Mass transport in Plants Flashcards
Explain five properties that make water important for organisms (5)
A solvent for metabolic reactions to occur
High heat capacity - buffer changes in temp
Large latent heat of vaporisation - cooling effect
Cohesion - Supports column of water
Metabolite in hydrolysis
Describe how water is moved through a plant according to cohesion-tension theory
Water is lost from the leaves via transpiration
Which reduces the water potential in the cell
Water drawn out of xylem
Which creates tension
Cohesive forces between water molecules
Describe the mass flow hypothesis
Sucrose lowers the water potential of the source cell - water enters by osmosis
The respiring cell is using up sucrose - more positive water potential so water leaves
This leads to an increased hydrostatic pressure in the source cell and decreased HP in the sink cell which forces the solution towards the sink cell in the phloem
Translocation - How is sucrose transported from the source to sieve tube?
Photosynthesis creates organic substances which creates a high conc of sucrose at the site of production
Sucrose diffuses down a conc gradient into the companion cells via facilitated diffusion
Active transport of H+ ions occurs from companion cells into spaces within the cell walls creating a conc gradient via carrier proteins into sieve tubes
Co transport of sucrose with the H+ ions occurs which transports sucrose into sieve tubes
Translocation - Movement of sucrose within sieve tube
The increase of sucrose in the sieve tube lowers water potential
Water enters sieve tube, the increase in water volume increases hydrostatic pressure causing the liquid to be forced towards the sink
Translocation - Transport of sucrose to the sink
Sucrose is used in respiration at the sink, more sucrose actively being transported decreases the water potential
This results in osmosis from the sieve tube into sink cell
The removal of water decreases the volume in the sieve tube therefore the HP decreases
Movement of soluble organic substances is due to the difference in hydrostatic pressure between the source and sink
Why does water having a high latent heat of vaporisation benefit living organisms?
Provides a cooling effect, little loss of water through evaporation
Measuring the rate of transpiration - practical
1) A leafy shoot cut under water
2) Potometer filled completely with water, make sure there is no air bubbles
3) Using a rubber tube leafy shoot is fitted to potometer under water
4) Potometer is removed from under the water, all joint’s sealed with waterproof jelly
5) Air bubble introduced into the capillary tube
6) As transpiration occurs water moves through capillary tube and the bubble of air moves with it
7) Distance moved over a period of time is recorded and the mean is calculated of a number of repeats
8) Vol of water lost of a period of time can be calculated
Puncture experiments
Phloem punctured with hollow tube, zap oozes out, high pressure in phloem
Xylem punctured, air sucked in, low pressure
Demonstrating they transport substances differently; water pulled up in xylem and sap pushed down in phloem
Ringing experiments
A ring is cut in a stem just deep enough to cut the phloem but not the xylem , swelling above the ring occurs, whilst there is reduced growth below the ring
Sugars are transported downwards from source to sink
Radioactive Tracer Experiments
One leaf is exposed to co2 containing isotope 14C
This will be taken up by photosynthesis and incorporated into glucose and sucrose
Plant then frozen in liquid nitrogen to kill it quickly and placed onto photographic film in the dark
Autoradiograph shows location of compounds containing the isotope
Shows organic substances are transported downwards from leaf to roots
Use your understanding of the mass flow hypothesis to explain how pressure is generated inside the phloem tube (3)
Sucrose is actively transported lowering water potential
water moves in via osmosis
