Mass transport

Question 1

Give two ways in which the structure of starch is similar to cellulose [2]

Answer 1

Both

1. Are polymers / polysaccharides / are made of monomers / of monosaccharides;
2. Contain glucose / carbon, hydrogen and oxygen;
3. Contain glycosidic bonds;
4. Have 1−4 links;

Question 2

Give two ways in which the structure of starch is different from cellulose. [2]

Answer 2

(Starch) Contains α / alpha glucose.

No H-bonds between molecules.

Question 3

Using the diagram, suggest and explain one other way in which sieve cells are adapted for mass transport. [2]

Answer 3

1) Very little cytoplasm / cytoplasm at edge so more hollow walls.
2. (So) easier / more flow.

Question 4

Using the diagram, suggest and explain one other way in which companion cells are adapted for the transport of sugars between cells. [2

Answer 4

1) Mitochondria release energy / ATP / site of respiration.

2. For active transport / uptake against concentration gradient

Question 5

Organic compounds synthesised in the leaves of a plant can be transported to the plant’s roots. This transport is called translocation and occurs in the phloem tissue of the plant

Answer 5

(a) One theory of translocation states that organic substances are pushed from a high pressure in the leaves to a lower pressure in the roots.

Describe how a high pressure is produced in the leaves. [3]

• Water potential becomes lower / becomes more negative (as sugar enters phloem);
  2. Water enters phloem by osmosis;
  3. Increased volume (of water) causes increased pressure.

Question 6

PCMBS is a substance that inhibits the uptake of sucrose by plant cells.
Scientists investigated the effect of PCMBS on the rate of translocation in sugar beet. The figure below shows their results.

During their experiment, the scientists ensured that the rate of photosynthesis of their plants remained constant. Explain why this was important. [2]

Answer 6

1) Rate of photosynthesis related to rate of sucrose production.
2. Rate of translocation higher when sucrose concentration is higher.

Question 7

The scientists concluded that some translocation must occur in the spaces in the cell walls. Explain how the information in the figure above supports this conclusion. [2

Answer 7

1) Rate of translocation does not fall to zero / translocation still occurs after 120 minutes;
2. But sucrose no longer able to enter cytoplasm of phloem cells

Question 8

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

Describe the mass flow hypothesis for the mechanism of translocation in plants. [4]

Answer 9

1) In source / leaf sugars actively transported into phloem;
2. By companion cells.
3. Lowers water potential of sieve cell / tube and water enters by osmosis;
4. Increase in pressure causes mass movement (towards sink / root);
5. Sugars used / converted in root for respiration for storage

Question 10

Name the process that produced the 14CO2 released from the trunk. [1]

Answer 10

Respiration.