4.4 Transport in Plants Flashcards
Xylem Function
Transport water and minerals as part of the transpiration stream
Xylem Structure
Long cylinders made out of dead tissue
Have long pits which allow water to move sideways between vessels
Thickened with lignin which allow plant to remain flexible
Phloem Structure
Made out of living cells
Have sieve tube elements and companion cells
Sieve tube elements transport sugar
Companion cells are involved in ATP production + loading sucrose into sieve tubes
Transpiration
1) Water moves into the plant through root hair cells
2) Water moves through by either
Symplast- Water moving though the cytoplasm
Apoplast- Water moving through cell walls + intracellular spaces
3) When water reaches the Casparian strip it has to travel via symplast
4) Water reaches the xylem, which has a continuous column of water maintained by cohesion and adhesion
5) Evaporation causes hydrostatic pressure + water moves up the plant
6) Water reaches the leaf
7) Water evaporates from mesophyll cell walls
8) Water diffuses from air spaces in leaf out through the stoma
Factors affecting transpiration
Air Movement
Humidity
Light Intensity
Temperature
Air Movement
Removes still air from around leaf
increases steepness of CG
increases rate of diffusion
Humidity
Decreases steepness of CG
Decreases rate of diffusion
Light Intensity
Increases number of stomata open for photosynthesis
Plateaus when all stomata are open
Temperature
Increases rate of evaporation
Warmer air holds more water vapour
Plateaus when something else becomes limited
Source
Area that produces more sugar than required
Sink
Area that consumes more sugar than it produces
Translocation
1) Sucrose is moved from source to sink
2) Can be loaded via symplast or apoplast pathway
3) Phloem is unloaded passively
Sucrose moves into companion cells + then into shoot by diffusion
Symplast pathway
1) Sucrose moves by diffusion from leaf to companion cell of phloem into phloem sieve tube
2) Decreases WP of phloem so water moves into phloem from xylem by osmosis
3) Generates HP in phloem so water moves down the sieve tube towards the sink down the pressure gradient
Apoplast pathway
1) Sucrose moves by diffusion from leaf to companion cell wall into the cytoplasm
It is moved by AT across the companion cell wall into the cytoplasm
2) It moves through the plasmodesmata
by diffusion into sieve tube
Mass flow Stengths
Radioactive isotopes
Steam
Aphids
Radioactive isotopes
Marks glucose
uses autoradiography to trace the resultant sucrose through phloem
Steam
Kills a ring of bark
stops movement in phloem but not xylem
Aphids
Feed of phloem tubes + exude sap
evidence for high HP in phloem
Mass flow Weaknesses
1) Doesn’t explain the 2 way movement in sieve tube
2) Doesn’t explain presence of sieve plates
3) Doesn’t explain why there’s movement at different speeds in sieve tube
