3.3.4 Mass Transport in Plants

Question 1

(Mass Transport in Plants) What does phloem tissue transport?

Answer 1

Organic solutes (mainly sugars like sucrose) around plants.

Question 2

(Mass Transport in Plants) What are phloem tissues formed from?

Answer 2

Cell arranged in tubes.

Question 3

(Mass Transport in Plants) Describe the structure of sieve tube elements, regarding phloem tissue.

Answer 3

Living cells that form a tube for transporting solutes. They have no nucleus and few organelles, so there is a companion cell for each sieve tube element.

Question 4

(Mass Transport in Plants) Describe the function of sieve tube elements, regarding phloem tissue.

Answer 4

They carry out living functions for sieve cells, e.g. providing the energy needed for active transport of solutes.

Question 5

(Mass Transport in Plants) What is translocation?

Answer 5

The movement of solutes (e.g. amino acids and sugars) to where they are needed in the plant.
This is an energy-requiring process.

Question 6

(Mass Transport in Plants) Where does translocation occur?

Answer 6

In the phloem.

Question 7

(Mass Transport in Plants) What are solutes sometimes called?

Answer 7

Assimilates.

Question 8

(Mass Transport in Plants) Translocation moves solutes from ‘sources’ to ‘sinks’. Describe what is meant by these terms.

Answer 8

‘Source’ is where assimilates are produced (high concentration).
‘Sink’ is where assimilates are used up (lower concentration).

Question 9

(Mass Transport in Plants) What is the function of enzymes regarding the phloem, and was are the effects of this?

Answer 9

Maintain a concentration gradient from the source to the sink by changing the solutes at the sink.
Makes sure there is always a lower concentration at the sink than the source

Question 10

(Mass Transport in Plants) Describe how to use a potometer to measure the rate of transpiration. (6)

Answer 10

• Cut a leafy shoot underwater to avoid air passing into the xylem vessels.
• Cut at an angle to avoid crushing the xylem.
• Fill the potometer ensuring there are no air bubbles.
• Use vaseline to create a tight seal with the stem and potometer.
• Introduce an air bubble to the capillary tubing.
• Ensure leaves are dry so stomata are not blocked.

Question 11

(Mass Transport in Plants) Describe the ‘source’ section of the mass flow hypothesis. (4)

Answer 11

• Companion cells actively transport solutes (e.g. sucrose) into the adjacent phloem.
• Lowers water potential in phloem’ sieve cells.
• Water diffuses in by osmosis.
• Increases the turgor pressure.

Question 12

(Mass Transport in Plants) Describe the ‘flow’ section of the mass flow hypothesis.

Answer 12

Mass flow (translocation) of solutes in the phloem, down a pressure gradient, from SOURCE to SINK.
The higher the concentration of sucrose at the source, the higher the rate of translocation.

Question 13

(Mass Transport in Plants) Describe the ‘sink’ section of the mass flow hypothesis. (4)

Answer 13

• Solutes (e.g. sucrose) are removed from the phloem.
• Raises water potential in phloem sieve cells.
• Water diffuses out by osmosis.
• Decreases the turgor pressure.

Question 14

(Mass Transport in Plants) What does xylem tissue transport?

Answer 14

Water and mineral ions in solution.

Question 15

(Mass Transport in Plants) Describe the structure of the xylem.

Answer 15

Very long, tube-like structures formed from dead cells (vessel elements) joined end to end. No end walls on these cells, making an uninterrupted tube that allows water to pass through the middle easily.

Question 16

(Mass Transport in Plants) Describe structures xylem as the tissue that transports water in the stem and leaves of plants.

Answer 16

• Facilitates the ‘transpiration stream’.
• Dead, hollow, open ended vessel elements.
• Narrow.
• Lignified, structurally strong.
• Water.

Question 17

(Mass Transport in Plants) What is transpiration?

Answer 17

The evaporation of water from the leaf (stomata) down a water potential gradient.

Question 18

(Mass Transport in Plants) What 4 factors affect transpiration rate?

Answer 18

Light intensity, temperature, humidity, and wind.

Question 19

(Mass Transport in Plants) Explain the cohesion-tension theory of water transport in the xylem. (5)

Answer 19

• Transpiration - water evaporates from leaves via the open stomata (down a water potential gradient).
• Creates a negative pressure in the leaves which pulls water particles forward from the xylem.
• Water particles are polar (slightly charged) and cohesive (hydrogen bonding).
• This “pull” or tension acts on the entire continuous column of water in the xylem.
• Water is also adhesive and attracted to the xylem walls resulting in upward capillary action.

Question 20

(Mass Transport in Plants) Outline the use of tracers and ringing experiments to investigate transport in plants as evidence for / against the mass flow hypothesis.

Answer 20

Ringing experiments that cut phloem in photosynthesising plants result in a build up of sucrose above the ring cut and stem swelling yet an absence of sucrose below the ring.

Plants exposed to radioactively labelled CO2 show the presence of radioactive organic compounds present in the phloem as evidenced by exposure of photographic film over the transverse section of a stem.

Question 21

(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘facilitates the ‘transpiration stream’’.

Answer 21

One way mass flow of water from the roots to the leaves to the air (down a water potential gradient).

Question 22

(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘dead, hollow, open ended vessel elements’.

Answer 22

Allows the continuous flow of water.

Question 23

(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘narrow’.

Answer 23

Aids capillary action.

Question 24

(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘lignified, structurally strong’.

Answer 24

Resist the high negative pressure created in the leaf by transpiration (evaporation).

Question 25

(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘water’. (3)

Answer 25

1) Used as a metabolite in photosynthesis
2) Maintains cell turgidity
3) Inorganic ions in the soil e.g. nitrates, phosphates are transported to the leaves