3.2.3 - Transport across cell membranes

Membrane structure

Two layers of lipid and a scatter of proteins decide what enters and leaves a cell.

Cell-surface membranes and the membranes around eukaryotic organelles share the same basic structure, described by the fluid-mosaic model. The proteins present vary with each membrane’s function.

The arrangement of components

Start with the bilayer in the diagram. Phosphate heads are hydrophilic and fatty acid tails are hydrophobic (3.1.3), so the heads face the watery environments on either side and the tails face inwards. Proteins lie within or across the bilayer, and cholesterol fits between phospholipids rather than forming a separate layer.

[DIAGRAM: asset_name: 2.3 - Transport across cell membranes - Diagram 1; asset_slug: 2.3 - Transport across cell membranes - Diagram 1; recommended_method: retained_png; caption: A labelled fluid-mosaic membrane showing phospholipid bilayer, channel protein, carrier protein, glycoprotein, glycolipid and cholesterol.; description: A labelled fluid-mosaic membrane showing phospholipid bilayer, channel protein, carrier protein, glycoprotein, glycolipid and cholesterol.]

The carbohydrate chains of glycoproteins and glycolipids project from the surface facing away from the cytoplasm.

What moves and what restricts it

The membrane is fluid because phospholipids move sideways (laterally) within their layer. Many proteins and other lipids drift sideways too, although some proteins are anchored.

Cholesterol may also be present, and it restricts the movement of the other molecules in the membrane. So the membrane becomes less fluid and more stable, and less water and fewer ions leak through it.

Diffusion and facilitated diffusion

Oxygen crosses a membrane with no help, but glucose barely crosses at all without a protein.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. It is passive: particles move using their own kinetic energy, so the cell does not have to supply any.

The simplest case is simple diffusion directly through the phospholipid bilayer. The hydrophobic middle of the bilayer limits what can cross this way:

• Small non-polar molecules, such as oxygen and carbon dioxide, pass through quite easily.
• Charged ions and polar molecules, such as glucose and amino acids, do not dissolve in the hydrophobic core, so they barely cross.
• Large molecules do not slip easily between the phospholipids.

Channel and carrier proteins

Substances the bilayer blocks can cross through proteins instead.

Two kinds of protein do this, each specific to particular substances:

• Channel proteins provide a hydrophilic pore through the membrane. This route is especially important for ions.
• Carrier proteins bind a specific molecule, such as glucose, then change shape and release it on the other side.

The protein provides a route, not energy. Like simple diffusion, facilitated diffusion moves a substance down its concentration gradient and uses no ATP.

What sets the rate

Particles cross in both directions, but many more cross from the high-concentration side than cross back. So a steeper concentration gradient gives more net movement per second. The rate also increases with:

• a larger membrane surface area, because more particles can cross at once
• a shorter diffusion distance, because particles have less far to travel
• more suitable channel or carrier proteins, because there are more routes across

Osmosis and water potential

Put a red blood cell in pure water and it bursts. Put a plant cell in the same water and it simply firms up.

Osmosis is a special case of diffusion because it involves only water.

A partially permeable membrane lets water cross but holds back many solutes. Which way the water moves depends on water potential.

Water potential is measured in kPa. Pure water at atmospheric pressure has a water potential of 0 kPa. Adding solute lowers it: solute particles attract water molecules, which leaves fewer free to move, so the water potentials of solutions are negative.

The less negative value is the higher water potential. So water moves from a solution at −100 kPa into a cell initially at −400 kPa.

How water crosses and how fast

Water molecules are polar, but they are small enough for some to pass through the bilayer. Water channels called aquaporins give a much faster route. Neither route requires ATP.

A larger water potential difference produces faster net movement through an otherwise unchanged membrane. As water moves, the difference shrinks, so net movement slows. At equal water potentials molecules still cross in both directions, but at equal overall rates.

Why animal and plant cells respond differently

The outcome differs because a plant cell has a cell wall.

An animal cell has no wall to resist expansion, so in a much higher water potential it swells and may burst. As a plant cell takes up water, its contents press against the wall. This pressure raises the cell’s water potential until it equals the external value, stopping net uptake.

A cell pressed firmly against its wall like this is turgid; a cell with no pressure against its wall is flaccid. So a turgid cell can be at water-potential equilibrium: equal water potentials do not automatically mean a flaccid cell.