17.01.04 · mol-cell-bio / biomolecules

Lipid chemistry: fatty acids, glycerophospholipids, sphingolipids, sterols, and their roles

stub3 tiersLean: nonepending prereqs

Anchor (Master): Vance & Vance, Biochemistry of Lipids, Lipoproteins and Membranes, 5th ed. (2008)

Intuition Beginner

Lipids are a diverse group of molecules that share one property: they do not dissolve in water. Unlike proteins and carbohydrates, which are built from repeating monomer units, lipids are defined by their physical behaviour rather than a common chemical structure. Their water-insolubility makes them ideal building blocks for the barriers that separate a cell from its environment.

The simplest lipids are fatty acids: long hydrocarbon chains with a carboxyl group ( $-COOH$ ) at one end. The chain can be saturated (every carbon carries the maximum number of hydrogens, with no double bonds) or unsaturated (containing one or more carbon-carbon double bonds that introduce kinks in the chain). Saturated chains pack tightly together and are solid at room temperature; unsaturated chains cannot pack as tightly and remain liquid.

Phospholipids are the most abundant lipids in cell membranes. Each phospholipid has a glycerol backbone connecting two fatty acid tails to a phosphate-containing head group. The head is water-soluble (hydrophilic) and the tails are water-insoluble (hydrophobic). When placed in water, phospholipids spontaneously arrange themselves into a bilayer: two parallel sheets with the tails facing inward and the heads facing the water on both sides. This bilayer is the fundamental architecture of every biological membrane.

Cholesterol is a sterol — a lipid built from four fused carbon rings rather than a flexible chain. Cholesterol inserts between phospholipids in animal cell membranes. At low temperatures it prevents fatty acid tails from packing too tightly, keeping the membrane fluid. At high temperatures it restrains excessive movement, stabilising the membrane. This dual regulatory role is essential for membrane function.

Visual Beginner

The major lipid classes and their structural features:

Class	Core structure	Key feature	Biological role
Fatty acid	Hydrocarbon chain + COOH	Saturated vs unsaturated	Energy storage, membrane component
Glycerophospholipid	Glycerol + 2 fatty acids + phosphate head	Amphipathic (head + tails)	Bilayer formation, membrane scaffold
Sphingolipid	Sphingosine + fatty acid + head group	Built on amino alcohol backbone	Cell recognition, myelin sheaths
Sterol	Four fused rings + hydroxyl group	Rigid planar ring system	Membrane fluidity, hormone precursor

Worked example Beginner

Consider oleic acid and stearic acid, two 18-carbon fatty acids found in olive oil and animal fat respectively.

Step 1. Compare the structures. Stearic acid is an 18:0 fatty acid (18 carbons, zero double bonds). Its chain is fully saturated, meaning every carbon along the chain carries two hydrogen atoms. Oleic acid is 18:1 (18 carbons, one double bond). The double bond occurs between carbons 9 and 10 and has a cis geometry, introducing a roughly 30-degree bend in the chain.

Step 2. Predict the physical properties. The straight chain of stearic acid allows neighbouring molecules to pack closely, with extensive van der Waals contacts between chains. Stearic acid therefore has a melting point of approximately 70 degrees Celsius and is solid at room temperature. The kink in oleic acid prevents tight packing, reducing the van der Waals interactions. Oleic acid melts at approximately 13 degrees Celsius and is liquid at room temperature.

Step 3. Biological consequence. Animal fat (rich in saturated fatty acids like stearic acid) is solid at body temperature and stored as semisolid deposits. Olive oil (rich in monounsaturated oleic acid) remains liquid. The degree of unsaturation in membrane phospholipids is a major determinant of membrane fluidity: organisms in cold environments increase the proportion of unsaturated fatty acids in their membranes to prevent them from becoming too rigid.

Step 4. What this illustrates. A single double bond in an 18-carbon chain changes the melting point by nearly 60 degrees Celsius. The geometry of lipid molecules at the atomic scale directly determines the macroscopic physical properties of fats, oils, and biological membranes.

Check your understanding Beginner

Exercise (easy, multiple choice).

Why do phospholipids spontaneously form bilayers in water?

A. The phosphate head groups attract each other through ionic bonds B. The hydrophobic tails are driven together by the hydrophobic effect, while the hydrophilic heads remain in contact with water C. Covalent bonds form between adjacent fatty acid tails D. Glycerol reacts with water to form a polymer

Hint

Each phospholipid has a polar head and nonpolar tails. Think about what happens to the nonpolar tails when surrounded by water molecules.

Answer

Option B. The hydrophobic effect drives the nonpolar fatty acid tails away from water and toward each other, while the polar head groups remain hydrated on the surface. The bilayer is the geometry that simultaneously minimises unfavourable tail-water contacts and maximises favourable head-water contacts. No covalent bonds form between phospholipids in the bilayer; the structure is held together entirely by noncovalent forces (the hydrophobic effect plus van der Waals interactions between tails).

Exercise (easy, true/false).

A bacterium living in a hot spring would be expected to have more saturated fatty acids in its membrane than a bacterium living in cold ocean water.

