Download BIO1019 Lecture 20 - phospholipids

Phosphoacylglycerols (Phospholipids)
• Phosphoacylglycerols are fatty acid esters of
glycerol which also contain a phosphate group
and other specific groups
• The phosphate group
replaces the fatty acid on
C number 3 of a
triacylglycerol molecule Phosphate group
O
CH 2
R C O CH
CH 2
X
O
O C R
O
O P O X
OH
The groups which can bind to the phosphate are all
alcohols. They are very polar molecules and are
often charged
HO CH 2 CH 2
NH 3 +
Ethanolamine
CO O HO CH 2 CH NH 3 +
Serine
HO CH 2CH 2
Choline
CH 3
N + CH 3
CH 3
H
H
H
H
OH
OH
OH
H
H
Inositol
OH
OH
H
When choline is part of the
phosphoacylglycerol it is called
phosphatidyl choline
A Phosphoacylglycerol
X
Non-polar “tail”
Polar “head”
• Molecules which have one polar end and one nonpolar end are called amphipathic molecules
• Phosphoacylglycerols are amphipathic and it is
that property on which their major biological
function is based
Triacylglycerol
From: Lehninger Principles of Biochemistry 4th edition. Figures 10-2 and 10-13
Phosphoacylglycerol
Amphipathic molecules spontaneously form a
number of specific structures including:
Monolayers at a liquid-air interface
Micelles
Bilayer
From: Molecular Cell Biology by Lodish 5th Edition. Based on Figure 2-20
• The phosphoacylglycerol or phospholipid bilayer
is the basis of biological membrane structure
From:Stryer Biochemistry 5th edition. Figures 12.11 and 12.12
• Membrane structure due to the amphipathic
nature of the phosphoacylglycerols
• Acylglycerols cannot form bilayers because they
are not amphipathic
• Sterols are also found in membranes. They are
amphipathic
Cholesterol
Stigmasterol
HO
Polar
HO
Non-Polar
Polar
Non-Polar
Proteins and carbohydrates are also components
of membranes
Described as a fluid mosaic with movement of
molecules within the plane of the membrane
From: Lehninger principles of Biochemistry 4th Edition. Figure 11-3
• Integral Proteins (a, b, c) – span the membrane (may
have multiple transmembrane segments) or partially
immersed in lipid layer
• Peripheral Proteins (d, e) – loosely attached:
electrostatic interaction, bonding to integral protein,
hydro-phobic anchor, bonding to phosphoacylglycerol
via carbohydrate chain
From:Stryer Biochemistry 5th Edition. Figure 12.17
Why have proteins and carbohydrates in
membranes?
• Cell recognition
• Extracellular enzyme activities
• Transport of compounds across the membrane
Three types of transport of solutes across
membranes:
• Diffusion
• Passive transport (facilitated diffusion)
• Active transport
Diffusion
• Solute passes across membrane from area of
high concentration to area of low concentration
until concentration equalised
• Driven by concentration gradient
• Small polar molecules diffuse through small gaps
in hydrophobic environment.
• Larger polar molecules (particularly if charged)
do not diffuse
• Uncharged, lipophilic molecules diffuse readily
Passive transport (facilitated diffusion)
• Depends on the presence of specific proteins
which transport the molecule from one side of
the membrane to the other
• Driven by concentration gradient
Active Transport
• Depends of the presence of specific transport
proteins
• Transport requires energy
• Transport can be against a concentration gradient
Osmosis and Tonicity
Osmosis
• The process by which a solvent passes across a
semi-permeable membrane from a solution with
a low osmotic pressure to one with a high
osmotic pressure
• Osmotic pressure is directly proportional to the
molar concentration of all the solutes and ions
which cannot pass across the membrane
• A semi-permeable membrane is one which allows
passage of solvent but not solute
Semi-permeable
membrane
High osmotic
pressure
Low osmotic
pressure
Both sides have equal osmotic pressure
Tonicity
• Biological membranes are selectively impermeable
not semi-permeable
• Allow free passage of solvent and some solutes
but not all
• Tonicity is due to the osmotic pressure exerted
only by the solutes which cannot pass across the
selectively permeable membrane
Urea passes across the membrane, sucrose cannot
0.5 M urea
1.0 M sucrose
0.5 M sucrose
Lower tonicity
Flow of water
0.5 M urea
Higher tonicity
0.5 M sucrose
0.5 M sucrose
Equal tonicity
No net flow of water
Equal tonicity