Cambridge International AS and A Level Biology 4: Cell membranes and transport

Cell Membranes and Transport is Cambridge International Biology 9700 Topic 4. It covers fluid mosaic structure, signalling, membrane transport, surface area to volume ratio and water potential. Diffusion, osmosis, dialysis tubing, agar-block and plant-tissue procedures remain in the dedicated practical hub; theory owns mechanisms, calculations and interpretation.

1. The fluid mosaic model

Phospholipids are amphipathic. Hydrophilic heads interact with water on both membrane surfaces, while hydrophobic tails turn inward away from water, forming a bilayer. Lipids and many proteins can move laterally, making the membrane fluid. In a typical animal cell-surface membrane, phospholipids, proteins, cholesterol, glycolipids and glycoproteins contribute to the mosaic. Membrane composition varies between organisms and organelles.

The hydrophobic core limits passage of ions and most polar molecules. Small non-polar molecules can cross the bilayer more readily. Selective permeability therefore arises from both lipid properties and specific transport proteins.

2. Membrane components and functions

Phospholipids form the basic barrier and permit membrane flexibility. In animal cell-surface membranes, cholesterol sits among phospholipid tails, stabilising the membrane, reducing excessive fluidity at high temperature and preventing tight packing at low temperature. It also reduces permeability to some small water-soluble substances.

Channel proteins provide hydrophilic pathways, often selective by size or charge. Carrier proteins bind particular solutes and change conformation. Some carriers mediate facilitated diffusion, while pumps use energy for active transport.

In the cell-surface membrane, glycoproteins and glycolipids project carbohydrate chains towards the extracellular side. They participate in cell recognition, antigens, adhesion and signalling. Receptor proteins bind specific ligands.

3. Cell signalling

In a secreted-ligand signalling model, one cell releases a ligand that reaches other cells. Binding to an appropriate receptor can initiate intracellular events and a response, provided the cell has the necessary downstream machinery. This model does not describe every signalling pathway.

Specificity depends on receptor structure and expression, together with the responding cell's signalling machinery. A ligand can pass many cells without affecting them if they lack the receptor. The ligand need not enter the cell when a surface receptor transduces the signal.

4. Simple and facilitated diffusion

For a neutral solute in the simple concentration-gradient model, diffusion is net movement from higher to lower concentration due to random molecular motion. Ions respond to an electrochemical gradient, which combines concentration and electrical effects. Simple diffusion crosses the bilayer directly and requires no direct ATP use. In the simple model, increasing surface area or concentration difference increases rate, while increasing diffusion distance reduces it, with other conditions fixed. Temperature also affects rate and membrane properties.