The cell membrane has a fluid mosaic structure, comprising a phospholipid bilayer with embedded proteins. The phospholipid tails face each other at the center of the bilayer to be protected from the watery environments inside and outside the cell, while the phosphate heads face outwards. Transmembrane and peripheral proteins embedded in the bilayer form channels to regulate what passes in and out of the cell. Cholesterol maintains the fluidity and stability of the membrane. The membrane acts as a selectively permeable barrier and facilitates cell structure, communication, recognition, mobility, and chemical reactions.

1. Cell membranes!
Remember that a membrane is not just a lipid bilayer, but
comprises the lipid, protein and carbohydrate parts.
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2. Cell membrane
Both the cell surface membrane and the
membranes surrounding certain organelles have
the same basic structure. Much of the
membrane is made up of a ‘sea’ of
phospholipids with protein molecules ‘floating’ in
between the phospholipids. Some of these proteins
span the whole width of the membrane.
Because the membrane is fluid, and because of
the mosaic arrangement of the protein
molecules, the structure of the membrane is
called the fluid mosaic model.

3. Fluid mosaic model of Cell
membrane

4. The phospholipids are arranged in two layers a
bilayer. The phosphate heads are polar
molecules and so are water-soluble. The lipid
tails are non-polar and therefore are not water-
soluble.
This means that the phospholipids are arranged
with the heads in contact with the cytoplasm or
extra-cellular fluid, both of which are watery
environments. The tails are protected from this,
by being as far from the cytoplasm and extra-
cellular fluid as possible.

5. The proteins in the membrane, line pores in the lipid
bilayer. The polar groups of the protein molecules mean
that substances that would not be able to penetrate the
lipid bilayer, (because they are insoluble in lipid), can still
move from one side of the membrane to the other.
There are also short polysaccharide chains that are
attached to the outer surface of the membrane. Most of
these carbohydrates are attached to proteins and are
called 'Glycoproteins'. They may help in the recognition
of, and interaction with, other cells. They may also play a
part in the recognition of hormones and foreign
molecules.

6. Cholesterol is also present in the
membrane. It maintains the fluidity and
increases the stability of the membrane.
Without cholesterol the membrane
would easily split apart.

7. Functions of a cell membrane
• Functions of a membrane it's:
• 1.Selectively permeable barrier.
• 2.Structural, keeping the cell contents together.
• 3.Allows communication with other cells.
• 4.Allows recognition of other external
substances.
• 5.Allows mobility in some organisms, e.g.
amoeba.
• 6.The site of various chemical reactions.