Summary of all lectures in the course Plant Physiology (BB069B) that is given in the 2nd year of the biology bachelor. Great if you struggle with large chucks of text. Lots of bullet points and images.
Lecture 2 Water and transport - Beware we know like 70% of this already
Water is essential for most plant functions
● firmness and growth
● solvent
● transport
Very high consumption necessary due to loss of water during CO2 uptake
Small decreases in water content lead to high amounts of stress
Primary production → The amount of carbon being fixed into
biomass
Higher water availability leads to higher primary production
● in agriculture
● in ecosystems
Why is water so suitable as a solvent and a transport
medium?
The characteristic physical properties of water are of key importance
● High Polarity
○ Ions and polar molecules dissolve easily
○ H-bridges (surface tension)
■ Causes cohesion and adhesion with allows for transport
Transport system inside plants
xylem → water and nutrients
phloem → carbohydrates
water transport inside and between cells
is also important
Water potential
Potential energy of water, or its ability to
perform mechanical work
water potential =
● osmotic potential (also expressed as
Ψπ)
● hydrostatic pressure
● gravitational potential
Ψw = Ψs + Ψp + Ψg
Water will move towards the more negative potential
● Ψp > 0 Pushing force
● Ψp < 0 Pulling force
1
,osmotic potential Ψs = pressure resulting from the property of water to move from hypotonic
to hypertonic solutions
● inside plants Ψs always < 0
● in pure water Ψs = 0
Hydrostatic pressure Ψp = physical pressure resulting from turgor (Ψp > 0) or tension
caused by cohesion of the rising water in a vessel (Ψp < 0)
Gravitational potential Ψg = pressure exerted by the position of the water column in the field
of gravity (Ψg > 0)
pure water in an open beaker at an atmospheric pressure of 1 atm
● Ψs = 0 MPa
● Ψp = 0 MPa
● Ψg = 0 MPa
Calculations
Ψs = - (n/V) · R ·T (derivative of the 't Hoff-equation)
● n = amount of dissolved particles (in mol) – be mindful of salts!
● V = volume of solution (in m3)
● R = gas constant = 8.314 J K-1mol-1
● T = temperature (in K)
Ψg = ρw · g · h
● ρw = density of the liquid (in kg m-3) = 0.998 · 103 kg m-3 for water
● g = Gravitational acceleration = 9.8 m s-2
● h = height (in m)
Ψp - Due to pressure exerted by the cell wall
Ψg - This component is set to 0 for water potentials inside
cells, as the height differences are very small
(plant cell = 10 -100 µm)
Ψg is important in things like trees that are high as fuck
2
,When water uptake and evaporation are unbalanced, turgor loss will happen
quickly
elastic modulus ε - describes the elasticity of
the cell wall depends on turgor pressure
Transport inside cells is mostly due to diffusion
tc=½ = time it takes until half the concentration of the starting point is reached
K ~ 1 at room temperature
d = distance
(for example: cell length = 50 µm)
Ds = diffusion constant
(for example: glucose in water: 10–9 m2 s–1)
Large distance transport cannot be brought about with diffusion. This can work for cells but
not for leaves.
How does water enter and leave the cell?
● Diffusion through the cell membrane
● Diffusion through channels in the membrane → aquaporins
○ Passive
○ Direction depends on Ψw
○ Often closable
3
, Water uptake and usage have to be balanced by plants
● Uptake mostly in the roots
● “Usage" mostly through evaporation
● Driver of water transport from the roots, through the xylem, to the leaf is the
difference in Ψp
The atmosphere (usually) has a low water vapour pressure
Diffusion of water vapour from the leaf to the atmosphere
● Depends mostly on the resistance of the stomata
How far they are open is partly
determined by the CO2 concentration
1. If Ψw is too low, the evaporation
resistance is increased by (partly)
closing the stomata
2. stomatal conductance gs and leaf
conductance gl decrease
This can be measured by measuring the
change in humidity of the air
Opening of guard cells due to an increase in Ψp
and asymmetrical enforcements of the cell walls
4
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