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The Structure And Role Of Proteins In Cell Membranes

1564 words - 6 pages

The Structure and Role of Proteins in Cell Membranes

Cells are the building blocks of which all living organisms are
composed. There are lots of different types of cells that make up
living organisms but they are all similar in structure. All cells are
surrounded by a cell membrane (or plasma membrane), which controls the
movement of substances into and out of the cell. Cell membranes are
described as partially permeable. It is also structural and keeps the
cell contents together and separate from other cells. Furthermore it
allows communication between cells and allows recognition of other
external substances. Membranes that surround cell organelles are also
very similar to the cell membrane. Membranes are composed of
phospholipids, which form the bulk of the membrane, proteins, which
are scattered around in the membrane. There may also be some molecules
of carbohydrates and cholesterol present.

Phospholipids are a special type of lipid. Phospholipids have two
fatty acid chains; these form a tail, which is hydrophobic. Fatty acid
tails do not have a charge, so they are non-polar and insoluble in
water. The phosphate group (head of the phospholipids) is hydrophilic
because it has a positive charge and is soluble in water. The
phospholipids are arranged in a bilayer, with their polar, hydrophilic
phosphate heads facing outwards, and their non-polar, hydrophobic
fatty acid tails facing each other in the middle of the bilayer. The
hydrophilic heads are in contact with the cytoplasm or extra-cellular
fluid, both of which are watery environments. The hydrophobic tails
are protected from this, by being as far from the cytoplasm and
extra-cellular fluid as possible. This hydrophobic layer acts as a
barrier to all but the smallest molecules, effectively isolating the
two sides of the membrane. Different kinds of membranes can contain
phospholipids with different fatty acids, affecting the strength and
flexibility of the membrane, and animal cell membranes also contain
cholesterol linking the fatty acids together and so stabilising and
strengthening the membrane.

There are many different sorts of proteins with complex tertiary
structures in a membrane. The proteins usually span from one side of
the phospholipid bilayer to the other (intrinsic proteins), these are
usually large proteins, but they can also sit on one of the surfaces
(extrinsic proteins), these are usually smaller proteins. They also
change their position and can slide around the membrane very quickly
and collide with each other, but can never flip from one side to the
other. The proteins have hydrophilic amino acids in contact with the
water on the outside of membranes, and hydrophobic amino acids in
contact with the fatty chains inside the membrane. Proteins make up
about 50% of the mass of membranes, and are...

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