B2.1 Membranes and membrane transport

The bilayer

Phospholipids are amphipathic. In water they arrange themselves into a double layer with hydrophilic phosphate heads facing the watery fluid on each side and hydrophobic tails facing inward. This creates a continuous barrier through which ions and large polar molecules cannot easily pass, while small non-polar molecules such as oxygen and carbon dioxide cross readily.

Fluid mosaic model

The membrane is described as a fluid mosaic: phospholipids move sideways within each layer, and proteins are scattered through it like tiles in a mosaic. Integral proteins are embedded in the bilayer, often spanning it; peripheral proteins sit on the surface. Proteins act as channels, carriers, pumps, receptors, enzymes and attachment points. Glycoproteins and glycolipids on the outer surface form a carbohydrate layer used in cell recognition and adhesion.

Passive transport

Passive transport needs no ATP; substances move down a concentration gradient.

• Simple diffusion: net movement from high to low concentration directly through the bilayer (oxygen, carbon dioxide, small lipid-soluble molecules).
• Facilitated diffusion: movement down a gradient through channel or carrier proteins, for ions and polar molecules such as glucose. Channels are specific; aquaporins are channels that speed up water movement.
• Osmosis: the net movement of water across a partially permeable membrane from a region of lower solute concentration to one of higher solute concentration. Solutions are described as hypotonic, isotonic or hypertonic relative to a cell.

flowchart TD
  A["Does the molecule cross the bilayer directly?"] -->|"Yes: small, non-polar"| B["Simple diffusion"]
  A -->|"No"| C["Moving down its gradient?"]
  C -->|"Yes"| D["Facilitated diffusion via channel or carrier"]
  C -->|"No: against gradient"| E["Active transport via pump using ATP"]

Active transport and pumps

Active transport moves substances against their concentration gradient using energy from ATP. Pump proteins bind the specific substance, ATP is hydrolysed and the energy changes the pump's shape so the substance is released on the other side. Cells such as root hair cells and kidney tubule cells, which do a lot of active transport, have many mitochondria.

Investigating osmosis

A classic practical places plant tissue (for example potato cylinders) in a range of solute concentrations and records percentage change in mass. Plotting percentage change against concentration and finding where the line crosses zero estimates the concentration that is isotonic with the tissue. Using percentage change allows fair comparison of samples with different starting masses.

Membrane fluidity and cholesterol (HL)

Unsaturated fatty acid tails with kinks increase fluidity, while saturated tails pack tightly. In animal cells, cholesterol sits between phospholipids: at warm temperatures it restricts movement and reduces fluidity; at cool temperatures it prevents tight packing and stops the membrane becoming rigid. It also reduces permeability to ions and small polar molecules.

Vesicles, gated channels and cotransport (HL)

• Endocytosis: the membrane folds inward and pinches off a vesicle containing material from outside. Exocytosis: a vesicle fuses with the membrane and releases its contents, for example digestive enzymes. Both depend on membrane fluidity.
• Gated channels open or close in response to a stimulus: voltage-gated sodium and potassium channels in neurons, and ligand-gated channels such as the nicotinic acetylcholine receptor at synapses.
• The sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP.
• Cotransport: a carrier moves two substances together, using the gradient of one to drive the other. In the small intestine, sodium moving back in down its gradient carries glucose with it against its own gradient.

Common mistakes

• Defining osmosis without mentioning a partially permeable membrane, or describing water moving towards lower solute concentration.
• Calling facilitated diffusion active because it uses proteins. It needs no ATP.
• Saying cholesterol always makes membranes more rigid; its effect depends on temperature.
• Using absolute mass change instead of percentage change in data analysis.

Exam tips

• Explain active transport by mentioning specificity, binding, ATP hydrolysis, shape change and release.
• Estimate the isotonic point from a graph by reading the x-intercept and stating the value with units.
• Distinguish channels from pumps by direction relative to the gradient and use of ATP.
• Label diagrams accurately: integral protein, peripheral protein, glycoprotein, cholesterol, phospholipid head and tail.