Water Potential and Osmosis: A Comprehensive Guide for Biology Students, Lecture notes of Thermodynamics

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Ψ = Ψp + Ψs

Calculating osmosis

What is it?

A measure of the tendency of water to LEAVE

a cell, system or solution when pressured by

either concentration OR literal pressure.

It helps explain, for example, why water

leaves the soil and enters root cells of plants.

Remember: osmosis is the diffusion of

water.

Water will move across a membrane to reach

equilibrium (homeostasis) if solutes

cannot or do not diffuse.

remember… osmosis occurs when the 2 solutions

differ in osmotic pressure

same osmolarity**

isoosmotic

NO net movement of water

differ in osmolarity**

greater solute concentration

more dilute

net flow of water hyperosmotic hypoosmotic

**moles solute/L solution

Water Balance

 Osmoregulation~ control of water balance  Hypertonic~ higher concentration of solutes

 Hypotonic~ lower concentration of solutes

 Isotonic~ equal concentrations of solutes

Cells with Walls:

 Turgid (very firm)

 Flaccid (limp)

 Plasmolysis~ plasma membrane pulls away from cell wall

 Osmosis = the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Osmosis is considered in terms of water potential and solute potential.

Water Potential

 Defined as: the measure of the kinetic (free)

energy of water molecules.

 Water molecules are constantly moving in a random fashion.

 The concepts of free energy and water potential are derived from the second law of thermodynamics.

In thermodynamics, free energy is defined as the potential for performing work.

Example: The water at the top of the fall has a higher potential for performing work than the water at the base of the fall. The water is moving from an area of higher free energy to an area of lower free energy. The free energy from water is the power source for waterwheels and hydroelectric facilities.

Ψ = Ψs + Ψp

Water potential is determined by the combined effect

of solute concentration and physical pressure.

Water potential is calculated using the formula:

Water moves from regions of high water potential to regions of low water potential.

Water Potential in Cells

higher water potential

lower water potential

higher solute concentration

lower solute concentration

Some Basic Principles

 Water moves spontaneously from places of higher water potential to places of lower water potential

 Between points of equal water potential, there is no net water movement

 Water potential values are ususally negative

 Ψw is increased by an increase in pressure potential (ΨP)

 Ψw is decreased by addition of solutes which lowers the solute potential (ΨS )

Water Potential, Ψ

 Ψ is measured in units of pressure  Pure water at standard temperature and pressure has a Ψ of zero  The addition of solutes to water lowers its Ψ (makes it more negative), just as an increase in pressure makes it more positive  Water will move from higher Ψ to lower Ψ

Pressure Potential in plant cells.

• Turgor pressure – forced

caused by cell membrane

pushing against cell wall.

• Wall pressure – an equal and

opposite force exerted by cell

wall. Counteracts the

movement of water due to

osmosis.

• Other pressures – tension,

cohesion, atmospheric, root,

etc.

Ψs - solute (osmotic) potential

• Remember: pure water has a solute potential

(Ψs) of zero.

• Solute potential can never be positive.
• Adding more solute is a negative experience!

the solute potential becomes negative.