2.3 Water Potential

Why is water a good solvent?

It is dipolar. This allows water molecules to form hydrogen bonds with other polar solute molecules and ions

Why is water dipolar?

The electrons are distributed unevenly between the hydrogen and oxygen atoms, therefore the hydrogen side is slightly positive while the oxygen side is slightly negative

Hydration shells

The sphere of water molecules around each dissolved ion

Solvation

The interaction between the ions of solute and solvent molecules, where the ions become surrounded by solvent molecules

Osmosis

The diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a selectively permeable membrane

Hypotonic

A solution that has a lower concentration of solute compared to the cytoplasm of the cell, meaning there will be net movement of water into the cell

Hypertonic

A solution that has a greater concentration of solute compared to the cytoplasm of the cell, meaning there will be net movement of water out of the cell

Isotonic solution

A solution that has the same solute concentration as the cytoplasm of the cell, meaning that there will be no net movement of water into or out of the cell

What happens if a cell is in an isotonic solution?

There will be no net movement of water into or out of the cell.The cell is in dynamic equilibrium

What happens if an animal cell is placed in a hypertonic solution?

It will lose water by osmosis and become crenated (shrivelled up)

What happens if an animal cell is placed in a hypotonic solution?

It will gain water by osmosis. Because it has no cell wall to create turgor pressure, it will continue to expand until the cell membrane is stretched too far and bursts

Why do multicellular organisms need to maintain isotonic tissue fluid around their cells?

To prevent harmful changes from happening to cells

How do unicellular organisms control osmosis?

Some unicellular organisms live in freshwater aquatic habitats that are hypotonic to their cytoplasm. Excess water will be continuously collected in the contractile vacuole and pumped out of the organism to maintain the osmotic concentration of the cytoplasm

Turgor pressure

The internal pressure exerted on the cell wall of a plant cell due to water entering the cell via osmosis

When is a plant cell turgid?

When the plant cell is fully inflated with water and has become rigid and firm

Why is turgidity important for plants?

It provides support and strength for the plant to stand upright

What happens if a plant does not receive enough water?

The cells cannot remain turgid and the plant wilts

Plasmolysis

The process of contraction or shrinkage of the protoplasm of a plant cell caused by the loss of water

Protoplasm

The part of a cell that is surrounded by a plasma membrane

What happens if a plant cell is placed in a hypertonic solution?

The protoplast gradually shrinks and no longer exerts pressure on the cell wall. It begins to pull away from the cell wall, this process is known as plasmolysis. The plant cell becomes flaccid and is said to be plasmolysed

Why is it important that the solution in an intravenous drip is isotonic in relation to the blood plasma?

If the solution was hypotonic, there would be net movement of water into red blood cells, causing them to burst. This would result in a decrease in the oxygen carrying capacity of the blood. If the solution was hypertonic, the red blood cells would shrivel and become crenated, increasing the risk of blood clots forming as they cannot move freely through capillaries

Why do donated human organs need to be kept in an isotonic solution?

To prevent damage to the cells due to the net movement of water by osmosis

Water potential (kPa)

The potential energy of water per unit volume

Why is water potential stated relative to pure water at atmospheric pressure and 20ºC?

Because it is impossible to measure the absolute quantity of the potential energy of water

What happens as solutes are added to a solution?

Water potential decreases. Energy is stored in hydrogen bonds between solute molecules and water molecules, meaning that the more solute there is, the less energy is available as potential energy

How does water potential affect water movement?

Solutions with a high water potential contain water molecules with a greater potential energy for movement, and therefore a greater tendency to move. Solutions with a low water potential contain water molecules with a lower potential energy for movement, and therefore their tendency to move is reduced

How does water potential determine the direction water moves in?

Water molecules move from an area of high water potential to an area of low water potential

Symbol for water potential

Ψ or Ψw

Solute potential

Symbol Ψs, it is the component of water potential that is due to the presence of solute molecules, it can take values of zero or below

Why does solute potential decrease as solutes are added?

Energy is stored in hydrogen bonds between solute molecules and water molecules, meaning that the more solute there is, the less energy is available as potential energy

Pressure potential

Symbol Ψp, it is the hydrostatic pressure exerted by water against the cell wall, therefore it is generally positive inside cells

When can a negative value of pressure potential occur?

In xylem vessels where sap is being transported under tension

Formula for total water potential

Ψw = Ψs + Ψp

What happens when plant tissue is placed in a hypotonic solution?

The cytoplasm contains dissolved substances which lower the solute potential of the plant cells, contributing to a lower water potential inside the plant cell. Water moves from the surrounding solution into the plant cell down its water potential gradient. The pressure potential increases as the cytoplasm presses against the cell wall. Eventually the pressure potential reaches a point where the water potential is equal inside and outside the cell, and the inward movement of water stops. Plant cells in this state are turgid

What happens when plant tissue is placed in a hypertonic solution?

The surrounding solution has a lower solute potential than that of the cell cytoplasm, contributing to a lower water potential. Water moves out of the plant cell into the solution down its water potential gradient. The loss of water results in a reduced volume of cytoplasm and a decreased pressure potential inside the cell. Plant cells will lose turgor pressure and the plant will begin to wilt