Section 5: Membranes and Membrane Function
Biomembranes: Membranes define what is a cell (biomembranes are only found in a cell). Allow
specialized cellular functions to occur in a localized manner.
Basic components: Lipids, Sterols, and Proteins
• Peripheral membrane proteins i.e. dystrophin.
Due to amphipathicity, phospholipids spontaneously form lipid bilayers in aqueous solution where
properties of the fatty acids confer properties onto the bilayers.
• Phosphoplipids spontaneously form structures to hide their hydrophobic tails in solution.
• Biomembranes are different from any phospholipid bilayer – they are not just a phospholipid
bilayer, it includes sterols and proteins.
• In a normal membrane bilayer, the proteins and lipids are both crucial. Proteins can move
within it unless anchored
Fatty Acid: Long hydrocarbon chain attached to a polar carboxyl head group. Acidic.
• Amphipathic. (part fat/part acid)
• Often Cx:y, where x = number of carbon molecules, and y = number of double bonds
― i.e. C16:0 – 16 carbons and no double bonds. The fatty acid is saturated.
• No double bonds = saturated.
• One double bond = unsaturated.
• More than one double bond = polyunsaturated
― Using fatty acids to build a membrane so you want to use fatty acids to give the
membrane certain properties.
Effect of double bond on structure
Biomembranes are semi-fluid: it is a certain consistency. Need to use types of fatty acids that form a
structure that is semi-fluid at 37 degrees.
C16:0 Tm = 63ºC (palmitate) – straight chain – at body T it is a solid.
C18:0 Tm = 70ºC (stearate) – straight- at body T it is a solid
C18:1 Tm = 13ºC (oleic acid) – one kink – at body T it is a liquid
• Tm increases with increasing chain length, but melting point decreases with increasing
• Saturated chains can tightly packed – numerous van der Waal’s forces interaction between the
chains at body temperature it would be a solid.
• cis double bonds common, trans rare
― cis double bonds allow things to be tightly packed at body temperature, low Tm so it is
Page 1 of5 ― Trans double bonds are very fatty by raising the Tm– they do not pack tightly so they are
solid at body temperature. They are not natural.
• Over 100 fatty acids known but only a few are common in membranes.
Three Classes of Membrane Lipids
• Phospholipid: does not tell you whether it is a phosphoglyceride or a sphingolipid. It tells you
that the head group is attached by a phosphate group.
• Glycolipid: the head group is a sugar
– 2 Fatty acids + glycerol backbone + head group
– Head groups are attached on with a phosphate
– 4 different head groups create 4 different types:
PE, PC, PS, PI.
– Most common type: 2 esterfied fatty acid tails
and polar head group
– Less common type: 2 fatty acid tails but only one
is attached by an ester (plasmalogen)
– Sphingosine backbone + 1 fatty acid
– Amino alcohol with long hydrocarbon chain and
fatty acyl chain attached via amide bond
– Can be a glycolipid (can have a sugar head
– Can be a phospholipid (can have a head group
attached by a phosphate group)
– Four ring hydrocarbon
– Sterols are also amphipathic. Polar charged
group + hydrophobic carbon tail.
– Most commonly understood is cholesterol.
– When they are put into the lipid bilayer, they can change the properties of that bilayer.
– Can intercalate between phospholipids, where they orient themselves with their polar hydroxyl
groups facing the same direction as the polar head groups of the fatty acids
– Lipid composition can affect thickness and curvature.
i. In some circumstances, the cholesterol can changed the diameter of the lipid
membrane but it doesn’t have to. It can make the membrane thicker or thinner by its
ii. If you use a certain phospholipid that is more cylindrical in structure in one leaflet and
you use another phospholipid that is more conical in structure in the other leaflet, you
can impart a curve in t