most reduced state being S0, and increasing in oxidation state until S4. At stage four a diatomic oxygen molecule is creatediv. The process begins with photons exciting a pair of chlorophyll named P680, this is due to absorption of light with a wavelength of 680nm, the chlorophyll is excited to P680+ this species is a strong oxidising agent, strong enough to oxidise water. P680+ is oxidised by Manganese ions in the water oxidising complex, Manganese has a variable oxidation state of 1 to 5 enabling this cycle to occur. This oxidation repeats four times until the water oxidising complex has a positive enough charge to split the water moleculev,vi.         An Mn4Ca cluster is the important component contained within the water oxidising complex, this has been attempted to be analysed at resolutions high enough to determine the exact structure of the water oxidising complex. X-ray spectroscopy was used but it is suggested that the X-rays may cause damage to the metal sites resulting in an unclear idea of its structurevii. The X-ray crystal structures nevertheless does show that the water oxidising complex is made up of several clusters involving Mn and Ca, at its core is CaMn3O4 ­with various arrays around it. Oxygen bridges the Mn and Ca atoms, with 3 Oxygen atoms bonded to eachviii.   The hydrogen bonding phenomenon in water also has its use in PS II where it is thought that hydrogen bonding networks can provide proton transfer pathways for the delivery of protons. Water has been observed forming a hydrogen bonding network around the water oxidising complex acting as a catalyst, leading to numerous exit paths for protonsix.   The purpose of photosystem II is to split two water molecules into four protons, four electrons and molecular oxygen. This has numerous results, with the electrons replacing those used by the water oxidising complex and the protons being released into the lumen to create a proton gradient across the membrane. The electrons reduce plastoquinone, an electron acceptor in PS II, via another electron acceptor, pheophytin. Plastoquinone requires two electrons along with two protons to be oxidised, therefore the splitting of one water molecule has the potential to oxidise two plastoquinone molecules, producing 2 plastoquinol molecules5.   Plastoquinol is later oxidised to reform plastoquinone by another protein, this protein reduces plastocynanin, another protein which acts as an electron carrier. Photosystem I is another complex molecule where photons absorbed by a pair of chlorophylls known as P700 reduce ferredoxin using the electron carrier plastocynanin. Ferredoxin-NADP+ reductase is an enzyme which takes an electron from two ferredoxin molecules to synthesise NADPH which is later used in the Calvin-Benson cycle5.   The protons produced from the splitting of water are released into the lumen. This increase in concentration of protons in the lumen creates a proton gradient from the lumen to the lesser concentrated stroma. ATP synthase is integrated into the thylakoid membrane, the membrane that separates the lumen and stroma. The passage of protons through ATP synthase is used as an energy source for the generation of ATP from ADP and inorganic phosphate. ATP or adenosine triphosphate is used as a short-term store of energy due to the relatively large amount of energy released when breaking the bond between the second and third phosphate group on ATP5.   The ATP created in this process, in addition to the NADPH synthesised from the reactions in PS I are both involved in the Calvin-Benson cycle (figure 2). In this chain of reactions, ribulose 5-phosphate molecules, a five-carbon sugar, is phosphorylated by an ATP molecule, catalysed by the enzyme phosphoribulose kinase. The reaction forms ADP and ribulose 1,5-biphosphate. This newly generated molecule is then converted by the enzyme rubisco into an unstable six carbon sugar in a process known as carbon fixation, this larger molecule is then broken down into two 3-phosphoglycerate molecules, more commonly known as GP. ATP produced earlier are again useful in the phosphorylation of GP into 1,3-biophosphoglycerate.   NADPH formed from NADP+ and a protonfrom the splitting of water is used in a reaction catalysed the enzyme glyceraldehyde 3-phosphate dehydrogenase where 1,3-biophosphoglycerate is reduced by NADPH producing the very useful glyceraldehyde 3-phosphate, known as GAP, with the remaining products NADP+, ADP and Pi going on to be used again in the earlier stages of photosynthesis. GAP is divided after production, with fivesixths of the produced molecules being recycled and eventually regenerating ribulose 5-phosphate which undergoes this cycle again. One sixth of the GAP produced does however go on to be used by the organism in a variety of ways; one such example is its immediate use as a food nutrient5,x.   Some consider the production of GAP to be the final useful product of photosynthesis, however most know it as thesugar glucose. Glucose could be considered more useful to the plant because it can be used to synthesise sucrose and starch, both long chain polymers of glucose. Sucrose is produced in the cytoplasm of the cell and can be transported around the organism to be broken down to provide energy or alternatively, be stored by the organism. This is an advantage over starch as starch is only produced in the chloroplasts of a cell, the cell membrane of this organelle is not permeable to starch and therefore is only stored in the stroma of chloroplastsxi.   In addition to the specific role in photosynthesis, water affects photosynthesis in much more obscure ways, such as the structure of cell which depends heavily on water. If a cell has too much water inside it will swell, some cells may undergo a process known as cytolysis where the cell ruptures, however the strong cell wall in plants prevent this from happening. The reverse can happen to a cell in hypertonic solutions,where water leaves the plant cell. This is much more dangerous for a plant as osmotic pressure of water is an extremely important factor of how a cell maintains its turgidity, this keeps the plant upright and able to stay in direct sunlightxii.   Water plays a vital role in photosynthesis, it is essential to photosystem II as it provides the necessary electrons for progression of photosynthesis. This allows the organism to produce glucose, which is vital for the growth and maintenance of the organism. Photosynthesis is also the reason earth can sustain aerobic life; the oxygen and food produced is vital. In addition to supporting life photosynthesis helps to maintain the global CO2 concentration which if left unchecked could cause drastic shifts in the earths temperatures

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