Explain bilayer self-assembly, membrane-protein roles, fluidity control and cell adhesion.
Estimated time: 72 minutes
IB syllabus: B2.1 · SL and HL
Every cellular membrane has a phospholipid bilayer roughly 7–10 nm thick. A phospholipid is amphipathic: its phosphate-containing head is polar and hydrophilic, while its fatty-acid tails are non-polar and hydrophobic. In water, heads interact with the aqueous cytosol or extracellular fluid and tails cluster away from water. A bilayer closes at exposed edges because closure removes energetically unfavourable tail–water contacts.
The bilayer is called fluid because phospholipids and many proteins diffuse laterally rather than occupying fixed coordinates. It is a mosaic because chemically different lipids, integral proteins, peripheral proteins, glycoproteins and glycolipids share the surface. The model predicts flexibility, self-sealing and local reorganization—properties required when vesicles bud, cells change shape or two membranes fuse.
Integral proteins contain hydrophobic regions that associate with lipid tails; transmembrane proteins also expose hydrophilic regions on both surfaces. They act as selective channels, carriers, ATP-driven pumps, receptors, adhesion molecules and immobilized enzymes. Peripheral proteins associate with a membrane surface or an integral protein and can anchor the cytoskeleton, organize signalling complexes or catalyse reactions.
Carbohydrate chains occur only on the non-cytosolic face when attached to proteins or lipids. Their varied shapes create a recognition code used in cell–cell identification, immune recognition and attachment. A receptor binds a ligand because of complementary chemistry, but binding is only the first step: a conformational change must transmit information to intracellular proteins.
Saturated fatty-acid tails are straight and pack closely, strengthening intermolecular attractions and lowering fluidity. Cis-unsaturated tails contain bends that frustrate packing and preserve movement, especially at low temperature. Organisms can adjust their lipid composition: a microorganism exposed to cold may increase unsaturation, whereas excessive fluidity at high temperature can be restrained by more saturated lipids.
Cholesterol in animal membranes and related sterols in plant membranes sit between phospholipids. At high temperature their rigid rings restrict phospholipid movement and reduce permeability. At low temperature they prevent neighbouring tails from packing into a rigid solid. Cholesterol is therefore a fluidity buffer, not simply a substance that always makes a membrane more or less fluid.
Cell adhesion molecules connect cells to cells, cells to extracellular matrix, or matrix to cytoskeleton. Cadherins contribute to cell–cell junctions, integrins link cells with matrix, selectins bind surface carbohydrates during inflammatory cell movement, and immunoglobulin-family CAMs have roles including neural adhesion. Their dual attachment and signalling functions allow tissue architecture to influence survival, movement, contact inhibition and differentiation.
Alter temperature, tail unsaturation and cholesterol to inspect packing, proteins and surface carbohydrates.
Exchange · gradients · inheritance
A cold-adapted bacterium is suddenly grown at a still lower temperature. Which membrane change most directly restores lateral mobility?
Practice focused questions or see how IB combines this topic with ideas from elsewhere in the course.
Matching parts: 3(c), 3(d), 3(e)
Matching part: 3