Topic 2---B: Cell Membranes- 2. Diffusion

Question 1

What is diffusion?

Answer 1

It is the net movement of particles from an area of higher concentration to an area of lower concentration

Question 2

What process is diffusion?

Answer 2

It’s a passive process so no energy is needed for it to happen

Question 3

How does diffusion work?

Answer 3

• Molecules will diffuse both ways but the net movement will be to the area of a lower concentration
• It continues until particles are evenly distributed throughout the liquid or gas
• Particles diffuse down a concentration gradient

Question 4

Simple diffusion

Answer 4

When molecules diffuse directly through a cell membrane

Question 5

What molecules can diffuse easily through the membrane by diffusion?

Answer 5

• Lipid soluble substances (hormones-glycerol)
• Small uncharged molecules (oxygen and carbon dioxide)
• Water can pass through membrane by osmosis

Question 6

Factors affecting the rate of diffusion

Answer 6

• Surface area- the larger the surface area the faster the rate of diffusion
• The thickness of the exchange surface- the thinner it is the shorter the distance the particles have to travel so the faster the rate of diffusion
• Concentration gradient- The higher it is the faster the rate of diffusion but as diffusion takes place the difference in concentration between the two sides of the membrane decreases until it reaches equilibrium so diffusion slows down over time

Question 7

Faciliated diffusion

Answer 7

• Some larger molecules (e.g. amino acids, glucose) would diffuse slowly through the phospholipid bilayer because they’re so big.
• Charged particles (e.g. ions, polar molecules) would also diffuse slowly because they’re water soluble and the centre of the bilayer is hydrophobic.
• So to speed things up large or charged particles diffuse through carrier or channel proteins in the cell membrane instead.
• Like diffusion, faciliated diffusion moves particles down a concentration gradient
• Its also a passive process

Question 8

Carrier protein

Answer 8

• They move large molecules across the membrane down a concentration gradient.
• Different carrier proteins faciliate the diffusion of different molecules and the rate at which a substance can diffuse in or out of the cell depends on the number of specific carrier proteins present.

Question 9

How do carrier proteins work?

Answer 9

• First, a large molecule attatches to a carrier protein in the membrane
• Then the protein changes shape
• This releases the molecule on the opposite side of the membrane

Question 10

Channel proteins

Answer 10

• Channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient.
• Different channel proteins faciliate the diffusion of different charged particles.

Question 11

Factors affecting the rate of faciliated diffusion

Answer 11

• The number of channel or carrier proteins- the more there is the faster faciliated diffusion will occur.
• The concentration gradient- The higher the concentration gradient, the faster the rate of faciliated diffusion but when equilibrium is reached the rate of faciliated diffusion will level off.

Question 12

How do you calculate the rate of diffusion?

Answer 12

By finding the gradient by dividing the change in y by the change in x