1Why a protein is needed

Whether a molecule needs help depends on its structure.

• Small and nonpolar (O₂, CO₂): dissolves in the bilayer core, crosses unaided (simple diffusion).
• Charged (Na⁺, K⁺, Cl⁻, Ca²⁺): surrounded by water, repelled by the nonpolar core. Needs a channel.
• Large and polar (glucose, amino acids): too large and too polar for the core. Needs a carrier or channel.
• Small, polar, uncharged (water): trickles through the bilayer, much faster through aquaporins.

A protein solves the problem by giving the molecule a path lined with polar and charged groups, so it never has to enter the hydrophobic core.

2Channels and carriers

Channel proteins form a water-filled pore through the membrane. Each channel is specific: the size and the charges lining the pore let only one kind of ion, or water, through. Many channels can open and close (they are gated), so the cell can switch the flow on and off. When open, a channel lets millions of ions per second pass.

Carrier proteins (transport proteins) bind the molecule on one side, change shape, and release it on the other. They are slower than channels, and they are also specific: a glucose carrier carries glucose and not amino acids.

In both cases the direction is set by the gradient, not by the protein. A glucose carrier moves glucose into a cell when there is more outside and out of the cell when there is more inside. No ATP is used.

Figure 1 puts a channel next to a pump. In (a), the particles are crowded outside and sparse inside, and they flow down that gradient through the channel with no ATP. In (b), the pump moves particles toward the side where they are already crowded, and it needs ATP to do it. Only (a) is facilitated diffusion; (b) is active transport (2.8).

Facilitated diffusion: a protein provides the route, the gradient provides the drive, and no ATP is spent.

3A rate that levels off

Simple diffusion speeds up in proportion to the concentration difference. Facilitated diffusion speeds up too, but only until every protein is busy. After that the rate cannot rise further, however high the concentration goes: the proteins are saturated.

An invented data set for glucose entering red blood cells:

From 0 to 2 mmol L⁻¹ the rate rises by 2.0 units per mmol L⁻¹; from 20 to 40 it rises by only 0.035 units per mmol L⁻¹. A curve that flattens like this is the sign of a protein with a limited number of copies. To go faster, the cell would have to put more carrier proteins in its membrane.

4Ion channels and a polarised membrane

When an ion moves, its charge moves with it. Suppose a cell has more K⁺ inside than outside, and its K⁺ channels open. K⁺ diffuses out, down its gradient. Each K⁺ that leaves carries a positive charge out, so the inside becomes more negative than the outside. The membrane is now polarised: there is a difference in charge, a membrane potential, across it.

That is why ion channels matter so much to nerve and muscle cells: opening and closing them changes the charge across the membrane, and those changes are signals (Unit 4). The gradients that the channels run down are built by active transport (2.8). Channels spend gradients; pumps build them.

5Aquaporins

Water is polar but small and uncharged, so some crosses the bilayer directly. Many cells need far more water movement than that allows. Aquaporins are channel proteins specific to water. Water molecules pass through them in single file, and each aquaporin can let through billions per second. Cells that move a lot of water, such as plant root cells and some kidney cells, have many aquaporins.

Aquaporins do not pump water. Water still moves by osmosis, from where water potential is higher to where it is lower (2.7). The aquaporin only makes that movement faster.

An invented experiment shows it. Egg cells that normally have almost no aquaporins were given the gene for one; others were not. Both were moved into dilute solution, and the time for each to swell by 20% was recorded:

Water entered more than 10 times faster when aquaporins were present.

6Predicting the effects of a change

AP asks you to predict the causes or effects of a disruption to a system. For facilitated diffusion, the chain is always: fewer working proteins → slower movement of that one substance → whatever depends on it suffers.

• A drug blocks K⁺ channels: K⁺ cannot leave, so the inside does not become as negative; the membrane's polarisation changes.
• A mutation removes the glucose carrier: glucose enters slowly, so the cell has less fuel for respiration.
• A cell makes extra aquaporins: water moves in or out faster when a gradient exists, but the direction and the final balance are still set by water potential.
• Raising the external concentration beyond saturation: no further increase in rate.

Note what stays the same: the substances that cross the bilayer directly (O₂, CO₂) are not affected by blocking a protein.