Transport processes Flashcards
Plants lecture 3
Over evolutionary time of plants, there has been a decreasing dependence on what?
Water
How did algal ancestors of land plants absorb water, minerals & CO2?
How did early nonvascular land plants absorb water?
Directly from surrounding water
Lived in shallow water & had aerial shoots
What are the 2 types of transport?
Short & long distance
What is transported short distances?
Basic nutrients from cells w/ access to nutrient to those w/out
What are the features of short distance transport?
Doesn’t require special tissue organisation
Occurs in vascular & non-vascular plants
Intercellular
What is transported long distances?
Nutrients around whole plant
What are the features of long distance transport?
Requires vascular tissue
Occurs only in vascular plants
What are the 2 major pathways through plants?
Apoplast
Symplast
What does the apoplast consist of?
Everything external to plasma membrane
- cell walls, extracellular spaces & interior of vessel elements + tracheids
What does the symplast consist of?
The cytosol of living cells in a plant
& plasmodesmata
How is membrane potential established in plants?
Pumping H+ out of cytoplasm via ATP-dependent proton pumps
What does pumping H+ out of cells establish?
What can these be used for?
Membrane potential
pH gradient
= 2 forms of potential energy that can drive the transport of solutes
What can be co-transported into plant cells with protons?
Neutral solutes e.g. sucrose loaded into phloem
Ions e.g. nitrate uptake in plant roots
How do ion channels in plant cells work?
Open & close in response to: voltage, membrane stretching & chemical factors
e.g. K+ ion channel involved in release of K+ from guard cells when stomata close
What 2 components affect water potential?
Solute conc
Pressure
Which way does water flow?
Higher water potential to lower
What does ‘potential’ refer to?
The capacity of water to do work
What is solute potential directly proportional to?
Molarity (no. of particles present in solution)
Why is water potential always -ve?
Pure water = 0
When solute added –> bind to water
–> reduces capacity to do work
so solute potential = always -ve
Give examples of when pressure potential is +ve or -ve relative to atmospheric pressure
+ve = in cells –> produces turgor pressure
-ve = in xylem vessels
What is plasmolysis?
When a cell shrinks & pulls away from cell wall
What is the role of aquaporins?
Facilitate water diffusion across membranes
- affect rate at which water moves osmotically across membrane
What causes the action of aquaporins to decrease?
Increases in cycstolic Ca2+
or
Decrease in pH
By what process does long-distance transport occur?
Bulk flow
via pressure gradient (high–>low)
(independent of solute conc)
What is the Casparian strip?
A belt of waxy material that blocks the passage of water & minerals
How can water by-pass the Casparian strip?
Only minerals already in symplast (or entering that pathway by crossing the plasma membrane of an endodermal cell) can detour the Casparian strip
How is xylem sap pushed up?
Root pressure
Describe the process that creates root pressure
At night - low transpiration 1.Accumulation of minerals in xylem (Casparian strip prevents leaking back) 2. Lowers water potential 3. Water flows in from root cortex 4. Generates root pressure = positive pressure on xylem sap (Can cause guttation)
How is xylem sap pulled up?
Cohesion-Tension
Describe the process of transpirational pull
- water vapour in airspaces diffuse down WP gradient & exit via stomata
- Water evaporates
- -> air-water interface retreats further into mesophyll cell walls - Surface tension creates -ve pressure potential
- pulls water in xylem into leaf
- pull on xylem sap transmitted from leaves to roots
What type of bonds cause adhesion & cohesion?
H bonds
What is adhesion?
Strong adhesion of water molecules to hydrophilic walls of xylem cells
What is cohesion?
H bonds between water molecules
What is phloem sap?
Aqueous solution high in sucrose
can also contain amino acids, hormones & minerals
What must happen before sugar can be exported to sinks?
Sugar must be loaded into sieve-tube elements via symplastic or both symplastic & apoplastic pathways
How is sugar loaded into sieve-tube elements at the source?
Active transport
- uses proton pumping & H+/sucrose co-transport
How is sugar unloaded from sieve-tube elements at the sink?
Diffusion
What causes bulk flow of phloem sap?
+ve pressure
= pressure flow
Describe bulk flow of phloem sap
- Higher sucrose conc at source than sink
- Water moves in from xylem –> increases pressure
- Higher pressure at source
- Sap flows down pressure gradient to sink
- Water moves back into xylem
What is self-thinning?
When sinks demand more than sources can provide
