Water Transport in Plants

Question 1

Describe the structure of Xyem vessels

Answer 1

-elongated tubes formed from dead cells joined together in bundles, and contain no cytoplasm and nucleus
-no end walls between cells forming a long continuous tubes
-thick walls made of lignin which provides support and withstands tension
-gaps called pits in the cell wall to allow lateral movement of water

Question 2

Movement of water can be explained by what?

Answer 2

Cohesion-Tension Theory

Question 3

what is cohesion tension theory?

Answer 3

1. Water evaporates from the mesophyll due to heat from the sun via transpiration creating a negative water potential, causing water to diffue in through osmosis
2. This increase in water tension pulls more water into the leaf through transpiration pull
3. Water molecules are cohesive due to they form hydrogen bonds, so when some water molecules are pulled into the leaf others follow, pulling a whole colum of water in the xylem up from the roots to mesophyll tissue in the leaves
4. Water enters the stem through the roots.

Question 4

What is the evidence to support cohesion-tension?

Answer 4

-If a trunk or stem is damaged and a xylem cell is broken water does not leak out which it would if the vessels were under pressure, and when air enters the tree can no longer draw up water because the continuous colom of water is broken
-trucks of trees reduce in diameter during the daytime when transpiration is at its greatest because adhesion of water molecules to the wall of xylem results in a tension which pulls the xylem walls in, and at night there is less tension so diameter increases

Question 5

How does increasing light intensity affect transpiration?

Answer 5

It increases transpiration because increased photosynthesis requires mor carbon dioxide via the stomata, so it needs to be open and more stomata open so more water is evaporated

Question 6

how does increase in temperature affect transpiration?

Answer 6

It increases it as there is an increased evaporation because there is more kinetic energy in molecules creating a water potential gradient

Question 7

how does increased wind intensity affect transpiration?

Answer 7

It increases transpiration as water molecules move away from the stomata quicker incresing the water potential gradient

Question 8

how does increased humidity affect transpiration?

Answer 8

It decreases it as there is a lower water potential gradient so less water is evaporated

Question 9

How is small leaf surface area an adaptation in Xerophytes?

Answer 9

It reduces the surface area for evapotation, and small surface area means fewer stomata.

Question 10

How is sunken stomata an adaptation of xerophytes?

Answer 10

Able to maintain humid air around the stomata to reduce the water potential gradient.

Question 11

How are stomatal hairs an adaptation of Xerophytes?

Answer 11

Able to maintain the humid air around the stomata to reduce the water potential gradient, reducing evapouration.

Question 12

How are rolled leaves an adaptation of xerophytes?

Answer 12

They reduce the affect of wind to reduce the water potential gradient, reducing evapouration.

Question 13

How are extensive root systems an adaptation of xerophytes?

Answer 13

They maximise water uptake, therefore increaseing the chances of contact with water as they build a wide area to absorb rain fall. They also often have swollen stems to store the collected water

Question 14

How is having a reduced number of stomata an adaptation of xerophytes?

Answer 14

Reduce the mount of places water can evaporate from.

Question 15

How is having a thicker and waxy cuticle an adaptation o xerophytes?

Answer 15

Waterproof leaves and stems reduce evaporation.

Question 16

What is translocation?

Answer 16

The movement of solutes such as sucrose from the source to the sink.

Question 17

what is the structure of the phloem?

Answer 17

-have a tube formed from elongated living cells called seive tubes
-have a thin layer of cytoplasm around te edge of the cell maximising space for mass flow
-seive tubes are supported by companion cell, contains mitochondria and a nucleus to produce ATP helping to carry out functions
-there are end walls between cells which contain holes to let solutes pass through (seive plates)

Question 18

Why is translocation an active process?

Answer 18

Its is bidirectional meaning sinks can be above the source and some can be below, therefore requiring energy as solutes are pushed around under high pressure

Question 19

Describe the active loading of sucrose into companion cells at the source.

Answer 19

H+ ions are actively pumped out of the companion cell using ATP, into the cells of the source tissue. This creates a hydrogen concentration gradient across the companion cell membrane. H+ ions can diffuse back down the conc. grad. through transport proteins requiring a sucrose molecule to be co-transported. Increaseing the conc. of sucrose in the compannion cell, which can diffuse into the seive tube.

Question 20

What is mass flow?

Answer 20

1).At the source sucrose is actively transported into the seive tubes by companion cells
2).This lowers the water potential in the seive tube, so water enters via osmosis from the xylem
3).This increases hydrostatic pressure in the seive tube creating a hydrostatic pressure gradient
4).So mass flow occurs
5).Sucrose is removed by active transport to be used by respiring cells

Question 21

What is the evidence to support mass flow hypothesis?

Answer 21

-Sucrose concentration is higher in the leaves than the roots
-When a seive tube is cut sap is released, the sap is under high positive pressure within the phloem

Question 22

What is th evidence that goes against mass flow hypothesis?

Answer 22

-The end plates of the seive tube would create a barrier to mass flow, or slow it down so why are the present?