Mass Transport In Plants Flashcards
(14 cards)
What is xylem and what is its function?
A tissue that transport water and ions up the plant stem.
Feature of xylem not important
It’s a tissue made up of Xylem are continuous tubes of dead
cells with no end walls so water can pass easily through the plant. They have rings of lignin to add extra support and prevent them collapsing.
What property of water ensures a continuous column of water
exists within the xylem vessel?
What is the cohesion theory?
Strong cohension between water molecules due to hydrogen bonding (so water pulls from roots transpiration)
As water leaves the leaf xylem, tension is transmitted down the continuous water column in the xylem and water is drawn up from the roots to stomata (transpiration steam)
What is transpiration?
loss of water by evaporation from the leaves via stomata.
(Transpiration occurs due to water potential gradient from inside to outside the plant if there is a greater water potential inside.)-water potential gradient
What piece of apparatus can be used to estimate the rate of
transpiration?
Potometer
The rate of transpiration is how much water is lost from a plant per unit time.So any factors that increase the rate of EVAPORATION increases the rate of transpiration
How do xylem and phloem vessels compare?
Xylem-transport water in one direction however phloem transports sugar in all directions
Xylem is often dead -tissue ; phloem is a living tissue and has companion cells
What is translocation in plants?
The transport of sugars and other substances from sources (e.g leaf cells) to sinks(cells that use or store the substances transported )
What is the structure of a phloem vessel?
1)Sieve tube
2)Sieve tube element
In between each other phloem (sieve tube element ) are 3)companion cells
Explain the process of translocation(the mass flow hypothesis)
The products of photosynthesis move from the companion cells (sourced.leaf) to the sieve tube by active transport.
This causes the water potential within the sieve tubes to decrease and water moves, by osmosis, from the companion cells and surrounding xylem vessels.
- Solutes are removed from the sieve tubes at the sink where they are used or stored. Water potential in the sieve tube increases and water leaves.
- The changes in water potential create a pressure gradient and this moves assimilates from source to sink
Which conditions affect the rate of transpiration?
Side note
( the best conditions to dry laundry: sunny, warm, dry, windy; then these are also the conditions in which plants lose water fastest through transpiration due to quicker evaporation)
Temperature - water molecules have more energy and so evaporate from the surface of the mesophyll cells faster. The water vapour diffuses out of the sub-stomatal space faster as there is an increased water potential gradient.
Wind - as the water vapour diffuses out of the stomata, wind movements blow the water molecules away so increasing the water potential gradient.
Light intensity – more stomata open as light intensity increases to allow more CO2 to enter the leaf for photosynthesis to take place.
Humidity - dry atmospheric air creates a steep water potential gradient between the sub-stomatal space and the
outside of the leaf thereby increasing transpiration. Humidity has a negative correlation with transpiration rate;
the others are positive.
1)What is the name of the hypothesis which attempts to explain this process?
2)Name two piece of evidence that supports this hypothesis?
1)Mass flow hypothesis
2)woody stem ringing experiments/radioactive tracers
Explain the ringing experiment?
outer bark of a woody stem contains phloem but not xylem. If the bark (and so phloem is removed) a bulge will appear
above the ring. This contains a higher concentration of sugars than below the ring suggesting the downward movement of sugars.
Why does the bark swell above the ring?
A. Water moves by osmosis, down its water potential gradient, to where sugar is highly concentrated
Explain the tracer experiment.
Plant grown in atmosphere containing 14C -> incorporated into sugars produced during
photosynthesis
• Radioactive sugars traced as they move through the plant -> take thin crosssections of plant stem and place on X-ray film -> blackened where exposed to radiation from 14C sugars
• Blackened regions correspond to where phloem is in stem,
What are challenge to the mass flow theory(. Name one piece of evidence that does not support this hypothesis.)?
organic substances move to all parts of the plant and not just
to where the pressure difference is the greatest.
• sieve plates create too much of an obstacle in relation to the
pressure created
What does correlation and casual mean?
Correlation=have a mutual relationship or connection
Caution= a person or thing that gives rise to an action, phenomenon, or condition
