This is an essay based on paper 3 past paper 25 mark question on the control of movement in cells and organisms, covering both year 12 and year 13 content.
The importance of the control of movement in cells and organisms.
The importance of the control of movement in cells is evident in the transmission of impulses
between neurons. When an action potential reaches the presynaptic neuron, sodium ion
channels open, causing an influx of sodium ions into the cell, making it more positive. This
causes calcium ion channels to open so calcium ions can diffuse in. The movement of calcium
ions into the presynaptic neuron is important because it helps the synaptic vesicles fuse with the
presynaptic membrane so that neurotransmitter (such as acetylcholine) is released.
Acetylcholine is then able to diffuse across the synaptic cleft and bind to receptors on the
postsynaptic neuron. This movement is controlled by the establishment of a concentration
gradient that allows acetylcholine to move from a high to low concentration. Sodium ions can
enter by facilitated diffusion into the postsynaptic neuron because acetylcholine binds to the
receptors on the membrane which causes sodium ion channels to open. This results in the
depolarisation of the postsynaptic neuron.
This control of movement is important, especially at neuromuscular junctions where the
entrance of sodium ions always causes an excitatory effect and enables muscle movement.
This process is outlined in the reflex arc which is a survival mechanism that protects our bodies
from harm. If the movement of sodium ions into the postsynaptic neuron was inhibited, sensory
neurons would not be able to pass on signals to relay neurons in the CNS which in turn would
not transfer the impulse to motor neurons that could trigger the appropriate response (such as
retracting your finger from a burning flame) in an effector (muscle or gland). This would also
severely hinder the autonomic nervous system as vital functions such as maintaining a regular
heartbeat would be affected. If neurotransmitters did not diffuse across the synapse and bind to
complementary receptors allowing Na+ ions to enter the postsynaptic neuron and cause
depolarisation, the SAN would not be able to cause the atria to contract or send a wave of
depolarisation that reaches the AVN causing the ventricles to contract. This would impede
oxygenated blood from reaching respiring tissues so that cell function would come to a halt.
Selective reabsorption in the kidneys relies on the control of movement between and within
cells. The active transport of Na+ ions into the capillaries establishes a concentration gradient
between the lumen of the proximal convoluted tubule and the epithelial cell. This allows Na+
ions to enter the epithelial cell by facilitated diffusion alongside other molecules such as glucose
and amino acids using co-transport. This movement into the epithelial cell lowers the water
potential so that water can move in by osmosis. These molecules can then diffuse into the blood
so that they’re reabsorbed rather than excreted in urine.
This process is vital because it ensures that the blood retains necessary biological molecules
such as glucose which otherwise cannot cross the tubule membrane. This glucose can then be
phosphorylated during glycolysis to form fructose bisphosphate which separates into two triose
phosphate molecules and is each phosphorylated again to produce 2 molecules of pyruvate.
Without the production of pyruvate, neither anaerobic respiration nor aerobic respiration could
occur. In aerobic respiration, pyruvate is necessary for the link reaction as it is oxidised and
decarboxylated to form acetyl which reacts with coenzyme A. This molecule is necessary
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