D2.3 Water Potential

What is a solution

Liquid that is a homogenous solution of substances

What is a solvent

Dissolving agent of a solution

What is a solute

Substance that is dissolved

What is an aqueous solution

Water is solvent

Why is water a good solvent

Water is a polar solvent, and can form hydrogen bonds easily
Ionic compounds dissociate easily in water
Nonionic polar molecules (proteins, sugars, alcohols) can also dissolve in water.

hydrogen bond formation between solute and water molecules, and attractions between both positively and negatively charged ions and polar water molecules.

How does water diffuse and why

Lower solute concentration to higher solute concentration
ie. hypotonic to hypertonic

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

Define osmosis

diffusion of water across a selectively permeable membrane

Define tonicity

Tonicity is the ability of a solution to cause a cell to gain or lose water

What is an
isotonic solution
hypertonic solution
hypotonic solution

• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Movement of water between isotonic solutions

There is no net movement of water between two isotonic solutions because there is no difference between the concentrations of osmotically active solutes, so equal numbers of water molecules move between them. This is known as dynamic equilibrium.

What are osmotically active solutions and examples

• Solutes are osmotically active if intermolecular attractions form between them and water.

• Sodium, potassium and chloride ions and glucose are all osmotically active.

How do cells maintain water balance

• Cells can change how rapidly osmosis occurs by changing the permeability to water of their plasma membrane.

• They can also change the direction of movement, but only by raising or lowering the concentration of osmotically active solutes inside the cell.

Hypertonic solution and animal cell

• If an animal cell is bathed in a hypertonic solution, water leaves the cell by osmosis, so the cytoplasm shrinks in volume.

• The area of plasma membrane does not change, so the cell develops indentations, which are sometimes called crenations.

Hypotonic solution and animal cell

• If an animal cell is bathed in a hypotonic solution, water enters the cell by osmosis making it swell.

• Because it lacks the support that a wall would provide, the cell easily bursts.

• This can be demonstrated by placing a small droplet of blood in pure water and then examining it using a microscope.

• The blood cells swell up to form a spherical shape and then burst, leaving ruptured plasma membranes

Adaptation of organisms to hypertonic/hypotonic environments + example

Osmoregulation (control of water balance)
The protist Paramecium, which is hypertonic to its pond water environment (thus water diffuses into the protist), has a contractile vacuole that acts as a pump. When full, the vacuole contracts, expelling excess water from the cell

Ideal environment for plants and why

Hypotonic environment
Turgid plant tissue can provide support because of its strength under compression.
The stems and leaves of non-woody plants resist gravity in this way.

Hypotonic solution and plant cell

• A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)

• High pressures due to entry of water by osmosis can build up inside plant cells because the cell wall is strong enough to prevent bursting. This is the normal state of healthy plant cells.

Isotonic environment and plant cell

• If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell and the pressure of the cytoplasm decreases.

• If it drops to atmospheric pressure, the plasma membrane no longer pushes against the cell wall and the cell is not turgid.

• Further water loss causes plant cells to become flaccid (limp). Leaves and stems bend downwards. This is called wilting and is seen in plants that have lost water by transpiration in hot weather or droughts

Plant response to isotonic/hypertonic environments

• The plant usually avoids further water loss by closing stomata.

Hypertonic environment and plant cell

• In a hypertonic environment, plant cells lose water; as the volume of cytoplasm decreases, the plasma membrane eventually pulls away from the cell wall, an effect called plasmolysis

• It is a damaging process and usually causes the death of the cell.

Natural causes of plasmolysis

• Plasmolysis sometimes happens naturally, for example when seawater floods terrestrial ecosystems as a result of high tides or a tsunami

Medical use of isotonic solutions

• Safely introduced to a patient’s blood system via an intravenous drip as part of treatment

• Used to rinse wounds and skin abrasions

• Used to keep areas of damaged skin moistened prior to skin grafts

• Used as the basis for eye drops

• Frozen to the consistency of slush for cooling hearts, kidneys and other donor organs to be transported to the hospital where the transplant operation is to be done.

What is water potential, what is water potential zero

It is a measure of the potential energy per unit volume.

• The absolute quantity of potential energy cannot be determined, so all values are relative.

• Pure water at standard atmospheric pressure and 20°C has been assigned a water potential of zero.

Unit for water potential

kilopascals (kPa) or megapascals (MPa)

Usual water potential values for cells

potential of 0kPa or lower

Water potential equation

ψ_w = ψ_s + ψ_p
where
ψ_w is water potential
ψ_s is solute potential
ψ_p is pressure potential

What affects water potential

solute potential (solute concentration)

• The potential energy of water changes if solutes dissolve in it

• When solutes dissolve, the potential energy of water is reduced.

• With no solutes dissolved, the solute potential is zero.

• Because it is impossible for water to hold less than no solutes, the only possible solute potentials are zero or negative.

pressure potential (hydrostatic pressure)

• Rises or falls in hydrostatic pressure also change the potential energy of water—pressure potential (Ψp).

• The higher the pressure, the more potential energy water has.

• Pressure potential can be negative or positive because it can be greater or less than atmospheric pressure.

Water potential when mixing a hypotonic solution and a plant cell

At atp, ψ_p is zero. water potential = solute potential
Assuming the water potential of the cell is initially lower, there will be a net movement of water from solution to the plant cells.

The cell will reach the same water potential of the solution.

Water enters the cell. This makes the solute potential less negative. At the same time, this increases the internal hydrostatic pressure of the cell. This increases the pressure potential. When the hydrostatic pressure of the cells cannot increase further, they are turgid.

A water potential of zero in a cell means that the solute potential and pressure potential are numerically equal (but opposite signs)

Water potential when mixing hypertonic solution and plant cells

Solute potential of solution more negative than tissue
Pressure potential of solution is zero. Pressure potential in plant cells likely to be above zero as the internal hydrostatic pressure is likely > atmospheric pressure.
Thus, cell has higher water potential. net movement of water out of tissue
Water loss from tissue recudes the pressure inside cells. When the pressure within the cell drops to atmospheric pressure, pressure in cells = pressure of solution, cells become flaccid.
Net movement of water from the cells would have caused the solute concentration of cells to increase. If it is still higher than the solution, water continues to leave the plant cells, reducing the volume of cytoplasm and causing plasma membrane and cytoplasm to detach from the cell wall, making the cell plasmolysed.