Facilitated diffusion

Facilitated diffusion is the passive movement of molecules or ions across a cell membrane, down their concentration gradient, through a channel protein or a carrier protein. It is used by substances that cannot pass through the phospholipid bilayer on their own, such as glucose, amino acids and ions. Like simple diffusion it needs no energy from ATP, but its rate is limited by how many proteins the membrane has.

What is facilitated diffusion?

The middle of a cell membrane is made of the fatty acid tails of phospholipids, which are hydrophobic. Small, uncharged molecules such as oxygen and carbon dioxide dissolve through it easily: that is simple diffusion. But ions are charged, and molecules such as glucose are large and polar, so the hydrophobic core stops them.

Facilitated diffusion gets round this. Proteins that span the membrane give these substances their own route across. The word "facilitated" means helped: the proteins make the crossing possible, but the movement still goes from a higher concentration to a lower one, driven by the random motion of the particles.

Channel proteins and carrier proteins

Channel proteins

A channel protein forms a water-filled pore through the membrane. Ions and some small polar molecules pass through it. Each channel is specific: its size and the charges lining it decide what can pass. Many channels are gated, so they can open and close. In a neurone, for example, voltage-gated sodium ion channels open during an action potential and sodium ions diffuse into the axon. Water passes through channel proteins called aquaporins.

Carrier proteins

A carrier protein has a binding site for one particular molecule. The molecule binds on one side of the membrane, the protein changes shape, and the molecule is released on the other side. Glucose enters many cells this way. In facilitated diffusion the shape change happens without ATP; in active transport, carrier proteins use ATP to move substances the other way.

Does facilitated diffusion require energy?

No. Facilitated diffusion is a form of passive transport. The particles move because of their own kinetic energy, from where there are more of them to where there are fewer, so the cell does not spend any ATP. This is the key difference from active transport.

Osmosis is water moving by diffusion through a partially permeable membrane, and much of it goes through aquaporins, so osmosis is partly facilitated diffusion too. See what is osmosis and what the cell membrane does for more. Sort examples into the right kind of transport in this question from the Cell Biology course.

Facilitated diffusion examples

• Glucose into red blood cells, through glucose carrier proteins. Red blood cells have no mitochondria and use glucose quickly in anaerobic respiration, which keeps the concentration inside low.
• Sodium ions into a neurone through voltage-gated channels during an action potential.
• Potassium ions out of a neurone through voltage-gated potassium channels during repolarisation.
• Water through aquaporins. In the kidney, ADH adds more aquaporins to the collecting duct membranes so more water is reabsorbed.
• Glucose out of ileum cells into the blood. Glucose builds up inside the epithelial cells after co-transport, then leaves through carrier proteins by facilitated diffusion.

The rate of facilitated diffusion

Three things decide how fast facilitated diffusion goes: the concentration gradient, the surface area of the membrane, and the number of channel or carrier proteins. The last one gives it a different graph from simple diffusion.

Here x is the difference in concentration across the membrane and y is the rate of movement, in arbitrary units. At first, a bigger difference means faster facilitated diffusion. Then the curve levels off: every carrier or channel is busy, so adding more molecules cannot speed it up. The proteins are now the limiting factor. For simple diffusion the graph would keep rising in a straight line, because nothing has to carry the molecules.

Check your understanding.

Exam questions with answers

Why can't glucose cross the membrane by simple diffusion?

Glucose is a large, polar molecule. It cannot pass through the hydrophobic fatty acid tails in the middle of the phospholipid bilayer, so it needs a carrier protein.

A drug stops respiration in a cell. Which transport stops?

Active transport stops, because it needs ATP from respiration. Facilitated diffusion carries on as long as there is a concentration gradient, because it is passive and uses no ATP.

How could a cell increase its rate of facilitated diffusion?

By putting more channel or carrier proteins into its membrane, or by having a larger membrane surface area, for example with microvilli. Keeping the concentration low on the inside, by using up the substance, also keeps the gradient steep.

Common mistakes

• Saying facilitated diffusion uses energy. It is passive. Only active transport uses ATP.
• Saying it moves substances against the gradient. It always goes down the concentration gradient.
• Saying channel proteins are used in active transport. Active transport uses carrier proteins.
• Forgetting the plateau. The rate levels off because the proteins are saturated, not because the gradient has gone.
• Writing "glucose is too big" without saying why it matters. Say it is large and polar, so it cannot cross the hydrophobic core.

Practise with a short timed paper

Transport across membranes is in the first year of A-level Biology. The daily A-level Biology: Year 1 paper is a short timed paper of 8 questions, with a new one every day. You can also watch diffusion and osmosis across a membrane in the Cell Simulator.

Read next

Biology · CellsCell simulator: organelles, diffusion and osmosisFree cell simulator: explore labelled animal, plant and bacterial cells, then run diffusion and osmosis across a membrane with live water potential.