Transport in Plants

Question 1

Define vascular bundle.

Answer 1

• Vascular system in herbaceous dicotyledonous plants.

- Consists of two transport vessels, the xylem and the phloem.

Question 2

Describe the structure and function of the vascular system in the roots of dicotyledons.

Answer 2

Xylem arranged in an X shape to provide resistance against force. Phloem found as patches between the arms. Surrounded by endodermis, aiding water passage.

Question 3

Describe the structure and function of the vascular system in the stem of dicotyledons.

Answer 3

Vascular bundles organised around a central pith. Xylem on the inside of the bundle to provide support and flexibility, phloem on the outside. Cambium is found between the two.

Question 4

What structure in plants is adapted for the uptake of water and minerals?

Answer 4

Root hair cells.

Question 5

How is water taken up from the soil?

Answer 5

• Root hair cells absorb minerals by active transport reducing the water potential of the root.
• Water potential of root hair cells is lower than that of the soil.
• Water moves into the root by osmosis.

Question 6

Outline how plant roots are adapted for the absorption of water and minerals.

Answer 6

Plant roots are composed of millions of root hair cells which have:

• Long hairs that extend from the cell body, increasing the surface area for absorption.
• Many mitochondria which produce energy for the active transport of mineral ions.

Question 7

State the three pathways by which water moves through the root.

Answer 7

• Apoplast
• Symplast
• Vacuolar.

Question 8

Describe the apoplast pathway.

Answer 8

Water moves through intercellular spaces between cellulose molecules in the cell wall. It diffuses down its water potential gradient by osmosis.

Question 9

Describe the symplast pathway.

Answer 9

Water enters the cytoplasm through the plasma membrane and moves between adjacent cells via plasmodesmata. Water diffuses down its water potential gradient by osmosis.

Question 10

Describe the vacuolar pathway.

Answer 10

Water enters the cytoplasm through the plasma membrane and moves between vacuoles of adjacent cells. Water diffuses down its water potential gradient by osmosis.

Question 11

Describe the structure and function of the the endodermis.

Answer 11

• Innermost layer of the cortex of a dicot root.
• Impregnated with suberin which forms the Casparian strip.
• Endodermal cells actively transport mineral ions into the xylem.

Question 12

What is the function of the Casparian strip?

Answer 12

• Blocks the apoplast pathway, forcing water through the symplast route.
• Enable control of the movement of water and minerals across the root into the xylem.

Question 13

What molecule makes the Casparian strip waterproof?

Answer 13

Suberin.

Question 14

Relate the structure of the xylem to its function.

Answer 14

• Long, continuous columns made of dead tissue, allowing the transportation of water.
• Contain bordered pits, allowing the sideways movement of water between vessels.
• Wall impregnated with lignin, providing structural support.

Question 15

Define transpiration.

Answer 15

• The loss of water vapour from the parts of a plant exposed to the air due to evaporation and diffusion.
• Consequence of gaseous exchange; occurs when the plant opens the stomata to exchange oxygen and carbon dioxide.

Question 16

What is the transpiration stream?

Answer 16

The flow of water from the roots to the leaves in plants, where it is lost by evaporation to the environment.

Question 17

How does water move up the stem?

Answer 17

• Root pressure.
• Cohesion tension theory.
• Capillarity.

Question 18

What is root pressure?

Answer 18

The force that drives water into and up the xylem by osmosis due to the active transport of minerals into the xylem by endodermal cells.

Question 19

Explain the cohesion-tension theory.

Answer 19

• Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together.
• Surface tension of the water also creates this sticking effect.
• Therefore as water is lost through transpiration, more is drawn up the stem from the roots.

Question 20

Define capillarity.

Answer 20

The tendency of water to move up the xylem, against gravity, due to adhesive forces that prevent the water column dropping back.

Question 21

State the factors that affect the rate of transpiration.

Answer 21

• Light
• Temperature
• Humidity
• Air movement

Question 22

How does temperature affect the rate of transpiration?

Answer 22

A higher temperature increases random motion and rate of evaporation, therefore increases the rate of transpiration.

Question 23

How does light affect the rate of transpiration?

Answer 23

A higher light intensity increases the rate of photosynthesis, causing more stomata to open for gas exchange, therefore increases the rate of transpiration.

Question 24

How does humidity affect the rate of transpiration?

Answer 24

High humidity means the water content of the air next the the leaf is high. This reduces the concentration gradient, therefore decreases the rate of transpiration.

Question 25

How does air movement affect the rate of transpiration?

Answer 25

Large amounts of air movement blow moist air away from the leaves, creating a steep concentration gradient, therefore increases the rate of transpiration.

Question 26

What is a hydrophyte?

Answer 26

A plant that is adapted to live and reproduce in very wet habitats, e.g water lilies.

Question 27

Give adaptations of hydrophytes that allow them to live in wet conditions.

Answer 27

• Thin or absent waxy cuticle.
• Stomata often open.
• Wide, flat leaves.
• Air spaces for buoyancy.

Question 28

What is a xerophyte?

Answer 28

A plant that is adapted to live and reproduce in dry habitats where water availability it low, e.g. cacti.

Question 29

Give adaptations of xerophytes that allow them to live in dry conditions.

Answer 29

• Small/rolled leaves.
• Densely packed mesophyll.
• Thick waxy cuticle.
• Stomata often closed.
• Hairs to trap moist air.

Question 30

What are mesophytes?

Answer 30

• Terrestrial plants adapted to live in environments with average conditions and an adequate water supply.
• They have features that enable their survival at unfavourable times of the year.

Question 31

Relate the structure of the phloem to its function.

Answer 31

• Sieve tube elements transport sugars around the plant.
• Companion cells designed for active transport of sugars into tubes.
• Plasmodesmata allow communication and the exchange of substances between sieve tubes and companion cells.

Question 32

What are cytoplasmic strands?

Answer 32

Small extensions of the cytoplasm between adjacent sieve tube elements and companion cells.

Question 33

Describe the function of cytoplasmic strands.

Answer 33

• Allow communication and the exchange of materials between sieve tubes elements and companion cells.
• Hold the nucleus in place.

Question 34

Define translocation.

Answer 34

The movement of organic compounds in the phloem, from sources to sinks.

Question 35

Summaries the mass-flow hypothesis of translocation.

Answer 35

• Sugar loaded into sieve tubes via active transport.
• Lowers water potential, causing water to move in from the xylem.
• Hydrostatic pressure causes sugars to move towards the sink.

Question 36

Give evidence for the mass-flow hypothesis.

Answer 36

• Sap is released when stem is cut so must be pressure in phloem.
• Sap exuding from the stylet of an aphid inserted into sieve tubes provides evidence that sugars are carried in the phloem.
• There is a higher sucrose concentration in the leaves than the roots.
• Autoradiographs produced using carbon dioxide labelled with radioactive carbon provide evidence for translocation in the phloem.

Question 37

What is autoradiography?

Answer 37

A technique to record the distribution of radioactive material within a specimen.

Question 38

What is a potometer?

Answer 38

An apparatus used to measure water uptake from a cut shoot.