Hint

Saturated fatty acids pack more tightly than unsaturated ones. What does tight packing do to membrane fluidity and stability at high temperatures?

Answer

True. Saturated fatty acid chains are straight and pack tightly, raising the melting temperature of the membrane. At the high temperatures found in hot springs (often 70-80 degrees Celsius), unsaturated membranes would be excessively fluid and leaky. Thermophilic organisms adapt by increasing the proportion of saturated and even branched fatty acids to maintain membrane integrity at elevated temperatures. Conversely, cold-adapted organisms increase unsaturation to prevent their membranes from becoming too rigid.

Exercise (easy, multiple choice).

Cholesterol modulates membrane fluidity by:

A. Forming covalent cross-links between phospholipid head groups B. Interacting with fatty acid tails through its rigid ring system, restraining movement at high temperature and disrupting tight packing at low temperature C. Dissolving the bilayer and forming micelles D. Catalysing the conversion of saturated fatty acids to unsaturated fatty acids

Hint

Cholesterol is a small, rigid molecule with a single hydroxyl group. Consider how this shape inserts among phospholipid tails.

Answer

Option B. Cholesterol's four fused rings form a rigid planar structure that inserts between phospholipid fatty acid tails, with its single hydroxyl group positioned near the head-group region. At high temperatures, the rigid ring system restricts the lateral movement of neighbouring tails, reducing excessive fluidity. At low temperatures, the bulky ring disrupts the regular packing of saturated tails, preventing the membrane from becoming too ordered and rigid. This dual buffering action is why cholesterol is called a "fluidity buffer."

Formal definition Intermediate+

Fatty acids are carboxylic acids with long unbranched aliphatic chains of 4 to 28 carbons (most commonly 16 or 18 in biological systems). They are described by a numerical notation $C D : n$ where $C$ is the number of carbon atoms, $D$ is the number of double bonds, and $n$ indicates the position of the first double bond counted from the methyl (omega) end. For example, linoleic acid is 18:2( $ω$ -6), meaning an 18-carbon chain with two cis double bonds, the first at carbon 6 from the omega end. The omega notation is important because mammals cannot synthesise double bonds beyond carbon 9, making $ω$ -6 and $ω$ -3 fatty acids essential dietary requirements.

Double bonds in biological fatty acids are almost exclusively cis. A cis double bond introduces a kink of approximately 30 degrees in the hydrocarbon chain, reducing the ability of adjacent chains to pack closely. Each cis double bond lowers the melting temperature by approximately 30-40 degrees Celsius for an 18-carbon chain. Trans double bonds, which do not introduce a kink, are rare in nature but are produced industrially during partial hydrogenation of vegetable oils; their straight-chain geometry allows tighter packing and raises the melting point, with established cardiovascular health consequences.

Glycerophospholipids (phosphoglycerides) are built on a glycerol backbone with fatty acids esterified at the sn-1 and sn-2 positions (where "sn" denotes stereospecific numbering, with sn-1 being the top carbon when the hydroxyl on carbon-2 points to the left in the Fischer projection). The sn-3 position bears a phosphate group, which in turn is esterified to one of several head-group alcohols:

Head group	Abbr.	Head-group alcohol	Net charge (pH 7)
Phosphatidylcholine	PC	Choline	Zwitterionic (0)
Phosphatidylethanolamine	PE	Ethanolamine	Zwitterionic (0)
Phosphatidylserine	PS	Serine	Negative (-1)
Phosphatidylglycerol	PG	Glycerol	Negative (-1)
Phosphatidylinositol	PI	Inositol	Negative (-1)
Cardiolipin	CL	Two PG units	Negative (-2)

The sn-1 position typically carries a saturated fatty acid while sn-2 carries an unsaturated one, producing an asymmetry that influences the packing geometry of the bilayer. PC is the most abundant phospholipid in most eukaryotic membranes (approximately 50% of the outer leaflet). PE and PS are enriched in the inner leaflet; the ATP-dependent enzyme flippase maintains this asymmetry ^{[Alberts 2022 Ch. 2]}.

Sphingolipids are built on sphingosine (trans-4-sphingenine), an 18-carbon amino alcohol with a trans double bond at C4. The amino group at C2 is acylated with a fatty acid to form ceramide, the simplest sphingolipid and the metabolic precursor to all others. Addition of a phosphocholine head group to ceramide yields sphingomyelin, the only sphingolipid that is also a phospholipid. Addition of one or more sugar residues to ceramide produces glycosphingolipids: a single glucose or galactose gives a cerebroside (abundant in myelin), while addition of an oligosaccharide chain terminated by sialic acid produces a ganglioside (concentrated in neuronal membranes).

Sterols are isoprenoid-derived lipids characterised by a fused four-ring steroid nucleus (three six-membered rings designated A, B, C and one five-membered ring D). Cholesterol ( $C_{27} H_{46} O$ ) is the dominant animal sterol. It orients in the bilayer with its single hydroxyl group at C3 near the head-group region and its hydrocarbon tail at C17 extending into the acyl-chain region. Plants produce phytosterols (sitosterol, stigmasterol) and fungi produce ergosterol, all with the same ring system but different side-chain modifications. The rigid ring system is the structural basis for cholesterol's modulation of membrane properties.

