D2.3 Water Potential

What is water as solvent?

What is water as solvent?

Aqueous solutions refer to any solutions that have water as the solvent.

Explain how water is able to dissolve many subsances

Water is known as the "universal solvent" because of its exceptional ability to dissolve a wide variety of substances.

1. Polarity
- Water molecules are polar, meaning they have an uneven distribution of electrons, resulting in a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. This creates a dipole moment in the molecule.
- The polarity of water allows it to interact with both positively and negatively charged ions and polar molecules, making it an effective solvent for a broad range of substances.

2. Hydrogen bonding
- Water molecules can form hydrogen bonds with other water molecules and with substances that contain polar covalent bonds. Hydrogen bonding occurs when the positively charged hydrogen of one water molecule is attracted to the negatively charged oxygen of another water molecule.

Medical applications of istonic solutions

Intravenous fluids are used during a variety of different medical applications. Adding IV fluid will ultimately expose body cells to the solute concentration of the added fluid. The IV fluid first comes in contact with the Extracellular fluids (fluids outside of the cell).

ICF and ECF must be isotonic to maintain dynamic equilibrium is maintained.
Therefore, most IV solutions are isotonic.

*ICF (intercelular fluid)

Explain the movement of water from less concentrated to more concentrated solutions.

high conc --> hypertonic
low conc sol --> hypotonic
same solution conc --> isotonic

If a cell is placed in a hypertonic solution:
- net movement of water out of the cell and into the surrounding solution
- leads to shriking and crenated nature of animal cell
- plasomylsis if it is a plant cell

If a cell is palced in ypotonic solution:
- a net movement of water from the solution into the cell
- this leads to the cell to swell due to high tugor pressure
- if animals cell it may burst due to lack of cell wall, which is called lysis

If a cell is placed in an isotonic solution, there will be equal movement of water into and out of the cell, resulting in a state called dynamic equilibrium

Define the term water potential

Water potential is a measure of potential energy per unit volume of water.
It is impossible to measure the absolute quantity of the potential energy of water.
Reference point is pure water at atmospheric pressure and 20°C. Units used are kilopascals (kPa)

The water potential of pure water at atmospheric pressure and 20℃ is zero.
Adding solute to water makes it more difficult for the water molecules to move, so the water potential decreases (becomes negative).

Explain the direction that water moves in terms of water potential

Lower solute concentration means more free water molecules and higher water potential

Whilst higher solute concentration means less free water molecules and lower water potential

Explain how solute potential and pressure potential affect the water potential within cells (water potential equation)

Solute Potential (Ψs):
- Definition: Solute potential is the component of water potential that is due to the presence of solute molecules in a solution. The more solute present, the lower the solute potential.
- Effect on Water Potential: As solute concentration increases, the solute potential becomes more negative. Water molecules tend to move from regions of higher water potential to regions of lower water potential. Therefore, a solution with a lower solute potential will have a more negative impact on the overall water potential.

Pressure Potential (Ψp):
- Definition: Pressure potential is the component of water potential associated with the pressure exerted on a solution. In plant cells, pressure potential is often positive due to the turgor pressure generated by the cell wall pushing against the cell's contents.
- Effect on Water Potential: Pressure potential can counteract the negative effects of solute potential. Turgor pressure in plant cells, for example, can make the overall water potential more positive. This pressure can prevent the uptake of water by osmosis or, if high enough, can even lead to the movement of water out of the cell.

Total Water Potential (Ψ):
- Calculation: The total water potential (Ψ) is the sum of the solute potential and the pressure potential.
- Mathematically, Ψ = Ψs + Ψp.

Effect on Water Movement: Water will move from a region of higher water potential to a region of lower water potential. Therefore, cells with a higher solute potential or pressure potential will tend to gain water, while cells with a lower solute potential or pressure potential will tend to lose water.

