3.2.3 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

Name and describe five ways
substances can move across the cell-
surface membrane into a cell. (5)

Answer 3

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.
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

Describe how 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

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

What two factors
affect the rate of facilitated diffusion (2)

Answer 6

1. (external) concentration
2. number of channel / carrier proteins

Question 7

Suggest and explain two ways the cell-surface
membranes of may be adapted to allow
rapid transport of nutrients. (2)

Answer 7

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 8

Describe how
substances move across cell-surface
membranes by facilitated diffusion. (3)

Answer 8

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

Question 9

Contrast the
processes of facilitated diffusion and
active transport. (3)

Answer 9

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 10

Why does inhibiting respiration/using cyanide prevent active transport? (4)

Answer 10

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 Transport

Question 11

Req prac 3- How do we find water potential
of plant tissue practically? (3)

Answer 11

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 12

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 12

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 13

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 13

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

Question 14

Req prac 4 – How does a high temperature
disrupt membranes (2)

Answer 14

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

Question 15

Req prac 4 - How does alcohol disrupt
membranes (1)

Answer 15

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

Question 16

Req prac 4 - Use your knowledge of membrane structure to explain how high temperature cause an increase in absorbance (1)

Answer 16

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

Question 17

Req prac 4 - Explain why it is important to control the volume of water in each test tube (1).

Answer 17

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;