Key mechanism Intermediate+

Mechanism (Amphiphile self-assembly and bilayer formation). Phospholipids are amphiphilic molecules with a polar head and nonpolar tails. Above a threshold concentration (the critical aggregation concentration), amphiphiles in aqueous solution spontaneously organise into structures that minimise unfavourable hydrophobic-water contacts while maintaining favourable head-group hydration. The geometry of the resulting aggregate is predicted by the critical packing parameter $P = v / (a \cdot l)$ , where $v$ is the tail volume, $a$ is the optimal head-group area, and $l$ is the critical tail length.

When $P \leq 1/3$ , the molecule is cone-shaped (large head relative to tails) and forms micelles (spherical aggregates with tails pointing inward). When $1/3 < P \leq 1/2$ , cylindrical or ellipsoidal micelles form. When $1/2 < P \leq 1$ , the molecule is approximately cylindrical and forms **bilayers** (planar or vesicular structures). When $P > 1$ , the molecule is inverted-cone-shaped (small head relative to tails) and forms inverted hexagonal phases ( $H_{II}$ ) or other non-lamellar structures.

Typical phospholipids with two fatty acid chains have $P$ values in the range 0.7-1.0, strongly favouring bilayer formation. Lysophospholipids (with one fatty acid chain removed) have $P \approx 0.3 - 0.4$ and form micelles. Diacylglycerol (no head group, $P > 1$ ) promotes inverted hexagonal phase formation. The packing parameter thus provides a quantitative framework for predicting how individual lipid molecular geometry dictates the architecture of the aggregate ^{[Tanford 1980 Ch. 6-7]}.

Bilayer physical properties

The lipid bilayer is approximately 5 nm thick. The interior (acyl-chain region) has the physical properties of a hydrocarbon liquid: permeability to water and small uncharged molecules is moderate, while ions and polar solutes cross very slowly. The bilayer is a two-dimensional fluid: individual phospholipids undergo rapid lateral diffusion within the plane of their own leaflet (lateral diffusion coefficient $D \approx 1 0^{- 8}$ $cm^{2} / s$ , meaning a phospholipid diffuses a distance equal to a bacterial cell length in roughly one second). Spontaneous transverse diffusion ("flip-flop") from one leaflet to the other is extremely slow for phospholipids ( $t_{1/2} \approx$ hours to days) because moving the polar head group through the hydrophobic interior is energetically unfavourable. This kinetic barrier is the physical basis for the maintenance of lipid asymmetry between the two leaflets.

The gel-to-liquid-crystalline phase transition temperature ( $T_{m}$ ) of a phospholipid bilayer depends on chain length and unsaturation. For a homologous series of saturated diacyl-PC lipids, $T_{m}$ increases by approximately 10-15 degrees Celsius per additional two methylene groups (e.g., DLPC C12:0 has $T_{m} \approx - 1$ degrees Celsius, DMPC C14:0 has $T_{m} \approx 23$ degrees Celsius, DPPC C16:0 has $T_{m} \approx 41$ degrees Celsius, DSPC C18:0 has $T_{m} \approx 55$ degrees Celsius). Each cis double bond lowers $T_{m}$ by approximately 30-40 degrees Celsius relative to the saturated analogue. Biological membranes maintain their composition such that $T_{m}$ is below the growth temperature, ensuring the membrane remains in the liquid-crystalline (fluid) state.

Worked example: predicting aggregate structure from molecular geometry

Consider three amphiphiles and determine the aggregate each forms.

Sodium dodecyl sulfate (SDS): single 12-carbon chain, large sulfate head group. Tail volume $v \approx 350$ $\overset{˚}{A}^{3}$ , head area $a \approx 60$ $\overset{˚}{A}^{2}$ , tail length $l \approx 17$ $\overset{˚}{A}$ . Packing parameter $P = 350/ (60 \times 17) \approx 0.34$ . This is at the micelle-bilayer boundary; SDS forms spherical micelles in dilute solution.

DPPC (dipalmitoylphosphatidylcholine): two 16-carbon chains, PC head group. Tail volume $v \approx 880$ $\overset{˚}{A}^{3}$ , head area $a \approx 71$ $\overset{˚}{A}^{2}$ , tail length $l \approx 17$ $\overset{˚}{A}$ . Packing parameter $P = 880/ (71 \times 17) \approx 0.73$ . This falls in the bilayer range; DPPC forms stable bilayer vesicles (liposomes) above its $T_{m}$ of 41 degrees Celsius.

DOPE (dioleoylphosphatidylethanolamine): two 18-carbon chains with one cis double bond each, small PE head group. The cis double bonds increase tail volume relative to tail length, and the PE head group is small. Packing parameter $P \approx 1.1 - 1.3$ . This exceeds 1.0, and DOPE forms the inverted hexagonal phase ( $H_{II}$ ) at physiological temperatures, adopting a structure in which the head groups surround narrow water channels and the tails extend outward.

