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Biology 25-mark essay: the properties of water and its importance to living organisms
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This is an example 25-marker A level biology essay on the properties of water and its importance to living organisms.
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Water is a dipolar molecule and this property ensures that water molecules stick together due tothe attraction of their positive and negative poles. This attraction forms hydrogen bonds and this
feature is incredibly important in mass transport within plants. The transpiration stream within a
plant is caused by the evaporation of water from mesophyll cells through the stomata into the
atmosphere. This reduces the water potential within the cells so that water moves in by
osmosis. This water movement up the stem to the leaves occurs via the xylem. The water within
forms an unbroken column that is pulled upwards due to the cohesion-tension of the hydrogen
bonds of water molecules. This places negative pressure on the plant and maintains turgidity to
remain upright.
This transpiration pull is important because it ensures that the plant stands tall so it’s more likely
to outcompete other plants in absorbing the sunlight for the light-dependent reaction of
photosynthesis. This means there is a higher production of NADPH and ATP for the reduction of
GP to TP in the Calvin cycle, allowing the formation of glucose and regeneration of RuBP for
carbon fixation. This reduces carbon dioxide levels in the atmosphere. Furthermore the water in
the xylem is necessary for translocation and the mass flow hypothesis, where water moves into
the phloem by osmosis, creating high hydrostatic pressure so that sucrose can move down the
phloem to the sink cell.
Water is also an important metabolite as it is often involved in hydrolysis reactions. For
example, in glycogenolysis, water is needed to hydrolyse the 1,4 and 1,6 glycosidic bonds
within glycogen to produce alpha glucose monomers. The highly branched nature of glycogen
increases its surface area and terminal ends so that water can easily react with the hydroxyl
group to break the bond. This is similarly necessary for proteolysis where the peptide bonds
within proteins are hydrolysed by water to produce amino acids. In triglycerides, water is used to
hydrolyse ester bonds to produce 3 fatty acids and one phosphate group. The breakdown of
these molecules allows for their reabsorption.
This process is important because without water, glycogenolysis could not occur. This means
that when the alpha cells in the islets of Langerhans within the pancreas detect low blood
glucose levels and release glucagon to bind to the liver receptors to trigger a cascade of
reactions for glucose release, no response occurs and the negative feedback mechanism is no
longer effective. This means that low blood glucose levels would not change and this reduces
respiration as glucose is necessary for both aerobic and anaerobic respiration. Glucose needs
to be phosphorylated to produce triose phosphate, which is oxidised to produce pyruvate and a
net ATP yield of 2. If this were unable to occur, less ATP would be produced and aerobic
respiration could not occur because it requires pyruvate in the link reaction. Less ATP would
prevent muscle contraction because ATP hydrolysis is necessary for the myosin head to bend,
pulling the actin along in the sarcomere.
Water is a solvent, often containing many dissolved substances within it. This is evident in
tissue fluid, which contains water as well as glucose, amino acids, ions and other molecules.
Water creates a high hydrostatic pressure in blood at the arteriole end, causing these small
molecules to be forced out the blood plasma, forming tissue fluid in a process called
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