Phospholipids and Cholesterol

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson develops the structure of phospholipids (glycerol + 2 fatty acids + phosphate head) and cholesterol (four fused rings), focusing on the amphipathic property of phospholipids that drives spontaneous bilayer self-assembly in water, and cholesterol's role as a membrane fluidity buffer. The content connects directly forward to Module 2.1.5 (biological membranes) (refer to the official OCR H420 specification document for exact wording).

Lipids are not only energy stores — they are essential components of every biological membrane. This lesson focuses on phospholipids and cholesterol, the two lipid classes that define the structure and properties of the cell surface membrane and internal membranes.

By the end of this lesson you should be able to: describe the structure of a phospholipid (glycerol + 2 fatty acids + phosphate) and of cholesterol (four fused rings + hydroxyl); explain the term amphipathic and how it drives spontaneous bilayer self-assembly via the hydrophobic effect; explain, both qualitatively and with a worked quantitative comparison, how the degree of fatty-acid saturation and the intercalation of cholesterol together set membrane fluidity; and evaluate the biological and clinical significance of cholesterol as both an essential molecule and a cardiovascular risk factor.

0. Membrane Fluidity Quantified — a Worked Comparison (Saturation and Phase Transition)

Before dissecting the molecules, it is worth grasping why their fine structure matters so much. Every biological membrane must be held in the liquid-crystalline state — fluid enough for lateral diffusion of proteins and lipids, ordered enough to remain an intact permeability barrier. Below a characteristic transition (melting) temperature, symbolised $T_m$ , the bilayer freezes into a rigid gel phase in which the fatty-acid tails pack into an ordered, extended, all-trans array; above $T_m$ the tails become disordered and mobile. The single most important determinant of $T_m$ for a given head-group is the fatty-acid tail composition, and this is where saturation becomes decisive.

Worked comparison. Consider three phosphatidylcholine species that differ only in their tails. Approximate transition temperatures (well-established textbook values) are:

Phospholipid	Tail description	Approx. $T_m$
Distearoylphosphatidylcholine (18:0 / 18:0)	two long, fully saturated C18 tails	$\approx 55\,^{\circ}\text{C}$
Dipalmitoylphosphatidylcholine (16:0 / 16:0)	two saturated C16 tails (two carbons shorter)	$\approx 41\,^{\circ}\text{C}$
Dioleoylphosphatidylcholine (18:1 / 18:1)	two C18 tails, each with one cis double bond (unsaturated)	$\approx -20\,^{\circ}\text{C}$

Read the table as a controlled experiment. Comparing the first two rows isolates tail length: removing two carbons from each tail (C18 → C16) lowers $T_m$ by roughly $14\,^{\circ}\text{C}$ , because shorter tails present less surface for van der Waals contact and so pack less tightly. Comparing rows one and three isolates saturation: introducing a single cis double bond per tail collapses $T_m$ by about $75\,^{\circ}\text{C}$ — a far larger effect than shortening the chain. The cis double bond puts a rigid ~30° kink in the tail; kinked tails cannot line up into the ordered gel lattice, van der Waals contacts are drastically reduced, and the bilayer stays fluid to much lower temperatures.

The biological pay-off is immediate. Human body temperature is $\approx 37\,^{\circ}\text{C}$ . A membrane built only of 18:0/18:0 phospholipids ( $T_m \approx 55\,^{\circ}\text{C}$ ) would be frozen solid and non-functional at body temperature; a membrane rich in 18:1/18:1 phospholipids ( $T_m \approx -20\,^{\circ}\text{C}$ ) stays comfortably fluid. Real membranes tune $T_m$ well below the operating temperature by blending saturated and unsaturated tails. This is exactly why Arctic fish and other cold-adapted organisms raise the proportion of unsaturated fatty acids in their membrane phospholipids — a process called homeoviscous adaptation — so that $T_m$ falls with the environmental temperature and the membrane never enters the gel phase. It also explains why the same organism can re-model its membrane lipids seasonally.

Cholesterol enters this quantitative picture as a broadening agent rather than a simple shifter of $T_m$ . By intercalating between the tails, it abolishes the sharp gel-to-fluid transition altogether: near $T_m$ it sits in what biophysicists call the liquid-ordered phase, restraining tails above the transition (reducing fluidity) yet spacing them apart below it (preventing the gel lock-up). The qualitative "fluidity buffer" description you will meet in Section 4 is, at the biophysical level, precisely this smearing-out of the transition. Keep this worked comparison in mind — it converts the vague phrase "unsaturated tails make membranes more fluid" into a defensible quantitative claim, which is exactly the kind of AO2 precision that separates A from A* answers.

1. Phospholipids — the Building Blocks of Membranes

A phospholipid is similar to a triglyceride, but with one key difference: one of the three fatty acids is replaced by a phosphate group. The result is a molecule with two hydrocarbon tails and a phosphate-containing head.

Key Definition — Phospholipid: A lipid molecule composed of glycerol, two fatty acid tails, and a phosphate group (often attached to a further polar group). It is amphipathic — having both hydrophilic and hydrophobic regions.

1.1 Structure

Glycerol backbone — three carbons
Two fatty acid tails (can be saturated or unsaturated)
Phosphate group on the third carbon of glycerol; typically bonded to an additional polar group such as choline (giving phosphatidylcholine, the commonest phospholipid in animal membranes)
Three ester bonds in total: two between fatty acids and glycerol, one between phosphate and glycerol (also called a phosphoester bond)

1.2 Amphipathic Nature

The phosphate head is charged and polar, so it is hydrophilic — it forms hydrogen bonds with water.