Exercises Intermediate+

Exercise 1 (easy, multiple choice).

An 18-carbon fatty acid with two cis double bonds at positions 9 and 12 (counted from the carboxyl end) is designated:

A. 18:2( $ω$ -6) B. 18:2( $ω$ -9) C. 18:2( $ω$ -12) D. 18:0( $ω$ -6)

Hint

The omega ( $ω$ ) number is counted from the methyl end, not the carboxyl end. For an 18-carbon chain, the methyl end is carbon 18. Count backward to find the position of the first double bond from the methyl end.

Answer

Option A. The first double bond from the methyl end is at position 18 - 12 = 6, meaning carbon 6 from the omega end. The second double bond is at carbon 9 from the omega end (18 - 9 = 9). This fatty acid is linoleic acid, an essential $ω$ -6 fatty acid. The omega notation highlights the fact that mammals cannot introduce double bonds beyond carbon 9 from the carboxyl end (carbons closer to the methyl terminus), making linoleic acid a dietary essential.

Exercise 2 (easy, short answer).

Distinguish between the structure of sphingomyelin and a typical glycerophospholipid (e.g., phosphatidylcholine). What is the backbone in each case?

Hint

Sphingomyelin is built on sphingosine, while glycerophospholipids are built on glycerol. Compare the number of hydroxyl groups available for acylation and head-group attachment.

Answer

Glycerophospholipids are built on a glycerol backbone (a three-carbon triol). Fatty acids are esterified to the sn-1 and sn-2 hydroxyls, and the sn-3 hydroxyl bears the phosphate-head-group moiety. Sphingomyelin is built on sphingosine (an 18-carbon amino alcohol). The amino group at C2 carries a fatty acid via an amide bond (not an ester), and the C1 hydroxyl bears a phosphocholine head group. The C4 trans double bond in sphingosine is an additional structural feature absent in glycerol-based lipids. Despite these differences, both sphingomyelin and PC share the same phosphocholine head group and are amphipathic bilayer-forming lipids.

Exercise 3 (medium, short answer).

Explain why flippase activity is necessary to maintain membrane asymmetry. What would happen to phospholipid distribution if flippases were inactivated?

Hint

Spontaneous flip-flop is very slow for phospholipids, but biosynthesis of new phospholipids occurs on the cytoplasmic (inner) leaflet. How do lipids reach the outer leaflet?

Answer

New phospholipids are synthesised on the cytoplasmic face of the endoplasmic reticulum, initially populating the inner leaflet. Spontaneous transverse diffusion (flip-flop) of phospholipids is negligibly slow because moving the polar head group through the hydrophobic bilayer interior is energetically unfavourable ( $t_{1/2}$ on the order of hours to days). Flippases are ATP-dependent transporters that selectively move specific phospholipids (particularly PS and PE) from the outer to the inner leaflet against their concentration gradient. Without flippase activity, the inner leaflet would gradually accumulate phospholipids while the outer leaflet would become depleted. scramblase activity (calcium-activated, bidirectional, ATP-independent) would eventually randomise the distribution, leading to loss of asymmetry. In platelets, controlled scramblase activation exposes PS on the outer surface, providing a catalytic surface for coagulation factor assembly.

Exercise 4 (medium, multiple choice).

Which of the following changes would increase the fluidity of a lipid bilayer at a given temperature?

A. Increasing the proportion of saturated fatty acids B. Increasing the chain length of the fatty acids C. Introducing cis double bonds into the fatty acid tails D. Removing cholesterol from a membrane that is already below its $T_{m}$

Hint

Fluidity is determined by how tightly fatty acid chains pack. Consider which change would disrupt tight packing.

Answer

Option C. Cis double bonds introduce kinks in the hydrocarbon chain that prevent close packing of adjacent chains, lowering $T_{m}$ and increasing fluidity. Option A would decrease fluidity (saturated chains pack tightly). Option B would decrease fluidity (longer chains have more van der Waals contacts). Option D would decrease fluidity (below $T_{m}$ , cholesterol disrupts tight packing of ordered chains; removing it makes the gel phase more rigid).

Exercise 6 (hard, short answer).

Tay-Sachs disease results from a deficiency in hexosaminidase A, which cleaves the terminal N-acetylgalactosamine (GalNAc) from the ganglioside GM2. Explain how this enzymatic defect leads to neurodegeneration, and relate the pathway to the general principle of sphingolipid catabolism.

Hint

Sphingolipids are degraded stepwise in lysosomes by specific hydrolases. Each enzyme removes one specific sugar or modification. What happens when one step is blocked?

