Topic 4: Cell membrane
4.1 Fluid mosaic membranes
Students should be able to:
1) describe the fluid mosaic model of membrane structure with reference to the hydrophobic and hydrophilic interactions that account for the formation of the phospholipid bilayer and the arrangement of proteins
2) describe the arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes
3) 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 receptors) and cell recognition
4) outline the main stages in the process of cell signalling leading to specific responses
4.1.1 — Fluid mosaic model of membrane structure: hydrophobic and hydrophilic interactions
The fluid mosaic model describes the cell membrane as a two-dimensional liquid in which a continuous phospholipid bilayer provides the basic matrix and proteins are embedded or attached, with lateral mobility of both lipids and many proteins; each phospholipid is amphipathic, having a polar, hydrophilic phosphate head that faces the aqueous exterior and interior and two non-polar, hydrophobic fatty-acid tails that associate in the bilayer core by hydrophobic interactions, and these opposing affinities (hydrophilic heads interfacing with water and hydrophobic tails avoiding water) drive spontaneous bilayer formation and account for its selective permeability, while membrane proteins — integral (spanning), peripheral (surface-associated) and lipid-anchored — are arranged within and on the surfaces of the bilayer where their positions are stabilised by hydrophobic interactions with the tails or by ionic/hydrogen bonds with head groups and aqueous surroundings; the result is a dynamic “mosaic” of lipids and proteins whose fluidity and organisation permit membrane deformation, lateral protein movement for signalling and transport, and the creation of functional microdomains.
4.1.2 — Arrangement of cholesterol, glycolipids and glycoproteins in cell surface membranes
Cholesterol molecules are interspersed among the fatty-acid tails in the inner region of animal cell membranes where each cholesterol has a small polar hydroxyl that aligns toward the polar head groups and a rigid hydrophobic steroid ring that lodges among the hydrocarbon tails, thereby modulating local packing and membrane fluidity; glycolipids have carbohydrate chains covalently attached to lipid molecules and are located mainly on the extracellular leaflet where the carbohydrate projects outward into the extracellular space, while glycoproteins are proteins with covalently attached oligosaccharide chains exposed on the outer surface of the membrane, producing a carbohydrate-rich glycocalyx; collectively cholesterol, glycolipids and glycoproteins are asymmetrically distributed between the two leaflets and contribute to membrane stability, fluidity control, cell recognition and extracellular interactions.
4.1.3 — Roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins in cell surface membranes
Phospholipids form the semipermeable barrier and determine basic membrane permeability because the hydrophobic core impedes passage of polar and charged molecules while permitting diffusion of small non-polar species; cholesterol fits between fatty-acid tails and reduces membrane permeability to small water-soluble molecules, increases mechanical stability and prevents solidification at low temperatures while restraining fluidity at high temperatures; integral membrane proteins function as carrier proteins that bind specific solutes and undergo conformational change to transport them across the bilayer (facilitated diffusion or active transport) or as channel proteins that form aqueous pores selective for particular ions or molecules, and peripheral proteins participate in signalling scaffolds and cytoskeletal attachment; glycoproteins and glycolipids on the external surface mediate cell recognition, cell–cell adhesion and act as receptors for signalling molecules, and together the varied components allow membranes to maintain internal composition, carry out selective transport, support cell signalling events and present surface antigens for recognition.
4.1.4 — Main stages in cell signalling leading to specific responses
Cell signalling leading to a specific cellular response involves three main stages: first, secretion of specific chemicals (ligands) by signalling cells (these ligands can be hormones, neurotransmitters, cytokines or local mediators); second, transport of ligands to target cells which may occur by diffusion over short distances, by bulk flow in extracellular fluid or blood for long-range signals, or via synaptic clefts in nervous transmission; third, binding of ligands to complementary cell surface receptors on target cells (these receptors are membrane proteins or glycoproteins) which induces receptor conformational change or receptor-mediated clustering and initiates intracellular transduction cascades that amplify the signal and produce specific responses such as altered enzyme activity, gene transcription or vesicle trafficking.
