Cell membranes and transport

📚The biology

The fluid mosaic model

Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails. In water they arrange themselves so the heads face the aqueous solutions on either side and the tails point inward, away from water: a bilayer forms spontaneously because that arrangement is the most stable. Proteins are scattered through it — some spanning the whole width (intrinsic or transmembrane), some on one surface (extrinsic).

Fluid, because the phospholipids move laterally within their layer and the membrane is flexible; mosaic, because the proteins are scattered irregularly like tiles.

• Phospholipids — form the bilayer; the hydrophobic core is the barrier that makes the membrane partially permeable, letting small non-polar molecules through and blocking large or charged ones.
• Cholesterol — sits between the phospholipids. It regulates fluidity: it stops the membrane becoming too fluid at high temperature and too rigid at low temperature, and it reduces permeability to water-soluble substances and ions. Also gives mechanical stability.
• Glycolipids and glycoproteins — carbohydrate chains on the outer surface only. They act as cell surface antigens for recognition (see Topic 11), as receptors, and in cell adhesion.
• Proteins — channel proteins (water-filled pores for specific ions), carrier proteins (change shape to move a specific solute), enzymes, and receptors for cell signalling.

Cell signalling

Three stages, and the syllabus wants them in order:

1. A cell secretes a specific chemical (a ligand), such as a hormone.
2. The ligand is transported to target cells, for example in the blood.
3. The ligand binds to a complementary cell surface receptor on the target cell, triggering a response inside it.

Specificity comes entirely from the receptor: only cells carrying a receptor complementary to that ligand can respond, which is why a hormone circulating everywhere affects only certain tissues. What happens next — G proteins, second messengers, enzyme cascades — is Topic 14.

Movement across membranes

• Simple diffusion — net movement of molecules from high to low concentration, down the gradient, passive. Small non-polar molecules (O₂, CO₂) pass straight through the bilayer.
• Facilitated diffusion — still down the gradient and still passive, but through channel or carrier proteins. Needed by polar molecules and ions, which cannot cross the hydrophobic core. Because it depends on proteins, it can be saturated.
• Osmosis — the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Much of it goes through aquaporin channel proteins.
• Active transport — movement against the concentration gradient, through a carrier protein, using ATP. The carrier changes shape as ATP is hydrolysed. Rate depends on respiration, so it stops if a respiratory inhibitor is added — the standard way to distinguish it experimentally from facilitated diffusion.
• Endocytosis — the membrane invaginates around bulk material and pinches off a vesicle inside the cell. Requires ATP.
• Exocytosis — a vesicle fuses with the cell surface membrane and releases its contents outside. Requires ATP.

Surface area to volume ratio

Diffusion alone supplies a cell only if the surface is large enough relative to the volume it must serve. Consider a cube of side \( l \):

Surface area rises with the square of the length while volume rises with the cube, so as an organism gets bigger the ratio falls. A 1 cm cube has a ratio of 6:1; a 2 cm cube, 3:1; a 3 cm cube, 2:1. That is the whole reason large organisms need specialised exchange surfaces and transport systems — the reason for Topics 7, 8 and 9.

For a cylinder of radius \( r \) and height \( h \): surface area \( = 2\pi r^{2} + 2\pi r h \), volume \( = \pi r^{2} h \). Agar blocks of different sizes soaked in a dye or an indicator show the effect directly — the small block is coloured through, the large one has an unstained core.

Water potential

Water potential (Ψ) is the tendency of water to move out of a system, measured in kilopascals (kPa). Pure water has the highest possible value, defined as zero. Adding solute lowers water potential, so all solutions have negative values.

“More concentrated” therefore means “more negative”: −800 kPa is a lower water potential than −200 kPa, and water flows from the −200 to the −800. The syllabus does not require solute potential or pressure potential separately — only Ψ.

Plant cells have a cell wall, so they behave differently from animal cells:

• In a solution of higher Ψ, water enters, the vacuole expands and pushes the membrane against the wall. The wall resists, so pressure builds and the cell becomes turgid. It does not burst.
• In a solution of lower Ψ, water leaves, the cell becomes flaccid, and eventually the membrane pulls away from the wall: plasmolysis. The point at which plasmolysis just begins is incipient plasmolysis.

Animal cells have no wall. In a solution of higher Ψ water enters, the cell swells and bursts — haemolysis in a red blood cell. In a solution of lower Ψ water leaves and the cell shrinks and crenates. This is why animals must osmoregulate (Topic 14) and plants need not.