3.3.4.2 Mass transport in plants

Question 1

What is the cohesion-tension theory?

Answer 1

The cohesion-tension theory explains the movement of water in the xylem due to cohesion between water molecules and tension created by transpiration.

Question 2

How does transpiration create tension in the xylem?

Answer 2

Transpiration causes water to evaporate from the mesophyll cells in the leaves, creating a negative pressure that pulls water up the xylem.

Question 3

What is cohesion in the context of the cohesion-tension theory?

Answer 3

Cohesion refers to the hydrogen bonding between water molecules that helps form a continuous column of water in the xylem.

Question 4

What is adhesion in the cohesion-tension theory?

Answer 4

Adhesion is the attraction between water molecules and the walls of the xylem vessels, aiding in the upward movement of water.

Question 5

What is the function of phloem in plants?

Answer 5

Phloem transports organic substances, such as sucrose and amino acids, from sources to sinks in plants.

Question 6

What is the mass flow hypothesis?

Answer 6

The mass flow hypothesis proposes that translocation occurs due to pressure differences created by the active loading and unloading of solutes in the phloem.

Question 7

How does sucrose enter the phloem?

Answer 7

Sucrose is actively transported into the sieve tube elements by companion cells, lowering water potential and causing water to enter by osmosis.

Question 8

What creates the pressure gradient in the phloem?

Answer 8

The active loading of sucrose at the source increases hydrostatic pressure, while unloading at the sink lowers it, creating a pressure gradient.

Question 9

What is a ringing experiment and what does it show?

Answer 9

A ringing experiment involves removing a ring of bark (including phloem) from a stem, which interrupts translocation and causes sugars to accumulate above the ring, demonstrating the role of phloem in transport.

Question 10

What is a tracer experiment in plant transport studies?

Answer 10

Tracer experiments use radioactive isotopes, such as 14C in CO2, to label sugars and track their movement in the phloem via autoradiography.

Question 11

What evidence supports the mass flow hypothesis?

Answer 11

Evidence includes:
(1) Pressure differences in the phloem shown by sap release when cut,
(2) radioactive tracers showing sugar movement, and
(3) ringing experiments demonstrating phloem transport.

Question 12

What evidence contradicts the mass flow hypothesis?

Answer 12

Evidence includes:
(1) The presence of bidirectional flow in the same sieve tube,
(2) the observation that sieve plates seem to obstruct flow, and
(3) not all solutes move at the same speed.

Question 13

What factors affect the rate of transpiration?

Answer 13

Factors affecting transpiration rate include: (1) Light intensity - increases stomatal opening for photosynthesis, (2) Temperature - increases evaporation and diffusion rates, (3) Humidity - reduces water potential gradient when high, (4) Wind speed - removes water vapor, maintaining the gradient,

Question 14

Use your understanding of the mass flow hypothesis to explain how pressure is generated inside the phloem tube. (3)

Answer 14

1. Sucrose actively transported (into phloem);
2. Lowering/reducing water potential
   OR
   More negative water potential;
3. Water moves (into phloem) by osmosis (from xylem);

Question 15

Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 15

1. Water lost from leaf because of transpiration / evaporation of water
   (molecules) / diffusion from mesophyll / leaf cells;
   OR
   Transpiration / evaporation / diffusion of water (molecules)
   through stomata / from leaves;
2. Lowers water potential of mesophyll / leaf cells;
3. Water pulled up xylem (creating tension);
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;

Question 16

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant.

Suggest one reason why. (1)

Answer 16

Plant has roots
OR
xylem cells very narrow;

Question 17

Describe the mass flow hypothesis for the mechanism of translocation in
plants. (4)

Answer 17

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.

Accept starch

Question 18

Give one similarity between the structure of xylem and phloem tissue.

Answer 18

both are made up of cells joined end to end

xylem vessels (in xylem) and sieve elements (in phloem) both lack nuclei

both are complex tissues

OR

both are made up of more than one cell type

Question 19

Describe differences between the structure of xylem and phloem tissue.

Answer 19

xylem has lignified walls, phloem does not

xylem has a wide lumen, phloem has a small lumen

xylem has no end walls OR sieve plates, phloem does

xylem has no companion cells, phloem does

xylem has vessels, phloem does not

xylem does not have sieve tube elements, phloem does

xylem has bordered pits, phloem does not

xylem has no cytoplasm/organelles, phloem does

Question 20

Suggest why plants that have a small height don’t need a xylem to survive.

Answer 20

have a large surface area to volume ratio

the distance that water/minerals need to travel is only short

Question 21

Explain why large multicellular plants require a specialised transport system.

Answer 21

large plants have a small surface area to volume ratio

there is a long distance from the external surface to the cells

diffusion is not fast enough

named example of a substance that is transported in plants (e.g. sucrose, water)

Question 22

Describe and explain one way in which sieve tube elements are adapted for the mass transport of sugars.

Answer 22

Approach 1:

no/few organelles/cytoplasm

more room for flow of sugars

OR

Approach 2:

thick walls

resist pressure

Question 23

Describe and explain one way in which companion cells are adapted for the mass transport of sugars.

Answer 23

contain mitochondria that release energy/ATP

for active transport

Question 24

Describe and explain ways in which the xylem vessels are specialised for their function.

Answer 24

cellulose/lignified cell walls to provide support under tension

cell walls lined with lignin for waterproofing OR to prevent water loss

cell wall made of cellulose to allow adhesion of water

no cytoplasm OR lack of cell contents OR hollow so the flow of water is easier

continuous tube/column so the flow of water is easier

pits/pores so water can be supplied to surrounding tissues/cells

wide/large diameter/lumen so a large volume of water can be transported

Question 25

Explain how hydrogen bonding facilitates the movement of water through the xylem.

Answer 25

hydrogen bonding allows cohesion and adhesion

makes water cohesive OR stick together

allows a transpiration stream OR allows a continuous column of water

allows adhesion of water molecules to the lining of the xylem OR to cellulose molecules

Question 26

What is transpiration?

Answer 26

the loss of water vapour from the leaves

Question 27

State the environmental factors that affect the rate of transpiration.

For each factor given, explain how it affects the rate of transpiration.

Answer 27

temperature

increased temperature increases the rate of evaporation/diffusion

OR

at high temperatures, stomata close so transpiration stops

humidity

increased humidity decreases rate of transpiration as the water potential gradient is less steep

wind speed OR air movement

higher wind speed causes a steeper water potential gradient

OR

at high wind speed, stomata close and transpiration stops

water availability

reduced water availability causes stomata to close and transpiration stops

OR

when more water is available, there is a steeper water potential gradient between roots and leaves, so transpiration is faster

light intensity

higher light intensity increases transpiration rate as more stomata open

OR

at very high light intensity, stomata close so transpiration stops

Question 28

Give two reasons why the potometer does not truly measure the rate of transpiration. (2)

Answer 28

1. Water used for support / turgidity;

Accept: water used in (the cell’s) hydrolysis or condensation (reactions) for one mark. Allow a named example of these reactions

2. Water used in photosynthesis;
3. Water produced in respiration;
4. Apparatus not sealed / ‘leaks’;

Question 29

What is the function of xylem in plants?

Answer 29

“Xylem transports water and dissolved mineral ions from the roots to the stem and leaves.”