AQA A-LEVEL BIOLOGY ESSAY: THE IMPORTANCE OF WATER AND THE REGULATION OF WATER CONTENT IN ORGANISMS
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Course
AQA BIOLOGY (7402)
Institution
UPPINGHAM SCHOOL
Top mark AQA A-Level biology essay of the importance of water and the regulation of water content in organisms.
includes the properties of water, photosynthesis, transport of water, regulation of water content, tissue, hydrolysis and condensation.
Useful for revision for this specific topic ...
TIO WATER AND THE REGULATION OF WATER CONTENT IN ORGANISMS
Water is important as it is a metabolite for photosynthesis. Photosynthesis is an important
process in plants as it is where plants convert light energy into sugars such as glucose which
can used to make cellulose. Cellulose makes up plant cell walls and ensures the plant is
stable and rigid so it can grow towards sunlight. Water is involved in the light-dependent
reaction of photosynthesis in the photolysis of water where a water molecule is split into
hydrogen ions, oxygen, and electrons. In the light-dependent reaction, light energy is
absorbed by chlorophyll in the photosystems. The light energy excites the electrons in the
chlorophyll, giving them more energy, which eventually causes them to be released from the
chlorophyll. This process is called photoionization. The electrons from the photolysis of water
replace lost electrons from chlorophyll. These electrons and hydrogen ions are used in the
electron transport chain resulting in NADP being reduced to NADPH and the hydrogen ions
are also involved in the electrochemical gradient across the thylakoid membrane so that H+
ions travel through ATP synthase and produce ATP. Water is important as without this, there
would be no replacement of electrons lost by chlorophyll so there would be no production of
ATP and NADPH for the light-independent reaction no conversion of glycerate-3-phosphate
to triose phosphate so sugars such as glucose cannot be produced so plants would not be
able to grow.
The properties of water allow it to carry out important functions such as mass transport in
plants. Water travels all the way up a plant through the xylem. Water leaves the leaves
through transpiration. This decreases the pressure and water potential in leaves so water will
move up the plant in a continuous column. Water can do this due to cohesion and adhesion
properties. The H2O molecule is polar and can form hydrogen bonds with itself as cohesion
and it can also adhere to the xylem wall. This makes water important as its properties allow
the transpiration stream to occur so water can be transported from the roots to the leaves
where it is used for photosynthesis in the light-dependent reaction. Water is also an
important component in transport within the phloem. This is where dissolved solutes are
transported from their source (photosynthesising leaf cells to the sink cells that require
glucose for respiration. Water is important in this process as it is a good solvent so many
substances are easily dissolved in it.
The regulation of water content in organisms such as humans is called osmoregulation. It is
important that the body cells have the correct water content as too much water in a cell
causes cell membranes to burst which is cell lysis, therefore killing the cell so it is unable to
carry out its required processes. Too little water in a cell would cause it to shrivel and
become non-functional. Blood water potential is controlled by Antidiuretic hormone (ADH).
Osmoreceptors in the hypothalamus detect changes as they shrink or shrivel as water
moves in or out of the osmoreceptors down a water potential gradient via osmosis. If blood
water potential is too low, water will leave by osmosis. Nerve impulses are sent along
sensory neurons to the posterior pituitary gland. These nerve impulses stimulate the release
of Antidiuretic hormone (ADH). ADH attaches to specific tertiary-shaped receptors on the
kidneys which are the target cells. That results in more aquaporins being embedded into the
walls of the collecting duct and DCT so more water is reabsorbed from the nephron back into
the blood. This brings blood water potential back to a normal level. It is important that blood
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