This is a practice Paper 3 AQA A Level Biology Essay. Mark achieved was an A* and this is a piece of the highest calibre.
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The importance of shapes fitting together in cells and organisms
One example that highlights the importance of how shapes fit together is in enzymes.
Enzymes are catalysts that are needed within the body to ensure that the body’s metabolism
works fast enough for life to continue, and without them, humans would be unable to survive.
Enzymes are proteins have a particular tertiary structure, built up from the amino acid
sequence. Each enzyme has a specific shape that is complimentary to the substrate on
which it acts. This ensures that, via the induced fit model, the enzyme substrate complex can
form and thus the enzyme can catalyse the reaction by lowering the activation energy. If the
enzyme substrate could not form then the reaction would not be catalysed and many of the
body’s processes would not function. This highlights the importance of shapes fitting together
in cells and how this is key to maintaining the key processes within organisms.
A further example highlighting the importance of shapes is the structure of DNA. Each
Deoxyribonucleic Acid molecular consists of three parts: a nitrogenous base (Thymine,
Guanine, Cytosine and Adenine), a deoxyribose molecule and a phosphate group. The
nitrogenous bases each have a specific shape, which means that they can complimentary
pair to each other, with A pairing with T and C pairing with G. This pairing between the
nitrogenous bases is held together by hydrogen bonds with 3 H bonds between C and G and
2 H bonds between A and T. These hydrogen bonds are very important in increasing the
stability of the DNA and thus helping to prevent mutation. Furthermore, the structure of the
phosphodiester backbone is also key to limit mutations occurring as the backbone protects
the genetic information and prevents it from being corrupted. This highlights the importance
of the shape of DNA in organisms as a whole.
An additional example highlighting the importance of shapes in organisms is antibodies.
Antibodies are made up of 4 proteins, 2 which are “heavy chains” and 2 which are “light
chains”. Antibodies have a variable region, which is specific and complimentary to certain
antigens found on pathogens. This allows the antibody to bind to the antigen, forming an
antigen-antibody complex, which is similar to the enzyme-substrate complex as described
above, and thus aids the immune system by agglutination, enabling phagocytes to act on
many antigens all at once. This is very important within organisms as it makes up a key
element of immune response, a process which is vital for the maintenance of life.
The use of shapes and how these shapes fit together is also evident in the transmission of
nerve impulses across a synapse. A synapse is a junction between a neurone and an
effector, another neurone or the central nervous system. In the presynaptic neurone are
neurotransmitters, stored within vesicles in the presynaptic knob. On the postsynaptic
neurone are a number of receptor sites that are a complimentary shape to the
neurotransmitters. The neurotransmitters are released into the synaptic cleft, whereby they
move across the cleft and bind to the receptor sites which are of a complimentary shape.
This leads to a generator potential forming in the postsynaptic neurone or the response seen
in the effector. This highlights the importance of shapes and how they fit together as this
complimentary shape is vital to the passing on of a nerve impulse across a synapse. If the
neurotransmitter and receptor sites were not of a complimentary shape and thus didn’t fit
together, then the nerve impulse would not be able to pass across a synapse and thus we
would not see the end response.
In addition, shapes are very important in how they fit together within the process of muscle
contraction. The process of muscle contraction relies on a cross bridge forming between the
bulbous head of the myosin filament and the actin filament at the actin-myosin binding site.
This means that the head of the myosin filament must have a complimentary shape to that of
the binding site found on the actin filament otherwise this cross-bridge would not form and
this would mean that there would be no movement of the actin filament, resulting in no
shortening of the sarcomere and thus there would be no muscle contraction. This highlights
the importance of shapes in biology as if the shape of the bulbous head and actin filament
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