HL Bio - Cellular Bio (II): D2.3 Water Potential

Define solvation

The process of dissolving involving a combination of a solvent and the molecules and ions of of a solute

Describe solvation of polar solutes and water

Dissolves due to the attraction between polar molecules and the partial positive and partial negative charges of water

Positive ions attract to the partial negative oxygen poles of water

Negative ions attract to the partial positive hydrogen poles of water

Water molecules forms shells around ions to prevent precipitation by clumping together

Define osmosis

The net movement of freely moving water molecules from a higher water potential to a lower water potential across a partially permeable membrane until dynamic equilibrium is reached

What is meant when solute is osmotically active?

When solutes form intermolecular forces of attraction between water and solutes

How does water moves through the cell membrane?

1. simple diffusion through the phospholipid bilayer

2. facilitated diffusion through aquaporins (transport proteins)

How do unicellular organisms regulate water intake?

Unicellular cells inevitably take in water as its cytoplasm is hypotonic to the environment

Uses contractile vacuoles to expel water to prevent bursting

How do animals regulate water intake?

Extracellular fluids must be isotonic to prevent swelling or shrinking

Humans: regulate solute concentration of extracellular fluids using the kidneys

Describe how turgor pressure is developed in plant cells

Cell in hypotonic solution - higher water potential in solution - water moves into the cell - cell wall protects the cell from bursting and allows turgor pressure to be developed

Describe plasmolysis

Cell in hypertonic solution - environment has lower water potential - water moves from the cell into the environment - creates a gap between plasma membrane and cell wall - plasmolysed; usually does not recover

Describe the medical applications of isotonic solutions

Intravenous fluid injected from a syringe or elevated bag must be close to isotonic to prevent osmotic damage to blood cells

Isotonic solutions are used to cool hearts and transport donor organs to hospitals to prevent osmotic damage

Describe the uses of a saline solution [4]

- safely introduced to blood system with IV drip

- basis of eye drops

- frozen to the consistency of slush to transport donor organs

- used to moisturise skin before skin grafts

Water potential can also be defined as a measure of:

Measure of potential energy per unit volume of water

What is the equation for water potential?

water potential = solute potential + pressure potential

Ψw = Ψp + Ψs

What is the range of solute potential?

0 (in pure water) or negative (the higher the solute concentration the more negative the solute potential)

Pressure potential is usually ______, with the exception of...

positive

exception of xylem vessels as sap is carried under tension

Why does water move from a higher water potential to a lower water potential?

to minimise potential energy

Explain how two factors affect the water potential of cells

1. Solute concentration: solvation - higher solute concentration - more IMFOA formed between polar solutes and partial positive and negative poles of water - bond formation releases energy - reduced potential energy held by water

2. Pressure exerted by cell wall (pressure potential) - rise in pressure exerted = higher potential energy

Explain the movement of water when plant tissue is placed in i) pure water ii) hypotonic solution iii) hypertonic solution

i) pure water: pure water Ψw = 0 - higher water potential in pure water than in plan tissue - water moves into the cell until Ψw = 0

ii) hypotonic: higher solute concentration in plant tissue - lower water potential in plant tissue than environment - water moves into the cell, turgor pressure is maintained

iii) hypertonic: lower solute concentration in plant tissue - higher water potential in plant tissue than environment - movement of water from the plant tissue into the environment - flaccid when Ψp = 0 - plasmolysis if water loss continues - however loss of water concentrates cytoplasm - dynamic equilibrium may be reached, turgor pressure is lost