E&T: Transport in Plants - Xylem and Phloem

Question 1

Xylem tissue transports what?

Answer 1

Water + mineral ions in solution that move up the plant from roots to leaves.

Question 2

Phloem tissue transports what?

Answer 2

Organic substances in solution both up and down the plant.

Question 3

Xylem vessels

Answer 3

Long, tube-like structures formed from dead cells joined end to end with no end walls.

Question 4

What helps water move up plants against the force of gravity?

Answer 4

Cohesion

Tension

Question 5

Explain how tension helps water move up a plant:

Answer 5

• Transpiration occurs from leaves
• Creates tension
• Pulls more water into the leaf

Question 6

Explain how cohesion helps water move up a plant:

Answer 6

• Cohesion means water molecules ‘stick’ together
• When some are pulled into the leaf, others follow.

Question 7

Transpiration

Answer 7

The evaporation of water from a plant’s surface, especially the leaves.

Question 8

Briefly describe transpiration:

Answer 8

• Water evaporates from the moist cell walls
• Accumulates in spaces between cells in the leaf
• When stomata open, water moves out of leaf + down concentration gradient.

Question 9

What 4 main factors affect transpiration rate?

Answer 9

Light

Temperature

Humidity

Wind

Question 10

How does light affect transpiration?

Answer 10

• Lighter = faster the transpiration rate
• Because stomata open in light to let in CO₂.

Question 11

How does temperature affect transpiration?

Answer 11

• Higher temperature = faster transpiration rate
• Water H2O molecules have more energy = evaporate faster

Question 12

How does humidity affect transpiration?

Answer 12

• Lower humidity = faster transpiration rate
• Dry air increases concentration gradient between leaf + air

Question 13

How does wind affect transpiration?

Answer 13

• High wind = faster transpiration rate
• Air movement blows away water molecules around stomata = increasing concentration gradient

Question 14

What instrument can be used to estimate transpiration rate?

Answer 14

Potometer

Question 15

Describe how to use a potometer to estimate transpiration rates:

Answer 15

• Cut shoot underwater at a slant - prevents air escaping + increases SA
• Assemble potometer in water + insert shoot underwater
• Remove apparatus from water but keep end of capillary tube submerged.
• Check apparatus is air+watertight
• Dry leaves, allow for shoot to acclimatise + shit tap
• Remove end of capillary tube until one airbubble forms - put end back in
• Record starting position of air bubble
• Stopwatch + record distance moved per unit time, eg. per hour.

Question 16

What are two important cell types in phoem tissue and why?

Answer 16

Sieve tube elements = form tube for transporting solutes

Companion cells = carry out living functions for sieve cells, eg. providing energy.

Question 17

Why do sieve tube elements need companion cells?

Answer 17

Sieve tube elements have no nucleus + few organelles

Question 18

Translocation

Answer 18

Movement of solutes from sources to sinks.

Question 19

What role do enzymes play in translocation?

Answer 19

Maintain a concentrtion gradient from source to sink by changing the solutes at the sink to make sure they are lower concentration.

Question 20

What is the best supported theory for transport in the phloem?

Answer 20

Mass flow hypothesis

Question 21

Describe the mass flow hypothesis:

Answer 21

• AT actively loads solutes from companion cells to sieve tubes at source
• Lowers WP inside sieve tubes, so water enters by osmosis.
• Creates high pressure inside sieve tubes at source end
• At sink end, solutes are removed
• Increases WP inside sieve tubes so water leaves by osmosis
• Lowers pressure in sieve tubes
• Creates pressure gradient between source + sink
• Pushes solutes along sieve tubes towards sink.

Question 22

What is the evidence for the mass flow hypothesis?

Answer 22

• Ring of bark (phloem, not xylem) removed from woody stem = bulge forms with higher conc. sugars than fluid lower down = evidence of downward flow of sugars
• Radioactive tracers can track movement of organic substances

Question 23

Describe how radioactive tracers can be used to demonstrate translocation of solutes:

Answer 23

• Radioactively-labelled CO₂supplied to a single leaf
• Incorporated into organic substances produced by the leaf, which will be transported via translocation
• Plant is killed by freezing in liquid nitrogen
• Placed on photographic film.
• Where film turns black = radioactive substance present.