Diffusion Notes

Cambridge IGCSE Biology: Revision notes

Key facts

Diffusion is the net movement of particles down a concentration gradient, caused by their random movement.
Diffusion is passive: it needs no energy from the cell.
Only small molecules and ions can diffuse through the cell membrane; large molecules such as starch cannot.
The rate rises with a steeper gradient, a larger surface area, a higher temperature and a shorter distance.
Exchange surfaces such as alveoli and villi are adapted to speed up diffusion.

What is diffusion

Particles move randomly, so more move from a crowded region to an empty one than the other way: a net movement down the gradient.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is caused by random kinetic movement, not because particles 'try' to spread out. It is passive and needs no energy from the cell.

High concentration

Low concentration

Particles move randomly, so there is a net movement from the high concentration region to the low concentration region.

1
Random movement
Particles move in all directions
2
More at high concentration
So more move out of that region
3
Net movement
From high to low concentration
4
Equilibrium
No net movement

Why diffusion happens

Does diffusion need energy from the cell?

Across membranes

Small molecules and ions can cross the membrane; large molecules cannot.

Oxygen and carbon dioxide are small and cross easily. Ions pass through channels. Water crosses by osmosis, a special case of diffusion. Large molecules such as proteins and starch cannot cross, so not all substances diffuse.

Small molecules (O₂, CO₂) Ions Large molecules (starch, protein) Crosses the membrane? Yes Yes No How Pass between the lipid molecules Through ion channels Too large to pass

	Small molecules (O₂, CO₂)	Ions	Large molecules (starch, protein)
Crosses the membrane?	Yes	Yes	No
How	Pass between the lipid molecules	Through ion channels	Too large to pass

Small molecules (O₂, CO₂)

Crosses the membrane?:: Yes
How:: Pass between the lipid molecules

Ions

Crosses the membrane?:: Yes
How:: Through ion channels

Large molecules (starch, protein)

Crosses the membrane?:: No
How:: Too large to pass

Which substance cannot diffuse through the cell membrane?

Why diffusion matters

Diffusion brings in oxygen and nutrients and removes carbon dioxide and wastes.

Single-celled organisms depend entirely on diffusion across their membrane. In the lungs oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses the opposite way, each down its own gradient. Large organisms need specialised exchange surfaces: lungs, gills and the small intestine. Link diffusion to a named process such as respiration.

1
Alveolus
High oxygen concentration
2
Oxygen diffuses
Into the blood and red blood cells
3
Carbon dioxide diffuses
From the blood into the alveolus
4
Both
Move down their own gradients at the same time

Gas exchange in the lungs

Why does carbon dioxide diffuse out of respiring cells?

Factors affecting rate

Diffusion is faster with a steeper gradient, a larger surface area, a higher temperature and a shorter distance.

Explain the mechanism. A higher temperature gives particles more kinetic energy, so they move faster and collide with the membrane more often. A thin wall means a shorter distance. Gills work well because they have a huge surface area, thin walls and a flow of water that keeps the gradient steep.

Diffusion rate against concentration difference

Diffusion rate $\propto \dfrac{\text{area} \times \text{gradient}}{\text{distance}}$

Concentration gradient Surface area Temperature Distance Faster when Larger Larger Higher Shorter Example Glucose absorbed faster from a 10% than a 2% solution Villi, alveoli Faster at 37 °C than 20 °C Capillary walls are one cell thick

	Concentration gradient	Surface area	Temperature	Distance
Faster when	Larger	Larger	Higher	Shorter
Example	Glucose absorbed faster from a 10% than a 2% solution	Villi, alveoli	Faster at 37 °C than 20 °C	Capillary walls are one cell thick

Concentration gradient

Faster when:: Larger
Example:: Glucose absorbed faster from a 10% than a 2% solution

Surface area

Faster when:: Larger
Example:: Villi, alveoli

Temperature

Faster when:: Higher
Example:: Faster at 37 °C than 20 °C

Distance

Faster when:: Shorter
Example:: Capillary walls are one cell thick

Why do thin walls speed up diffusion?

Exchange surfaces

Exchange surfaces have a large area, thin walls, a good blood supply and ventilation to keep the gradient steep.

In the lungs, alveoli give a huge surface area, have thin walls and are surrounded by capillaries, and breathing brings fresh air. In the small intestine, villi increase the surface area and have a blood supply. Single-celled organisms have a high surface area to volume ratio, so they need none of these.

Large surface area Thin walls Good blood supply Ventilation or movement Where Alveoli, villi, gill filaments Capillaries, alveoli All exchange surfaces Lungs, gills Effect More area for particles to cross One cell thick: short distance Removes the substance, keeping a steep gradient Refreshes the gradient

	Large surface area	Thin walls	Good blood supply	Ventilation or movement
Where	Alveoli, villi, gill filaments	Capillaries, alveoli	All exchange surfaces	Lungs, gills
Effect	More area for particles to cross	One cell thick: short distance	Removes the substance, keeping a steep gradient	Refreshes the gradient

Large surface area

Where:: Alveoli, villi, gill filaments
Effect:: More area for particles to cross

Thin walls

Where:: Capillaries, alveoli
Effect:: One cell thick: short distance

Good blood supply

Where:: All exchange surfaces
Effect:: Removes the substance, keeping a steep gradient

Ventilation or movement

Where:: Lungs, gills
Effect:: Refreshes the gradient

How does a good blood supply increase the rate of diffusion at an exchange surface?

Try an exam question

Explain how the structure of the alveoli makes gas exchange efficient.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.