Biological membranes and their proteins
General functions of cell membranes membrane composition
• Form a continuous highly selective • 40% lipid
permeability barrier
• 60% protein
• Allow control of the enclosed chemical
environment • 1-10% carbohydrate (dry weight)
• Facilitate communication between cells • 20% of the total membrane weight is
and their environment water
• Allow recognition through the presence of
signalling molecules, adhesion proteins,
and allow immune surveillance
• Generate signals in response to electrical and chemical
stimuli
Phospholipids
Lipids
• Most abundant
• Provide basic structure
• Hydrophilic (polar) group or the head
• Support for the membranes group composed of phosphate and
and regulate their function chorine, amines, amino acids or sugars.
This group is attached to glycerol.
• Composed of
phospholipids, cholesterol • Hydrophobic (non polar) group or the tail
and glycolipids group composed of two fatty acid chains
attached to glycerol. (Length of C16 and
C18 are most common). Unsaturated fatty
acid side chains (with double bonds) in the
cis conformation introduce a kink in the
chain which prevents the fatty acids
packing tightly together.
• Glycerol backbone- The only
phospholipid which contains sphingosine
instead of glycerol is sphingomyelin.
This phospholipid consists of
sphingosine which contains one fatty
acid chain (ceramide) and choline with
phosphate group. Sphingomyelin is
found most commonly in the myelin
sheaths surrounding axons.
, Structure of biological membrane
• Phospholipid bilayer- antiparallel sheets of
phospholipids
• Glycolipids- attached carbohydrate
• Cholesterol- make it easy to ip- op
between the two layers
• The uid model by Singer and Nicholson-
ability of lipids and proteins to move in
membranes.
Dynamic structure
Lipid molecules posses four permitted moles of mobility in a
lipid bilayer
• Intra-chain motion - kink formation in the fatty acyl
chains
• Fast axial rotation
• Fast lateral diffusion within the plane of the bilayer
Restrains on mobility:
• Flip- op - movement of lipid molecules from one half of • lipid mediated effects -
the bilayer to the other on a one for one exchange basis proteins tend to separate
out into the uid phase or
Three modes of protein motion: cholesterol poor regions
• Conformational change • membrane protein
associations
• Rotational
• association with extra-
• Lateral membranous proteins
(peripheral proteins) e.g.
• No ip- op cytoskeleton
Movement of ions across the membrane is important because:
• the maintenance of intracellular pH
• the maintenance of ionic composition
• regulation of cell volume
• the concentration of metabolic fuels and building blocks
• the extrusion of waste products of metabolism and toxic substances
• the generation of ionic gradients necessary for the electrical excitability of nerve and
muscle
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General functions of cell membranes membrane composition
• Form a continuous highly selective • 40% lipid
permeability barrier
• 60% protein
• Allow control of the enclosed chemical
environment • 1-10% carbohydrate (dry weight)
• Facilitate communication between cells • 20% of the total membrane weight is
and their environment water
• Allow recognition through the presence of
signalling molecules, adhesion proteins,
and allow immune surveillance
• Generate signals in response to electrical and chemical
stimuli
Phospholipids
Lipids
• Most abundant
• Provide basic structure
• Hydrophilic (polar) group or the head
• Support for the membranes group composed of phosphate and
and regulate their function chorine, amines, amino acids or sugars.
This group is attached to glycerol.
• Composed of
phospholipids, cholesterol • Hydrophobic (non polar) group or the tail
and glycolipids group composed of two fatty acid chains
attached to glycerol. (Length of C16 and
C18 are most common). Unsaturated fatty
acid side chains (with double bonds) in the
cis conformation introduce a kink in the
chain which prevents the fatty acids
packing tightly together.
• Glycerol backbone- The only
phospholipid which contains sphingosine
instead of glycerol is sphingomyelin.
This phospholipid consists of
sphingosine which contains one fatty
acid chain (ceramide) and choline with
phosphate group. Sphingomyelin is
found most commonly in the myelin
sheaths surrounding axons.
, Structure of biological membrane
• Phospholipid bilayer- antiparallel sheets of
phospholipids
• Glycolipids- attached carbohydrate
• Cholesterol- make it easy to ip- op
between the two layers
• The uid model by Singer and Nicholson-
ability of lipids and proteins to move in
membranes.
Dynamic structure
Lipid molecules posses four permitted moles of mobility in a
lipid bilayer
• Intra-chain motion - kink formation in the fatty acyl
chains
• Fast axial rotation
• Fast lateral diffusion within the plane of the bilayer
Restrains on mobility:
• Flip- op - movement of lipid molecules from one half of • lipid mediated effects -
the bilayer to the other on a one for one exchange basis proteins tend to separate
out into the uid phase or
Three modes of protein motion: cholesterol poor regions
• Conformational change • membrane protein
associations
• Rotational
• association with extra-
• Lateral membranous proteins
(peripheral proteins) e.g.
• No ip- op cytoskeleton
Movement of ions across the membrane is important because:
• the maintenance of intracellular pH
• the maintenance of ionic composition
• regulation of cell volume
• the concentration of metabolic fuels and building blocks
• the extrusion of waste products of metabolism and toxic substances
• the generation of ionic gradients necessary for the electrical excitability of nerve and
muscle
flflfl fl
fl fl fl
