Facilitated diffusion

Where it happens

Facilitated diffusion happens at the plasma membrane, at carrier proteins and channel proteins embedded in the phospholipid bilayer. It is used for molecules that are too large or too polar to pass directly through the membrane, such as glucose, amino acids and ions.

Three sites appear again and again in exam questions. Glucose enters red blood cells and muscle cells through glucose carrier proteins.

Sodium and potassium ions cross the membrane of a neurone through ion channels when an impulse passes. Glucose and amino acids leave the epithelial cells of the villus into the blood capillary by facilitated diffusion, once they have built up inside the cell.

The membranes of organelles such as the mitochondrion also carry channel and carrier proteins for the same purpose.

Inputs and outputs

• Input: a specific molecule or ion, such as glucose, an amino acid, a sodium ion or a chloride ion, on the high-concentration side of the membrane.
• Input: a carrier protein with a binding site of matching shape, or a channel protein forming a water-filled pore of matching size and charge.
• Input: a concentration gradient across the membrane, which is the only source of driving force.
• Output: the same molecule or ion, unchanged, on the low-concentration side.
• Output: the carrier or channel protein returned to its original state, ready for the next molecule.
• Output: no ATP is used and no ADP is produced, which separates this process from active transport.

The steps

1. A specific molecule, such as glucose, is present in higher concentration on one side of the membrane.
2. The molecule collides with the membrane by random movement, but cannot pass through the phospholipid bilayer because it is too large or too polar.
3. The molecule binds to a carrier protein, or enters an open channel protein, that matches its shape.
4. A carrier protein changes shape to move the molecule across the membrane; a channel protein simply allows the molecule through.
5. The molecule is released on the other side, where its concentration is lower.
6. The protein is unchanged and can transport further molecules.
7. Net movement continues until the concentration is equal on both sides, after which molecules still cross but at the same rate in each direction.

Why it matters and how it is controlled

Facilitated diffusion lets cells take up molecules they need, such as glucose entering red blood cells or intestinal cells, without spending energy. Because it depends on a limited number of carrier and channel proteins, its rate levels off once all the proteins are in constant use, unlike simple diffusion.

The rate is controlled by three things. The steepness of the concentration gradient sets how fast molecules reach the proteins.

The number of carrier or channel proteins in the membrane sets the maximum rate; a cell can raise this number by inserting more proteins, which is how insulin increases glucose uptake by muscle cells. Temperature affects the kinetic energy of the molecules and therefore the rate of collision with the proteins.

Channel proteins add a further control: some are gated, opening or closing in response to a signal. The sodium and potassium channels of a neurone open only when the membrane potential changes, which is what allows a nerve impulse to travel as a wave rather than leaking continuously.

How it is examined

Questions often ask you to compare facilitated diffusion with simple diffusion and with active transport, to identify which one is at work in a given scenario, or to explain why a molecule such as glucose needs a carrier protein to cross the membrane.

The graph question is the one to prepare for. A graph of rate against concentration difference shows simple diffusion as a straight line, while facilitated diffusion rises and then flattens to a plateau.

You are expected to explain the plateau in terms of all carrier proteins being occupied. Multiple-choice items test the direction of movement (down the gradient) and the energy requirement (none).

Structured items give a cell such as a red blood cell, a villus epithelial cell or a neurone and ask you to name the transport process for a labelled molecule and justify the choice.

Common misconceptions

Worked exam-style question

Question. A student measured the rate of glucose uptake by red blood cells placed in glucose solutions of increasing concentration. The rate rose steeply at first, then flattened and stayed constant even when the concentration was doubled again.

When the experiment was repeated with a respiratory inhibitor added, the results were the same. (a) Name the process by which glucose enters the red blood cells.

(b) Explain why the rate flattened at high concentrations. (c) Explain what the result with the respiratory inhibitor shows about this process.

Model answer. (a) Facilitated diffusion. (b) Glucose is too large and polar to cross the phospholipid bilayer, so it enters through specific carrier proteins.

At low concentrations, raising the concentration increases the number of glucose molecules reaching the carriers, so the rate rises. At high concentrations, all carrier proteins are occupied at every moment, so the rate is limited by the number of carrier proteins and cannot rise further.

(c) The inhibitor stops respiration, so no ATP is made. Since uptake was unchanged, the process does not use ATP: it is passive, driven only by the concentration gradient, which rules out active transport.

Source:SRC-DSKP-EN

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