2.5.2 diffusion across membranes

Question 1

simple diffusion

Answer 1

movement of molecules from area of high concentration to area of low concentration
- passive process
- doesn’t involve ATP

Question 2

factors affecting rate of simple diffusion

Answer 2

1. temperature –> increases = molecules have more kinetic energy = rate of diffusion increases
2. diffusion distance –> thicker membrane = slower rate of diffusion
3. surface area –> large SA = increased rate of diffusion
4. size of diffusing molecules –> smaller ions/molecules diffuse more rapidly
5. concentration gradient –> steeper gradient = faster rate of diffusion

Question 3

describe facilitated diffusion (referring to phospholipid bilayer)

Answer 3

• small polar molecules are insoluble in the lipid bilayer as cannot interact with hydrophobic tails
• they diffuse through water-filled channel proteins (pores)
• cholesterol molecules reduce membrane permeability to small water-soluble molecules
• glucose molecules are too large for water-filled channel proteins = bind to transmembrane carrier protein
• specific carrier proteins for different molecules