Lipids & Membrane Biochemistry

Overview

Lipids are defined by a shared physical property (poor water solubility) rather than a shared chemical scaffold the way amino acids or sugars are — which is why “lipid” covers everything from triglycerides to cholesterol to signalling eicosanoids. This page covers the chemistry that matters for membrane structure specifically: fatty acid saturation, the amphipathic phospholipid, and why cholesterol has the paradoxical dual effect of both stiffening and fluidising membranes depending on temperature. Plasma Membrane Structure & Transport covers the resulting bilayer’s transport behaviour; this page covers the chemistry underneath it.

Key Concepts

Fatty acid structure and saturation

A fatty acid is a long hydrocarbon chain terminating in a carboxylic acid group. Saturated fatty acids have no carbon-carbon double bonds, allowing the chain to lie fully extended and pack tightly against neighbouring chains via van der Waals interactions — this tight packing is why saturated fats are typically solid at room temperature. Unsaturated fatty acids contain one (monounsaturated) or more (polyunsaturated) C=C double bonds, almost always in the cis configuration in natural lipids, which introduces a rigid ~30° kink in the chain. That kink prevents tight packing, lowering the melting point — unsaturated fats are typically liquid at room temperature.

Source: tutorchase.com

Triglycerides: pure energy storage

A triglyceride (triacylglycerol) is glycerol esterified to three fatty acids. Because the molecule is almost entirely hydrocarbon by mass and carries no polar head group, triglycerides pack into anhydrous, extremely energy-dense storage droplets — roughly twice the energy density per gram of glycogen, which by contrast must be stored hydrated (each gram of glycogen holds substantial associated water), making fat the far more mass-efficient long-term reserve, at the cost of slower mobilisation kinetics than glycogen.

Phospholipids: the amphipathic membrane building block

A phospholipid replaces one fatty acid of a triglyceride-like backbone with a phosphate-linked polar head group (e.g. choline, serine, ethanolamine, inositol), producing a molecule with a hydrophilic head and two hydrophobic tails — amphipathic. In water, this amphipathicity drives spontaneous self-assembly into a bilayer: tails face inward, away from water; heads face outward, into the aqueous environment on both sides. This is not an energetically “designed” outcome so much as the thermodynamically favoured configuration — burying the hydrophobic tails minimises the disruption to water’s hydrogen-bond network (the same hydrophobic-effect logic that drives protein tertiary-structure folding, see Protein Structure, Folding & Function).

Head group identity determines membrane leaflet asymmetry and signalling function, not just electrostatics — e.g. phosphatidylserine is normally kept on the cytoplasmic leaflet by ATP-dependent flippases, and its appearance on the outer leaflet is a recognised “eat me” signal for phagocytes during apoptosis (see Cell Junctions, Extracellular Matrix & Cell Death).

Source: bio.libretexts.org (Boundless Biology)

Cholesterol and membrane fluidity

Cholesterol is a rigid, planar four-ring sterol that intercalates between phospholipid tails, with its single polar hydroxyl oriented toward the bilayer surface. Its effect on fluidity is temperature-dependent and bidirectional:

• Above the phase-transition temperature (fluid, disordered membrane): cholesterol’s rigid ring system restricts the motion of adjacent fatty acid tails, decreasing fluidity.
• Below the phase-transition temperature (membrane tending toward a rigid, ordered gel state): cholesterol prevents fatty acid tails from packing into a fully ordered crystalline array, increasing fluidity.

Net effect: cholesterol acts as a fluidity buffer, narrowing the range of temperatures over which the membrane’s physical state changes sharply — a frequently mis-simplified concept (“cholesterol makes membranes more fluid”) that is only half correct.

Source: Chegg homework-help question

Source: ScienceDirect (research article)

Lipids as signalling molecules

Not all lipid biology is structural. Eicosanoids (prostaglandins, thromboxanes, leukotrienes), derived from the membrane phospholipid-released fatty acid arachidonic acid via the cyclooxygenase (COX) and lipoxygenase pathways, act as short-range paracrine signals in inflammation, pain, and clotting — the pharmacological target of aspirin and other NSAIDs (irreversible/reversible COX inhibitors, respectively — see the irreversible-inhibition mechanism in Enzyme Kinetics & Regulation). Steroid hormones (derived from cholesterol) and phosphoinositide second messengers (e.g. IP₃/DAG, generated by PLC cleavage of PIP₂) extend lipid signalling into gene regulation and intracellular second-messenger cascades (see Cell Signaling & Communication).

Comparative Structures

Common Exam Questions

• “Why are unsaturated fats liquid at room temperature?” — always cite the cis double bond kink preventing tight packing, not a vague “different structure” answer.
• “Does cholesterol increase or decrease membrane fluidity?” — the trick answer depends on temperature; a well-posed question will specify above or below the transition temperature, and a well-prepared answer should note the dependency even if not explicitly asked.
• Distinguishing why fat (not glycogen) is the long-term storage molecule of choice hinges on energy density and the anhydrous-vs-hydrated storage argument together, not either alone.

Visual Reference

Interactive

• A phospholipid bilayer self-assembly simulator: scatter phospholipid icons in a 2D “water” field and animate their spontaneous reorganisation into a bilayer, illustrating the hydrophobic-effect argument without requiring a thermodynamics derivation.

Static

(Static images are placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here.)

Practice Problems

1. A membrane sample is enriched in polyunsaturated phospholipids compared to a control sample of otherwise identical composition. Predict how its fluidity at a fixed temperature compares to the control, and explain the chemical basis.

2. Explain why storing the same amount of chemical energy as fat rather than glycogen requires roughly half the mass, referencing both the chemistry of the molecules and how each is stored physically in the body.

3. Phosphatidylserine normally resides on the inner (cytoplasmic) leaflet of the plasma membrane. During apoptosis, it appears on the outer leaflet. What class of membrane protein is most directly implicated in maintaining the normal asymmetric distribution, and what does its loss of function during apoptosis signal to the surrounding tissue?