Page 20 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

2 Veterinary Histology of Domestic Mammals and Birds

VetBooks.ir Glycolipid Glycoprotein

Peripheral protein

Lipid bilayer

Integral protein

Peripheral protein

1.1 Structure of the cell membrane (schematic). The membrane consists of a lipid bilayer with associated
membrane proteins. Carbohydrate side chains attach to the outer surface of the membrane, giving rise to the
glycolipids and glycoproteins that make up the glycocalyx. During uptake of ligands (e.g. proteins), the mem-
brane invaginates to form an endocytotic vesicle.

At a molecular level, the fluidity of the membrane is
· a lipid bilayer, determined by the ability of the phospholipids to rotate
· membrane proteins and and diffuse laterally within the membrane. Less fre-
· surface polysaccharides.
quently, they also flip-flop between individual layers of
the bilayer.
The fluidity of the cell membrane gives rise to the plas-
Lipid bilayer ticity of the cell as a whole. Components of the membrane
The lipid molecules of the cell membrane are arranged may be pinched off (e.g. by exocytosis or endocytosis) or
in a double layer. Phospholipids are the main component replaced. Following interruption by external influences or
of the bilayer, with cholesterol and glycolipids present in cellular secretory activity, the lipid bilayer is re-established
smaller quantities. The lipids are amphipathic, with one spontaneously within the aqueous microenvironment,
hydrophilic (affinity for water, polar), electrically charged thus restoring the continuity of the membrane.
head group and two hydrophobic (water-repelling, non- Cholesterol is an important functional component of
polar) hydrocarbon tails. The hydrophobic tails consist the membrane, serving to regulate the intramembranous
alternately of saturated and unsaturated fatty acids. The movement of lipid chains and to increase the mechanical
hydrophilic head groups are oriented towards the outer strength of the bilayer.
surfaces of the membrane while the hydrophobic tails Glycolipids are present in the outer layer of the plas-
face towards each other. Due to their specific lipid mol- malemma of all animal cells and contribute significantly to
ecule content, and the concentration of electric charge in the asymmetry of the cell membrane. Their head groups,
the outer membrane components, most membranes are consisting of one or more polar sugar residues (oligosac-
asymmetrical (Figure 1.1). charides), project beyond the surface of the membrane.
The structure of the cell membrane has important func- Glycolipids consist predominantly of neutral glycolipids
tional implications. The variable length of the saturated (e.g. galactocerebroside within myelin sheaths) or gan-
and unsaturated hydrocarbon chains, and their ability to gliosides (e.g. sialic acids in cell receptors) (for further
move, influences the dynamics of the membrane, giving information refer to ‘Membrane polysaccharides’, below,
it a fluid quality. and biochemistry texts).

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