4.2 Movement into and out of cells
Students should be able to:
1)describe and explain the processes of simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
2) investigate simple diffusion and osmosis using plant tissue and non-living materials, including dialysis (Visking) tubing and agar
3) illustrate the principle that surface area to volume ratios decrease with increasing size by calculating surface areas and volumes of simple 3-D shapes
4) investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes
5) investigate the effects of immersing plant tissues in solutions of different water potentials, using the results to estimate the water potential of the tissues
6) explain the movement of water between cells and solutions in terms of water potential and explain the different effects of the movement of water on plant cells and animal cells
4.2.1 — Describe and explain: simple diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis
Simple diffusion is passive movement of molecules down their concentration gradient directly through the lipid bilayer or between molecules in the membrane and occurs chiefly for small non-polar substances (e.g. O₂, CO₂). Facilitated diffusion is passive transport down a concentration gradient via membrane proteins: channel proteins provide hydrophilic pores for rapid ion flow, while carrier proteins bind specific solutes and change conformation to shuttle them across. Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Active transport uses membrane proteins (pumps) and metabolic energy (usually ATP) to move solutes against their electrochemical gradients (for example the Na⁺/K⁺ pump), producing net accumulation of substances on one side of the membrane. Endocytosis is bulk uptake of extracellular material when the plasma membrane invaginates and pinches off to form vesicles (phagocytosis for particles, pinocytosis for fluid), and exocytosis is vesicle-mediated export of materials when intracellular vesicles fuse with the plasma membrane, releasing contents to the outside and adding membrane components to the cell surface.
4.2.2 — Investigating simple diffusion and osmosis using plant tissue and non-living materials (dialysis tubing and agar)
Investigations commonly use dialysis (Visking) tubing to model a partially permeable membrane and agar blocks to visualise diffusion: for example, fill dialysis tubing with starch solution and immerse it in iodine solution — iodine diffuses into the tubing producing a blue-black colour if starch is present, whereas large starch molecules do not diffuse out; to study osmosis with non-living models place agar cubes impregnated with dye into a concentration gradient and measure the rate or distance of dye penetration for cubes of identical composition but different sizes to observe diffusion dependence on surface area and distance. With plant tissue (e.g. potato cores) osmosis can be investigated by placing cores in solutions of different solute concentration, measuring mass change to quantify net water uptake or loss, and using controls; careful experimental design (replicates, identical sample size, timed intervals, blotting to remove surface solution before weighing) allows estimation of relative rates of diffusion and osmotic movement.
4.2.3 — Surface area to volume ratios and the effect of size (illustration and calculation)
Surface area to volume ratio (SA:V) decreases as three-dimensional objects increase in size because surface area scales with length squared while volume scales with length cubed; calculate SA and V for simple shapes to illustrate this: for a cube of side 1 cm, surface area = 6 × 1² = 6 cm² and volume = 1³ = 1 cm³ so SA:V = 6:1; for a cube of side 2 cm, surface area = 6 × 2² = 24 cm² and volume = 2³ = 8 cm³ so SA:V = 24:8 = 3:1; thus the larger cube has a smaller SA:V ratio, demonstrating why small cells (high SA:V) exchange materials with their environment more rapidly per unit volume than large cells, and why organisms adopt features (flattened shapes, branching, or internal folding) to increase effective surface area for exchange.
4.2.4 — Investigating the effect of changing surface area to volume ratio on diffusion using agar blocks
To investigate SA:V effects use agar blocks containing a visible tracer (dye or pH indicator) prepared to identical composition but different sizes; immerse blocks in a solution that reacts with the tracer (or vice versa) and measure either the time taken for colour change at the centre or the depth of penetration after fixed times, plotting rate or distance versus block size to show diffusion is slower over greater distances and that blocks with lower SA:V exhibit reduced effective diffusion per unit volume; ensure standardisation of concentration, temperature and agitation, and use repeated measures to calculate mean diffusion rates and reduce error.
4.2.5 — Investigating the effects of immersing plant tissues in solutions of different water potentials and estimating tissue water potential
Estimate tissue water potential experimentally by incubating uniform plant tissue samples (for example potato cores) in a series of known solute concentrations (e.g. sucrose solutions) for a fixed period, blotting and weighing samples before and after to calculate percentage change in mass; plot percentage mass change against external solution concentration and interpolate the concentration at which there is no net mass change — this external concentration corresponds to the tissue’s water potential (the point of equilibrium), and careful experimental control (identical sample size, replicate samples, consistent temperature and timing, and accurate concentration preparation) yields an estimate of the solute concentration that balances the tissue water potential.
4.2.6 — Explain movement of water between cells and solutions in terms of water potential and different effects on plant and animal cells
Water moves by osmosis from regions of higher water potential to regions of lower water potential across partially permeable membranes; in plant cells the rigid cell wall limits expansion so when water enters by osmosis cells become turgid (internal hydrostatic pressure resists further water intake and cells are firm), while when water leaves cells become flaccid and if severe they plasmolyse — the protoplast shrinks away from the cell wall; in animal cells which lack cell walls, net water intake can cause swelling and eventually lysis (bursting) if uncompensated, and net water loss causes cells to shrink (crenation), so the presence of a cell wall in plants results in different physical outcomes for similar osmotic movements compared with animal cells.