Answer

Sphingolipid degradation occurs in lysosomes through a stepwise pathway in which each enzyme removes one specific moiety from the oligosaccharide head group. Hexosaminidase A normally removes the terminal GalNAc residue from GM2, converting it to GM3. In Tay-Sachs disease, mutations in the HEXA gene (encoding the alpha subunit of hexosaminidase A) abolish this activity. GM2 accumulates in lysosomes, particularly in neurons, because the brain is rich in gangliosides and neuronal turnover of membrane lipids is high. The accumulated GM2 distends lysosomes, displacing normal cytoplasm and interfering with neuronal function. Progressive neurodegeneration manifests as developmental regression, seizures, and death typically by age 4-5 in the infantile form. This is a sphingolipid storage disease (sphingolipidosis), one of several lysosomal storage disorders that illustrate the requirement for complete enzymatic degradation of complex lipids. Related disorders include Gaucher disease (glucocerebrosidase deficiency, glucosylceramide accumulation) and Niemann-Pick disease (sphingomyelinase deficiency, sphingomyelin accumulation).

Exercise 7 (hard, symbolic).

The gel-to-liquid-crystalline transition temperature $T_{m}$ for a series of saturated diacylphosphatidylcholines scales approximately linearly with chain length. Given that DLPC (C12:0) has $T_{m} \approx - 1$ degrees Celsius and DSPC (C18:0) has $T_{m} \approx 55$ degrees Celsius, estimate $T_{m}$ for DMPC (C14:0) and DPPC (C16:0). What does this predict about the physical state of a DPPC bilayer at human body temperature (37 degrees Celsius)?

Hint

Assume a linear relationship between the number of carbons per chain and $T_{m}$ . DLPC has 12 carbons, DPPC has 16, DSPC has 18. The slope is $(55 - (- 1)) / (18 - 12) = 56/6 \approx 9.3$ degrees per carbon.

Answer

Assuming a linear relationship: the slope is $(55 - (- 1)) / (18 - 12) = 56/6 \approx 9.33$ degrees Celsius per additional carbon atom per chain.

DMPC (C14:0): $T_{m} \approx - 1 + 2 \times 9.33 \approx 17.7$ degrees Celsius. The experimentally measured value is approximately 23 degrees Celsius, so the linear approximation is reasonable but not exact (the relationship has slight curvature due to chain-length-dependent changes in van der Waals cooperativity).

DPPC (C16:0): $T_{m} \approx - 1 + 4 \times 9.33 \approx 36.3$ degrees Celsius. The experimental value is approximately 41 degrees Celsius. The discrepancy arises because the true relationship is not perfectly linear; longer chains show slightly steeper increases in $T_{m}$ due to enhanced cooperativity of the chain-melting transition.

At 37 degrees Celsius, DPPC is near its $T_{m}$ and would exist as a mixture of gel and liquid-crystalline domains (phase coexistence). The biological significance is that pure DPPC membranes would be at the borderline of fluidity at body temperature. Real biological membranes avoid this problem by incorporating a mixture of chain lengths and unsaturated fatty acids, lowering the effective $T_{m}$ well below physiological temperature to ensure the membrane remains fully fluid.

Lipid signalling, lipoproteins, and membrane biophysics Master

Beyond their structural role, lipids function as signalling molecules, energy transport vehicles, and determinants of membrane mechanical properties. This section treats three advanced topics: lipid-derived second messengers, lipoprotein particle structure, and the quantitative biophysics of bilayer elasticity.

Lipid signalling: PIP2, DAG, and ceramide

Phosphatidylinositol 4,5-bisphosphate (PIP2) is a minor phospholipid (less than 1% of total membrane lipid) with an outsized signalling role. The inositol head group of PI is phosphorylated at positions 4 and 5 by lipid kinases to generate PIP2. Phospholipase C (PLC), activated by G-protein-coupled receptors, hydrolyses PIP2 into two second messengers: diacylglycerol (DAG), which remains in the membrane and activates protein kinase C (PKC), and inositol 1,4,5-trisphosphate (IP3), which diffuses into the cytoplasm and opens calcium channels on the endoplasmic reticulum. The PIP2-PLC pathway is one of the most widely used signalling cascades in eukaryotic cells, regulating secretion, contraction, proliferation, and synaptic transmission ^{[Berg Tymoczko Stryer 2019 Ch. 12]}.

PIP2 also serves as a substrate for PI 3-kinase (PI3K), which phosphorylates the 3-position of the inositol ring to generate PIP3. PIP3 recruits proteins containing pleckstrin homology (PH) domains to the membrane, including Akt/PKB, a central regulator of cell survival and metabolism. The localisation and degradation of PIP3 by the phosphatase PTEN (a tumour suppressor) is a critical control point in oncogenic signalling.

Ceramide is both a metabolic intermediate in sphingolipid biosynthesis and a signalling molecule in its own right. Ceramide is generated by sphingomyelinase (which cleaves the phosphocholine head group from sphingomyelin) in response to stress signals including TNF-alpha, Fas ligand, and ionising radiation. Ceramide promotes apoptosis through several downstream effectors, including ceramide-activated protein phosphatases and the reorganisation of membrane rafts into large signalling platforms. The ceramide-membrane raft interaction is nontrivial: ceramide displaces cholesterol from ordered membrane domains, altering the biophysical properties of the raft and facilitating the clustering of death receptors.

