Mass Transport: Plants Flashcards
What are the 3 different aspects of the cohesion-tension theory?
- Cohesion
- Adhesion
- Root pressure
What is the idea of “cohesion” in cohesion-tension theory?
- Water = di-polar (slight negative O/ slight positive H’s.)
- Hydrogen bonds form between different water molecules.
- Creates COHESION between water molecules ie. they can stick together.
- Water travels up xylem as a continuous column.
What is the idea of “ahesion” in cohesion-tension theory?
- Ahesion of water is when water sticks to other molecules. The water will adhere (stick) to the xyelm walls.
- Narrower the xylem, the bigger the impact of adhesion (water sticks to a larger area of xylem walls- think of it like a straw.)
() - extra info for clarification.
What is the idea of “root-pressure” in cohesion-tension theory?
- Water moves into roots - by osmosis - increases volume of water in root, pressure within root increases.
- Increase in pressure in roots forces water above it (ie. in the xylem) to move upwards (ie. pushing it upwards) –> positive pressure.
What is the “xylem?”
- Xylem is a tissue that transports water in the stem and leaves of
plants.
Difference between positive/ negative pressure?
- Positive pressure- pushing water up- : water moves up xylem due to increase in pressure (of water)
- Negative pressure- pulling water up- : water moves up xylem due to decrease in pressure (of water vapour.)
Steps for movement of water up the xylem in plants.
1.) Water vapour evaporates out of stomata (transpiration.) Loss in water volume creates lower pressure.
2.) Water pulled up xylem to replace water lost - moves due to negative pressure.
3.) H bonds between H20 molecules, they are cohesive. Creates column of water.
4.) H20 molecules adhere to walls of xylem. Pulls water column upwards.
5.) Column of water pulled- creates tension - pulling xylem in to be narrower.
What is the phloem?
- Phloem is tissue that transports organic substances in plants.
What 2 key cells does the phloem tissue contain?
- Sieve tube elements.
- Companion cells.
Adaptations of “seive tube elements” cells in phloem.
- Seive plates: allow for organic substances to constantly be transported.
- Living cells.
- Contain no nucleus
- Contain few organelles.
Function of companion cells in phloem.
- Provide ATP required for active transport of organic substances.
Mass flow hypothesis (1)
Transport of sucrose from source to seive tube element, movement of sucrose within phloem seive tube element.
- Photosynthesis in chloroplasts creates organic substances.
- Sucrose actively transported into seive tube element - using companion cell.
- Increase of sucrose in seive tube element lowers water potential.
- Water enters seive tube elements from surrounding xylem by osmosis.
- Increase in volume in seive element increases hydrostatic pressure, causing liquid to be forced down to sink.
Mass flow hypthothesis (2)
Transport of sucrose/ sugars to the sink.
- Sucrose used by the sink cells in respiration.
- Sucrose actively-transported into sink cell, water potential decreases.
- Osmosis: from seive tube element into sink cell.
- Removal of water decreases hydrostatic pressure by decreasing volume.
What are the source cells and sink cells in the mass flow hypothesis?
- Source cells: photosynthesising leaf cells–> produce sugars.
- Sink cells: respiring cells.
How do soluble organic substances actually move up and down the plant?
- Movement of soluble organic substances –> difference in hydrostatic pressure between the source and sink end of sieve tube element.
- Higher hydrostatic pressure in source end of sieve tube element than sink end, solution forced towards sink cell.
How is the companion cell used to actively transport sucrose into seive tube element from the source?
- Provides the ATP needed for active transport of sucrose.
What is the idea of “mass flow hypothesis?”
- Mass flow from source of production, the leaves, to the sink - the site where organic substances (such as glucose and sucrose) are used up in respiring tissues.
- Requires energy (active-transport.)
How are “tracers” used to investigate sugar transport in plants?
- Plants ONLY provided with radioactive CO₂.
- Radioactive CO₂ absorbed by plant/ used in photosynthesis –> making sugars that all contain radioactively-labelled carbon.
- Thin slices of stems cut/ placed on x-ray films.
- X-ray film will turn black when exposed to radioactive carbon, hilights where phloem are/ proves sugars are transported in phloem.
How are “ringing experiments” used to investigate sugar transport in plants?
- Ring of bark/ phloem peeled and removed from trunk.
- Removing phloem causes trunk above (removed part) to swell.
- Analysis of liquid in this swelling shows it contains sugar.
- Proves that when phloem removed, sugars can’t be transported –> phloem must transport sugars.
() - extra info clarification.
What are the 2 things that would investigate sugar transport in plants?
1.) Tracers.
2.) Ringing experiments.
Evidence against mass flow hypothesis.
1.) Sugar doesn’t always travel to sink with lowest h. pressure as mass flow hypothesis suggests.
2.) Sieve plates would slow down mass flow, why are they there?
