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Summary Biology A Level Essay: Shapes
Extended and high scoring essay example for AQA A Level Biology on the question 'The Importance of Shapes in Biology'.
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The importance of shapes fitting together in cells and moleculesThroughout the natural world and all of its various ecosystems and unique forms of life, specific
shapes hold an extremely important role in all aspects of cellular biology, whether in the form of
biological catalysis as a result of enzymes or in the acquisition and destruction of pathogens in
the body; the shape and specific structure of molecules are crucial to life in any situation.
Perhaps the most obvious example of shapes fitting together in biology comes in the form
of enzymic action. In this process, enzymes provide an alternative lower energy pathway for
reactions to follow, thus allowing for specific chemical reactions to take place at significantly
lower temperatures, for example, the digestion of proteins in the stomach is catalysed by the
protease pepsin, an enzyme that allows humans to break down proteins into amino acids at our
body temperature instead of at a considerably higher temperature at which we could not survive.
The shape of an enzyme or its tertiary protein structure, allows it to form enzyme substrate
complexes with specific molecules which have a complementary structure to that of the active
site of the enzyme. The active site of an enzyme can also adapt its shape to better complement
that of the substrate, a process known as, “induced fit”, which results in the enzyme-substrate
complex changing shape. The process of enzymic catalysis can be interrupted by the introduction
of substrate molecules with a similar tertiary structure to that of the intended substrate, however
these molecules cannot be broken down by the enzyme and instead block the enzyme’s active
site, thus preventing further molecules from undergoing catalysis and limiting the rate of reaction.
Another method of interrupting this process is known as non-competitive inhibition, where an
inhibitor molecule bind to the enzyme on a site other than its active site, known as an allosteric
site, and changes the shape of the enzyme thus preventing the reaction from taking place. The
allosteric site of the enzyme can also be involved in its own form of inhibition where a molecule
binds to this site and blocks the active site of the enzyme, thus preventing any substrate
molecules from binding to the enzyme and being broken down.
In this way, enzymes are perhaps the most prevalent example of shapes fitting together in
biology and therefore are heavily involved in the majority of biological processes regardless of the
organism in question. A particularly intriguing example of enzymic activity is the process whereby
cells who have reached their Hayflick limit (the number of times individual human cells can divide
before they become senescent) self-terminate. At this point in the cell’s life, inactive molecules
known as zymogens are converted into apoptotic caspases (hydrolytic endonucleases) which
cause a cascade of signalling events resulting the controlled demolition of crucial cell
components such as mitochondria and the endoplasmic reticulum. However, in cases where a
mutation in the P53 gene has altered the tertiary structure of the caspase and therefore changed
its shape, the enzyme is unable to begin the process of apoptosis and the cell therefore will
continue to divide. This uncontrolled cell division, or hyperplasia, can result in the cell transitioning
into a cancerous state and thus the formation of a tumour.
Another pertinent example of how shape affects biological processes appears in the
digestive system, where enzymes break down digestive substrates into molecules that can be
assimilated into the blood. The enzyme maltase, for example, breaks down maltose into glucose
which is the primary respiratory substrate for both aerobic and anaerobic respiration in humans. A
similar enzyme to maltase, lactase, is involved in breaking down the disaccharide lactose into the
monosaccharides glucose and galactose. However, if a mutation causes a change in the tertiary
structure of lactose, the body is unable to break it down and therefore suffers with lactose
intolerance.
The process of shapes fitting together is also integral to another key biological process;
the immune response. In this process, the specific leukocytes (white blood cells) seek out and
destroy both any foreign material that has entered the body or any pathogenic organisms that
have gained entry. The process begins when cells known as phagocytes (literally, “cell eaters”)
encounter a pathogen or its antigen, whether in the form of a membrane bound antigen or a free-
floating toxin. These cells have receptors on their cell surface membrane which are
complementary to the 3D structure of the antigen and on the formation of an antigen-receptor
complex the process of phagocytosis is intimated. Once the process has taken place and the
pathogen has been destroyed by lysozymes through the process of hydrolysis, the phagocyte
then presents the antigens of the now terminated pathogen on its surface. The phagocyte is now
known as an antigen-presenting cell and is prepared to kick start the main immune response
which involves T helper cells. T helper cells have complementary receptors on their cell
membrane that bind to the pathogenic antigens on the antigen-presenting cell. Once the T helper
cell is bound to the antigen-presenting cell it releases chemical messengers known as cytokines
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