Lipoprotein particles: HDL, LDL, VLDL

Lipids are insoluble in aqueous plasma and are transported as lipoprotein particles — spherical complexes with a surface monolayer of phospholipids and apolipoproteins surrounding a hydrophobic core of cholesteryl esters and triglycerides. The major classes differ in density, size, and lipid composition:

Particle	Diameter (nm)	Core lipids	Primary apolipoproteins	Function
Chylomicron	75-1200	Dietary triglycerides	B-48, A-I, A-IV	Transport dietary fat from intestine to tissues
VLDL	30-80	Endogenous triglycerides	B-100, E	Transport synthesised triglycerides from liver
LDL	18-25	Cholesteryl esters	B-100	Deliver cholesterol to peripheral tissues
HDL	5-12	Cholesteryl esters	A-I, A-II	Reverse cholesterol transport (periphery to liver)

LDL is the primary cholesterol carrier in blood. LDL particles bind the LDL receptor on cell surfaces, are internalised by receptor-mediated endocytosis, and are degraded in lysosomes to release free cholesterol. Mutations in the LDL receptor gene cause familial hypercholesterolaemia, in which LDL accumulates in plasma and deposits in arterial walls, forming atherosclerotic plaques. LDL is often called "bad cholesterol" in clinical contexts, while HDL ("good cholesterol") mediates reverse cholesterol transport via the ABCA1 transporter and lecithin-cholesterol acyltransferase (LCAT).

HDL biogenesis begins with the secretion of apolipoprotein A-I by the liver and intestine. Lipid-poor apoA-I acquires phospholipid and cholesterol from peripheral cells via ABCA1, forming nascent discoidal HDL. LCAT on the HDL surface esterifies cholesterol to cholesteryl ester, which partitions into the core, converting the disc to a spherical particle. Cholesteryl ester transfer protein (CETP) exchanges cholesteryl esters from HDL for triglycerides from VLDL and LDL, completing the shuttle of peripheral cholesterol back to the liver for excretion as bile acids.

Bilayer elastic properties

The lipid bilayer is not a passive barrier but a mechanically responsive structure whose elastic properties determine vesicle shape, membrane fusion, and the function of mechanosensitive ion channels.

Area compressibility modulus ( $K_{a}$ ). This quantifies the energy required to change the area per lipid molecule in the plane of the bilayer. For a typical phospholipid bilayer, $K_{a} \approx 200 - 250$ mN/m. The bilayer can stretch by only approximately 2-5% before rupture, because the elastic energy stored in area stretching is dominated by the increase in hydrophobic exposure at the bilayer-water interface. The area per lipid molecule in a fluid DPPC bilayer is approximately 64 $\overset{˚}{A}^{2}$ (head-group region) to 28 $\overset{˚}{A}^{2}$ (tail region at the bilayer midplane), as determined by X-ray scattering and molecular dynamics simulations.

Bending modulus ( $κ$ ). This quantifies the resistance of the bilayer to curvature. For typical phospholipid bilayers, $κ \approx 10 - 20$ $k_{B} T$ (where $k_{B}$ is Boltzmann's constant and $T$ is temperature). The bending modulus is related to the area compressibility modulus and the bilayer thickness $d$ by $κ \approx K_{a} \cdot d^{2} /12$ for a uniform elastic sheet, though real bilayers deviate from this simple model because the two leaflets can slide relative to each other (the "squeeze" mode).

Spontaneous curvature ( $C_{0}$ ). A bilayer composed of identical leaflets has zero spontaneous curvature ( $C_{0} = 0$ ). If the two leaflets differ in lipid composition, the bilayer adopts a preferred curvature. For example, a leaflet enriched in PE (small head, large tails, inverted-cone tendency) curves toward the PE-rich side. This spontaneous curvature is central to membrane budding and vesicle formation: proteins of the dynamin superfamily and BAR domains sense or induce curvature, and the ESCRT machinery uses curvature generation to form intralumenal vesicles during multivesicular body biogenesis.

Lipid rafts and membrane heterogeneity

The fluid mosaic model of Singer and Nicolson (1972) portrayed the membrane as a uniform two-dimensional solution of proteins in a lipid bilayer. Decades of biophysical evidence have refined this picture: membranes are laterally heterogeneous, containing transient nanoscale domains enriched in cholesterol, sphingolipids, and specific membrane proteins. These lipid rafts are proposed to function as platforms for signal transduction, membrane trafficking, and pathogen entry.

The biophysical basis for raft formation is the preferential interaction between cholesterol and sphingolipids. Sphingolipids have long, saturated acyl chains that pack more tightly than the unsaturated chains typical of glycerophospholipids. Cholesterol fills the voids created by packing mismatches, forming a liquid-ordered ( $L_{o}$ ) phase that is more ordered than the surrounding liquid-disordered ( $L_{d}$ ) phase but still laterally fluid (unlike the gel phase, $L_{β}$ ). The $L_{o}$ phase coexists with the $L_{d}$ phase over a range of temperatures and compositions, as mapped by ternary phase diagrams of cholesterol/sphingomyelin/unsaturated-PC mixtures.