The two fatty acid tails are non-polar hydrocarbons, so they are hydrophobic — they repel water but interact favourably with other non-polar molecules.

A molecule with both hydrophilic and hydrophobic regions is called amphipathic (or amphiphilic). This dual nature is the reason phospholipids spontaneously form membranes.

2. Phospholipid Behaviour in Water

When phospholipids are placed in water, they spontaneously organise themselves to minimise unfavourable interactions between water and the hydrophobic tails. Several arrangements are possible:

2.1 Monolayer

At the air-water interface, phospholipids form a single layer with heads in the water and tails projecting into the air.

2.2 Micelle

In the bulk of water, phospholipids can form micelles — small spherical clusters with tails in the core and heads facing outward.

2.3 Bilayer (Membrane)

The most biologically important arrangement is the phospholipid bilayer. Two layers of phospholipids orient with their heads facing the watery environments on each side of the membrane and their tails buried in the centre of the bilayer, shielded from water.

This bilayer structure is the fundamental architecture of all biological membranes, including the plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, mitochondrial membranes and thylakoid membranes.

2.4 Fluid Mosaic Model

Phospholipid molecules in a bilayer are not fixed — they can move laterally within each layer, giving the membrane a fluid quality (like a two-dimensional liquid). Proteins are embedded in or attached to the bilayer, creating the fluid mosaic model of membrane structure (Singer and Nicolson, 1972).

2.5 Role of Unsaturated Fatty Acids

Unsaturated fatty acid tails have kinks (due to C=C double bonds) that prevent tight packing.
Membranes with more unsaturated phospholipids are more fluid.
Organisms in cold environments (e.g., Arctic fish) have more unsaturated fatty acids in their membranes to maintain fluidity at low temperatures.

3. Biological Functions of Phospholipids

Form membranes — bilayer structure is the backbone of every biological membrane.
Barrier to water-soluble molecules — polar and charged molecules cannot cross the hydrophobic core without assistance.
Allow selective transport — in conjunction with embedded transport proteins.
Cell signalling — specific phospholipids (e.g., phosphatidylinositol) generate second messengers.
Lung surfactant — phospholipids lining alveoli reduce surface tension, preventing collapse.

4. Cholesterol — a Membrane Steroid

Cholesterol is a steroid lipid — it belongs to a class of lipids with a characteristic four-fused-ring structure. It is not built from glycerol and fatty acids, so it is structurally very different from triglycerides and phospholipids. Nevertheless, it plays crucial roles in animal cell membranes.

4.1 Structure

Cholesterol consists of:

Four fused hydrocarbon rings (three six-membered and one five-membered) — the steroid backbone.
A short hydrocarbon tail at one end.
A single hydroxyl group (–OH) at the other end, providing a small hydrophilic region.

The hydroxyl group makes cholesterol weakly amphipathic: mostly hydrophobic but with one polar end. This allows cholesterol to orient itself within a phospholipid bilayer with the –OH close to the phosphate heads and the rest of the molecule buried among the fatty acid tails.

4.2 Roles of Cholesterol in Membranes

Cholesterol has two main structural roles, both related to membrane fluidity:

At high temperatures: cholesterol reduces fluidity by binding to adjacent phospholipid tails and restricting their movement. This prevents the membrane from becoming too fluid or leaky.
At low temperatures: cholesterol maintains fluidity by preventing fatty acid tails from packing too tightly together. This stops the membrane from becoming rigid and cracked in the cold.

Cholesterol therefore acts as a fluidity buffer, keeping the membrane at a stable fluidity across a range of temperatures.

4.3 Other Roles of Cholesterol

Beyond its membrane role, cholesterol is the precursor for:

Steroid hormones — testosterone, oestrogen, progesterone, cortisol, aldosterone.
Bile salts — emulsify fats in the small intestine.
Vitamin D — synthesised in the skin from a cholesterol derivative by UV light.

4.4 Cholesterol and Health

While cholesterol is essential, excess blood cholesterol — especially as low-density lipoprotein (LDL) — is associated with the build-up of plaques in artery walls (atherosclerosis) and increased risk of coronary heart disease and stroke. High-density lipoprotein (HDL) cholesterol is considered protective because it transports cholesterol back to the liver for excretion.

5. Comparing Triglycerides, Phospholipids and Cholesterol

Feature	Triglyceride	Phospholipid	Cholesterol
Glycerol backbone?	Yes	Yes	No
Fatty acids	3	2	0
Phosphate group?	No	Yes	No
Hydrophilic/hydrophobic	Entirely hydrophobic	Amphipathic	Mostly hydrophobic, small polar –OH
Bonds	3 ester	2 ester + 1 phosphoester	Fused ring structure
Main role	Energy storage	Membrane structure	Membrane fluidity + steroid precursor

graph TD
  L[Lipids] --> T["Triglycerides<br/>glycerol + 3 fatty acids"]
  L --> P["Phospholipids<br/>glycerol + 2 fatty acids + phosphate"]
  L --> S["Steroids<br/>4 fused rings"]
  T --> T1[Energy storage]
  P --> P1[Membrane bilayer]
  S --> S1["Cholesterol<br/>membrane fluidity"]
  S --> S2[Steroid hormones]
  S --> S3[Bile salts, Vitamin D]

6. Experimental Identification of Lipids — the Emulsion Test

The emulsion test is the standard biochemical test for lipids:

Lesson 7: Phospholipids and Cholesterol