3.4 - Mass transport in plants Flashcards
What are the components of the vascular bundle? (tissues)
xylem and phloem
What is the purpose of the xylem?
- provide support and flexibility to stem
- transport H2O and minerals in transpiration
Describe the structure of xylem vessels
- long cylinders of dead tissue = continuous column
- pits
- lignin
Describe the process of transpiration
- H2O moves from xylem to mesophyll cells by osmosis
- H2O evaporates from mesophyll cell surface into intercellular spaces
- H2O vapour diffuses out of stomata down WP gradient
How can the rate of transpiration be investigated?
- use potometer
- measure bubble movement
Give the factors that can affect the rate of transpiration
- no. of leaves
- no./size/position of stomata
- waxy cuticle
- light intensity
- temperature
- humidity
- air movement
- H2O availability
What are xerophytes and give their adaptations
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
Describe the purpose and structure of phloem
Tissue that transports organic substances
- living cells
- sieve tube elements and companion cells
- plasmodesmata
Evidence for and against mass transport
For:
- pressure in sieve tube elements
- higher sucrose conc in source than sink
- increases in sucrose conc in leaves followed by increase 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 conc gradient
Outline the cohesion-tension theory of water transport in the xylem
- H2O evaporates from stomata in leaves via transpiration
- lowers WP of mesophyll cells
- H2O pulled up xylem creating tension
- H2O molecules cohere together by hydrogen bonds
- forming continuous H2O column
- adhesion of H2O molecules to xylem walls
Describe the mass flow hypothesis for translocation in the phloem
- sucrose AT into phloem at source by companion cells
- lowers WP of sieve tube and H2O enters phloem by osmosis
- increase in pressure causes mass movement towards sink
- H2O moves down sieve tube from area of high to low hydrostatic pressure
- sucrose AT back into companion cells at sink and used in respiration or stored
- H2O moves back into xylem by osmosis
Describe the ringing experiment
- removal of ring of surface tissues from plant stem while leaving core intact
- removes phloem but xylem intact
Describe the tracer experiment
- 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
