describe the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in cell surface membranes, with reference to stability, fluidity, permeability, transport (carrier proteins and channel proteins), cell signalling (cell surface

Published by Patrick Mutisya · 8 days ago

Fluid Mosaic Membrane – Cambridge A-Level Biology 9700

Fluid Mosaic Model – Overview

The fluid mosaic model describes the cell surface membrane as a dynamic, semi‑permeable sheet composed of a phospholipid bilayer in which proteins, cholesterol, glycolipids and glycoproteins are interspersed. The “fluid” aspect refers to the lateral movement of lipids and proteins, while the “mosaic” aspect reflects the heterogeneous distribution of different molecular species.

Key Molecular Components

ComponentStructural RoleFunctional Contributions
PhospholipidsForm the basic bilayer matrix; amphipathic molecules with hydrophilic heads outward and hydrophobic tails inward.

  • Provide structural stability through hydrophobic interactions.

  • Determine baseline membrane fluidity – saturated tails increase rigidity, unsaturated tails increase fluidity.

  • Control passive permeability – small non‑polar molecules diffuse readily, ions and polar molecules are largely excluded.

CholesterolIntercalates between phospholipid tails within the inner leaflet.

  • Modulates fluidity: at high temperatures it restrains phospholipid movement, at low temperatures it prevents tight packing.

  • Adds mechanical stability, reducing membrane brittleness.

  • Creates micro‑domains (lipid rafts) that concentrate certain proteins.

GlycolipidsLocated mainly in the outer leaflet; consist of a lipid anchor covalently linked to carbohydrate chains.

  • Contribute to membrane stability by forming hydrogen bonds with extracellular water.

  • Involved in cell‑cell recognition and adhesion.

  • Participate in the formation of lipid rafts.

Integral (intrinsic) proteinsSpan the bilayer, often with one or more transmembrane α‑helices.

  • Form channels and carriers for selective transport of ions, nutrients and waste.

  • Act as receptors for hormones, neurotransmitters and growth factors.

  • Provide anchoring points for the cytoskeleton.

Peripheral (extrinsic) proteinsAssociate loosely with the membrane surface, usually via interactions with integral proteins or phospholipid head groups.

  • Serve as enzymes, structural supports, or signaling adaptors.

  • Facilitate cytoskeletal attachment and intracellular signaling cascades.

GlycoproteinsProteins (integral or peripheral) covalently linked to carbohydrate chains.

  • Key players in cell‑surface receptors and signal transduction.

  • Mediate cell‑cell recognition, immune response (antigens) and tissue compatibility.

  • Increase membrane hydration and protect proteins from proteolysis.

Membrane Stability and Fluidity

Stability is achieved through the hydrophobic core of phospholipids and the reinforcing effect of cholesterol. Fluidity is a balance between:

  1. Temperature – higher temperatures increase kinetic energy, reducing van der Waals forces.

  2. Fatty‑acid composition – unsaturated (cis‑double bonds) create kinks that prevent tight packing.

  3. Cholesterol content – acts as a “fluidity buffer”.

Permeability

Passive diffusion across the membrane depends on:

  • Size – molecules < ≈ 500 Da can cross more easily.

  • Polarity – non‑polar molecules (O₂, CO₂) diffuse readily; polar molecules (glucose) require transport proteins.

  • Charge – ions are excluded unless specific channels are present.

Transport Mechanisms

Channel Proteins

Form aqueous pores that allow rapid, selective passage of ions or water. They can be gated (voltage‑gated, ligand‑gated, mechanically gated) and often exhibit high specificity, e.g., aquaporins for H₂O.

Carrier (Transport) Proteins

Bind specific solutes on one side of the membrane, undergo a conformational change, and release the solute on the opposite side. They mediate:

  • Facilitated diffusion – movement down a concentration gradient without energy input.

  • Active transport – movement against a gradient, powered by ATP or ion gradients (e.g., Na⁺/K⁺‑ATPase).

Cell Signalling – Surface Receptors

Membrane‑bound receptors detect extracellular signals and initiate intracellular responses. Major classes include:

  • G‑protein‑coupled receptors (GPCRs) – transduce signals via heterotrimeric G proteins.

  • Receptor tyrosine kinases (RTKs) – autophosphorylate tyrosine residues upon ligand binding.

  • Ion‑channel linked receptors – open ion channels directly in response to ligand binding.

Glycoproteins are often the extracellular domain of these receptors, providing ligand specificity and protecting the protein core.

Cell Recognition – Antigens and Glycoconjugates

Cell‑surface antigens are typically carbohydrate chains on glycolipids or glycoproteins. They enable:

  • Self‑non‑self discrimination by the immune system.

  • Blood‑type determination (ABO antigens).

  • Cell‑cell adhesion during tissue formation.

These glycoconjugates are highly variable, allowing a vast repertoire of recognition patterns.

Suggested diagram: A cross‑section of the fluid mosaic membrane showing the arrangement of phospholipids, cholesterol, glycolipids, integral and peripheral proteins, and glycoprotein receptors.

Summary Table – Functional Themes

ThemeKey ComponentsRepresentative Functions
StabilityPhospholipids, CholesterolMaintain structural integrity; prevent rupture under mechanical stress.
FluidityPhospholipid tail saturation, CholesterolAllow lateral movement of proteins; adapt to temperature changes.
PermeabilityPhospholipid bilayer, Channel proteinsRegulate passive diffusion of gases, water, and small non‑polar molecules.
TransportCarrier proteins, Channel proteinsFacilitated diffusion, active transport of ions, nutrients, and metabolites.
Cell signallingGlycoprotein receptors, Integral proteinsDetect hormones, neurotransmitters; trigger intracellular cascades.
Cell recognitionGlycolipids, Glycoproteins (antigens)Immune identification, blood group antigens, tissue adhesion.