unit 2 transport across cell membranes

Question 1

Membrane structure – Describe how proteins arrange themselves in the membrane (2)

Answer 1

1. Hydrophobic parts of helix/AP (to the outside) to sit within the (hydrophobic) fatty acid (tails) of the phospholipids;
2. Hydrophilic parts of helix/AP (to the inside) as ions are charged/polar/water soluble;

Question 2

Membrane structure – Describe the role of

cholesterol (1)

Answer 2

Cholesterol stabilises the membrane OR Cholesterol restricts the movement of molecules/phospholipids (making up the membrane);

Question 3

Transport methods -

Name and describe five ways

substances can move across the cell-

surface membrane into a cell. (5)

Answer 3

Transport methods -

Name and describe five ways

substances can move across the cell-

surface membrane into a cell. (5)
1. (Simple) diffusion of small/non-polar molecules down a concentration gradient; If no reference to ‘small/ non-polar’ for 1. accept this idea from ‘large/charged’ given in description of
2. Facilitated diffusion down a concentration gradient via protein carrier/channel; Reject if active rather than passive
3. Osmosis of water down a water potential gradient;
4. Active transport against a concentration gradient via protein carrier using ATP;
5. Co-transport of 2 different substances using a carrier protein;

Question 4

Transport methods - The movement of

substances across cell membranes is

affected by membrane structure.

Describe how. (5)

Answer 4

1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
2. Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) 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/carriers determines how much movement;
7. Membrane surface area determines how much diffusion/movement;
8. Cholesterol affects fluidity/rigidity/permeability;

Question 5

Transport methods - Give two similarities

in the movement of substances by

diffusion and by osmosis. (2)

Answer 5

1. (Movement) down a gradient / from high concentration to low concentration;
2. Passive / not active processes; OR Do not use energy from respiration / from ATP / from metabolism;

OR Use energy from the solution;

Question 6

Transport methods – What two factors

affect the rate of facilitated diffusion (2)

Answer 6

1. Membrane folded so increased / large surface area; OR Membrane has increased / large surface area for (fast) diffusion / facilitated diffusion / active transport / co-transport;
2. Large number of protein channels / carriers (in membrane) for facilitated diffusion;
3. Large number of protein carriers (in membrane) for active transport;
4. Large number of protein (channels / carriers in membrane) for co-transport;

Question 7

Transport methods - Describe how

substances move across cell-surface

membranes by facilitated diffusion. (3)

Answer 7

1. Carrier / channel protein;
2. (Protein) specific / complementary to substance;
3. Substance moves down concentration gradient;

Question 8

Transport methods - Contrast the

processes of facilitated diffusion and

active transport. (3)

Answer 8

1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins;
2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP;
3. Facilitated diffusion takes place down a concentration gradient whereas active transport can occur against a concentration gradient.

Question 9

Transport methods - Why does inhibiting

respiration/using cyanide prevent

active transport? (4)

Answer 9

1. Oxygen is required for aerobic respiration which releases ATP
2. ATP is needed to change the shape of the protein carrier
3. Which would cause the release of the transported ion/molecule
4. So no ATP, no Active Transpor

Question 10

Req prac 3- How do we find water potential

of

plant tissue practically? (3)

Answer 10

1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis;
2. Find concentration where curve crosses the x-axis/where percentage change is zero;
3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis);

Question 11

plant tissue practically? (3) 1. Plot a graph with concentration on the x-axis and percentage change in mass on the y-axis; 2. Find concentration where curve crosses the x-axis/where percentage change is zero; 3. Use (another) resource to find water potential of sucrose concentration (where curve crosses x-axis);

Req prac 4 - Describe an experiment that

you could do to investigate whether the

mangrove root cells have a lower water

potential than sea water.

You are given:

• a piece of fresh mangrove root
• sea water
• access to laboratory equipment.

Answer 11

1. Record mass/length before and after;
2. Place in sea water for specified/equal time;
3. Remove surface water;
4. Increase in mass/length will show water has been absorbed by osmosis; 5. Repeat minimum of three times;

Question 12

Req prac 4 - Give one way in which the student could ensure the first three beetroot cylinders were kept at 25 °C throughout her experiment. (1)

Answer 12

Measure temperature (in tube) at intervals and use appropriate corrective measure (if temperature has fluctuated);

Question 13

Req prac 4 – How does a high temperature

disrupt membranes (2)

Answer 13

(By) 70oC denaturing/altering membrane protein OR (By) 70oC increasing fluidity/permeability of membrane;

Question 14

Req prac 4 - How does alcohol disrupt

membranes (1)

Answer 14

(By) ethanol dissolving phospholipid bilayer OR (By acid) altering membrane protein;

Question 15

Req prac 4 - Use your knowledge of

membrane structure to explain how

high temperature cause an increase in

absorbance 1)

Answer 15

Higher absorbance indicates more pigment (released/in solution) OR Higher absorbance indicates more membrane damage/permeability

Question 16

Req prac 4 - Explain why it is important

to control the volume of water in each

test tube (1).

Answer 16

Req prac 4 - Explain why it is important

to control the volume of water in each

test tube (1). 1. (If) too much water the concentration of pigment (in solution) will be lower / solution will appear lighter / more light passes through (than expected); OR (If) too little water the concentration of pigment (in solution) will be greater / solution will appear darker / less light passes through (than expected);
2. So results (from different temperatures) are comparable;