Biology 2601A/B Lecture Notes - Lecture 11: Interphase, Lignin, Tracheid

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Lecture 11 - Xylem & Phloem
October 24, 2017
6:03 PM
Water and fluid transport: Moving fluids in plants
Xylem and Phloem
Xylem
o Translocates water and inorganic nutrients from the roots to the leaves
Phloem
o Translocates sugars, proteins and signaling molecules (thin outer ring)
From source tissues (e.g. leaves) to sink tissues (e.g. roots)
Source tissue - creating/generating materials
Sink tissue - consuming materials
How does the water move in xylem?
In animal systems, fluids are generally moved through the creation of positive pressures where
pressure can push water
In plant systems, water is pulled rather than pushed
o Example; sugar maples, trees pump sugar into the xylem which increases the solute
concentration which reduces your solute potential which pushes water up
o This is an exception
Adhesion, Cohesion, & Air-water interface
Adhesion: hydrogen bonding between water molecules and polar groups of carbohydrate
molecules
o Adhesion between water and soil particles anchors the water column in the soil
o Adhesion between water and cell walls anchors the water column at the site of evaporation
in leaves
Cohesion: hydrogen bonding between water molecules which maintains the water column up
through the plant itself
o Coheads two liked things
Air-water interface: Surface of water where evaporation occurs
o Water column meets the air
Cohesion and adhesion of water
Water can cohere and adhere
In a chem lab
o The eisus of that ate ues upads ad that’s eause ou hae adhesio of ate
molecules to the surface and cohesion of the water molecules to each other which creates
surface tension
Surface tension allows some organisms to walk across the surface
Capillary action: the more narrow the tube is, the higher the water will move up
o Only if there is adhesion
o This is how the water column is able to sustain against gravity
Water moves down a pressure gradient
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Desied as a ate potetial Ψ easued i MPa pessue uits i plats
Water moves towards the lower water potential (so from 0 to -1MPa)
o Water potentials never go above 0
Water will move from something that has water potential that is almost zero to something that is
more negative such as -2
o Down a concentration gradient
Water moves down a pressure gradient
Wate potetials ust e oe egatie at the esophll ell all tha i the ste
ad oe egatie at the ste tha at the oot…
o Otherwise it wont move from the root to the stem
ad oe egatie at the oot tha i the soil…
for water to move from the soil to the leaf
o Need to have a continuous gradient of water potential
Example ith tee that I do’t hae a pitue of:
o Soil is -0.3 Mpa (fairly close to 0)
o Trunk xylem is -0.6 MPa
o Leaf (cell walls) is -1 MPa
o Leaf (air spaces) is -7.0 Mpa
Even fully saturated air has a really negative water potential
o Outside air is -100 MPa
Figure:
o If we just look at a cross section of the leaf
o The cell walls are fully saturated
o If we are just looking at the cell wall
The bottom left picture is what the cell wall looks like
o Bunch of cellulose microfibrils make up the cell wall (net or network of overlapped
microfibrils)
o In between the microfibrils are little pores
o In each of the pores is where you get the air water interphase
Where water is evaporating out of the leaf into the air spaces
o There is cohesion of the water molecules to each other at the air-water surface creating
surface tension
o There is adhesion of those water molecules that are bonding to the cell walls (cellulose)
o As water evaporates and the pores get drier and drier and water moves down into the pore,
the radius of the air water interphase decreases (the picture on the bottom right)
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o The radius of curvature of the air water interphase dictates the water potential
Chart on middle left
Water moves down a pressure gradient
Energy from the sun breaks the hydrogen bonds at the air-water interface
o This allows the transportation and evaporation occur
As the water in the pores at the air-water interface evaporates, the radius of the water surface
decreases
This generates more and more negative water potentials that pull in water molecules due to
cohesive forces
o Drives the gradient
Water moves down a pressure gradient
The apoplast is generally incompressible because of lignin and cellulose in cell walls
o Apoplast not living part of the cell
o Has to be strong enough to withstand really strong pulls
o Example; sucking really hard on a straw, the straw will collapse
Apoplast volume (xylem cells) is generally fixed
Water must fill this fixed volume, so evaporating water must be replaced
Water moves down a pressure gradient
If ate eapoates oe uikl tha it’s eplaed, fee ate oleules i the le
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