Simple vs Facilitated Diffusion
How substances cross membranes by simple diffusion and facilitated diffusion using channel and carrier proteins, and the factors that affect the rate of diffusion. Includes the 4-mark exam question, a worked model answer and the examiner trap on saying facilitated diffusion uses no ATP.
Chapters (9)
Exam question [4 marks]
Oxygen crosses a cell-surface membrane by simple diffusion. Sodium ions need facilitated diffusion. Explain why, and suggest how a cell could increase the rate at which sodium ions enter.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Oxygen is small and non-polar / lipid-soluble, so it dissolves in / passes between the phospholipids (hydrophobic tails) | "can pass through the bilayer", "passes between phospholipids" | "oxygen is small" alone; "oxygen is polar" |
| 2 | Sodium ions are charged / polar, so cannot pass through the hydrophobic core / tails of the bilayer | "repelled by the fatty acid tails", "not lipid-soluble" | "ions are too big"; "ions are blocked by the membrane" with no reason |
| 3 | Ions pass through channel proteins or carrier proteins, down the concentration gradient, without ATP (passive) | "through protein channels / carriers", "from high to low concentration" | "ions use ATP"; "ions go against the gradient" |
| 4 | Increase the number of channel proteins or carrier proteins in the membrane (so more routes for ions) | "more transport proteins", "more carriers / channels", "steeper concentration gradient (for example removing Na+ from inside the cell)", "more surface area (for example microvilli)" | "make the membrane thinner"; "increase ATP" |
Marks 1 and 2 explain the difference between the molecules by the nature of the bilayer; mark 3 needs the protein AND the passive detail; mark 4 is credited for the proteins. Increasing surface area (for example more microvilli) or maintaining a steeper concentration gradient is also accepted for mark 4. Channel proteins do not change shape; carrier proteins do (not needed for the marks here).
Model answer (4/4)
Oxygen is small and non-polar, so it dissolves in the hydrophobic fatty acid tails and passes between the phospholipids. Sodium ions are charged, so they cannot pass through the hydrophobic core of the bilayer. They pass through channel or carrier proteins, down their concentration gradient, without using ATP. The cell could increase the number of channel or carrier proteins in its membrane.
Examiner trap
- Saying facilitated diffusion uses ATP
- Saying channel proteins change shape (carriers do)
- Saying large polar molecules and ions pass through the bilayer directly
Key terms
- simple diffusion
- facilitated diffusion
- channel protein
- carrier protein
- concentration gradient
- non-polar
- polar
- ion
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26What diffusion is
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. We say they move down a concentration gradient. It is passive, which means it needs no energy from the cell, because the particles are simply moving at random with their own kinetic energy.
0:47Simple diffusion
First, simple diffusion. Think about the middle of the bilayer. It is made of the fatty acid tails, and those are hydrophobic and non-polar. So small, non-polar molecules, like oxygen, carbon dioxide and lipid-soluble substances, dissolve in that layer and slip straight between the phospholipids. No protein is needed, and no energy. That is simple diffusion.
1:13The problem for ions
Now the problem. Ions are charged, and large polar molecules, like glucose, have charged regions. The hydrophobic tails repel them, so they cannot pass through the bilayer directly. Saying ions just pass through the membrane is a classic way to lose marks.
1:30Channel proteins
So the cell uses proteins. This is facilitated diffusion. Channel proteins form a water-filled pore through the membrane. Each channel is specific, so only ions of the right size and charge fit through. The ions move down their concentration gradient, so it is still passive. No ATP.
1:52Carrier proteins
The other type is the carrier protein. A molecule binds to a specific site on the carrier. The carrier then changes shape, and releases the molecule on the other side. Again, the movement is down the concentration gradient, so no ATP is used.
Remember, carriers change shape, channels do not.
2:13What affects the rate
What decides how fast this happens? A larger surface area, such as microvilli, gives more membrane for diffusion to occur across. More channel or carrier proteins give more routes through the membrane. And a steeper concentration gradient makes diffusion faster.
Cells can be adapted by having more surface area, or more channels and carriers, in the membrane.
3:09Examiner trap and recap
The trap. Facilitated diffusion does not use ATP. It is passive, and the proteins just give ions a way through.
Recap. Small non-polar molecules diffuse through the bilayer. Ions and large polar molecules need channels or carriers.
Next lesson, we follow water itself across the membrane, in osmosis and water potential.
Watch first
This lesson builds on:
- Fluid-Mosaic Model: Cell Membrane Structure · AQA 7402 · 3.2.3
Keep going
More from 3.2 Cells
- Fluid-Mosaic Model: Cell Membrane Structure · AQA 7402 · 3.2.3
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
- Active Transport and Co-transport (Ileum) · AQA 7402 · 3.2.3
- Why Cells Transport Faster: Microvilli · AQA 7402 · 3.2.3
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-035.