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TOP BAND/A* AQA A Level Biology Synoptic Essay - How does the shape of a cell relate to its function
marked A* syntoptic essay example -Full response for AQA Level Biology essay in paper 3
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Cells are easy to distinguish by their shape: how does the shape of a cell relate to its function?Cells that have been specialised for a given function have characteristic shapes and features
that enable them to perform that function more efficiently.
Epithelial cells lining the ileum have a series of finger-like projections called microvilli that
increase their surface area, enabling more efficient absorption of digestion products. Embedded
within the plasma membranes of these epithelial cells are numerous carrier and channel
proteins that allow the passage of polar, non-lipid-soluble, and larger molecules through the
membrane. These molecules would be unable to otherwise diffuse freely through the
phospholipid bilayer, which only allows the passage of small, non-polar, lipid-soluble
substances. The role of epithelial cells lining the ileum is to enable the passage of molecules
such as glucose and amino acids by co-transport into the bloodstream. If these cells do not
possess a shape that maximises efficient absorption, these products cannot be absorbed and
used by cells. Glucose, for example, is absorbed into the bloodstream by epithelial cells and is a
respiratory substrate in glycolysis. Since glycolysis occurs in both aerobic and anaerobic
respiration to produce ATP, inefficient absorption of this molecule will hinder the rate of
respiration occurring in cells.
Red blood cells present in blood plasma have the role of binding to oxygen in the lungs and
transporting this oxygen to respiring tissue. Red blood cells possess a biconcave shape, which
is only enabled due to the cells not having a nucleus. This increases the surface area of the
cells, making it more likely for oxygen to bind to one of the four haemoglobin molecules present
in red blood cells. Another key feature of red blood cells is that their shape enables them to be
flexible. This means they can be flattened against capillary walls as they pass through them.
This reduces the diffusion pathway of substances such as oxygen, which are exchanged
between cells and the blood. It is important that red blood cells are able to efficiently supply
respiring cells with oxygen. Oxygen is the terminal electron acceptor in oxidative
phosphorylation; therefore, in its absence, aerobic respiration cannot occur to produce sufficient
ATP used in cellular processes.
The cells in the xylem tissue are important in supplying water and inorganic ions from the soil to
the leaves. Xylem cells have no end walls; their long shape enables the water that is drawn up
the xylem to form a continuous, unbroken column. These cells also lack organelles and
cytoplasm, both of which would otherwise impede or obstruct the flow of water up the plant.
The presence of lignin in the xylem enables the vessel to withstand the higher hydrostatic
pressure that occurs due to water. As water evaporates from the leaves, this establishes a water
potential gradient in which water is pulled up the soil, into the roots, up the xylem, and into the
leaves. This movement of water creates a negative pressure or tension inside the xylem vessel,
which the lignin must be able to withstand in order for the xylem cells to carry out their function
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