Transport across cell membranes

Question 1

Give two ways in which pathogens can cause disease.

Answer 1

1. (Releases) toxins;
2. Kills cells / tissues.
3. Accept any reference to cell / tissue damage
   Ignore infecting / invading cells

Question 2

Putting bee honey on a cut kills bacteria. Honey contains a high concentration of
sugar.
Use your knowledge of water potential to suggest how putting honey on a cut kills
bacteria.

Answer 2

1. Water potential in (bacterial) cells higher (than in honey) / water
   potential in honey lower (than in bacterial cells);
   Q candidates must express themselves clearly
2. Must be comparative e.g. high WP in cell and low WP in
   honey
3. Water leaves bacteria / cells by osmosis;
4. (Loss of water) stops (metabolic) reactions.

Question 3

LDL enters the liver cells (lines 3−4).
Using your knowledge of the structure of the cell-surface membrane, suggest how
LDL enters the cell.

Answer 3

3. (Protein part of) LDL attaches to receptor

4. Goes through carrier / channel protein.

Question 4

Explain how the monoclonal antibody would prevent the regulator protein from
working (lines 7−8).
………………………………………..

Answer 4

(c) Any two from:
1. (Monoclonal antibody) has a specific tertiary structure / variable region /
is complementary to regulator protein
Do not award MP1 if reference to active site.
2. Binds to / forms complex with (regulator protein)
“It” refers to monoclonal antibody in MP1 and MP2
3. (So regulator protein) would not fit / bind to the receptor / is not
complementary to receptor

Question 5

Describe how the control group should have been treated.

Answer 5

Injection with salt solution

1. Accept inject placebo in salt solution
2. Otherwise treated the same.

Question 6

The hydrostatic pressure falls from the arteriole end of the capillary to the venule
end of the capillary. Explain why.

Answer 6

Loss of water / loss of fluid / friction (against capillary lining)

Question 7

(c) High blood pressure leads to an accumulation of tissue fluid. Explain how.

Answer 7

(c) 1. High blood pressure = high hydrostatic pressure;
2. Increases outward pressure from (arterial) end of capillary / reduces
inward pressure at (venule) end of capillary;
3. (So) more tissue fluid formed / less tissue fluid is reabsorbed.
Allow lymph system not able to drain tissues fast enough

Question 8

The water potential of the blood plasma is more negative at the venule end of the
capillary than at the arteriole end of the capillary. Explain why.

Answer 8

d) 1. Water has left the capillary;
2. Proteins (in blood) too large to leave capillary;
3. Increasing / giving higher concentration of blood proteins (and thus wp).

Question 9

Percentage of lipid in plasma

membrane by mass.

Answer 9

1. Divide mass of each lipid by total mass of all lipids (in that type of cell);
2. Multiply answer by 100.

Question 10

Suggest one advantage of the different percentage of cholesterol in red blood cells
compared with cells lining the ileum.

Answer 10

Red blood cells free in blood / not supported by other cells so cholesterol helps
to maintain shape;
Allow converse for cell from ileum – cell supported by others
in endothelium so cholesterol has less effect on maintaining
shape.

Question 11

E. coli has no cholesterol in its cell-surface membrane. Despite this, the cell
maintains a constant shape. Explain why.

Answer 11

1. Cell unable to change shape;
2. (Because) cell has a cell wall;
3. (Wall is) rigid / made of peptidoglycan / murein.

Question 12

Q6.A group of students carried out an investigation to find the water potential of potato tissue.
The students were each given a potato and 50 cm3
of a 1.0 mol dm−3 solution of sucrose.
• They used the 1.0 mol dm−3 solution of sucrose to make a series of different
concentrations.
• They cut and weighed discs of potato tissue and left them in the sucrose solutions
for a set time.
• They then removed the discs of potato tissue and reweighed them.
The table below shows how one student presented his processed results.

Concentration of
sucrose solution / mol
dm−3 / Percentage change
in mass of potato
tissue
0.15 +4.7
0.20 +4.1
0.25 +3.0
0.30 +1.9
0.35 −0.9
0.40 −3.8
(a) Explain why the data in the table above are described as processed results.

Answer 12

Calculations made (from raw data) / raw data would have recorded initial and final
masses.

Question 13

Describe how you would use the student’s results in the table above to find the
water potential of the potato tissue.

