Transport Mechanisms in Plants

What is the structure labelled T?

• A companion cell

• B sieve plate

• C sieve tube element

• D xylem vessel element

Sucrose moves from a phloem sieve tube element into a root cell.

Which changes to the water potential and the volume of liquid in the phloem sieve tube element

are correct?

• A

• B

• C

• D

What contributes to the movement of water through the xylem vessel elements?

• A cohesion of water molecules through hydrogen bonding

• B ion movement followed by passive osmosis

• C negative water potential in the xylem

• D surface tension at the top of the plant

Which statement explains these observations?

• A During the day, cohesion between water molecules and lignin in the walls of the xylem vessels pulls the walls of the vessels inwards.

• B During the day, the tension developed in a moving column of water molecules reduces the pressure inside the xylem vessels, so that their walls move inwards.

• C During the night, the adhesion of water molecules to the lignin in the walls of the xylem vessels pushes the walls of the vessels outwards.

• D During the night, the pressure of the column of water molecules adhering to one another increases and pushes the walls of the xylem vessels outwards.

Transpiration is the consequence of which feature of plants?

• A the presence of symplast and apoplast pathways

• B the requirement for a transport system in plants

• C the requirement for leaves to exchange gases

• D the thin waxy cuticle on the lower epidermis of leaves

What could increase the rate of transpiration?

• A increasing the humidity

• B increasing the light intensity

• C decreasing the temperature

• D decreasing the wind speed

Which statement correctly describes transport pathways in dicotyledonous plants?

• A In the apoplast pathway, water may move through plasmodesmata.

• B In the symplast pathway, water may move through intercellular spaces.

• C The apoplast pathway may be blocked by the Casparian strip.

• D The symplast pathway may be blocked by the tonoplast.

What feature of the experiment could bring this conclusion into doubt?

• A The student did not control air flow or temperature surrounding the plants.

• B The student did not leave the plants to acclimatise to their surroundings.

• C The student did not take measurements at 0 minutes.

• D The student did not take account of the area of leaves.

What is the main function of a companion cell in phloem tissue?

• A providing cytoplasmic contact with the sieve tube element for loading

• B providing structural support for the sieve tube element

• C providing the nucleus for cell division in the phloem

• D providing the source of assimilates for storage

Which feature of transport in plants is correct for both xylem and phloem?

• A It is passive.

• B It occurs by mass flow.

• C It occurs from source to sink.

• D It occurs only in one direction.