Topic 2.7 Facilitated Diffusion: explain how the structure of channel and carrier proteins allows the facilitated diffusion of polar molecules and ions across a membrane.

What this topic is asking

The College Board (Topic 2.7) wants you to explain how the structure of channel and carrier proteins allows the facilitated diffusion of polar molecules and ions across the membrane, and why this transport is passive (no ATP).

Why facilitated diffusion is needed

The hydrophobic core of the bilayer blocks polar molecules and charged ions. To let these essential substances cross without using energy, the membrane provides transport proteins that give a hydrophilic route. The substance still moves down its concentration gradient, so no ATP is required; the proteins simply make the crossing possible.

Channel proteins and carrier proteins

Both are specific: a glucose transporter carries glucose, not amino acids; a potassium channel passes potassium, not sodium. Specificity comes from the protein's shape and the chemistry of its binding site or pore, another structure-to-function example.

Specificity and regulation

Because each transport protein recognizes a particular substance, facilitated diffusion is selective: the cell controls which polar molecules and ions can cross simply by which proteins it expresses in its membrane. This is a powerful regulatory tool. A cell can change its uptake of a nutrient by inserting more carrier proteins into the membrane or removing them, without altering the lipid bilayer itself. Some channels are also gated: they open or close in response to a signal such as a voltage change or a bound molecule, so the cell can switch transport on and off. Gated ion channels are central to nerve and muscle function, where rapid, controlled movement of ions down their gradients generates electrical signals. Facilitated diffusion therefore combines speed (no waiting for ATP) with selectivity and control.

How channels and carriers actually move a substance

The two protein types solve the same problem in different ways, and the College Board rewards describing the mechanism, not just the name. A channel protein is essentially a hydrophilic pore lined with polar amino-acid side chains, so an ion or small molecule that matches the pore can slip straight through while the hydrophobic core stays intact. Because the path is open (or opens briefly when gated), channels move substances very fast, which is why aquaporins can pass water far more quickly than it leaks across the bare bilayer, and why ion channels can carry millions of ions per second. A carrier protein instead has a binding site that the substance occupies on one face; binding triggers a conformational (shape) change that exposes the site to the other face, where the substance is released and the carrier resets. This bind-flip-release cycle is slower than an open channel, and it is the step that limits the maximum rate.

Selectivity arises directly from this structure: a channel's pore size and the charge of its lining decide what fits, while a carrier's binding site fits one substance, like an enzyme and its substrate.

Saturation and rate

The rate of simple diffusion keeps rising as the concentration gradient steepens, but facilitated diffusion is limited by the number of transport proteins. As concentration increases, the proteins fill up; once every protein is working at maximum, the rate plateaus (saturation). A graph of rate against concentration therefore levels off for facilitated diffusion but keeps rising for simple diffusion, a common AP data question.

Try this

Q1. Identify the type of protein that allows water to cross the membrane rapidly and state whether energy is used. [2 points]

• Cue. Aquaporins (channel proteins); no energy is used because water moves down its concentration gradient (osmosis is passive).

Q2. Explain why the rate of facilitated diffusion reaches a maximum but the rate of simple diffusion does not. [2 points]

• Cue. Facilitated diffusion depends on a limited number of transport proteins that become saturated, while simple diffusion is limited only by the gradient and keeps increasing.