4.1 Fluid‑mosaic membranes – Phospholipids
Learning Objective
Describe the molecular structure of phospholipids, emphasising the hydrophilic (polar) phosphate head and the hydrophobic (non‑polar) fatty‑acid tails, and relate these features to membrane function.
Why Phospholipids are Central to Membranes
- Amphipathic molecules: each molecule possesses a polar (water‑loving) region and a non‑polar (water‑repelling) region.
- In aqueous environments they self‑assemble into a bilayer, forming the basic scaffold of the fluid‑mosaic membrane.
Core Architecture of a Phospholipid
Glycerol Backbone
- Three‑carbon scaffold (C₃H₈O₃) with a hydroxyl (‑OH) on each carbon.
- Provides three attachment points:
- Carbon‑1 → fatty‑acid 1 (ester bond)
- Carbon‑2 → fatty‑acid 2 (ester bond)
- Carbon‑3 → phosphate group (phospho‑ester bond)
Fatty‑Acid Tails – Hydrophobic (non‑polar) region
- Long hydrocarbon chains (R₁‑COO⁻ and R₂‑COO⁻) attached to C‑1 and C‑2 of glycerol by ester linkages.
- Two common types:
- Saturated – only single C–C bonds; straight chains pack tightly → reduces membrane fluidity.
- Unsaturated – one or more C=C double bonds; kinks prevent tight packing → increases fluidity.
- Because they contain only C–C and C–H bonds, the tails are non‑polar and avoid water.
Phosphate Head Group – Hydrophilic (polar) region
- Phosphate (‑PO₄²⁻) attached to C‑3 of glycerol via a phospho‑ester bond.
- Often further linked to a small polar/charged moiety, giving rise to different phospholipids:
- Choline → phosphatidylcholine (PC)
- Ethanolamine → phosphatidylethanolamine (PE)
- Serine → phosphatidylserine (PS)
- Inositol → phosphatidylinositol (PI)
- The combination of the negatively charged phosphate and the usually positively charged head‑group makes this region strongly polar, enabling interaction with water and with membrane proteins.
Other Major Membrane Components (Syllabus Requirement 4.1)
- Cholesterol
- Planar sterol molecule with a small polar hydroxyl group.
- Intercalates between phospholipid tails, reducing excessive fluidity at high temperatures and preventing tight packing at low temperatures.
- Glycolipids
- Lipid molecules (often phospholipids or sphingolipids) bearing one or more carbohydrate chains on the outer leaflet.
- Function in cell‑surface recognition, signalling and protection.
- Glycoproteins (integral & peripheral)
- Proteins covalently linked to carbohydrate side‑chains.
- Serve as receptors, channels, enzymes and markers for cell‑cell interaction.
Functional Highlights (Linking Structure to Function)
- Membrane stability: the hydrophobic fatty‑acid core forms a barrier that excludes water‑soluble molecules, giving the membrane its integrity.
- Selective permeability: the non‑polar interior allows diffusion of small, non‑polar gases (O₂, CO₂) but blocks ions and polar solutes; protein channels embedded in the bilayer provide regulated pathways.
- Fluidity control: tail saturation, chain length and cholesterol content fine‑tune membrane fluidity, which is essential for the proper functioning of embedded proteins.
- Cell‑surface interactions: the polar head groups (especially those of glycolipids/glycoproteins) interact with the extracellular environment, mediating signalling, adhesion and immune recognition.
Influence of Phospholipid Composition on Transport (Syllabus 4.2)
The degree of fatty‑acid unsaturation and the amount of cholesterol directly affect membrane fluidity. A more fluid membrane (high unsaturation, low cholesterol) permits faster passive diffusion of small, non‑polar molecules and allows carrier proteins to change conformation more readily, enhancing facilitated diffusion. Conversely, a rigid membrane (high saturation, high cholesterol) slows passive diffusion and can limit the activity of transport proteins, making active transport comparatively more important for moving substances across the membrane.
Summary Table
| Component | General Structure | Polarity | Role in Membrane |
|---|
| Glycerol Backbone | C₃H₈O₃ | Amphipathic (central scaffold) | Anchors two fatty‑acid tails and the phosphate head |
| Fatty‑Acid Tail 1 | R₁‑COO⁻ (e.g., saturated C₁₆) | Non‑polar (hydrophobic) | Forms inner leaflet; contributes to barrier properties |
| Fatty‑Acid Tail 2 | R₂‑COO⁻ (e.g., unsaturated C₁₈) | Non‑polar (hydrophobic) | Same as Tail 1; unsaturation modulates fluidity |
| Phosphate Head | ‑PO₄²⁻ + Rhead (e.g., choline) | Polar/charged (hydrophilic) | Faces aqueous environment; interacts with proteins & water |
| Cholesterol | Planar sterol ring + OH group | Amphipathic (small polar OH) | Modulates fluidity & membrane thickness |
| Glycolipid | Lipid + carbohydrate chain | Polar carbohydrate exterior | Cell‑surface recognition & signalling |
Key Points to Remember
- Phospholipids consist of a glycerol backbone, two fatty‑acid tails (hydrophobic) and a phosphate‑containing head group (hydrophilic).
- The amphipathic nature drives spontaneous bilayer formation, the foundation of the fluid‑mosaic membrane.
- Tail saturation and cholesterol content together regulate membrane fluidity, influencing diffusion rates and protein activity.
- Additional membrane components – cholesterol, glycolipids and glycoproteins – are required by the syllabus and contribute to stability, fluidity and cell‑surface functions.
Practical Idea (AO3 – Experimental Skills)
Extract phospholipids from fresh spinach leaves, separate the different head‑group types by thin‑layer chromatography (TLC) using a suitable solvent system, and calculate Rf values. Staining the plate with iodine or phosphomolybdic acid visualises the spots, providing hands‑on experience with lipid identification – a skill that aligns with Paper 3 practical questions.