Answer 13

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 14

The structure of a plasma membrane is described as a fluid mosaic.
Explain why.
___________________________________________________________________

Answer 14

Fluid = molecules move around;
Mosaic = proteins floating among phospholipids/not just phospholipids/
other molecules in it/made of different sorts of molecules;

Question 15

Give two functions of proteins in plasma membranes.

Answer 15

Any two from
Enzymes;
Antigens/cell recognition/cell markers;
Receptors;
Carriers;
Channels;
Any 2
Accept active transport and facilitated diffusion for 1 mark each

Question 16

By what process did the calcium ions leave the cells after 10 minutes? Use evidence from the graph to support your answer.

Answer 16

Active transport;
Calcium ions move against a concentration gradient/calcium ion concentration
in solution is (much) higher than concentration inside cells;
2

Question 17

Describe two differences between active transport and facilitated diffusion.

Answer 17

Active transport against / facilitated down with concentration gradient;
Accept answers in terms of water potentials
Active transport uses ATP/energy, /facilitated doesn’t;
Reject along/across gradient
Active uses carrier (proteins), / facilitated (often) uses channel (proteins);

Question 18

Explain why molecules of oxygen and carbon dioxide are able to diffuse across membranes.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________.

Answer 18

Lipid/fatty acid part of membrane is non-polar/hydrophobic;
Accept lipid/fatty acid bilayer
Oxygen and carbon dioxide small/ non-polar (molecules);
Oxygen/carbon dioxide can diffuse through/dissolve in/
get between molecules in this layer;
Down a concentration gradient;

Question 19

Explain why ventilation increases the efficiency of gas exchange.

Answer 19

Brings more oxygen/removes carbon dioxide;
Maintains diffusion/concentration gradients;
Between alveoli and blood/capillaries;

Question 20

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

Answer 20

By diffusion - substances move DOWN a concentration gradient

• Simple diffusion - through bilayer is small and/or non-polar
• Facilitated diffusion - substances diffuse though transport proteins
• By osmosis - movement of water only down a concentration gradient
• By active transport - ATP used in transporting substances against concentration gradient
• Co-transport of glucose and amino acids using Na+ ions
• Endocytosis and exocytosis - bulk transport of substances into/out of a cell

Question 21

Describe how the structure of a phospholipid molecule is different from the structure of a triglyceride molecule. (2)

Answer 21

Phosphilipid has a phosphate ion bonded to glycerol

- Phospholipid has two fatty acids bonded to glycerol

Question 22

Describe the structure of a cell membrane. (5)

Answer 22

1. Double layer of phospholipid molecules;
2. Detail of arrangement of phospholipids;
3. Intrinsic proteins/protein molecules passing right through;
4. Some with channels/pores;
5. Extrinsic proteins/proteins only in one layer/on surface;
6. Molecules can move in membrane/dynamic/membrane contains cholesterol;
7. Glycocalyx/carbohydrates attached to lipids/proteins;

Question 23

Describe the part played by cell surface membranes in regulating the movement of substances into and out of cells. (6)

Answer 23

1. Non-polar/lipid soluble molecules move through phospholipid layer/bilayer;
2. Small molecules/water/gases move through phospholipid layer/bilayer;
3. Ions/water soluble substances move through channels in proteins;
4. Some proteins are gated;
5. Reference to diffusion;
6. Carriers identified as proteins;
7. Carriers associated with facilitated diffusion;
8. Carriers associated with active transport/transport wi

Question 24

Describe how proteins are arranged in a plasma membrane and the part they play in transporting substances into and out of cells. (6)

Answer 24

1 Some proteins pass right through membrane;
2 Some proteins associated with one layer;
3 Involved in facilitated diffusion;
4 Involved in active transport;
5 Proteins act as carriers;
6 Carrier changes shape / position;
7 Proteins form channels / pores;
8 Protein allows passage of water soluble molecules / charged particles / correct named example;

Question 25

Describe the processes involved in the absorption of the products of starch digestion. (5)

Answer 25

Glucose moves in with sodium (into epithelial cell);
Via (carrier / channel) protein / symport;
Sodium removed (from epithelial cell) by active transport / sodium- potassium pump;
Into blood;
Maintaining low concentration of sodium (in epithelial cell) / maintaining sodium
concentration gradient (between lumen and epithelial cell);
Glucose moves into blood;
By (facilitated) diffusion;