Unit 6 - Organisms respond to changes in their internal and external environments (A100)
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25/25 A Level biology essay: the importance of complementary shapes of molecules in organisms
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Unit 6 - Organisms respond to changes in their internal and external environments (A100)
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AQA
25/25 A Level biology essay written by a student who achieved A* in Biology at A Level. This essay is a great example of how to integrate beyond the specification content to get into the highest marking bracket.
Unit 6 - Organisms respond to changes in their internal and external environments (A100)
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The importance of complementary shapes of molecules in organisms.
Complementary shapes fitting together in organisms is the key mechanism by which a wide
range of different metabolic reactions and processes take place. In this essay, I will explore the
roles of enzymes such as ATP hydrolase and rubisco as well as the importance of
complementarity in muscle contraction, blood glucose concentration and translocation of sugars
in plants.
This idea of complementarity is very important in synaptic transmission and I will be looking at
the neuromuscular junction in particular. At the neuromuscular junction when the
neurotransmitter acetylcholine is released into the synaptic cleft from the pre-synaptic
membrane, it diffuses across the neuromuscular junction and binds to complementary receptors
on the post-synaptic membrane. The binding of acetylcholine to complementary receptors is
what enables sodium ion channels in the post-synaptic neurone to open. If enough sodium ions
enter the post-synaptic neurone and the -55mV threshold is met, an action potential can be
generated and the sarcolemma is depolarised. This depolarisation spreads down the t-tubules
to the sarcoplasmic reticulum, causing it to release calcium ions into the sarcoplasm.
Acetylcholine binding to those complementary receptors is very important because ultimately
without it, calcium ions wouldn’t be released into the sarcoplasm, tropomyosin would not be
moved and the myosin heads would not be able to bind to the binding sites on actin to form
actin-myosin cross bridges. As a result, the myosin heads would not be able to pull the actin
filament. Muscle contraction is extremely vital in increasing our chances of survival because it
allows us to respond quickly to stimuli. Therefore, it’s evident how important complementarity is
in allowing the sliding filament mechanism and therefore muscle contraction to occur.
Alongside acetylcholine being able to bind to complementary receptors on the post-synaptic
membrane, ATP hydrolase is also essential in facilitating the sliding filament mechanism. ATP
hydrolase is a crucial enzyme and not only important in muscle contraction but in multiple other
reactions as well. It would not be able to catalyse these other reactions without some key
features it has, one being its specific shape as an enzyme. Enzymes’ active sites are specific
and unique in shape because of the specific folding and bonding in their tertiary structure. This
allows particular substrates to bind to the active site to form an enzyme-substrate complex,
causing the active site to change shape slightly and therefore make the enzyme complementary
to the substrate, as explained by the induced fit model. ATP hydrolase hydrolyses ATP to ADP +
Pi using this mechanism. This hydrolysis reaction is extremely important because it releases
energy which is important in the active transport of sodium ions from the epithelial cells lining
the ileum into the bloodstream for example. Active transport is not a passive process and
therefore, requires the energy released from ATP hydrolysis. Once the sodium ions are actively
transported out, this creates a concentration gradient where the concentration of sodium ions in
the ileum is lower compared to in the blood. Sodium ions can then enter the epithelial cells from
the lumen by diffusion, along with glucose via co-transporter proteins. The co-transport of
glucose into the epithelial cells is essential for the absorption of glucose into the blood. Glucose
can now be transported around the blood, delivered to respiring tissue and used in glycolysis of
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