Biological membranes — OCR A-Level Biology

Membranes are partially permeable barriers formed from a phospholipid bilayer with proteins, cholesterol and glycoproteins. At the cell surface, the membrane separates the cytoplasm from the external environment, controls which substances enter and leave, and carries receptor molecules for cell signalling. Within cells, membranes surround organelles such as the nucleus, mitochondria and chloroplasts, compartmentalising reactions so incompatible processes occur in separate regions. Internal membranes also increase surface area for reactions, for example the folded inner membrane (cristae) of mitochondria, and form transport vesicles. Membranes therefore act as selective barriers, sites of signalling and sites of enzyme-catalysed reactions.

The fluid mosaic model describes membranes as a phospholipid bilayer in which proteins are scattered like tiles in a mosaic and can move laterally, giving fluidity. Teach the arrangement: hydrophilic phosphate heads face the aqueous exterior and cytoplasm, hydrophobic fatty acid tails point inwards, forming a barrier to water-soluble substances. Components and roles: phospholipids create the bilayer and its selective barrier; cholesterol sits between tails, restricting movement at high temperature and preventing packing at low temperature, so it buffers fluidity; intrinsic (integral) proteins span the bilayer, often acting as channel or carrier proteins for transport; extrinsic (peripheral) proteins sit on one surface, often as enzymes or receptors; glycoproteins and glycolipids on the outer surface act in cell recognition, receptors and as antigens. Glycocalyx aids adhesion.

Membrane structure is described by the fluid mosaic model: a phospholipid bilayer with proteins, glycoproteins and glycolipids, held together by hydrophobic interactions. Permeability depends on this structure. Temperature raises kinetic energy, so phospholipids move more and gaps open; moderate warming increases permeability, while high temperature denatures membrane proteins and disrupts the bilayer. Solvents such as ethanol dissolve lipids, disrupting the bilayer and increasing permeability. Increasing the proportion of unsaturated fatty acids creates kinks that increase fluidity. Cholesterol fits between phospholipids, restricting movement at moderate temperatures but preventing rigid packing at low temperatures. Small non-polar molecules cross most easily; large or charged molecules need channels or carriers.

Practical work on membrane permeability uses beetroot tissue: pigment (betalain) leaks from damaged vacuoles, and the colour intensity of the bathing solution indicates how much membrane damage has occurred. You vary one factor — temperature, ethanol concentration or detergent concentration — while keeping tissue mass, piece size, volume of solution, timing and rinsing constant. Pieces are blotted and rinsed to remove pigment released at the cut surfaces, then placed in water or the test solution for a fixed time. The solution is decanted or the tissue removed, and colour is compared by eye against a colour standard or measured with a colorimeter, whose absorbance you convert to a transmission or concentration value. Higher temperature, higher ethanol concentration and higher detergent concentration each increase permeability by disrupting the phospholipid bilayer and membrane proteins.

Molecules cross membranes by several distinct routes, and the specification clause asks you to know how each works. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the phospholipid bilayer down a concentration gradient. Water moves by osmosis, largely through channel proteins called aquaporins. Ions and polar molecules such as glucose cannot pass freely through the hydrophobic core, so they use facilitated diffusion through carrier or channel proteins, again down a gradient and without ATP. Active transport uses carrier proteins and ATP hydrolysis to move substances against a concentration gradient, as in the sodium-potassium pump. Bulk transport moves large quantities in vesicles: endocytosis brings material in, exocytosis releases it, both requiring ATP.

Model cells let you measure diffusion without living tissue. A common method uses cubes of agar jelly containing a pH indicator such as phenolphthalein, set with dilute alkali so the jelly is pink. When acid diffuses in from the surrounding solution, the indicator turns colourless, so the distance the colourless zone has moved into the cube is a measure of diffusion. You can vary surface area to volume ratio by cutting cubes of different side length, vary the concentration gradient by changing the acid concentration, or vary temperature. For each factor, change only that variable, keep the others constant, and measure the colourless depth from the outside of the cube inwards, for example with a ruler after slicing the cube. Repeat and calculate a mean. A larger surface area to volume ratio, a steeper concentration gradient or a higher temperature each increases the rate of diffusion.

Osmosis is the movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential. Water potential is measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and solutions have negative values. If a plant cell sits in a solution with higher water potential than its contents, water enters, the vacuole swells and the cell becomes turgid, which supports the plant. In a solution with lower water potential, water leaves, the vacuole shrinks and the membrane pulls away from the cell wall, giving plasmolysis. Animal cells lack a cell wall: in a higher water potential solution they swell and may burst (lysis); in a lower water potential solution they shrink and become crenated.

Water potential (Ψ) is the tendency of water to move; pure water at atmospheric pressure is 0 kPa, and solutions have negative values. In a practical investigation you prepare a series of solutions of known water potential, for example sucrose solutions, and place samples of plant tissue (such as potato cylinders or onion epidermis) and animal cells (such as red blood cells or cheek epithelial cells) into them. You record mass change in plant tissue or observe lysis and crenation in animal cells. Plant cells with a cellulose wall resist bursting: in a solution of higher (less negative) water potential they take up water and become turgid, while in a solution of lower (more negative) water potential they lose water and become plasmolysed. Animal cells lack a wall, so in pure water or very dilute solution they swell and lyse, and in concentrated solution they shrink and crenate.