Explain the changes that occur when a plant tissue is bathed in either a hypotonic or hypertonic solution in terms of solute and pressure potentials

1. Hypotonic Solution:

Solute Potential (Ψs):
The external solution has a lower solute concentration than the plant cell's cytoplasm.
Water moves into the cell from the hypotonic solution, following the concentration gradient.
The influx of water dilutes the cytoplasm, making the solute potential of the cell more negative.

Pressure Potential (Ψp):
As water enters the cell, the central vacuole in plant cells fills with water, exerting pressure on the cell wall.This pressure potential becomes more positive.

Overall Effect on Water Potential (Ψ):
The combined effect of increased solute potential (more negative) and increased pressure potential (more positive) may result in a net increase in water potential.The cell becomes turgid (firm and swollen) due to the pressure exerted by the cell wall against the influx of water.

2. Hypertonic solution:

Solute Potential (Ψs):
The external solution has a higher solute concentration than the plant cell's cytoplasm.Water moves out of the cell into the hypertonic solution, following the concentration gradient.The loss of water increases the solute concentration in the cytoplasm, making the solute potential of the cell more negative.

Pressure Potential (Ψp):

As water leaves the cell, the central vacuole loses water, and the cell membrane pulls away from the cell wall.The pressure potential becomes more negative.

Overall Effect on Water Potential (Ψ):

The combined effect of decreased solute potential (more negative) and decreased pressure potential (more negative) results in a net decrease in water potential.The cell becomes flaccid (soft and wilted) due to the loss of water and decreased turgor pressure.

Water potenital equation

The solute potentail of a solution

Ψs = -iCRT

i = ionisation constant
C = molar cocentration
R = presure constant (0.0831)
T = temp (kevlin 273)

Ψp = 0
--> open container

Ψ = "psi"

Osmosis

movement of water across a semipermeable membrane from areas of HIGH water concentration TO areas of LOW water concentration

Hypotonic SOLUTION

Solution: Low solute & High water
Cell: High solute & Low water
water enters cell more than leaves
Animal=bursts, Plant=turgid

Isotonic

Solution concentrations = Cell concentrations
equal water exchange
no change in cell behavior

Hypertonic SOLUTION

Solution: High solute & Low water
Cell: Low solute & High water
water leaves cell more than entering
Animal=shrivels/shrinks, Plant=plasmolysis

Plasmolysis

plant cell membrane shrinks away from cell wall as the cell shrinks in a hypertonic solution

Water Potential

= Ψ
water's capacity to do work for a plant
combined effects of: solute concentration + physical pressure

Water Potential Equation

Ψ = Ψp + Ψs
water potential = solute potential + pressure potential

Ψ = 0

Isotonic solution
equal exchange in water across membrane = equilibrium

Ψ = negative

Hypotonic solution
water will enter cell (towards the more negative Ψ)

Ψ = positive

Hypertonic solution
water will exit cell (towards the more negative Ψ)

Solute Potential

= Ψs
% of H2O molecules in a solution free to do work
solutes bind with H2O molecules = fewer free H2O to do work

Ψs = 0

100% water (no solutes)

Ψs = negative

at least one particle of solute in solution
(concentration of H2O lowered, less than 100%)

Pressure Potential

= Ψp
physical pressure on a solution

Ψp = 0

open container, no pressure

Ψp = negative

water tension - sucking/pulling
water leaves cell
EX: transpiration, xylem

Ψp = positive

pressure inside a cell against membrane - pushing
water enters cell
EX: turger pressure

Standard for Water Potential

pure water (Ψs = 0) in an open container (Ψp = 0)

Solute Potential Equation

--need solute concentration--
Ψs = - iCRT
i=# particles molecules make in water
C=molar concentration of material (if C=0 then Ψs=0)
R=pressure constant (0.0831)
T=temperature in degrees Kelvin

Water Movement Rule

High to Low
Hypo to Hyper

Pressure Potential in Animal Cells

= 0