First 304 words of the document:
The cell membrane surrounds all living cells and controls the movement in and out of substances
and is responsible for other properties of the cell too. Membranes are composed of proteins,
phospholipids and carbohydrates arranged in a fluid mosaic structure, as shown below:
The cell membrane is given the term 'fluid mosaic' as the phospholipids form a thin but flexible
sheet which is both fluid and rigid enough to perform its function. The 'mosaic' comes from the
appearance of the proteins and carbohydrates from a bird's eye view.
The phospholipids are arranged in a 'bilayer' with the hydrophobic fatty acid tails facing inward and
the hydrophilic phosphate heads facing outward; the meeting of tails is an effective barrier against
all but the smallest molecules.
The proteins can be integral (span from one side of layer to other) or peripheral (sit on one of the
surfaces.) These can slide along the membrane easily but do not collide.
Parts of Membrane
· Proteins that span the membrane are usually involved in the transportation of substances,
these are called protein channels
· Proteins on the outside can be used as receptors, by having a specific binding site, much like
enzyme-substrate bonds. They can also be used in cell recognition and cell
· Proteins with attached carbohydrates are called glycoproteins, the carbohydrates are short
polysaccharides and from a cell coat (glycocalyx) outside the cell membrane. The glycocalyx
are involved as receptors bringing cells together, act as an adhesive, enzymes anchored to
membrane and they play an immunological role, i.e., blood type.
· Phospholipids with carbohydrates attached are known as glycolipids; these provide energy
and act as a marker for cellular recognition.