Transport across membranes

Question 1

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

Answer 1

• The position of the molecules within the membrane is fluid – they are able to move around within the membrane.
• 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.

Answer 2

• Bilayer OR Water is present inside and outside a cell;
• Hydrophobic (fatty acid) tails point away/are repelled from water OR Hydrophilic (phosphate) heads point to/are in/are attracted to water;

Question 3

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

Answer 3

1 (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
2 Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
OR
Large / polar / water-soluble molecules go through proteins;
3 Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
4 Active transport is movement from low to high concentration / against concentration gradient;
5 Active transport / facilitated diffusion involves proteins / carriers;
6 Active transport requires energy / ATP;
7 Ref. to Na+ / glucose co-transport;

Question 4

The movement of substances across cell membranes is affected by membrane structure.

Describe how.

Answer 4

• Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
• Phospholipid (bilayer) prevents movement/diffusion of polar/ charged/lipid-insoluble substances OR (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
• Carrier proteins allow active transport;
• Channel/carrier proteins allow facilitated diffusion/co-transport;
• Shape/charge of channel / carrier determines which substances move;
• Number of channels/carriers determines how much movement;
• Membrane surface area determines how much diffusion/movement;
• Cholesterol affects fluidity/rigidity/permeability;

Question 5

Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.

Other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose.

Answer 5

Similarity

1. Both contain galactose / a glycosidic bond;

Ignore references to hydrolysis and / or condensation

Difference

2. Lactulose contains fructose, whereas lactose contains glucose;

Ignore alpha / beta prefix for glucose

Difference must be stated, not implied

Question 6

A student investigated the effect of surface area on osmosis in cubes of potato.
* He cut two cubes of potato tissue, each with sides of 35 mm in length.
* He put one cube into a concentrated sucrose solution.
* He cut the other cube into eight equal-sized smaller cubes and put them into a sucrose solution of the same concentration as the solution used for the large cube.
* He recorded the masses of the cubes at intervals.

Describe the method the student would have used to obtain the results in the graph. Start after all of the cubes of potato have been cut. Also consider variables he should have controlled. (4)

Answer 6

Method to ensure all cut surfaces of the eight cubes are exposed to the sucrose solution;

Credit valid method descriptions to fulfil mp1, 2 and 3 (no explanation is required).

2. Method of controlling temperature;

Accept ‘at room temperature’ for method

3. Method of drying cubes before measuring;
4. Measure mass of cubes at stated time intervals;

Accept time intervals between every 5 minutes with maximum of every 40 minutes.

Accept ‘weigh the cubes at stated time intervals’

Question 7

STATE 3 properties of water that are important in biology

Answer 7

Accept any three suitable properties e.g.:
* Is a metabolite
* Is a solvent
* Has a (relatively) high heat capacity
* Has a (relatively) large latent heat of vaporisation / evaporation
* Has cohesion / hydrogen bonds between molecules;

Question 8

Describe the structure of glycogen

Answer 8

Polysaccharide of α-glucose;
OR
polyMer of α-glucose;
2. (Joined by) glycosidic bonds
OR
Branched structure;

Question 9

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients.

Answer 9

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 10

Contrast the processes of facilitated diffusion and active transport.

Answer 10

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.
Since ‘contrast’, both sides of the differences needed

Question 11

During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.

Suggest how glycogen acts as a source of energy.

Do not include transport across membranes in your answer.

Answer 11

Hydrolysed (to glucose);

2. Glucose used in respiration;
3. Ignore ‘Broken down’