MEMBRANE TRANSPORTBiologyCells & Biomolecules

Mr Goodwin

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)

Diffusion

• Passive = does not require energy

Diffusion is the passive movement of particles from areas of high concentration to low concentration.

Animation • http://highered.mheducation.com/sites/0072495855/

student_view0/chapter2/animation__how_diffusion_works.html

Factors affecting diffusionThere are 3 main factors that affect the rate of diffusion: 1. Concentration gradient2. Surface area3. Length of diffusion path

Concentration Gradient• Concentration gradient = the difference in concentration

of a substance between two locations (eg, across a membrane)

Surface area

Higher surface area = faster diffusion

Length of Diffusion Path

Longer path for diffusion = slower diffusion

Adaptations in NatureFind 3 examples of how nature maximises the rate of diffusion. Use diagrams to explain how each example works.

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.

Animation• http://highered.mheducation.com/sites/0072495855/

student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Questions1. 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.

Osmosis• This is the passive movement of water molecules from areas of low solute concentration to areas of high solute concentration.

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.

Animations• http://education.uoit.ca/lordec/ID_LORDEC/

diffusion_osmosis/garib_diffusion_osmosis.swf

Summary Questions1. 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?