3.4 - Mass transport in plants

Question 1

What are the components of the vascular bundle?

Answer 1

xylem and phloem

Question 2

What is the purpose of the xylem?

Answer 2

to provide support and flexibility to the stem and transport water and minerals in transpiration

Question 3

Describe the structure of xylem vessels

Answer 3

• long cylinders of dead tissue = continuous column
• pits
• lignin

Question 4

Describe the process of transpiration

Answer 4

• water moves from xylem to mesophyll cells by osmosis
• water evaporates from mesophyll cell surface into intercellular spaces
• water vapour diffuses out of stomata down water potential gradient

Question 5

How can the rate of transpiration be investigated?

Answer 5

Using a potometer
Measure movement of bubble

Question 6

Give the factors that can affect the rate of transpiration

Answer 6

• number of leaves
• number/size/position of stomata
• waxy cuticle
• light intensity
• temperature
• humidity
• air movement
• water availability

Question 7

What are xerophytes and give their adaptations

Answer 7

Plants adapted to living in dry conditions because they minimise their water loss
- smaller leaves = reduced SA for water loss
- densely packed mesophyll and waxy cuticle = prevent water loss via evaporation
- close stomata
- hairs and pits to trap moist air to reduce water potential gradient
- roll leaves to reduce exposure of lower epidermis so trap moist air

Question 8

Describe the structure of phloem

Answer 8

• living cells
• sieve tube elements and companion cells
• plasmodesmata

Question 9

Evidence for and against mass transport

Answer 9

For:
- pressure in sieve tube elements
- higher sucrose concentration in source than sink
- increases in sucrose concentration in leaves followed by increase in sucrose concentration in phloem
- lack of O2 inhibits translocation in phloem

Against:
- sieve plates have unclear function as they would appear to hinder mass flow
- not all solutes move at same speed
- sucrose delivered at similar rate to all regions instead of quicker/slower depending on concentration gradient

Question 10

Outline the cohesion-tension theory of water transport in the xylem

Answer 10

Water evaporates from stomata in leaves via transpiration
Lowers water potential of mesophyll cells
Water pulled up xylem creating tension
Water molecules cohere together by hydrogen bonds
Forming continuous water column
Adhesion of water molecules to walls of xylem

Question 11

Describe the mass flow hypothesis for translocation in the phloem

Answer 11

Sucrose actively transported into phloem at source
By companion cells
Lowers water potential of sieve tube and water enters phloem by osmosis
Increase in pressure causes mass movement towards sink
Water moves down sieve tube from area of high to low hydrostatic pressure
Sugars used in root for respiration for storage
Water moves back into xylem by osmosis

Question 12

Describe the ringing experiment

Answer 12

Removal of ring of surface tissues from the stem of the plant while leaving the stem core intact
Removes phloem by xylem remains intact

Question 13

Describe the tracer experiment

Answer 13

Radioactive substance added that is readily absorbed by leaves and used in photosynthesis to produce sucrose (usually 14CO2)
Sucrose formed will be radioactive so its subsequent movement around the plant via translocation can be traced