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.
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 network1. 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.
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
Once water molecules reach
chloroplasts they undergo multiple changes leading to the splitting of the
This process of splitting water is part of Photosystem II, a complex protein
structure found embedded in the thylakoid membrane, as a part of four redox
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 created4.
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 molecule5,6.
13Figure 1: The
Kok cycle, showing the cycle of water oxidation from S0 through
S1, S2 and S3 to S4. In the centre,
a representation of the water oxidising complex with Calcium in yellow,
Oxygen in red and Manganese in purple.
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 structure7.
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 each8.
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 protons9.
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
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
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
from NADP+ and a proton from 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 five sixths 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
14Figure 2: The Calvin-Benson cycle shows
the steps of Ru5P to G3P.
the production of GAP to be the final useful product of photosynthesis, however
most know it as the sugar 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 chloroplasts11.
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 sunlight12.
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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