mass transport in plants

Question 1

(i) Give two ways in which the structure of starch is similar to cellulose.
(ii) Give two ways in which the structure of starch is different from cellulose.

Answer 1

(i) (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; Neutral: references to ‘unbranched’, insoluble, formed by condensation, flexible and rigid Are made of the monomer glucose
5. Hydrogen bonding (within structure).
(ii) (Starch)
1. Contains α / alpha glucose; A
2. Helical / coiled / compact / branched / not straight;
3. 1,6 bonds / 1,6 branching;
4. Glucoses / monomers same way up;
5. No H-bonds between molecules;
6. No (micro / macro) fibres / fibrils.

Question 2

(i) Using the diagram, suggest and explain one other way in which sieve cells are adapted for mass transport.
(ii) Using the diagram, suggest and explain one other way in which companion cells are adapted for the transport of sugars between cells.

Answer 2

(i) 1. No / few organelles / very little cytoplasm / cytoplasm at edge / more room / hollow / large vacuole / large space / thick walls;
2. (So) easier / more flow / (thick / strong walls) resist pressure. Easier flow may be expressed in other ways e.g. lower resistance to flow
(ii) 1. Mitochondria release energy / ATP / site of respiration;
2. For active transport / uptake against concentration gradient.

OR

3. Ribosomes / rough endoplasmic reticulum produce(s) proteins;
4. (Proteins) linked to transport e.g. carrier proteins / enzymes.

Question 3

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.

Answer 3

1. 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 4

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

Answer 4

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

Question 5

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.

Answer 5

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 6

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

Answer 6

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 7

(b) Name the process that produced the 14CO2 released from the trunk.
(c) How long did it take the 14C label to get from the top of the trunk to the bottom of the trunk? Explain how you reached your answer.
(d) What other information is required in order to calculate the mean rate of movement of the 14C down the trunk?

Answer 7

(b) Respiration.
(c) 1. (About) 30 hours;
2. Time between peak 14C at top of trunk and bottom.
(d) Length of trunk (between top and bottom).

Question 8

Root pressure is a force that is partly responsible for the movement of water through xylem in stems. Explain how the active transport of mineral ions into xylem vessels in the roots results in water entering these vessels and then being moved up the xylem tissue.

Answer 8

Any fivefrom:

Water potential in xylem reduced (by entry of ions);

Water potential gradient established between xylem and surrounding cells;

Plasma membranes of surrounding cells are partially permeable;

Water enters xylem by osmosis;

Volume of water in xylem increases;

Cannot move back due to gradient;

Pressure in xylem increases (and forces water upwards);

Question 9

The presence of an air bubble in a xylem vessel in the stem blocks the movement of water through that vessel. Use the cohesion-tension theory to explain why.

Answer 9

Any fourfrom:

Evaporation from leaves / transpiration;

Water in xylem under tension*/negative pressure/pulled up;

Water molecules cohere*/stick together/form hydrogen bonds;

So water a single column;

Air bubble breaks column / prevents cohesion;

Question 10

Water vapour diffuses through open stomata into the atmosphere. Describe two structural adaptations of the leaves of xerophytes that reduce this loss. Using Fick’s law, explain how these two adaptations reduce the rate of diffusion of water vapour intothe atmosphere.

Answer 10

reduced number of stomata;

reduced surface area;

OR

thick waxy cuticle;

increases diffusion distance;

OR

leaves reduced to spines;

reduced surface area ;

OR

sunken stomata;

reduce concentration difference;

OR

curled leaves;

reduced concentration gradient

Question 11

Explain how water enters a plant root from the soil and travels through to the endodermis.

Answer 11

water enters root hair cells;

by osmosis;

because active uptake of mineral ions has created a WP gradient;

water moves through the cortex;

(by osmosis) down a WP gradient;

through cell vacuoles and cytoplasms / symplastic pathway;through cell walls / apoplastic pathway;

Question 12

From the root, water is transported upwards through the stem. Explain how evaporation from the leaves can cause the water to move upwards.

Answer 12

WP in leaf cellsdecreases / becomes more negative;

therefore water moves out of xylem (into surrounding tissues) by osmosis;

this creates a pull/tension on the water in xylem;

Question 13

In daylight, most of the water evaporates from the leaves but some is used by the plant.Describe the ways in which this water could be used by the plant.

Answer 13

which is in a continuous column / water molecules cohere;

cohesion due to H bonding;

column doesn’t break because of adhesion with xylem walls;

(water is used in) the light-dependent reactions of photosynthesis;

electrons from water enable ATP production / H+are used to reduceNADP / produces O2;

(water can be used in) hydrolysis reactions within the plant;

to create turgor;as a solvent for transport;

as a medium for chemical reactions;

component of cells / cytoplasm;

Question 14

Two theories have been put forward to explain the upward movement of water in the xylem in a tree.·the cohesion-tension theory·the root pressure theory. Describe one piece of evidence that supports the root pressure theory and explain how it supports this theory.

Answer 14

EITHER

e.g.guttation(only) upward pressure could force liquid water out of leaves;

OR

Sap exuding from a cut, rooted stem;

(only) upward force could make this happen;

Question 15

The diameter of a tree is less during the day, when the tree is transpiring, than it is at night. Explain how this

(i) supports the cohesion-tension theory;
(ii) does not support the root pressure theory.

Answer 15

(i)Evaporation from leaves during daytime only/mainly;

tension/negative pressure (on water) in xylem creates inward pull(on walls of xylem vessel);

xylem vessels become narrower;

due to adhesion of water molecules (to walls of xylem vessels);

(ii)root pressure gives outward force/push on walls of xylem vessels;

tree would become wider/stay same diameter;

xylem vessels become wider/stay same diameter;