Transport Across Membranes A1

Question 1

What is the membrane made up of?
(3)

Answer 1

1. phospholipids
2. carbohydrates
3. proteins

Question 2

Describe the structure of a phospholipid molecule and explain how phospholipids are arranged in a plasma membrane.
(3)

Answer 2

1. glycerol joined to two fatty acid tails. Phosphate group joined to glycerol on opposite side. (condensation reaction, ester bond formed)
2. phospholipid has hydrophilic head (phosphate ad glycerol) and hydrophobic tail (fatty acid chains)
3. arrange to form a phospholipid bilayer (hydrophilic head facing out. Hydrophobic fatty acid chains facing in)

Question 3

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

Answer 3

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 of a variety of different molecules arranged into a mosaic

Question 4

Many different substances enter and leave a cell by crossing its cell surface membrane.
Describe how substances can cross a cell surface membrane. (7)

Answer 4

1. simple/facilitated diffusion from high to low concentration (down concentration gradient)
2. small/non-polar molecules go through proteins
3. water moves by osmosis from high to low water potential
4. active transport is movement from low to high concentration
5. active transport / facilitated diffusion involves proteins/carriers
6. active transport requires energy from ATP
7. Na+ / glucose co-transport

Question 5

Define diffusion.

Answer 5

The net movement of molecules from a higher concentration to a lower concentration across a partially permeable membrane.

Question 6

What type of process is diffusion?

Answer 6

a passive process (doesn’t require ATP)

Question 7

What is the equation for Fick’s law?

Answer 7

rate of diffusion
=
(SA x concentration gradient)
÷
diffusion distance

Question 8

Define osmosis.

Answer 8

Net movement of water molecules from a high to low concentration of solution through a selectively (partially) permeable membrane.

Question 9

Define hypotonic.

Answer 9

1. results in cell swelling and lysis (bursting - cell contents are lost)
2. high WP outside cell
3. low WP inside cell

Question 10

Define hypertonic.

Answer 10

1. results in cell shrivelling
2. high WP inside cell
3. low WP outside cell
4. plasmolysis (cell membrane has so little water it peels from cell wall)

Question 11

Define isotonic.

Answer 11

1. no net movement of water in or out the cells
2. WP inside cell = WP outside cell

Question 12

Describe active transport.

Answer 12

1. used to transport molecules across membrane against concentration gradient
2. from low to high concentration
3. requires a specifically shaped carrier protein
4. requires a source of energy supplied by ATP (so usually occurs at mitochondria)
5. which is produced during respiration

Question 13

What happens if the tertiary structure of a carrier protein changes?

Answer 13

1. facilitated diffusion / active transport can’t occur
2. as binding site has changed so no longer complementary to active site

Question 14

Students were advised to take additional readings.
Explain why. - application
(2)

Answer 14

1. more points plotted therefore more reliable line of best fit
2. determines intercept (where it crosses the x axis)

Question 15

Why would a student make several repeats of a concentration? - application
(2)

Answer 15

1. identify and remove anomalies
2. so variation in data is minimised

Question 16

People with coeliac disease don’t absorb products of digestion very well.
Explain why.
(3)

Answer 16

1. their micro-villi are damaged
2. which reduces the SA
3. for active transport and facilitated diffusion to occur

Question 17

What is the formula for calculating dilutions?

Answer 17

Volume of stock solution required

volume required
x
(concentration you want ÷ concentration provided)

Question 18

Representative results are…

Answer 18

1. reliable - big enough sample size (reduces impact of anomalies)
2. free from bias - selected at random