CELL MEMBRANE
1.1 Membrane Lipids
1.1.1 Major lipids and distribution
▪ Phospholipids
o Phosphoglycerides – more common class
▪ Glycerol backbone
▪ Attached are two fatty acids in ester linkage + phosphorylated alcohol
▪ Fatty acid constituents – even numbered carbon molecules w/ 16 or 18 carbons
▪ Unbranched, saturated / unsaturated with one or more cis double bonds
▪ Phosphatidic acid – simplest (1,2-diacylglycerol 3-phosphate) – key intermediate in formation of other
phosphoglycerides
▪ in most phosphoglycerides, 3-phosphate is esterified into an alcohol s/a ethanolamine, choline, serine, glycerol or
inositol
o Sphingomyelin
▪ Sphingosine backbone
▪ Fatty acid attached by amide linkage of amino group of sphingosine, forming ceramide
▪ Primary hydroxyl group is esterified to phosphorylcholine, forming sphingomyelin
▪ Myelin sheath
▪ Glycosphingolipids (GSL)
o Sugar containing
o Ceramide backbone
o Galactosyl- and glycosyl-ceramide (cerebrosides) and gangliosides
o In plasma membranes
o Cerebrosides
▪ Contain single hexone moiety either glucose or galactose
o Gangliosides
▪ contain a chain of 3 or more sugars, at least 1 of which is a sialic acid, attached to the primary alcohol of sphingosine
▪ Sterol
o Cholesterol
o In plasma membranes; also in mitochondria, Golgi complex, nuclear membranes
o Intercalates among phospholipids of membrane with hydroxyl group at aqueous interface and remainder of the molecule
within the leaflet
o Separated by: column, thin-layer and gas-liquid chromatography
o Structures established by: mass spectrometry
1.1.2 Amphipathic nature
▪ Hydrophilic, hydrophobic
▪ Polar head groups of phospholipids, hydroxyl group of cholesterol and sugar moieties of GSL interface with aqueous environment
▪ Saturated fatty acids – straight tails
▪ Unsaturated fatty acids – kinked tails; cis form in membranes
o Kinks – more kinks = more space = more movement = more fluid
▪ Polar head – hydrophilic; Hydrogen tails – hydrophobic / lipophilic
▪ Detergents*
o Used to solubilize membrane proteins – 1st step in purification
o Hydrophobic end binds to hydrophobic regions, displacing most of their bound lipids. Polar end is free, bringing proteins into
solution as detergent-protein complexes
1.1.3 Lipid bilayer and importance
▪ Micelle
o Serves solubility requirements
o Small and limited
o Hydrophobic shielded from water, hydrophilic in aqueous environment
▪ Bilayer
o Key structure
o Exposed edges eliminated by folding sheet back → enclosed vesicle / closed bilayer
o Impermeable to most water-soluble molecules
o Self-assembly by hydrophobic effect*
o Lipid soluble
▪ O2, CO2, N
o (more slowly diffuse through)
▪ Na, K, Cl
o Permeability coefficient – measure of the ability of a molecule to diffuse across barrier. Rapid = high. Least permeable = Na;
most permeable = H2O
o Non-lipid soluble – contain proteins that form channels for transport
1.2 Membrane Proteins
1.2.1 Types of membrane proteins, structural features
▪ Major functional molecules: enzymes, transport proteins, structural proteins, antigen, receptors
▪ Membrane phospholipids – solvent for membrane proteins
▪ Alpha-helical structure of proteins minimizes hydrophilic character
1.2.2 Specific biochemical roles in membrane
▪ interrupt the continuity of the lipid bilayer & constitute an alternate pathway through the cell membrane
1.2.3 Examples of membrane proteins
1.1 Membrane Lipids
1.1.1 Major lipids and distribution
▪ Phospholipids
o Phosphoglycerides – more common class
▪ Glycerol backbone
▪ Attached are two fatty acids in ester linkage + phosphorylated alcohol
▪ Fatty acid constituents – even numbered carbon molecules w/ 16 or 18 carbons
▪ Unbranched, saturated / unsaturated with one or more cis double bonds
▪ Phosphatidic acid – simplest (1,2-diacylglycerol 3-phosphate) – key intermediate in formation of other
phosphoglycerides
▪ in most phosphoglycerides, 3-phosphate is esterified into an alcohol s/a ethanolamine, choline, serine, glycerol or
inositol
o Sphingomyelin
▪ Sphingosine backbone
▪ Fatty acid attached by amide linkage of amino group of sphingosine, forming ceramide
▪ Primary hydroxyl group is esterified to phosphorylcholine, forming sphingomyelin
▪ Myelin sheath
▪ Glycosphingolipids (GSL)
o Sugar containing
o Ceramide backbone
o Galactosyl- and glycosyl-ceramide (cerebrosides) and gangliosides
o In plasma membranes
o Cerebrosides
▪ Contain single hexone moiety either glucose or galactose
o Gangliosides
▪ contain a chain of 3 or more sugars, at least 1 of which is a sialic acid, attached to the primary alcohol of sphingosine
▪ Sterol
o Cholesterol
o In plasma membranes; also in mitochondria, Golgi complex, nuclear membranes
o Intercalates among phospholipids of membrane with hydroxyl group at aqueous interface and remainder of the molecule
within the leaflet
o Separated by: column, thin-layer and gas-liquid chromatography
o Structures established by: mass spectrometry
1.1.2 Amphipathic nature
▪ Hydrophilic, hydrophobic
▪ Polar head groups of phospholipids, hydroxyl group of cholesterol and sugar moieties of GSL interface with aqueous environment
▪ Saturated fatty acids – straight tails
▪ Unsaturated fatty acids – kinked tails; cis form in membranes
o Kinks – more kinks = more space = more movement = more fluid
▪ Polar head – hydrophilic; Hydrogen tails – hydrophobic / lipophilic
▪ Detergents*
o Used to solubilize membrane proteins – 1st step in purification
o Hydrophobic end binds to hydrophobic regions, displacing most of their bound lipids. Polar end is free, bringing proteins into
solution as detergent-protein complexes
1.1.3 Lipid bilayer and importance
▪ Micelle
o Serves solubility requirements
o Small and limited
o Hydrophobic shielded from water, hydrophilic in aqueous environment
▪ Bilayer
o Key structure
o Exposed edges eliminated by folding sheet back → enclosed vesicle / closed bilayer
o Impermeable to most water-soluble molecules
o Self-assembly by hydrophobic effect*
o Lipid soluble
▪ O2, CO2, N
o (more slowly diffuse through)
▪ Na, K, Cl
o Permeability coefficient – measure of the ability of a molecule to diffuse across barrier. Rapid = high. Least permeable = Na;
most permeable = H2O
o Non-lipid soluble – contain proteins that form channels for transport
1.2 Membrane Proteins
1.2.1 Types of membrane proteins, structural features
▪ Major functional molecules: enzymes, transport proteins, structural proteins, antigen, receptors
▪ Membrane phospholipids – solvent for membrane proteins
▪ Alpha-helical structure of proteins minimizes hydrophilic character
1.2.2 Specific biochemical roles in membrane
▪ interrupt the continuity of the lipid bilayer & constitute an alternate pathway through the cell membrane
1.2.3 Examples of membrane proteins
