Transport across membranes

Question 1

Explain why a cell membrane may be described as a fluid-mosaic? (2)

Answer 1

1. The position of the molecules within the membrane is fluid - they are able to move around within the membrane
2. Membrane is made up from a variety of different molecules arranged into a mosaic

Question 2

Explain the arrangement of phospholipids in a cell-surface membrane. (2)

Answer 2

1. Bilayer
2. Hydrophobic fatty acid tails point away/repelled from water OR hydrophilic phosphate head point towards water

Question 3

What is Fick’s Law?

Answer 3

A
Rate of Diffusion = (surface area x concentration gradient) / diffusion distance

Question 4

What are 4 factors affecting the rate of diffusion?

Answer 4

1. Temperature
2. Surface Area
3. Concentration Gradient
4. Diffusion Distance / Pathway

Question 5

Q
How does increasing temperature affect the rate of diffusion? (1)

Answer 5

A
increased kinetic energy so faster rate of diffusion of molecules

Question 6

How does increasing surface area affect the rate of diffusion? (1)

Answer 6

larger surface area provides for space for molecules so faster rate of diffusion

Question 7

How does increasing concentration gradient affect the rate of diffusion? (1)

Answer 7

as concentration difference increases, rate of diffusion increases

Question 8

How does increasing diffusion distance affect the rate of diffusion? (1)

Answer 8

A
longer diffusion distance, slower rate of diffusion

Question 9

What happens if the tertiary structure of the carrier protein changes? (1)

Answer 9

Facilitated diffusion / active transport can’t occur (because the binding site has changed shape and is no longer complementary to the molecule, therefore there will not fit/bind)

Question 10

Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substance can cross a cell surface membrane. (5)

Answer 10

1. Simple / Facilitated diffusion from high concentration to low concentration
2. Small / non-polar molecules pass through phospholipid bilayer
   OR
3. Large / polar molecules pass through proteins
4. Water moves by osmosis / high water potential to low water potential / less to more negative water potential
5. Active transport is movement from low to high concentration
6. Active transport / facilitated diffusion involves proteins / carriers
7. Active transport requires ATP / energy
8. Ref to co-transport of Na+ and glucose

Question 11

Q
The movement of substances across cell membranes is affected by membrane structure. Describe how. (5)

Answer 11

1. Phospholipid (bilayer) allows movement/diffusion of non-polar substances
2. Phospholipid (bilayer) prevents movement/diffusion of polar/charged substances OR proteins allow polar/charged substances to cross the membrane/bilayer
3. Carrier proteins allow active transport
4. Channel/carrier proteins allow facilitated diffusion / co-transport
5. Shape/charge of channel/ carrier determines which substances move
6. Number of channels/carrier determines how much movement
7. Membrane surface area determines how much diffusion/movement
8. Cholesterol affects fluidity/rigidity/permeability

Question 12

Describe the relationship between size and surface area to volume ratio of organisms (1)

Answer 12

As the size of an organism increases, it’s surface area to volume ratio decreases

Question 13

Give two similarities in the movement of substances by diffusion and by osmosis. (2)

Answer 13

1. (Movement) down a gradient / from high concentration to low concentration
2. Passive / not active processes
   OR
3. Don’t use energy from respiration / ATP