Water And Transport In Plants

Question 1

What are the two main transport systems in vascular plants?

Answer 1

Xylem and phloem.

Question 2

What is the function of xylem? What do they look like in a vascular bundle image?

Answer 2

Transports water from the roots up to the leaves. They are pictured as the largest dots in the centre of the vascular bundle.

Question 3

What is the function of phloem? What do they look like in a vascular bundle image?

Answer 3

Transports sugars produced in photosynthesis from leaves, to sites where they are used or stored. They are the smaller holes on the outer edge of the vascular bundle

Question 4

What is the most abundant compound in plant cells?

Answer 4

Water.

Question 5

Describe the main functions of water.

Answer 5

• provides some support in the form of turgor pressure
• used to make organic compounds (donates electrons for photosynthesis, and H+ for structural elements)
• solvent for reactions and medium for solute movement

Question 6

What is the cause for the greatest loss of water from plants?

Answer 6

Transpiration.

Question 7

What is transpired water replaced with?

Answer 7

It is replaced with water taken up by roots.

Question 8

What kinds of cells do xylem tissue contain?

Answer 8

Water conducting cells:

• vessels (angiosperms only)
• tracheids (angiosperms and conifers)

Other cell types:

• parenchyma
• fibres

Question 9

What are the thick cell walls within the xylem reinforced with?

Answer 9

Lignin

Question 10

What kind of pressure can xylem conducting cells withstand?

Answer 10

They can withstand hydrostatic pressure.

Question 11

What allows xylem to stretch?

Answer 11

Xylem elements that mature in elongating stems have spiral thickening that allows them to stretch.

Question 12

Describe the appearance of tracheids.

Answer 12

Long, narrow, entire cells that overlap one another.

Question 13

Describe the appearance of vessels.

Answer 13

Wide cells with degenerated end walls.

Look like empty gaps

Question 14

Are vessels or tracheids more efficient at transporting water? Explain why.

Answer 14

Vessels are more efficient at transporting water because they contain perforations which allow for the continuous flow of water into xylem vessels.

Question 15

What is water movement through xylem aided by?

Answer 15

Cohesion and adhesion

Question 16

What is cohesion?

Answer 16

Interactions between water molecules via hydrogen bonding that holds them together.

Question 17

What is adhesion?

Answer 17

Interactions between water molecules and xylem walls, also known as capillary action.

Question 18

What is cavitation and when/how does it occur?

Answer 18

Formation of air bubble in the xylem vessel. It occurs when the water tension in a xylem vessel exceeds capillary action, causing water column to break and fill with water vapour creating an air bubble (embolism).

Question 19

What is water potential?

Answer 19

A measure of the potential energy of water molecules.

Question 20

What are the two main forms of potential relevant to the movement of water in plants?

Answer 20

• pressure potential

- solute potential (always negative)

Question 21

What is the water potential of the plant cell affected by?

Answer 21

• changes in solute concentrations (decrease in solute concentration leads to increase in water potential)
• hydrostatic pressure (increase in pressure leads to increase in water potential)

Question 22

What is water potential the sum of?

Answer 22

Osmotic potential and pressure potential.

Question 23

What is the transpiration-cohesion theory?

Answer 23

1. Transpiration through the stomata dries out mesophyll cells, lowering water potential. (Very negative)
2. water is then replaced from adjacent cells
3. Cells bordering tracheids take water from xylem
4. Negative pressure water potential lifts water column towards the tip of plant
5. Effect cascades to roots
6. Water flows in from soil

Question 24

What are lianes in trees?

Answer 24

• trees produce a lot of narrow vessels

- lianes produce a few wide vessels which LOWERS cavitation risks as more of stem is horizontal.

Question 25

When do angiosperms rely on tracheids as back-up?

Answer 25

When vessels are at risk of cavitation.

Question 26

Why is it difficult to establish gradient from roots to the leaves at the tips of tall trees?

Answer 26

Because leaf water potential is very low.

Question 27

What must stomata do in the leaves of very tall trees and why? Why is this an issue?

Answer 27

Stomata must be partially closed to prevent water loss and maintain leaf cell turgor. This is an issue because CO2 is unable to diffuse easily via the stomata and therefore photosynthesis is compromised. Leaves are in a state of effective drought.

Question 28

What does water enter the plant through? How is this efficiently achieved?

Answer 28

Water enters the plant through root hairs which is efficient because they have a large surface area for water to enter.

Question 29

What structure is water controlled by in the roots and what does it look like?

Answer 29

Endodermis. It is the circular border surrounding the xylem and phloem in the vascular bundle.

Question 30

Name and describe the two pathways through which water enters the root from soil and gets to the vascular tissue.

Answer 30

Symplastic pathway: water enters cell through plasma membrane and moves through cytoplasm of adjacent cells via plasmodesmata.

Apoplastic pathway: water moves between cells through cell wall and intercellular spaces

Question 31

What is the casparian strip and where is it located?

Answer 31

Casparian strip is made of suberin and water passes through it and therefore the endodermis to reach the vascular cells.

Question 32

What is the pericycle? What type of cells does it contain?

Answer 32

Thin layer of plant tissue between the endodermis and the phloem. It contains parenchyma or sclerenchyma cells.

Question 33

What kind of cells do phloem tissue contain?

Answer 33

Sugar conducting cells: sieve cells

Other cell types: companion cells (parenchyma) and transfer cells (parenchyma)

Question 34

What are aphids?

Answer 34

Phloem feeders that insert proboscis into phloem.

Question 35

Describe a characteristic of sieve cells.

Answer 35

Pores in sieve plates are open, allowing movement between connected cells.

Question 36

Describe the relationship between sieve cells and companion cells.

Answer 36

They form the same “mother” cell. Sieve cells lose most organelles except mitochondria. Companion cells retain an active nucleus and all organelles, therefore ribosome rich

Question 37

Loading and unloading sugars into and out of sieve cells is mediated by what?

Answer 37

Transfer cells.

Question 38

Is the process of loading and unloading sugars active or passive?

Answer 38

It is an active process. Sucrose is transported across sieve cells using H+ gradient.

Question 39

What is the Mass Flow hypothesis?

Answer 39

1. Loading sucrose decreases solute potential.
2. Water flows in from xylem, increasing pressure potential.
3. Increase in pressure causes mass in phloem to flow away from pressure.
4. Pressure gradient reinforced as sucrose is unloaded.

Question 40

Compare xylem and phloem in terms of their similarities and differences.

Answer 40

Similarities:
- pressure driven flow in both xylem vessels and phloem sieve cells. Sugars pushed in phloem (mass flow) while water pulled up in xylem (transpiration-cohesion).

Differences:

• active loading and unloading of solutes in phloem
• water movement is passive at all times