Mass transport in Plants Flashcards
Explain the cohesion-tension theory of water transport in the xylem.
Transpiration - water evaporates from leaves via the open stomata (down a water potential gradient)
Creates a negative pressure in the leaves which pulls water particles forward from the xylem
Water particles are polar (slightly charged) and cohesive (hydrogen bonding)
This “pull” or tension acts on the entire continuous column of water in the xylem
Water is also adhesive and attracted to the xylem walls resulting in upward capillary action
Describe xylem as the tissue that transports water in the stem and leaves of plants.
facilitates the ‘transpiration stream’ - one way mass flow of water from the roots to the leaves to the air (down a water potential gradient)
dead, hollow, open ended vessel elements - allows the continuous flow of water
narrow (aids capillary action)
lignified, structurally strong - resist the high negative pressure created in the leaf by transpiration (evaporation)
water 1) used as a metabolite in photosynthesis 2) maintains cell turgidity 3) inorganic ions in the soil e.g. nitrates, phosphates are transported to the leaves
Describe how to use a potometer to measure the rate of transpiration
Cut a leafy shoot underwater to avoid air passing into the xylem vessels
Cut at an angle to avoid crushing the xylem
Fill the potometer ensuring there are no air bubbles
Use vaseline to create a tight seal with the stem and potometer
Introduce an air bubble to the capillary tubing
Ensure leaves are dry so stomata are not blocked
Calculations for rate
Rate of transpiration = distance bubble moves / time
Rate of transpiration = volume of capillary tube / time (mm3 / min)
Measure the leaf surface area (area x 2)…
Rate of transpiration = vol / area / time
Describe xylem as the tissue that transports water in the stem and leaves of plants
facilitates the ‘transpiration stream’ - one way mass flow of water from the roots to the leaves to the air (down a water potential gradient)
dead, hollow, open ended vessel elements - allows the continuous flow of water
narrow (aids capillary action)
lignified, structurally strong - resist the high negative pressure created in the leaf by transpiration (evaporation)
water 1) used as a metabolite in photosynthesis 2) maintains cell turgidity 3) inorganic ions in the soil e.g. nitrates, phosphates are transported to the leaves
Detail the mass flow hypothesis for the mechanism of translocation in plants.
SOURCE - tissue that is producing an excess of / releasing organic molecules e.g. photosynthesising leaves
Companion cells actively transport solutes (e.g. sucrose) into the adjacent phloem
Lowers water potential in phloem’ sieve cells
Water diffuses in by osmosis
Increases the turgor pressure
SINK - tissue that is using / storing organic molecules e.g growing root tips
Solutes (e.g. sucrose) are removed from the phloem
Raises water potential in phloem sieve cells
Water diffuses out by osmosis
Decreases the turgor pressure
Mass flow (translocation) of solutes in the phloem, down a pressure gradient, from SOURCE to SINK.
Outline the use of tracers and ringing experiments to investigate transport in plants as evidence for / against the mass flow hypothesis.
Ringing experiments that cut phloem in photosynthesising plants result in a build up of sucrose above the ring cut and stem swelling yet an absence of sucrose below the ring.
Plants exposed to radioactively labelled CO2 show the presence of radioactive organic compounds present in the phloem as evidenced by exposure of photographic film over the transverse section of a stem.
