mass transport in plants Flashcards
How does water move up the xylem?
- mesophyll cells lose water to the air by evaporation
- water moved from the xylem into the leaf by osmosis
- this applies tension to the column of water
- the water moves up as one because of cohesion (H bonding)
Why does the diameter of a tree decrease during the day?
- more light more heat
- more evaporation
- increased transpiration
- water is pulled up the xylem by cohesion-tension
- was of xylem are pulled inwards
what are two pieces of evidence to support cohesion tension theory?
- when a xylem vessel is broken, water does not leak out but air is drawn in
- if a xylem vessel is broken and air enters the tree cannot draw water because the column is broken
what are factors that affect transpiration?
-light = more stomata open more transpiration
- temperature = more evaporation
- wind = maintains concentration gradient
How do you measure the rate of transpiration?
distance bubble moves in time
cross sectional area of tube
how does translocation work?
sucrose is made at the source
sucrose diffuses into companion cells
Hydrogen ions are actively transported out of companion cells
sucrose and H+ are co transported into the phloem
This lowers the water potential of the phloem
water moves in by osmosis from the xylem
this increases the hydrostatic pressure
at the sink sucrose is being used up
sucrose is actively transported from the sieve tubes which lowers their water potential
water moves into the cells by osmosis
the hydrostatic pressure is also lowered
there is a mass flow down the pressure gradient
what is some evidence questioning mass flow hypothesis
-function of sieve plates seem to hinder mass flow
