Diffusion and facilitated diffusionAQA A-Level Biology: Revision notes
Section 1
Simple 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. It is passive, needing no ATP.
In simple diffusion molecules cross the phospholipid bilayer directly. Small, non-polar (lipid-soluble) molecules such as oxygen, carbon dioxide and steroid hormones cross easily. Ions and polar molecules, such as glucose, are repelled by the hydrophobic fatty acid tails and cross only very slowly.
Do not say that ions cannot cross at all. They cross very slowly through the bilayer, so use 'cannot easily'.
Section 2
Facilitated diffusion
In facilitated diffusion large, polar or charged substances cross the membrane through protein channels or carrier proteins. It is passive and goes down a concentration (or electrochemical) gradient.
- Channel proteins form water-filled pores that are specific to certain ions and may open or close (gated).
- Carrier proteins bind a specific molecule on one side, change shape and release it on the other side.
Section 3
Factors affecting the rate of diffusion
- Concentration (or water potential) gradient: a steeper gradient gives a faster rate.
- Surface area of the membrane: a larger area gives a faster rate.
- Number of channel or carrier proteins: more proteins give a faster rate of facilitated diffusion.
- Thickness of the exchange surface: a thinner surface gives a faster rate.
Simple diffusion rate keeps increasing in proportion to the gradient. Facilitated diffusion levels off when all proteins are saturated, because the number of proteins is then the limiting factor.
In graph questions, a plateau means that something other than concentration is limiting, usually the number of carrier proteins.
Section 4
Adaptations of specialised cells
Cells that transport substances rapidly have adaptations:
- Folded membranes (e.g. microvilli) to increase surface area.
- A large number of channel or carrier proteins in the membrane.
Example: if microvilli increase the membrane area from 1500 µm² to 18 000 µm², the area is 12 times larger. With 40 carrier proteins per µm² the cell has 12 times as many carriers (720 000 instead of 60 000), so the rate of transport increases.
That's the notes covered.
Carry on to the next subtopic.
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
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).