The size and lifetime of lipid rafts in living cells remain debated. In model membranes (giant unilamellar vesicles), $L_{o}$ domains are micron-sized and stable. In living cells, fluctuations and cytoskeletal corralling may confine rafts to nanoscale (10-200 nm) transient clusters with lifetimes of milliseconds to seconds. The nontrivial challenge of observing nanoscale domains in living membranes has driven the development of super-resolution microscopy, fluorescence correlation spectroscopy, and single-particle tracking methods.

Connections Master

To cell membrane structure (17.02.01). The phospholipids and cholesterol described in this unit are the molecular building blocks of the bilayer treated in 17.02.01. The critical packing parameter introduced here predicts which lipids form bilayers and which form non-lamellar structures. Membrane fluidity, asymmetric lipid distribution, and lateral heterogeneity (rafts) are all direct consequences of the physical chemistry of the lipid molecules discussed in this unit. The bilayer is the substrate upon which membrane proteins operate, and its physical state (fluid vs gel, ordered vs disordered) modulates protein function.

To cellular respiration and energy metabolism (17.04.01). Fatty acids are a major metabolic fuel. Beta-oxidation cleaves two-carbon units from the carboxyl end of fatty acids to generate acetyl-CoA, which enters the citric acid cycle. The yield per carbon is higher for fatty acids than for glucose: complete oxidation of palmitate (C16:0) produces 106 ATP compared to 30-32 ATP for glucose, because fatty acids are more reduced (carry more hydrogens per carbon) and therefore yield more electrons for the electron transport chain. Triglycerides (glycerol esterified to three fatty acids) are the most energy-dense storage molecules in biology.

To cell signalling (17.07.01). PIP2, DAG, and ceramide are lipid second messengers that link extracellular signals to intracellular responses. The GPCR-phospholipase C-IP3/DAG pathway introduced in 17.07.01 begins with the hydrolysis of a single phospholipid molecule. The spatial and temporal precision of lipid signalling depends on the restricted diffusion of lipid messengers within the two-dimensional plane of the membrane, which concentrates the signal near the site of generation.

To immunology (17.10.01). Lipid antigens presented by CD1 molecules (structurally related to MHC class I but dedicated to lipid presentation) activate NKT cells, a subset of T lymphocytes with roles in infection, autoimmunity, and tumour surveillance. Sphingolipid storage diseases (Tay-Sachs, Gaucher, Niemann-Pick) have immune manifestations because accumulated lipids alter macrophage function. Lipopolysaccharide (LPS), the endotoxin of Gram-negative bacteria, is a glycolipid (lipid A anchored to a polysaccharide chain) that activates Toll-like receptor 4, initiating the innate immune response.

To neurobiology (17.09.01). The myelin sheath is a multilamellar membrane extension rich in sphingolipids (galactocerebroside, sulfatide, sphingomyelin) and cholesterol. The exceptionally high lipid-to-protein ratio in myelin provides the electrical insulation that enables saltatory conduction. Demyelinating diseases (multiple sclerosis, Guillain-Barre syndrome) involve the loss of these specialised membrane lipids. Gangliosides GM1 and GD1a are concentrated in neuronal synaptic membranes and serve as receptors for bacterial toxins (cholera toxin binds GM1).

Historical notes Master

The recognition of lipids as a distinct chemical class dates to the early 19th century, when Michel Eugene Chevreul established that fats are composed of glycerol esterified to fatty acids (1823). Chevreul's work on the saponification of fats — the reaction that produces soap — laid the foundation for lipid chemistry. His 1823 treatise Recherches chimiques sur les corps gras d'origine animale classified fats by their physical properties and chemical behaviour.

The structure of cholesterol was one of the great challenges of organic chemistry. Isolated from gallstones in the 18th century, cholesterol's molecular formula ( $C_{27} H_{46} O$ ) was established by 1859, but its ring structure resisted determination for decades. Heinrich Wieland and Adolf Windaus independently deduced the tetracyclic structure between 1900 and 1932, work for which they shared the 1927 and 1928 Nobel Prizes in Chemistry respectively. The complete stereochemistry was confirmed by X-ray crystallography in the 1950s, and Robert Burns Woodward completed the total synthesis of cholesterol in 1951, a landmark in synthetic organic chemistry.

The term "phospholipid" was introduced by the French biochemist Maurice Gobley in 1874, who isolated lecithin (phosphatidylcholine) from egg yolk and recognised the presence of phosphate. The amphipathic nature of phospholipids and their tendency to form films at air-water interfaces were studied by Agnes Pockels in the 1890s and by Irving Langmuir in the 1910s. Langmuir's trough, which measures the surface pressure of lipid monolayers, remains a standard biophysical tool. Langmuir received the 1932 Nobel Prize in Chemistry for his work on surface chemistry.

The spontaneous formation of lipid bilayers in water was demonstrated by Alec Bangham in 1965. Bangham observed that phospholipids dispersed in aqueous solution form closed vesicles ("liposomes"), and that these vesicles could encapsulate soluble molecules. This discovery had two profound consequences: it provided an experimental model for the biological membrane, and it inaugurated the field of liposome-based drug delivery. The first FDA-approved liposomal drug (Doxil, a doxorubicin-encapsulating liposome for cancer therapy) was approved in 1995.

