Mass Transport in Plants Flashcards
What is the role of the xylem in plants?
- a tissue that transports water + dissolved minerals ions in stem + leaves of plants
What are the 2 pathways that water can take to move across the cortex?
- apoplast pathway
- symplast pathway
Describe how water can move across the cortex through the apoplast pathway.
- water travels through the series of spaces running through cellulose cell walls, dead cells + hollow tubes of xylem by diffusion
Describe how water can move across the cortex through the symplast pathway.
- smaller volume of water travels into cell, possibly into vacuole + between cells through plasmodesmata by osmosis
What is transpiration?
- loss of water vapour via stomata by diffusion/evaporation
Explain the cohesion-tension theory of water transport in the xylem.
- when water vapour evaporates out of stomata, it creates a lower pressure, that creates tension + pulls more water up xylem to replace it (transpirational pull)
- due to cohesion, the water flows in a continuous column, as hydrogen bonds form between water molecules
- water molecules also adhere to walls of xylem, which helps pull water column upwards bc creates tension that pulls xylem in so is narrower
- as water moves into roots by osmosis, volume in roots inc, causing pressure to inc which also forces water above it upwards
What are the factors effecting rate of transpiration?
- light intensity
- T°C
- humidity
- wind
How does light intensity affect the rate of transpiration?
- positive correlation
- bc more light cases more stomata to open = larger SA for evaporation
How does T°C affect the rate of transpiration?
- positive correlation
- bc more heat means more KE, so faster moving molecules, so more evaporation
How does humidity affect the rate of transpiration?
- negative correlation
- bc more water vapour in air = higher water potential outside leaf, so dec water potential gradient
How does wind affect the rate of transpiration?
- positive correlation
- bc more wind will blow away humid air containing water vapour, so maintains water potential gradient
Describe a method to investigate the rate of transpiration using a potometer.
- cut a shoot underwater (to prevent air entering xylem + breaking water column) + place shoot in tube
- set up apparatus for a potometer + use petroleum jelly to seal any gaps (to prevent water leaking out) so it’s airtight
- dry leaves of shoot + introduce 1 air bubble to apparatus
- set up a envi factor to investigate
- measure + record distance air bubble moves towards plant so can be used to measure volume of water in tube that evaporated
- volume is divided by time to find rate
What is the role of phloem in plants?
- transports organic substances to all cells in a plant
What are the 2 key cells that make up the phloem tissue?
- sieve tube elements: living cells, containing no nucleus + few organelles
- companion cells: provides ATP required for active transport of organic substances
What is the mass flow hypothesis (used for translocation in plants)?
- mass flow of organic substances (e.g. sucrose) in a plant, from source (e.g. photosynthesising leaf), to sink (e.g.respiring cell), which requires energy
- occurs bc source cell produces sucrose, dec water potential + causing water to enter by osmosis, so hydrostatic pressure inc
- whereas, sink cell uses up sucrose, inc water potential + causing water to leave by osmosis, so hydrostatic pressure dec, forcing solution towards sink cell via phloem
What are the 3 translocations involved in the mass flow hypothesis in plants?
- transport of sucrose from source to sieve tube element
- movement of sucrose within sieve tube element of phloem
- transport of sucrose to sink
Describe the transport of sucrose from source to sieve tube element in the mass flow hypothesis.
- photosynthesis, occurring in chloroplast of leaves, creates organic molecules (e.g. sucrose) which is actively transported into sieve tube element, using companion cell
Describe the movement of sucrose within sieve tube element of phloem in the mass flow hypothesis.
- this inc of sucrose in sieve tube element dec water potential, so water enters from surrounding xylem vessels via osmosis, inc the volume of water
- this inc hydrostatic pressure in sieve tube element, causing liquid to be forced towards sink, where sucrose is used is respiration or stored as insoluble starch
Describe the transport of sucrose to sink in the mass flow hypothesis.
- more sucrose is actively transported into sink cell, so water potential dec + water moves from sieve tube element into sink cell by osmosis
- removal of water in sieve tube element, dec volume, so hydrostatic pressure dec
- movement of soluble organic substances is due to diff in hydrostatic pressure between source + sink end of sieve tube element
Explain the use of tracer experiments in investigating transport in plants.
- plants are provided w only radioactively labelled CO2, which is absorbed into plant + incorporated into sugars during photosynthesis
- thin slices from stems are then cut + placed on x-ray film that turns black when exposed to radioactive material
- when stems are placed on x-ray film, sections of stem containing sugars turn black, showing where phloem are + that sugars are transported in phloem
Explain the use of ringing experiments in investigating transport in plants.
- a ring of bark + phloem are peeled + removed off a tree trunk, leaving xylem intact
- removing phloem causes trunk above ring to swell w a sugar solution + trunk below to die
- this is bc when phloem is removed, sugars can’t be transported below ring proving phloem transports sugars
What is the evidence for + against the mass flow hypothesis?
For:
- when phloem sieve tube is punctured, phloem sap oozes out suggested it’s under pressure
- phloem sap taken near source has a higher sucrose conc than sap taken near sink
Against:
- suggested some sieve tubes translocate at diff times
- some scientists conducted experiments that detected diff substances move in opposite directions
