4. Selective Permeability
• The phospholipid bilayer is selectively permeable.
• Some molecules can pass through easily (diffusion)
• Others use a ‘tunnel’ (facilitated diffusion)
• Others need energy to cross the membrane (active
transport)
5. Diffusion
• Passive = does not
require energy
Diffusion is the passive movement of particles
from areas of high concentration to low
concentration.
7. Factors affecting diffusion
There are 3 main factors that affect the rate of diffusion:
1. Concentration gradient
2. Surface area
3. Length of diffusion path
11. Adaptations in Nature
Find 3 examples of how nature maximises the rate of
diffusion. Use diagrams to explain how each example
works.
12. Facilitated diffusion
• This is the passive movement of molecules across the
membrane, down the concentration gradient.
• It does not use energy, so it is therefore still passive.
• It uses a carrier protein. This is because the molecule would
not otherwise be able to pass through the phospholipid bilayer
• The carrier protein is specific to the type of molecule; it cannot
transport other molecules.
14. Questions
1. Explain how facilitated diffusion is different from simple
diffusion.
2. Explain why some molecules require transport proteins
to cross the cell membrane, despite having a strong
concentration gradient.
3. Find and describe two examples of facilitated diffusion.
15. Osmosis
• This is the passive movement of water molecules
from areas of low solute concentration to areas
of high solute concentration.
16. Osmosis
• Water moves by osmosis from an area of low solute
concentration (more water) to high solute concentration
(less water).
• The solute particles cannot move to balance the
concentrations as they are too large to fit through the
pores of the selectively permeable membrane.
18. Summary Questions
1. Construct a table to compare the 3 types of passive
transport.
2. Explain why the term “passive” transport is used.
3. Identify 3 ways of increasing the rate of diffusion.
4. Compare osmosis and simple diffusion. What are the
similarities and differences?
