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TOP BAND/A* AQA A Level Biology Synoptic Essay - The importance of shapes fitting together in cells and organisms
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The importance of shapes fitting together in cells and organismsIn order for enzymes to catalyse specific biochemical reactions by lowering their activation
energy, they must have specific 3D shapes. Enzymes are globular proteins. Their specific amino
acid sequence in their primary structure results in the folding of the polypeptide chain into an
a-helix or b-pleated sheet according to the hydrogen bonds that form in the protein's secondary
structure. This ultimately determines the protein's tertiary structure due to the specific positions
of the hydrogen bonds, ionic bonds, and disulfide bridges that allow further folding of the
polypeptide into its 3D shape. An enzyme’s tertiary structure is essential as it allows the
molecule to have a specific active site shape. Only a substrate that is complementary to this
active site can bind to it by induced fit, forming an enzyme-substrate complex. If there was an
alteration in this specific 3D shape (such as due to a mutation causing an alteration in the
amino acid sequence). The active site shape will ultimately change to no longer be
complementary to the substrate. The specific shape of the enzyme DNA polymerase enables it
to synthesise DNA strands in the 5’ to 3’ direction, which no other enzyme or protein can do due
to their differences in structure.
When an antigen enters the blood plasma, a specific B cell will take up the antigen by
endocytosis. Once this antigen is processed and presented on the B-cell’s cell surface
membrane, a specific T helper cell with a complementary receptor to this processed antigen will
bind to it. This attachment between receptor and antigen is important as it enables the now
activated B-cell to divide rapidly by mitosis. This is known as clonal expansion, where large
quantities of a clone of B-cells are produced. This clone will develop into plasma cells and
memory cells to bring about a sufficient immune response. The plasma cells produced all
secrete one specific antibody. This antibody will have a specific tertiary structure and, therefore,
a specific variable region that enables it to bind to its complementary antigen. It is important
that the antigen can bind to the antibody so it can be destroyed by agglutination before it
damages healthy cells and tissue.
In a cholinergic synapse, the arrival of an action potential at the presynaptic neuron causes
calcium ion (Ca2+) channels to open and enable an influx of Ca2+ into the presynaptic knob.
This results in the neurotransmitter acetylcholine being released into the synaptic cleft by
vesicles. It is essential that acetylcholine has a specific shape, as upon its diffusion down a
concentration gradient across the synaptic cleft, it must bind to its complementary
neuroreceptors on the postsynaptic neuron membrane. This binding results in the opening of
sodium ion (Na+) voltage-gated channels and an influx of Na+ into the postsynaptic neuron. It is
important that both the receptor and acetylcholine are complementary in shape, as without their
binding, the voltage-gated Na+ channels cannot open to enable depolarisation of the
postsynaptic neuron, and the action potential cannot go past the presynaptic neuron. Moreover,
the specific shape of acetylcholine enables it to bind to the active site of the enzyme
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