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

• provide support and flexibility to stem
• 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

• use potometer
• measure bubble movement

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 reduces SA for water loss
• densely packed mesophyll + waxy cuticle prevents water loss via evaporation
• shut stomata
• hairs and pits trap moist air to reduce water potential gradient
• rolled leaves 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 plant stem while leaving core intact
• removes phloem but xylem intact

Question 13

Describe the tracer experiment

Answer 13

• radioactive substance readily absorbed by leaves added + 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