An example essay I wrote in a mock exam last year on the importance of ions in biology, this essay shows clearly the structure and level of complexity required to achieve a high mark in the paper 3 essays.
Example essay: Explain the importance of ions in biology
Ions play an important role in allowing organisms to absorb nutrients from food. When glucose is digested it needs to be
absorbed through the ileum. Firstly, sodium ions are pumped out of the ileum epithelial cells through the
sodium/potassium pump and potassium ions are pumped in. This causes a low concentration of sodium ions in the
epithelial cells. So sodium diffuses into the cell through a membrane protein, sodium brings glucose into the cell with it.
There is then a high concentration of glucose in the cell, so this diffuses out through another membrane protein into the
bloodstream where the concentration of glucose is lower.
The role of ions is important in this instance as, without the movement of the sodium ions, glucose couldn’t be absorbed
into the bloodstream. Glucose is needed to produce ATP through respiration, if glucose couldn’t be absorbed ATP
wouldn’t be available to release energy for movement.
Furthermore, calcium ions and sodium ions are important in the production of nerve impulses across a synapse. When an
impulse reaches a synaptic knob, it causes calcium ion channels to open causing calcium ions to diffuse into a synaptic
knob. Calcium ions then bind to synaptic vesicles causing them to fuse with the presynaptic membrane and release a
neurotransmitter such as acetylcholine. In this case, acetylcholine binds to a receptor on the postsynaptic membrane and
causes sodium ion channels to open and ions to diffuse into the neurone. This depolarises the postsynaptic membrane, if
the potential difference produced reaches the threshold value then an action potential is fired.
Ions are important in this instance, as they allow synapses to have unidirectionality this is important as it means nerve
impulses only travel in one direction. If this were not to happen then multiple neurones could fire all at once. This would
mean that organisms can’t control their movements so would be more at risk of being injured or killed by predators.
Also, phosphate ions serve various important roles in cells. Firstly, phosphate ions are constituent parts of the sugar-
phosphate backbones of DNA, this backbone is important as it gives DNA it’s double helix shape, this shape makes DNA
a strong and stable molecule and also allows it to be stored compactly. If this backbone was changed, for example, it
didn’t contain phosphate, DNA may not be as a compact a molecule so less of it could be stored in cells so large
mammals like humans couldn’t exist.
Furthermore, phosphate ions form the hydrophilic heads of phospholipids in the phospholipid bilayer that makes up the
cell membrane. The charge of these ions makes them hydrophilic and this means the hydrophobic tails of phospholipids
face inwards. This makes the cell membrane-impermeable to many charged or large molecules. This prevents substances
from entering the cells that may affect cell function. If the cell membrane didn’t consist of hydrophobic and hydrophilic
many molecules could enter the cell affecting its water potential, which could cause osmotic lysis to occur meaning
organisms cells would die and the body couldn’t perform any functions.
Ions also play a key role in the nitrogen cycle. Nitrogen in the air is fixed by nitrogen-fixing bacteria in legumes. When
these plants die, or the organisms that eat them die, the nitrogen in their amino acids and nucleic acids are broken down
by saprobionts (a type of bacteria) into ammonium ions. These ions are then converted into nitrite and nitrate ions by
nitrifying bacteria and be absorbed by plants, The plants then convert the nitrates into amino acids and nucleic acids or
other nitrogen-containing biological molecules.
The role of the ions in the nitrogen cycle is important as it allows for nitrogen to be passed along the food chains from
decomposers to producers. Without the production of ammonium ions, nitrite ions and nitrate ions, nitrogen would be
trapped in dead organisms meaning plants and animals would run out of the nitrogen they need to produce amino acids
and nucleic acids essential in growth.
Finally, ions are important in the production of ATP in respiration NAD and FAD are co-enzymes that are reduced in
glycolysis, the link reaction and the Kreb’s cycle. These reduced co-enzymes then lose their hydrogen atoms to the
electron transport chain where the hydrogen is split into an electron and a hydrogen ion. Hydrogen ions are the then
pumped across the inner mitochondrial membrane, this creates an electrochemical gradient across the membrane. The
hydrogen ions then diffuse through the ATP synthase complex in the membrane. The movement of hydrogen through the
complex releases energy which is used to phosphorylate ADP into ATP. If not for the movement of hydrogen ions ATP
could not be produced so no energy would be available for movement or chemical reaction in organisms.
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