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.
The importance of interactions between cells and between organisms:
Synaptic transmission in the neuromuscular junction is vital to enabling the sliding filament
mechanism of muscle contraction. Although neurones don’t interact directly with one another,
their interaction is facilitated by the diffusion of acetylcholine across this cholinergic synapse.
Depolarisation of the presynaptic membrane causes voltage-gated calcium ion channels to
open, allowing calcium ions to enter the synaptic knob. Calcium ions stimulate vesicles
containing acetylcholine to move up to and fuse with the membrane, releasing the
neurotransmitter into the cholinergic synapse via exocytosis. Acetylcholine can now bind to
complementary nicotinic and muscarinic receptors located on the postsynaptic membrane,
therefore, facilitating the transmission of that electrical impulse from one nerve cell to the next.
Without this interaction, there would be no depolarisation of the sarcolemma or the muscle fibre.
Calcium ions would not diffuse into the myofibril from the sarcoplasmic reticulum and move
tropomyosin, resulting in myosin heads being unable to bind to actin and form cross bridges.
Muscle contraction is integral to allowing living organisms to respond rapidly to stimuli. But it’s
also a key component in our homeostatic response to extreme cold, whereby muscle
contraction is activated, generating heat as a by-product of that muscular activity. If not for the
interaction of the presynaptic and postsynaptic neurones, muscle contraction would not be
possible, proving detrimental to our survival.
Alongside synaptic transmission, the transport of oxygen to skeletal muscle is also crucial in
enabling muscle contraction. Haemoglobin facilitates this interaction between red blood cells
and muscle tissue. Haemoglobin is a globular protein found in red blood cells. It has specific
properties which allow it to carry out its function. Haemoglobin is an octahedral complex with a
coordination number of 6. The central transition metal ion is coordinately bonded to 4 nitrogens
from the multidentate ligand haem. Another coordinate bond is with nitrogen from the protein
globin, and the final coordinate bond is with an oxygen or water molecule. Due to this unique
bonding, it can undergo ligand substitution whereby in areas with a lower partial pressure of
oxygen, oxygen is unloaded from oxyhaemoglobin and substituted with a water ligand. This
unique bonding also explains the cooperative nature of oxygen binding and therefore, changes
to haemoglobin’s affinity for oxygen, both of which can affect the rate of oxygen exchange in this
specific interaction. This interaction is important because oxygen can be used by respiring
tissue as the terminal electron acceptor during oxidative phosphorylation. Without oxygen to
accept them, there would be no movement of electrons down the electron transport chain,
significantly impacting the rate of ATP production via chemiosmosis.
A constant supply of glucose, as well as oxygen, is also required for aerobic respiration to take
place. While we get our glucose from the food we eat, plants are capable of producing their own
carbohydrates and organic substances which are then transported on mass from the source to
the sink as outlined by the mass-flow hypothesis. However, the interaction between companion
cells and sieve elements is necessary for this transport to occur. Sieve elements are unable to
sustain independent metabolic activity without the support of companion cells because sieve
elements have no nuclei and fewer cellular organelles, an adaptation to maximise the flow rate
of the assimilates. Therefore, companion cells regulate the activity of the adjacent sieve
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