Membranes and its Functions essay

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The membranes of different types of cells are involved in many different functions
(25 marks)
Membranes have many different functions, with the most prominent being in eukaryotic cells,
due to the fact that they have membrane bound organelles, however, prokaryotes also have
membranes with specific functions, which I will come onto later on, all of which are partially
permeable.
Membranes are phospholipid bilayers, whereby they are similar to triglycerides due to having
3 fatty acids, however, the bilayer replaces on of the fatty acids with a phosphate group,
which is polar, thus when placed in water it becomes hydrophilic, moving away from the
water, thus forming a bilayer. This is known as a micelle. Across the membrane there are
intrinsic and extrinsic proteins. Extrinsic proteins can join with carbohydrates to form
glycocalyx, which allows for cell communication, as well as for recognition of own cells in
order to prevent an autoimmune response. Intrinsic proteins include carrier proteins and
channel proteins, which allow for specific substances to enter and exit the cell. Substances
that aren't small enough to diffuse across the bilayer, or aren't lipid soluble like hormones,
can't pass the membrane unless they go through channel/carrier proteins via facilitated
diffusion. This is the case with the passing of Na+ ions out of the neurone axon. Neurones
also have a sodiumpotassium pump embedded into the neurone membrane, which allows
for the neurone's resting potential to be maintained. A similar pump that is on the cristae of
the mitochondria, called the ATP synthase pump, which allows for the production of ATP
during respiration. Hydrogen ions from the electron transport chain are diffused into the
intermembrane space between the cristae and outer membrane of the mitochondria, creating
a concentration gradient, so H+ ions rush through the pump, joining ADP and Phosphate. To
use the ATP, another pump (ATPase), ATP binds to the carrier and dephosphorylates,
releasing energy, Cristae is a folded membrane, as are the thylakoid membranes in
chloroplasts, which increase the surface area, so respiration (cristae electron transfer chain)
and photosynthesis (thylakoids) can occur quickly and efficiently. Bacteria also have this
increased membrane surface are for respiration: mesosomes.
Membranes are key for reflex actions, as pressure sensors, Pacinian corpuscle, rely on
pressure to distort its membrane, which stimulates the naked nerve ending, which allows for
Ca2+ ions to be released in order to stimulate an action potential. The postsynaptic neurone
has receptor proteins on its membrane which allows for the neurotransmitter complimentary
fit for the release of Na+ ions. Other membrane receptors include those complimentary to
bacteria and viruses (lymphocytes), which therefore allows for an immune response.
Membranes can also have enzymes embedded into them, such as the epithelial cells in the
gut, as they have maltase, which allows for final carbohydrate absorption. Talking of gut
epithelial cells, they also have folded membranes, forming microvilli, which just like cristae,
increases the surface areas for digestion.
The main purpose of membranes is to separate different areas, which allows for different
biological processes to take place, eg: if there was no membrane around lysosomes, the
hydrolytic enzymes would break down the cell. As said previously, if the molecule isn't very
small or lipid soluble, it can't diffuse into the cell, a prime example being the breakdown of

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However, and exception to this movement across membranes is water, as it can pass the
membranes easily via osmosis. Another purpose of membranes, especially in plants, is for
maintain structure, as the cell wall for instance, which is made from cellulose, prevents the
membrane from lysis, as too much water could move into the cell via osmosis down the
water potential gradient, therefore protecting the cell.
Finally, as said previously, some organelles in eukaryotes are membrane bound, and this
includes the nucleus.…read more

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