Passive transport: diffusion, facilitated diffusion, channel and carrier proteins

Membranes, Proteins, DNA and Gene Expression · Gas exchange, membranes and transport · note 7 of 8

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Passive transport: diffusion, facilitated diffusion, channel and carrier proteinsSpec 2.5

In short

Passive transport moves substances across a membrane down a concentration gradient without using ATP. Diffusion occurs directly through the phospholipid bilayer; facilitated diffusion uses channel proteins or carrier proteins for ions and polar molecules, which cannot cross the hydrophobic core of the bilayer.

Passive transport is the movement of substances across a membrane without the cell supplying metabolic energy (no ATP is used). The substance moves down its concentration (or electrochemical) gradient, using the kinetic energy of its own molecules. There are two kinds: diffusion and facilitated diffusion.

Diffusion

Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. Small non-polar molecules such as oxygen and carbon dioxide diffuse directly through the phospholipid bilayer.

Facilitated diffusion

Ions and polar molecules (such as glucose) cannot cross the hydrophobic centre of the bilayer. Facilitated diffusion is passive transport through the membrane with the help of transport proteins. There are two types of protein.

Channel proteins compared with carrier proteins
Feature	Channel protein	Carrier protein
Structure	Water-filled pore through the membrane, lined with hydrophilic amino acids; often gated (opens and closes).	Has a binding site specific to one molecule or ion, on one side of the membrane.
How it works	Ions or small polar molecules of the right size and charge pass through the pore.	The molecule binds, the protein changes shape (conformation) and releases the molecule on the other side.
Specificity	Specific to particular ions by pore size and charge.	Specific to a particular molecule by the shape of the binding site.
Energy	Passive: no ATP.	Passive in facilitated diffusion: no ATP.

The rate of facilitated diffusion rises with the concentration gradient, but only up to a point. When all the carrier proteins are working at their maximum rate (saturated), adding more of the substance does not increase the rate. The rate also depends on the number of transport proteins in the membrane.

Exam tip:

'Facilitated diffusion' still needs a concentration gradient and still uses no ATP. Say 'down a concentration gradient through a channel or carrier protein' to earn the marks.

Passive transport: all three move down a concentration gradient and use no ATP.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What are the adaptations of the alveoli for gas exchange?

Alveoli give a large surface area to volume ratio through their huge number. Their walls and the capillary walls are one cell thick, so diffusion distance is short. A dense capillary network and ventilation maintain steep concentration gradients, and elastic fibres help air to be breathed out.

What is Fick's law of diffusion?

Fick's law says the rate of diffusion is proportional to the surface area multiplied by the concentration difference, divided by the thickness of the exchange surface. A large, thin surface with a steep gradient therefore gives rapid diffusion, which is why gas exchange surfaces have all three features.

What is the fluid mosaic model of the cell membrane?

The fluid mosaic model describes a membrane as a phospholipid bilayer in which proteins are scattered like a mosaic. It is fluid because the phospholipids and many proteins can move sideways. It is a model built from experimental evidence, such as freeze-fracture images and cell-fusion experiments.

All 5 questions on Gas exchange, membranes and transport