The critical packing parameter was formalised by Jacob Israelachvili and colleagues in the 1970s, building on the earlier work of Charles Tanford on the hydrophobic effect ^{[Tanford 1980 Ch. 6-7]}. Israelachvili's 1980 textbook Intermolecular and Surface Forces provided the quantitative framework that connects molecular geometry to aggregate structure, unifying micelles, bilayers, and non-lamellar phases under a single predictive parameter.

The fluid mosaic model of membrane structure was proposed by S. Jonathan Singer and Garth Nicolson in 1972, replacing earlier models that depicted membranes as static protein-lipid sandwiches. The Singer-Nicolson model emphasised the lateral mobility of both lipids and proteins within the two-dimensional fluid of the bilayer. The subsequent discovery of lipid rafts (Kai Simons and Elina Ikonen, 1997) introduced the concept of lateral heterogeneity, refining but not replacing the fluid mosaic framework.

Lipidomics — the comprehensive analysis of all lipids in a biological sample — emerged as a discipline in the 2000s, enabled by advances in mass spectrometry (particularly electrospray ionisation and tandem MS). Edward Dennis proposed the comprehensive classification of lipids into eight categories (fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, sterol lipids, prenol lipids, saccharolipids, and polyketides) in 2005, and the LIPID MAPS consortium maintains the standard nomenclature and database. Modern lipidomics can identify and quantify over 1,000 distinct lipid species from a single tissue sample, revealing lipid composition changes associated with disease states.

Bibliography Master

Alberts, B. et al. Molecular Biology of the Cell, 7th ed. Garland Science, 2022. Ch. 2: The Chemical Components of the Cell — Lipids. The standard cell-biology treatment of fatty acids, phospholipids, sphingolipids, sterols, and membrane structure.
Berg, J. M., Tymoczko, J. L. & Stryer, L. Biochemistry, 9th ed. W. H. Freeman, 2019. Ch. 12: Lipids and Cell Membranes. Thorough coverage of lipid classification, membrane physical properties, and lipid signalling with quantitative detail.
Vance, D. E. & Vance, J. E. Biochemistry of Lipids, Lipoproteins and Membranes, 5th ed. Elsevier, 2008. Ch. 1-3: Structural Lipids, Membrane Dynamics, Cholesterol. The advanced reference for lipid metabolism, membrane biophysics, and lipoprotein structure.
Tanford, C. The Hydrophobic Effect, 2nd ed. Wiley, 1980. Ch. 6-7: Micelles and Bilayers. The foundational treatment of the thermodynamic basis for amphiphile self-assembly, still cited as the standard reference for the physics of micelle and bilayer formation.
Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed. Academic Press, 2011. Ch. 18-20. The quantitative framework for the critical packing parameter, lipid aggregate geometry, and membrane elastic properties.
Simons, K. & Ikonen, E. Functional rafts in cell membranes. Nature 387 (1997) 569-572. The paper that introduced the lipid raft hypothesis and proposed a functional role for sphingolipid-cholesterol microdomains in membrane trafficking and signalling.
Singer, S. J. & Nicolson, G. L. The fluid mosaic model of the structure of cell membranes. Science 175 (1972) 720-731. The landmark paper establishing the modern conceptual framework for membrane structure.
Bangham, A. D., Standish, M. M. & Watkins, J. C. Diffusion of univalent ions across the lamellae of swollen phospholipids. J. Mol. Biol. 13 (1965) 238-252. The discovery of liposomes and the demonstration that phospholipid bilayers form sealed compartments.
Chevreul, M. E. Recherches chimiques sur les corps gras d'origine animale. Levrault, Paris, 1823. The foundational work establishing that fats are glycerol-fatty acid esters.
Dennis, E. A. Lipidomics joins the omics evolution. Proc. Natl. Acad. Sci. USA 106 (2009) 2089-2090. An overview of the lipidomics field and the LIPID MAPS classification system.

Prerequisites

17.01.01 pending

Used in

17.02.01

Tier anchors

beginner: Alberts et al., Molecular Biology of the Cell, 7th ed. (2022), Ch. 2
intermediate: Berg, Tymoczko & Stryer, Biochemistry, 9th ed. (2019), Ch. 12
master: Vance & Vance, Biochemistry of Lipids, Lipoproteins and Membranes, 5th ed. (2008)

References

Alberts et al. — Molecular Biology of the Cell, 7th ed. (Garland Science, 2022) · Ch. 2 The Chemical Components of the Cell — Lipids
Berg, Tymoczko & Stryer — Biochemistry, 9th ed. (W. H. Freeman, 2019) · Ch. 12 Lipids and Cell Membranes
Vance, D. E. & Vance, J. E. — Biochemistry of Lipids, Lipoproteins and Membranes, 5th ed. (Elsevier, 2008) · Ch. 1-3 Structural Lipids; Membrane Dynamics; Cholesterol
Tanford, C. — The Hydrophobic Effect, 2nd ed. (Wiley, 1980) · Ch. 6-7 Micelles and Bilayers

Estimated time

beginner: 15m
intermediate: 30m
master: 45m