HL Bio - Unit 1: Water Potential

Solvation with Water

Combination of a solvent with the molecules of a solute

• The more electronegative (oxygen) side will be attracted to positive ions/molecules enclosing them preventing them to interact with other negative ions/molecules

• The less electronegative (hydrogen) side will be attracted to negative ions/molecules enclosing them preventing them to interact with other positive ions/molecules

Osmosis

Passive movement of water

• Transport of water from an area of high water concentration to an area of low water concentration

  • It’s also going from a low solute concentration to high solute concentration

Osmolarity

measure of solute concentration

Hypertonic

High solute concentration within water (will take water)

Water enters the cell and the cytoplasm swells

• plants prefer

Hypotonic

Low solute concentration (will give away water)

When cell lose a lot of water and it will shrinks/ swivels

Isotonic

Same solute concentration

When water is lost and gained at the same rate

Medical Application of Isotonic solutions

• Isotonic sodium chloride solution

  • Safely introduce new blood system via Intravenous drip 

  • Rinse wounds

  • Keep skin moisturized prior to skin grafts

  • Base for eye drops 

  • Frozen to consistency of slush for organ donors 

Cholesterol

• Can also change how permeable their semi-permeable membrane is to water with cholesterols 

Cell walls

• Main constituent: Cellulose

• Thickness: Thick - 250 nm

• State: Solid - changes are limited 

• Tensile Strength: High - comparable to steel 

• Permeability: Freely permeable unless impregnated with a waterproof material 

• Turgidity - only for plants

Plasma Membrane

• Main constituent: Phospholipids

• Thickness: thin - 5nm

• State: Liquid, allows change of its position 

  • Formation of vesicles, binding cite

• Tensile Strength: Very low

Permeability: Semi-permeable

Water Potential as Potential Energy

• Pure water at Standard atmospheric pressure and 20^oC = Water potential 0

  • Absolute value can’t be determined so all of it is relative 

Water potential

• Water potential is the sum of solute potential and pressure potential. 

  • water potential (Ψw) = solute potential (Ψs ) + pressure potential (Ψp ) 

Factors influencing water potential

1. RIse or fall in hydrostatic pressure changes the potential energy 

   • Higher pressure = more potential energy in water

2. Solute dissolving in the water will reduce its potential energy 

   • Higher solute concentration = less potential energy

Movement of high to low water potential

• Water moves from a higher to a lower water potential because this minimizes its potential energy

  • Less strenuous on the system 

• Water potential found in cells reach a maximum of zero - This is because that it is impossible for water to have less than zero solutes 

  • Lower water potentials are therefore more negative

Solute Potential

• when solutes dissolve in the water 

  • Higher solute potential reduced the potential energy of water

pressure potential

• Changes in hydrostatic 

  • More pressure, the more potential water has (can be both positive and negative - greater or less than atmospheric pressure)

Bathing plants in hypotonic solutions

Assuming that the water potential of the plant tissue is initially lower (more negative) there will be a net movement of water from the solution to the plant cells. This will raise the water potential of the plant tissue by making the solute potential less negative and the pressure potential more positive.

Bathing in Hypertonic Solution

Both solute (more negative) and pressure potentials (solution=0, tissue=above zero) therefore give the cells a higher water potential than the bathing solution and there will be a net movement of water out of the tissue.