Revise Membranes and membrane transport for IB Biology (B2.1)

The foundation of every membrane is the phospholipid. Each phospholipid molecule is amphipathic - it has two chemically opposite ends. One end is a phosphate-containing head that is hydrophilic (polar, water-attracting); the other end is a pair of fatty-acid tails that are hydrophobic (nonpolar, water-repelling). Place many phospholipids in a watery environment and they arrange themselves so that the hydrophilic heads face the water while the hydrophobic tails huddle together away from it. The lowest-energy, most stable arrangement is a bilayer: two sheets of phospholipids tail-to-tail, with heads facing the aqueous fluids inside and outside the cell and the tails forming a nonpolar core in the middle. This assembly is spontaneous - it needs no energy - and it is self-sealing, because any gap exposes tails to water and is immediately closed.

The accepted description of membrane structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972. 'Fluid' captures the fact that the phospholipids, and many of the proteins, are not fixed - they drift sideways within the plane of the membrane, so the whole sheet behaves like a two-dimensional liquid. 'Mosaic' captures the fact that the membrane is a patchwork of many different components set into the bilayer. Alongside the phospholipids sit integral proteins, embedded in the hydrophobic core and often spanning the whole membrane as transmembrane transporters; peripheral proteins, attached loosely to a surface or to an integral protein without entering the core; cholesterol, a lipid wedged between phospholipids in animal cells; and glycoproteins, proteins bearing carbohydrate chains that project from the outer surface as cell-recognition markers.

Fluidity is not an incidental detail - it is essential for transport, for vesicle formation and for membranes to fuse and reseal. Two main factors control it. The first is the fatty-acid tails: tails with double bonds (unsaturated) have kinks that stop the phospholipids packing tightly, keeping the membrane more fluid, while straight saturated tails pack closely and make it more rigid. The second is cholesterol, which acts as a fluidity buffer in animal cells. At higher temperatures cholesterol restrains the phospholipids and stops the membrane becoming too fluid; at lower temperatures it keeps them apart and stops the membrane solidifying. The result is a membrane that stays workable across the range of temperatures a cell experiences.

Because the middle of the bilayer is hydrophobic, the membrane is selectively permeable: it lets some substances cross freely and restricts others. Small nonpolar molecules such as oxygen and carbon dioxide dissolve into the hydrophobic core and pass straight through. Small uncharged polar molecules like water cross slowly. But ions and large polar molecules such as glucose are repelled by the core and can only cross with the help of transport proteins. This selectivity is the whole point of the membrane: by controlling which substances cross, and by using proteins that can be regulated, the cell maintains an internal composition quite different from its surroundings.

Transport across the membrane falls into two broad categories. Passive transport moves substances DOWN a gradient and needs no metabolic energy; it includes simple diffusion, facilitated diffusion and osmosis. Active transport moves substances AGAINST a gradient and therefore requires energy from ATP together with a protein pump. Bulk transport handles materials too large for either route by packaging them into vesicles.

Passive transport is driven by the random thermal motion of particles. Because particles spread out from where they are crowded to where they are sparse, the net movement is always DOWN the concentration gradient, from higher to lower concentration, until the two sides are equal. No ATP is spent - the energy comes free from the particles' own motion. There are three passive routes across the plasma membrane.

Osmosis is best described using water potential. Water potential is a measure of the tendency of water to move; pure water has the highest value, and adding solute lowers it. Water always moves by osmosis from higher water potential to lower water potential - which means, in everyday terms, toward the more concentrated solution. Comparing a cell with its surroundings gives three cases, described by tonicity. In a HYPERTONIC solution the outside has lower water potential than the cell, so water leaves the cell. In a HYPOTONIC solution the outside has higher water potential, so water enters the cell. In an ISOTONIC solution the water potentials are equal, so there is no net movement.

Sometimes a cell needs to move a substance the 'wrong' way - against its concentration gradient, from where it is scarce to where it is already abundant. This does not happen spontaneously, so it must be powered: active transport uses energy from ATP together with a specific carrier protein, called a pump. The ATP causes the pump to change shape, carrying the bound substance across and releasing it on the far side, then resetting. Because it depends on ATP, active transport stops if respiration is inhibited - a useful experimental way to tell it apart from passive transport, which continues regardless.

The classic example is the sodium-potassium pump, found in the membranes of animal cells. Using the energy from one ATP, it pumps 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell, both against their concentration gradients. This maintains a low internal Na⁺ and high internal K⁺, setting up the electrochemical gradients that nerve cells use to transmit impulses and that other cells use to drive the co-transport of substances such as glucose.

Some materials are simply too large to cross the membrane through the bilayer or a protein - whole proteins, polysaccharides, or even entire microorganisms. These are moved by bulk transport, which relies on the membrane's fluidity to form and fuse vesicles, and which requires energy. In endocytosis the membrane folds inward around the material, pinches off, and forms a vesicle that carries the material into the cytoplasm; when the material is solid this is called phagocytosis ('cell eating') and when it is dissolved it is pinocytosis ('cell drinking'). Exocytosis is the reverse: a vesicle carrying material - for example, a secreted protein - moves to the plasma membrane, fuses with it, and releases its contents to the outside. Both processes let cells handle cargo far larger than any single molecule that diffusion or pumping could manage.

B2.1 asks you to explain processes, and explanation marks are awarded analytically - each distinct valid biological point is worth one mark. Method-style points (M) credit correct reasoning about a mechanism, answer-style points (A) credit a correct named example or conclusion, and error-carried-forward (ECF) means an early slip does not cost the marks that follow, provided your reasoning is written down. Notice how every mark below is tied to a specific named idea, not to loose phrasing, and how the engine accepts equivalent correct wording.

The transport principles here underpin much of the rest of biology. Facilitated diffusion and active transport reappear in the absorption of nutrients in the gut, in reabsorption in the kidney, and in the loading of ions in plant roots. The sodium-potassium pump is the engine behind the resting potential and nerve impulses. Osmosis and water potential govern water uptake in plants and the fate of cells in medical fluids. Master the two questions - which way is the substance moving relative to its gradient, and does the cell have to spend ATP - and you have a framework that explains membrane transport in every one of those contexts.