CELLS: Diffusion

Question 1

What is diffusion?

Answer 1

The net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until they are evenly distributed.

Question 2

In diffusion, can particles diffuse both ways?

Answer 2

Yes, but the net movement will be to the area of lower concentration.

Question 3

What is the concentration gradient?

Answer 3

The path from an area of higher concentration to an area of lower concentration.

Question 4

In diffusion, how do particles diffuse across the concentration gradient?

Answer 4

They diffuse down the concentration gradient.

Question 5

Can particles diffuse across cell membranes?

Give an example.

Answer 5

Yes, as long as they can move freely through the membrane.

• Eg. oxygen and carbon dioxide can diffuse easily through cell membranes because they’re small, so they can pass through spaces between the phospholipids.*
• Also, they are non-polar so are soluble in lipids, so can dissolve in hydrophobic bilayer.*

Question 6

When molecules diffuse directly through a cell membrane.

Answer 6

Simple diffusion.

Question 7

Simple diffusion.

Answer 7

When molecules diffuse directly through a cell membrane.

Question 8

Why is diffusion considered passive?

Answer 8

It does not rely on energy from an external source, for example ATP.

Instead energy comes from the natural, inbuilt motion of the particles.

Question 9

What molecules might find it harder to diffuse through the phospholipid bilayer?

Answer 9

Charged particles and larger molecules.

Question 10

Why might some molecules find it harder to diffuse through the phospholipid bilayer?

How might these molecules diffuse instead?

Answer 10

Larger molcules = due to size would diffuse extremely slowly.

Charged particles = would diffuse slowly, as thye are water soluble and the centre of the bilayer is hydrophobic.

They might diffuse through carrier protiens or channel proteins in a process known as facilitated diffusion.

Question 11

What are the similarities and differences between simple diffusion and facilitated diffusion?

Answer 11

Similarities:

• Passive process - does not rely on external source like ATP.
• Moves particles down concentration gradient.

Differences:

• Facilitated diffusion uses carrier proteins and channel proteins.

Question 12

What is the role of carrier proteins in facilitated diffusion?

Answer 12

To move large molecules across membranes.

Different carrier proteins faciliatate diffusion of different molecules.

Question 13

What are the two proteins involved in facilitated diffusion?

Answer 13

Carrier proteins and channel proteins.

Question 14

Explain the mechanisms involved in large molecules diffusing across the cell membrane:

Answer 14

• Large molecule attaches to carrier protein in membrane.
• Protein changes shape.
• This releases molecule on the opposite side of the membrane.

Question 15

Explain the mechanisms involved with diffusing charged particles across the cell membrane:

Answer 15

• Channel proteins form pores in cell membrane.
• Charged particle diffuses though this.

Question 16

What is the role of channel proteins in facilitated diffusion?

Answer 16

Assist in the diffusion of charged particles.

Question 17

In simple diffusion, what does the rate depend on?

Answer 17

• Concentration gradient = higher it is, faster the rate (therefore, as diffusion takes place, the rate decreases as difference in concentration decreases).
• Thickness of exchange surface = thinner, the faster the rate as particles have a shorter distance to travel through.
• Surface area = larger, the faster the rate, as more particles can be exchanged in the same amount of time.

Question 18

Give an example of how increased surface area increases diffusion rate:

Answer 18

Microvilli on epithelial cells in small intestine

• Projections formed by the cell-surface membrane folding up on itself.
• Give the cell a larger surface area.
• Larger surface area means more particles can be exchanged in the same amount of time.

Question 19

What factors affect the rate of facilitated diffusion?

Answer 19

• Concentration gradient = higher it is, faster the rate (therefore, as diffusion takes place, the rate decreases as difference in concentration decreases).
• Number of channel or carrier proteins = once all proteins in membrane are in use, facilitated diffusion can’t happen any faster. So greater the number of channel/carrier proteins, the faster the rate.

Question 20

Give an example of how having more channel proteins increases the rate of facilitated diffusion:

Answer 20

• Kidney cells are adapted to have lots of aquaporins.
• These allow the cells to reabsorb a lot of water that would have otherwise been excreted from the body.

Question 21

What are aquaporins?

Answer 21

Special channel proteins that allow the facilitated diffusion of water through cell membranes.