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TOP BAND/A* AQA A Level Biology Synoptic Essay - The importance of diffusion in living organisms
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The importance of diffusion in living organismsDuring synaptic transmission, the arrival of an action potential at the presynaptic neuron causes
calcium ion (Ca2+) channels to open, leading to an influx of Ca2+ at the presynaptic knob.
Vesicles containing neurotransmitters (such as acetylcholine at the cholinergic synapse) move
to and fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic
cleft. Acetylcholine must now diffuse down a concentration gradient in order to bind to its
complementary neuroreceptors on the postsynaptic neuron. In doing so, voltage-gated sodium
ion (Na+) channels, which are otherwise closed, can now open. The diffusion of Na+ into the
postsynaptic neuron through its channel protein by facilitated diffusion allows depolarisation to
occur. If the threshold value in membrane potential is reached, an action potential will occur at
the postsynaptic neuron. Therefore, the diffusion of acetylcholine across the synapse and Na+
into the postsynaptic neuron is crucial in order to allow the transmission of electrical impulses
in the nervous system. Without this diffusion, the electrical impulse cannot go past the
presynaptic neuron, and as a result, the nervous system will not be able to carry out its function
of effectively responding to environmental stimuli. The process of diffusion that occurs in the
synapse is important as it enables the survival of living organisms.
In the light-dependent reaction of photosynthesis, chemiosmosis relies on diffusion in order to
synthesise ATP and NADPH. Photons of light that strike chlorophyll molecules (in the thylakoid
membrane of the chloroplast) cause a pair of electrons to become excited to a higher energy
level and leave the chlorophyll molecule. These electrons move along the electron transfer chain
in a series of REDOX reactions, releasing energy. This energy is used to pump protons from the
stroma into the thylakoid space. As the photolysis of water in the thylakoid space also produces
protons (along with electrons and oxygen), there will be a higher concentration of protons in the
thylakoid space than in the stroma. The thylakoid membrane is impermeable to protons;
therefore, protons can only pass through the ATP synthase channel, down their electrochemical
gradient, and into the stroma by facilitated diffusion. This diffusion of protons through ATP
synthase is important because it causes the enzyme to change shape in such a way that it
catalyses the combination of ADP (adenine diphosphate) with Pi (inorganic phosphate) to
produce ATP in a condensation reaction. Protons in the stroma now combine with NADP to
produce NADPH. Facilitated diffusion is vital in chemiosmosis because the ATP and NADPH
generated can go into the stroma (through diffusion) and be used in the Calvin cycle to make
organic compounds. These organic compounds may include nucleotides, which are essential
for plant growth.
A gas exchange system such as that in fish relies on diffusion to absorb oxygen from water into
deoxygenated blood as well as remove waste carbon dioxide. In the gills, blood within the gill
lamellae and water flowing over the gills travel in opposite directions. This countercurrent flow
ensures a steep concentration gradient is maintained across all points of the gill lamellae.
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