describe the arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes

Fluid‑Mosaic Model of the Plasma Membrane (Cambridge AS & A‑Level 9700 – Topic 4.1)

1. The phospholipid bilayer

• Basic structure: two sheets of phospholipids. Each molecule has a
  • hydrophilic head‑group (e.g. phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine) that faces the aqueous environments (extracellular fluid and cytosol)
  • two non‑polar fatty‑acid tails that form the interior hydrophobic core.
• Bilayer asymmetry: the composition of the inner and outer leaflets differs.
  • Outer leaflet – rich in phosphatidylcholine, sphingomyelin and glycolipids.
  • Inner leaflet – enriched in phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol.
• Lateral mobility: phospholipids and most membrane proteins can move sideways, giving the membrane its fluid character.

2. Major surface‑membrane components and their arrangement

2.1 Cholesterol

• Location: intercalated **throughout both leaflets** of the bilayer; the rigid sterol ring lies parallel to the fatty‑acid tails.
• Examples: cholesterol (animal cells), ergosterol (fungi).
• Functional roles (mapped to syllabus):
  • Stability – the rigid ring provides structural support.
  • Fluidity – buffers membrane fluidity; restricts movement at high temperature, prevents tight packing at low temperature.
  • Permeability – reduces passive diffusion of small water‑soluble molecules.
  • Signalling & transport – concentrates in lipid‑raft micro‑domains, helping to organise receptors and transport proteins.

2.2 Sphingolipids (including glycolipids)

• Location: almost exclusively in the **outer leaflet**; the long saturated sphingosine backbone anchors the molecule in the bilayer.
• Types:
  • Glycolipids – sphingolipid backbone + covalently attached carbohydrate chain (e.g. cerebrosides, gangliosides).
  • Sphingomyelin – phosphocholine head‑group, no carbohydrate.
• Functional roles (mapped to syllabus):
  • Recognition – carbohydrate chains form the “sugar coat” that mediates cell‑cell recognition (blood‑group antigens, pathogen binding).
  • Protection – shields the membrane from mechanical damage and enzymatic attack.
  • Stability & fluidity – saturated sphingosine tails pack tightly, contributing to membrane order.
  • Signalling & transport – together with cholesterol they create **lipid‑raft micro‑domains** that concentrate receptors, ion channels and transporters.

2.3 Glycoproteins

• Location:
  • Integral (transmembrane) glycoproteins – span the bilayer via one or more α‑helical segments.
  • Peripheral glycoproteins – attached to the membrane surface by lipid anchors (e.g. GPI‑anchor) or by non‑covalent interactions with integral proteins.
• Carbohydrate attachment:
  • N‑linked (to asparagine) – added in the endoplasmic reticulum.
  • O‑linked (to serine/threonine) – added in the Golgi apparatus.
• Functional roles (mapped to syllabus):
  • Recognition – extracellular carbohydrate chains act as specific binding sites for hormones, antibodies, lectins.
  • Signalling – receptor glycoproteins bind ligands and trigger intracellular cascades (e.g. insulin receptor, GPCRs).
  • Transport – carrier glycoproteins undergo conformational changes to move solutes across the membrane (e.g. GLUT1, Na⁺/K⁺‑ATPase).
  • Channel activity – form pores that allow rapid passage of ions or water (e.g. voltage‑gated Na⁺ channels, aquaporins).
  • Adhesion – adhesion glycoproteins mediate cell‑cell or cell‑matrix contacts (e.g. integrins, selectins).
  • Stability & fluidity – interactions with the cytoskeleton and with cholesterol help maintain membrane integrity.

3. Lipid‑raft micro‑domains

• Enriched in cholesterol and sphingolipids (including glycolipids).
• Form relatively ordered, less‑fluid patches that compartmentalise membrane proteins such as receptors, ion channels and transporters.
• Play a key role in signal transduction, endocytosis and pathogen entry – all mentioned in the syllabus under “cell‑surface receptors”.

4. Summary table – component, location & syllabus‑linked functions

5. Key points to remember (exam‑style checklist)

• The membrane is a **dynamic fluid‑mosaic** of phospholipids, cholesterol, sphingolipids and proteins.
• Phospholipid heads are polar (phosphate‑choline, phosphate‑ethanolamine, etc.) and the two leaflets are **asymmetric** in composition.
• **Cholesterol** is present in both leaflets, modulating fluidity, stability and permeability.
• **Sphingolipids/glycolipids** are restricted to the outer leaflet; their carbohydrate chains form the extracellular sugar coat and participate in cell‑cell recognition.
• **Glycoproteins** may be integral or peripheral; they perform the five major membrane functions required by the syllabus:
  1. Stability
  2. Fluidity
  3. Permeability
  4. Transport (carriers & channels)
  5. Signalling & recognition (receptors, adhesion molecules)
• **Lipid rafts** are cholesterol‑ and sphingolipid‑rich micro‑domains that organise receptors and transport proteins for efficient signalling.