Chapter 3 – Water and plant cells
Water in plant life
- 97% of the water evaporates from leaf surfaces called transpiration. Only 2% for growth and 1% for
- 500 water molecules lost for every carbon dioxide molecule gained.
The structure and properties of water
The polarity of water makes it an excellent solvent
- Hydrogen bonding decreases electrostatic interaction between charged substances, thereby
increasing their solubility.
- Water molecules can orient themselves next to a charged or partially charged group to form shells
The thermal properties of water result from hydrogen bonding
- Hydrogen bonding results in high specific heat capacity
- Latent heat of vaporization is the energy needed to separate molecules from the liquid phase and
move them into the gas phase at a constant temperature.
- The high latent heat of vaporization enables plants cool themselves by evaporating water from leaf
The cohesive and adhesive properties of water are due to hydrogen bonding
- The energy required to increase the surface area of the gas liquid interface is known as the surface
- Surface tension and adhesion generate the physical forces that pull water through the plant’s
- Cohesion is the mutual attraction between molecules. Adhesion is the attraction of water to a solid
face phase as the cell wall. This attraction can be measured by the contact angle. Wettable surfaces
have contact angles of less than 90°.
- Cohesion, adhesion and surface tension give rise to capillarity. Water is drawn into capillary due to
attraction of water to the polar surface of glass (adhesion) and surface tension of water. Narrower
tube = higher equilibrium water level.
Water has a high tensile strength
- Cohesion gives water high tensile strength, the maximum force per unit area that a continuous
column can withstand before breaking.
- Water in a capped syringe is compressed and has a positive hydrostatic pressure when we push on
the plunger. If we pull on the plunger, a negative hydrostatic pressure develops in the water to
resist the pull.
- Gas bubbles reduce tensile strength. The gas bubble may expand until the tension in the liquid
phase collapses, a process called cavitation.
Water transport processes
- Two major processes in water transport: molecular diffusion and bulk flow.
Diffusion is the movement of molecules by random thermal agitation
- Js= -D Δs / sx this is Fick’s law where the flux density (J ) is asount of substance s crossing a unit
area per unit time in units of mol m -2s . The diffusion coefficient D is s proportionality constant
measures how easily substance s moves through a medium, depends on substance, size (large
atom = slow diffusion) and temp. Δc is thesconcentration gradient.
Diffusion is rapid over short distances but extremely slow over long distances
- Average time needed for a particle to defuse the distance L is equal to L /D s. Pressure-driven bulk flow drives long-distance water transport
- Bulk flow is the concerted movement of groups of molecules en masse usually in response to a
pressure gradient. I.e. water moving through a hose, river flowing, rain falling.
- See pg. 42 for equation. Pressure driven bulk flow is sensitive to radius of tube. Responsible for long
distance transport of water in the xylem and water flow through soil and cell walls. Independent of
solute concentration, as long as viscosity changes is negligible.
Osmosis is driven by a water potential gradient
- Diffusion is driven by a concentration gradient, pressure-driven bulk flow is driven by a pressure
gradient, where as osmosis is driven by both these forces.
The chemical potential of water represents the free-energy status of water
- The chemical potential of water is a quantitative expression of free energy associated with water.
Water potential is the chemical potential of water divided by the partial molal volume of water (1
-6 3 -1
mol = 18x10 m mol )
Three major factors contribute to cell water potential
- Water potential is Ψ anw Ψ = Ψ +wΨ + Ψswhereps,p,ggrepresent solutes, pressure and gravity.
- Ψ ss the solute potential or osmotic potential effect of dissolved solutes on water potential.
Primarily an entropy effect. Increases disorder of system and lowers free energy.
- Ψ ps the hydrostatic pressure of solution. + pressure raises water potential, - pressure reduces.
Positive pressure = turgor pressure, can also be negative as is the case in xylem and in walls
between cells where tension exists or negativ