Net movement of molecules or ions down a concentration gradient, from higher to lower concentration.

Which molecules cross the bilayer by simple diffusion?

Small, non-polar, lipid-soluble molecules such as oxygen and carbon dioxide.

Why can ions not easily cross the phospholipid bilayer?

They are charged and are repelled by the hydrophobic fatty acid tails.

What is facilitated diffusion?

Passive movement of polar or charged substances through channel or carrier proteins.

How does a channel protein work?

It forms a water-filled pore specific to certain ions.

How does a carrier protein work?

It binds a specific molecule, changes shape and releases it on the other side.

Name three factors that affect the rate of diffusion.

Concentration gradient, surface area, number of channel or carrier proteins (also thickness).

Why does the rate of facilitated diffusion level off at high concentration?

All the carrier proteins are saturated, so their number limits the rate.

How do microvilli increase the rate of transport?

They increase membrane surface area, giving more carrier and channel proteins.

Why is the rate of simple diffusion proportional to the gradient?

Nothing limits it, so a steeper gradient always gives a faster rate.

Diffusion and facilitated diffusionAQA A-Level Biology: Flashcards

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What is diffusion?: Net movement of molecules or ions down a concentration gradient, from higher to lower concentration.
Does diffusion need ATP?: No, it is passive.
Which molecules cross the bilayer by simple diffusion?: Small, non-polar, lipid-soluble molecules such as oxygen and carbon dioxide.
Why can ions not easily cross the phospholipid bilayer?: They are charged and are repelled by the hydrophobic fatty acid tails.
What is facilitated diffusion?: Passive movement of polar or charged substances through channel or carrier proteins.
How does a channel protein work?: It forms a water-filled pore specific to certain ions.
How does a carrier protein work?: It binds a specific molecule, changes shape and releases it on the other side.
Name three factors that affect the rate of diffusion.: Concentration gradient, surface area, number of channel or carrier proteins (also thickness).
Why does the rate of facilitated diffusion level off at high concentration?: All the carrier proteins are saturated, so their number limits the rate.
How do microvilli increase the rate of transport?: They increase membrane surface area, giving more carrier and channel proteins.
Why is the rate of simple diffusion proportional to the gradient?: Nothing limits it, so a steeper gradient always gives a faster rate.
If a membrane area increases 12 times with the same protein density, what happens to the number of carriers?: It increases 12 times.

Exam questions on Diffusion and facilitated diffusion

Glucose enters red blood cells through carrier proteins in the cell-surface membrane, and no ATP is used. Oxygen also enters red blood cells, but without the help of any membrane protein. In both cases the concentration outside the cell is higher than inside.
Describe how a carrier protein moves glucose into the cell.2 marks
A cell takes up substance X by facilitated diffusion through carrier proteins in its cell-surface membrane. The concentration of X outside the cell is higher than inside the cell.
The rate of uptake of X increased as the outside concentration rose from 0 to 6 mmol dm⁻³, but stayed constant at higher concentrations. Explain why the rate stayed constant.2 marks
Cells lining the small intestine have microvilli, which are folds of the cell-surface membrane. A researcher compared one cell with microvilli and a similar cell without. The absorptive membrane area is 1500 µm² for the cell without microvilli and 18 000 µm² for the cell with microvilli. Both membranes have 40 carrier proteins per µm².
Explain how the adaptations of the cell with microvilli allow rapid transport across its membrane.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).