Types of Membrane Lipids
•
Lipids make up ~50% of most animal cell membrane mass.
All membrane lipids are amphiphilic — they have a hydrophilic (polar) head and
hydrophobic (nonpolar) tails. A typical red blood cell membrane contains ~7 ×
10⁸ lipid molecules.
|
Lipid Class |
Backbone |
Key Features |
|
Glycerophospholipids |
Glycerol |
Two fatty acid tails via
ester bonds; examples: PC, PE, PS, PI, PG |
|
Sphingolipids |
Sphingosine |
One fatty acid via amide
bond; sphingomyelin (SM) has phosphocholine head |
|
Glycolipids |
Sphingosine (animals) |
Sugar attached instead of
phosphate; exclusively on outer leaflet |
|
Sterols |
Rigid 4-ring structure |
Cholesterol (animals),
ergosterol (fungi), sitosterol/brassicosterol (plants) |
|
Plasmalogens |
Glycerol |
Ether-linked alkene at sn-1
position (not ester-linked) |
Fatty Acid Tails and Melting Temperature (Tm)
•
The length and saturation of fatty acid tails determine
the membrane's phase transition temperature (Tm).
•
Longer chains → higher Tm (more van der Waals
interactions). Unsaturated chains (cis-double bonds) → kinks → lower Tm
(prevent tight packing).
|
Phosphatidylcholine Type |
Tm (°C) |
|
DSPC (Di 18:0 — two
saturated C18) |
+58 |
|
DPPC (Di 16:0 — two
saturated C16) |
+41 |
|
SOPC (18:0, 18:1 — one
unsaturated) |
+3 |
|
DOPC (Di 18:1 — two
unsaturated) |
−22 |
Cholesterol — The Fluidity Buffer
•
Cholesterol is the most abundant sterol in animal cell
membranes (35–45% of plasma membrane lipid). It inserts between phospholipids
with its hydroxyl group near the polar heads and its rigid ring system among
the fatty acid tails.
•
At high temperature: cholesterol stiffens the membrane
by restricting phospholipid movement → reduces fluidity.
•
At low temperature: cholesterol prevents
crystallisation by disrupting regular packing → maintains fluidity.
•
Overall effect: cholesterol buffers fluidity — broadens
the phase transition and decreases permeability to small water-soluble
molecules.
CHOLESTEROL DISTRIBUTION across organelles:
•
Plasma membrane (35–45%) >> Lysosomal membrane
> Smooth ER (5–8%) >> Inner mitochondrial membrane (lowest)
•
Smooth ER is the SITE of cholesterol synthesis but
maintains LOW levels to preserve fluidity for biosynthesis.
•
StAR protein transports cholesterol from outer to inner
mitochondrial membrane in steroidogenic cells.
•
Cholesterol is ABSENT from most prokaryotic and plant
cell membranes (exception: Mycoplasma membranes contain sterols).
Phospholipid Composition Varies by Membrane
|
Membrane |
Major Phospholipids |
Distinctive Feature |
|
Rat liver plasma membrane |
PC > SM > PS > PE
> PI |
High cholesterol,
sphingomyelin |
|
E. coli plasma membrane |
PE > PG > cardiolipin
(DPG) > PS |
No phosphatidylcholine, No
sphingomyelin |
|
Inner mitochondrial
membrane |
PC > PE > PS > PI |
High cardiolipin (DPG) —
essential for ETC Complexes |
Phospholipid Asymmetry
•
The two leaflets of the bilayer have different lipid
compositions. In red blood cells:
|
Leaflet |
Major Phospholipids |
Enriched Feature / Functions |
|
Outer (exoplasmic) |
PC + SM
(choline-containing) |
Glycolipids always outer;
forms lipid rafts |
|
Inner (cytosolic) |
PS + PE (charged) |
PS activates PKC; PI
phosphorylation → signalling → PIP2 → IP3 + DAG via PLC |
MAINTAINING ASYMMETRY — Three enzymes:
•
Flippase: ATP-dependent; moves PS and PE from outer →
inner leaflet (inward)
•
Floppase: ATP-dependent; moves lipids from inner →
outer leaflet (outward)
•
Scramblase: ATP-independent; moves any lipid
bidirectionally down concentration gradient
During APOPTOSIS: scramblase is
activated, flippase is inactivated → PS is exposed on the outer surface →
"eat me" signal → recognised by macrophages → phagocytosis.
Glycolipids
•
Glycolipids are sugar-containing lipids found
exclusively on the outer leaflet of the plasma membrane. In animal cells, they
are built from sphingosine (like sphingomyelin). They self-associate via
hydrogen bonds and van der Waals forces, partitioning into lipid rafts with
cholesterol.
|
Type |
Details |
|
Cerebrosides |
Simple glycolipids with a
single sugar (galactose or glucose) linked to ceramide |
|
Gangliosides |
Complex glycolipids with
sialic acid (negative charge); abundant in nerve cells (5–10% of total
lipid); GM1 is the receptor for cholera toxin |
•
Cholera toxin binds GM1 ganglioside → activates
adenylyl cyclase → ↑cAMP → opens CFTR Cl⁻ channels → Cl⁻ and water loss →
dehydration.
•
ORT (oral rehydration therapy) uses glucose + NaCl to
leverage the Na⁺-glucose cotransporter (SGLT), creating an osmotic gradient
that drives water absorption back into the blood.
Lipid Rafts and Caveolae
•
Lipid rafts are dynamic membrane microdomains enriched
in sphingolipids + cholesterol. Cholesterol fills the voids under the large
sphingolipid head groups, stabilising the raft. Sphingolipids have slightly
higher mutual affinity for cholesterol than other lipids, causing them to
aggregate. PS is NOT a raft component (it resides on the inner leaflet).
•
Caveolae are small flask-shaped invaginations of the
plasma membrane composed of cholesterol + sphingolipids + caveolins (integral
membrane proteins, on the inner leaflet). Functions: endocytosis, signal
transduction, calcium signalling in cardiac muscle cells.
Lipid Droplets and Lipoproteins
•
Lipid droplets form in the ER membrane and have a
phospholipid monolayer (not bilayer) surrounding a hydrophobic core.
•
Similarly, lipoproteins (VLDL, IDL, LDL, HDL,
chylomicrons) are covered by a phospholipid monolayer + apolipoproteins.
•
In contrast, endocytic vesicles and other
membrane-bound organelles are enclosed by a standard lipid bilayer.