3.4.2: Mass transport in plants Flashcards
Structure and function of xylem
1) Dead cells that form hollow tubes with no organelles - Faster water flow
2) End walls break down forming a continuous tube with no end walls - continuous column of water
3) Cell wall strengthened with lignin - makes xylem waterproof and provides support to enable adhesion of water
4) Xylem pits - little holes which allow water to move laterally around blocked vessels
How does water go from the xylem to the leaf? (5)
1) Water evaporates out of stomata on the leaves causing lower pressure, transpiration
2) Water is lost, lowers water potential in leaves so more water is pulled up the xylem to replace it
3) Creates cohesion - hydrogen bonds between water molecules creates continuous columns of water
4) Adhesion - stuck to the walls of the xylem
5) As columns of water is moved up, it creates tension pulling xylem in
Describe the mass flow hypothesis for the mechanism of translocation in plants
- In source (leaf) sucrose is actively transported into phloem
- By companion cells
- Lowers water potential of sieve tubes and water enters by osmosis
- Increase in pressure in sieve tubes causes sucrose to be actively transported into sink cells
- Sucrose is converted in roots for respiration or storage
What is the observation for the ringing experiment?
If a ring of bark is removed from a tree trunk. The trunk swells above the removed section.
Evidence for ringing experiment (3)
Liquid in swelling contains sugars showing that when phloem is removed sugars can be transported
What is tracers?
Plants provided with radioactively labelled carbon dioxide which is absorbed by plants and used in photosynthesis to create sugars with radioactively labelled carbon. Thin slices of stems are cut and placed on x ray that turns black when exposed to radioactive material.
Evidence for tracers
Stems placed on x-ray from stem containing the sugar turn black highlighting where there phloem us and shows sugars are transported in the phloem.
State and explain the four factors that affect transpiration
1) Light intensity - Higher light, more stomata open, larger surface area for evaporation
2) Temperature - higher temperature, more kinetic energy, faster moving molecules so more evaporation
3) Humidity - More humidity, lower water potential outside the leaf, reduces water potential gradient, less evaporation
4) Wind speed - More wind, moves water potential away, maintains water potential gradient for evaporation
Describe functions of sieve tubes (3)
- Living cells
- No nucleus
- Few organelles
Describe function of companion cells
Provide ATP for active transport of organic substances
What is transpiration?
Loss of water vapour from the stomata by evaporation
