Oxygen slips straight through a membrane, glucose needs a protein to help it, and sodium ions can be pumped against their gradient only while the cell is respiring. Size, charge and energy decide the route.
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, as a result of their random motion. It is passive. Small non-polar molecules such as O2 and CO2 diffuse straight through the bilayer (simple diffusion); ions and polar molecules need channel or carrier proteins (facilitated diffusion). Larger organisms and cells have a smaller surface area to volume ratio, so diffusion alone serves them less well. Active transport moves ions or molecules against their concentration gradient through carrier proteins, using energy from ATP made in respiration. Large particles and liquids cross in bulk inside vesicles: endocytosis into the cell and exocytosis out of it, both also using ATP.
Fig. 1: how the rate of entry changes with the concentration difference for simple and facilitated diffusion
Section 01
Simple and facilitated diffusion.
Try each molecule in turn. Start with oxygen, then glucose and Na+ ions, and change the Proteins slider. Leave the sim running until the outside and inside counts match, then read the net rate.
Substance
Route
Why
O2, CO2
through the bilayer: simple diffusion
small and non-polar, so they dissolve in the hydrophobic core
glucose, amino acids
carrier proteins: facilitated diffusion
polar, so repelled by the hydrophobic core
Na+, K+, Cl−
channel proteins: facilitated diffusion
charged, so cannot cross the hydrophobic core
proteins
endocytosis
far too large for any channel or carrier
Rate of diffusion increases with: a steeper concentration gradient, a larger surface area, a higher temperature and a thinner surface (shorter diffusion distance).
Facilitated diffusion also depends on the number of channel or carrier proteins. When every carrier is in use the rate stops rising (Fig. 1).
Equilibrium: when the concentrations are equal, molecules still cross both ways but there is no net movement.
Non-living models: Visking (dialysis) tubing is partially permeable, so a bag of starch and glucose in water shows that glucose diffuses out while starch stays in.
Examiner trap
Facilitated diffusion is still passive. The protein helps the substance across, but it only moves down its concentration gradient and no ATP is used. Do not write that carrier proteins always mean active transport.
Section 02
Surface area to volume.
Drop the three cubes into acid and watch the pink fade from the outside inward. Read the time each cube takes to clear, then switch to "same volume, three shapes".
Cube side / cm
Surface area / cm2
Volume / cm3
SA : V
Time to clear (sim)
1
6
1
6 : 1
about 17 min
2
24
8
3 : 1
about 69 min
3
54
27
2 : 1
about 156 min
The rule: as an object gets bigger, its surface area to volume ratio decreases. Surface area scales with length2, volume with length3.
The investigation: agar containing an alkali and an indicator (phenolphthalein, pink) is cut into cubes and put into dilute acid. The acid diffuses in at the same speed in every cube, so the time for the colour to disappear measures how quickly diffusion reaches the centre.
Control variables: concentration and volume of acid, temperature, the same batch of agar, and cutting the cubes accurately.
Same volume, different shape: a flattened 4 × 4 × 0.5 cm block (SA : V = 5 : 1) clears much faster than a 2 cm cube (3 : 1), because no point is far from the surface. Thin, flat or folded shapes are how large organisms keep diffusion distances short.
Examiner trap
The ratio decreases as size increases; it does not increase. And quote SA : V as a ratio to 1 (for example 3 : 1), showing the surface area and volume you used to get it.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the diffusion sim, oxygen crosses the membrane anywhere, without using a protein. Why?
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In the diffusion sim, glucose enters through carrier proteins and the rate stops rising when the gradient is very steep. Why?
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A 2 cm agar cube has a surface area of 24 cm2 and a volume of 8 cm3. What is its surface area to volume ratio?
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In the sim, what does the 1 cm cube clearing well before the 3 cm cube show?
Section 03
Active and bulk transport.
Run the sodium-potassium pump and count the ATP used. Then tick the cyanide box and run it again. Finally, run endocytosis and exocytosis and watch where the membrane goes.
Active transport: movement of ions or molecules across a membrane against a concentration gradient, through specific carrier proteins, using energy from the hydrolysis of ATP.
The Na+/K+ pump: for each ATP, the carrier moves 3 Na+ out of the cell and 2 K+ in. ATP phosphorylates the carrier, which changes its shape.
Evidence: a respiratory inhibitor such as cyanide stops ATP production, so active transport stops, but diffusion carries on.
Endocytosis: the membrane surrounds a large particle or droplet of liquid and pinches off as a vesicle inside the cell. Phagocytosis takes in solids (a phagocyte engulfing a bacterium); pinocytosis takes in liquids.
Exocytosis: vesicles, often from the Golgi body, move to and fuse with the cell surface membrane and release their contents outside, for example digestive enzymes or hormones such as insulin.
Energy: endocytosis and exocytosis need ATP, to move vesicles along the cytoskeleton and to change the shape of the membrane.
Stage 2 · Exam
Exam-style questions
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Exam style+20 XP
Which feature is shared by facilitated diffusion and active transport?
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In the active transport sim, what happens to the sodium-potassium pump soon after cyanide is added?
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A cell takes up an ion from a solution in which the ion's concentration is lower than inside the cell. The uptake stops when a respiratory inhibitor is added. Which process is this?
Exam style+20 XP
Cells in the pancreas release digestive enzymes into a duct. By which process do the enzymes leave the cells?
Skill unlocked
Diffusion, active and bulk transport, mastered.
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