A* marked biology essay (graded 24/25 by tutor and teacher), titled ' The importance of diffusion in organisms'. 6 Paragraphs with 1640 words.
This essay was written earlier this year.
One example of why diffusion is important in organisms is due to gas exchange in
the lungs. The medulla oblongata sends a signal through the phrenic nerve to the
diaphragm. This causes the diaphragm to flatten hence the thoracic volume
increases and the lung pressure drops below the atmospheric pressure. Air flows
down the pressure gradient, (inspiration occurs) down the trachea, then
entering the bronchus, then the bronchioles and finally entering the alveoli.
The alveoli are highly adapted for gas exchange, there’s many alveoli to provide a
large surface area, they are also surrounded by a large network of capillaries.
Oxygen diffuses out of the alveoli across the alveolar epithelium and the capillary
endothelium and into the haemoglobin in the red blood cells this occurs down a
diffusion gradient, as there’s a higher concentration of oxygen in the alveoli than
the blood. Diffusion is important here as oxygen is provided to red blood cells,
which carry oxygen to other cells in the body to be used in respiration. (O2 +
C6H12O6-> H2O +CO2. ) In respiration oxygen acts as a final electron acceptor,
during respiration ATP is synthesised from ADP + Pi via ATP synthase. ATP
hydrolysis provides energy for processes to occur such as the active transport of
sucrose into sieve cells in translocation, or the replication of DNA during
interphase which allows mitosis to occur. Hence diffusion is important in
organisms as it allows oxygen to be supplied to cells in organisms which is one of
the molecules needed to drive respiration.
Another reason why diffusion is important in organisms is that it supplies cells
with glucose (which as mentioned above is the other substance needed for
respiration to occur.) and amino acids. For example cotransport of glucose (and
also amino acid) molecules occurs from the lumen of the ileum into the
bloodstream in the small intestine. Sodium ions are actively transported out of
the epithelial cells through the cell membrane into the blood by sodium
potassium pump carrier proteins, this lowers the concentration of sodium ions in
the cell creating a concentration gradient. Sodium ions then diffuse from the
lumen of the ileum into the epithelial cell through sodium glucose cotransporter
proteins situated in the epithelial cell membrane. Therefore the concentration of
glucose inside the cell increases. Glucose can then diffuse out of the cell through
the cell membrane by a glucose protein channel into the blood. A concentration
gradient is maintained by the blood capillary carrying the glucose and sodium
ions away from the cell. Glucose can be taken to cells where it forms 2x pyruvate
molecules during glycolysis in respiration, the end product of respiration is ATP,
which as mentioned above provides energy (via hydrolysis) for processes such
as active transport to occur, hence the diffusion of glucose in the small intestines
in organisms is important. Amino acids can be used in processes such as the
translation of proteins for specific functions in the body e.g. enzymes, or they can
be used to form glucose via gluconeogenesis.
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