Water is key toalmost every organism existence, as humans, we need water to keep our bloodflow stable, our muscles healthy and so much more. Humans also rely on oxygenfor aerobic respiration, which in turn provides energy for everything we do.
The single biggest producer of oxygen are plants, they photosynthesise andproduce the oxygen we need to survive, a process which is dependent on waterbeing available. This discussion will detail how plants use water inphotosynthesis, how this affects later stages of photosynthesis and other moreniche aspects how plants make use of water. Given itsimportance water is a surprisingly simple molecule consisting of 2 hydrogenatoms covalently bonded to s single oxygen atom. This does however lead to someimportant characteristics, an example of this is the hydrogen bonding network.Water forms weak H-bonds, primarily electrostatic attraction of the oxygen atomto a hydrogen atom of nearby a water molecule. Many water molecules formH-bonds resulting in a relatively strong network of constantly rearrangingH-bond network. This forms an important part of the delivery of water to partsof the plant where photosynthesis occurs for instance in the xylem, thecohesion-tension theory suggests the transpiration of water out of a leafcauses a pull on water molecules further down the xylem, creating a chaineffect where the H-bond cohesion aids the transport of water up the stem of aplant.
Osmoticpotential results in net movement of water in or out of a cell, if the insideof a cell has a more negative water potential than on the outside, water willtravel through the cell wall and cell membrane into the cell via osmosis. Thisis usually due to higher concentrations of solute in the cell compared to that outsidethe cell. Once water moleculesreach chloroplasts they undergo multiple changes leading to the splitting ofthe molecules. This process of splitting water is part of Photosystem II, acomplex protein structure found embedded in the thylakoid membrane, as a partof four redox reactions. These so-called S-states describe the oxidation stateof the water oxidising complex and can be shown using the Kok cycle (see figure1), the most reduced state being S0, and increasing in oxidationstate until S4.
At stage four a diatomic oxygen molecule is createdi.The process begins with photons exciting a pair of chlorophyll named P680, thisis due to absorption of light with a wavelength of 680nm, the chlorophyll isexcited to P680+ this species is a strong oxidising agent, strongenough to oxidise water. P680+ is oxidised by Manganese ions in thewater oxidising complex, Manganese has a variable oxidation state of 1 to 5enabling this cycle to occur. This oxidation repeats four times until the wateroxidising complex has a positive enough charge to split the water molecule. An Mn4Cacluster is the important component contained within the water oxidisingcomplex, this has been attempted to be analysed at resolutions high enough todetermine the exact structure of the water oxidising complex. X-ray spectroscopywas used but it is suggested that the X-rays may cause damage to the metalsites resulting in an unclear idea of its structureii.
The X-ray crystal structures nevertheless does show that the water oxidisingcomplex is made up of several clusters involving Mn and Ca, at its core is CaMn3O4with various arrays around it. Oxygen bridges the Mn and Ca atoms, with3 Oxygen atoms bonded to each. The hydrogenbonding phenomenon in water also has its use in PS II where it is thought thathydrogen bonding networks can provide proton transfer pathways for the deliveryof protons.
Water has been observed forming a hydrogen bonding network aroundthe water oxidising complex acting as a catalyst, leading to numerous exitpaths for protons. The purpose ofphotosystem II is to split two water molecules into four protons, fourelectrons and molecular oxygen. This has numerous results, with the electronsreplacing those used by the water oxidising complex and the protons being releasedinto the lumen to create a proton gradient across the membrane. The electronsreduce plastoquinone, an electron acceptor in PS II, via another electronacceptor, pheophytin. Plastoquinone requires two electrons along with twoprotons to be oxidised, therefore the splitting of one water molecule has thepotential to oxidise two plastoquinone molecules, producing 2 plastoquinolmolecules. Plastoquinol islater oxidised to reform plastoquinone by another protein, this protein reducesplastocynanin, another protein which acts as an electron carrier. Photosystem Iis another complex molecule where photons absorbed by a pair of chlorophyllsknown as P700 reduce ferredoxin using the electron carrier plastocynanin.Ferredoxin-NADP+ reductase is an enzyme which takes an electron fromtwo ferredoxin molecules to synthesise NADPH which is later used in the Calvin-Bensoncycle.
The protonsproduced from the splitting of water are released into the lumen. This increasein concentration of protons in the lumen creates a proton gradient from thelumen to the lesser concentrated stroma. ATP synthase is integrated into thethylakoid membrane, the membrane that separates the lumen and stroma. Thepassage of protons through ATP synthase is used as an energy source for thegeneration of ATP from ADP and inorganic phosphate. ATP or adenosinetriphosphate is used as a short-term store of energy due to the relativelylarge amount of energy released when breaking the bond between the second andthird phosphate group on ATP. The ATP createdin this process, in addition to the NADPH synthesised from the reactions in PSI are both involved in the Calvin-Benson cycle (figure 2). In this chain ofreactions, ribulose 5-phosphate molecules, a five-carbon sugar, isphosphorylated by an ATP molecule, catalysed by the enzyme phosphoribulosekinase.
The reaction forms ADP and ribulose 1,5-biphosphate. This newlygenerated molecule is then converted by the enzyme rubisco into an unstable sixcarbon sugar in a process known as carbon fixation, this larger molecule isthen broken down into two 3-phosphoglycerate molecules, more commonly known asGP. ATP produced earlier are again useful in the phosphorylation of GP into1,3-biophosphoglycerate. NADPH formedfrom NADP+ and a proton from the splitting of water is used in areaction catalysed the enzyme glyceraldehyde 3-phosphate dehydrogenase where1,3-biophosphoglycerate is reduced by NADPH producing the very usefulglyceraldehyde 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. GAPis divided after production, with five sixths of the produced molecules beingrecycled and eventually regenerating ribulose 5-phosphate which undergoes thiscycle again.
One sixth of the GAP produced does however go on to be used by theorganism in a variety of ways; one such example is its immediate use as a foodnutrient. Some considerthe production of GAP to be the final useful product of photosynthesis, howevermost know it as the sugar glucose. Glucose could be considered more useful tothe plant because it can be used to synthesise sucrose and starch, both longchain polymers of glucose. Sucrose is produced in the cytoplasm of the cell andcan be transported around the organism to be broken down to provide energy oralternatively, be stored by the organism. This is an advantage over starch asstarch is only produced in the chloroplasts of a cell, the cell membrane ofthis organelle is not permeable to starch and therefore is only stored in thestroma of chloroplasts.
In addition tothe specific role in photosynthesis, water affects photosynthesis in much moreobscure ways, such as the structure of cell which depends heavily on water. Ifa cell has too much water inside it will swell, some cells may undergo aprocess known as cytolysis where the cell ruptures, however the strong cellwall in plants prevent this from happening. The reverse can happen to a cell inhypertonic solutions, where water leaves the plant cell. This is much moredangerous for a plant as osmotic pressure of water is an extremely important factorof how a cell maintains its turgidity, this keeps the plant upright and able tostay in direct sunlight. Water plays avital role in photosynthesis, it is essential to photosystem II as it providesthe necessary electrons for progression of photosynthesis. This allows theorganism to produce glucose, which is vital for the growth and maintenance ofthe organism.
Photosynthesis is also the reason earth can sustain aerobic life;the oxygen and food produced is vital. In addition to supporting life photosynthesishelps to maintain the global CO2 concentration which if leftunchecked could cause drastic shifts in the earths temperatures.