Page 21 - Biology_F5

Biology for Advanced Level Secondary Schools

Glycoprotein (a branch of
carbohydrate attached to protein) Glycolipid (a branch of carbohydrate
Peripheral attached to lipid)
protein Phospholipid head
Phospholipid layer
Phospholipid tail
Phospholipid
bilayer Cholesterol
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Channel protein with pore
Protein on one side of the membrane

Trasmembrane protein

Figure 1.7: Structure of a plasma membrane as per Fluid Mosaic Model

Lipids and lipid-soluble substances are Functions of the plasma membrane
allowed to cross the hydrophobic region (a) It covers the surface of every living cell
while non-lipids are not allowed to and acts as a barrier which separates
cross. Large molecules, such as glucose, the cytoplasm from extracellular
fatty acids, glycerol and amino acids environment.
are repelled by the hydrophobic region (b) It allows passage of some materials
and therefore, they diffuse through in and out of the cells (selective
special transport proteins called channel permeability).
proteins and carrier proteins by facilitated (c) It facilitates the transmission of nerve
diffusion.
impulse in the nervous system.
The strength of the model is that, it (d) It functions as a receptor site for
realises the presence of the phospholipid hormonal and neural transmission of
bilayer and protein layer. It also realises chemical stimuli.
the presence of pores in the membrane (e) It aids cell-to-cell recognition when
and the presence of cholesterol in the membranes of two cells come together.
membrane. The model explains the (f) It serves as a base for attachment of
presence of polar head (hydrophilic) the cytoskeleton in some organisms
and non-polar (hydrophobic) tail in and as the cell wall in others. Thus, the
phospholipids. Furthermore, the model cell membrane supports the cell and
explains the presence of glycolipid, as maintains its shape.
receptors that allow the cell to respond
to chemical messengers which regulate Adaptations of the cell surface
the activities of the cell. It is true that the membrane
membrane is a dynamic (ever-changing its The cell membrane is specialised to its
structure) with phospholipids in constant functions as follows:
motion. The weakness of the model is that (a) Presence of glycolipids, glycoproteins
it does not indicate or explain the presence and phospholipids for detection of
of electrolytes in the plasmalemma. stimuli, such as antigens and antibodies.

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