Osmosis

What is osmosis

The net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane down a water potential gradient.

When will no further net movement of water molecules occur

Until the water potential is the same on both sides of the membrane and a dynamic equilibrium is established

What is water potential

A measure of the tendency of water molecules to move from one region to another.

What is water potential measure in

KPa

What is the water potential of pure water

0 KPa

Solute potential

Decrease in water potential due to the presence of dissolved solutes

Pressure potential

An increase in the water potential due to applied pressure

Why do dissolved solutes lower water potential

Because water molecules form a hydration shell around the solutes as they dissolve. Therefore there are fewer free water molecules to collide with the partially permeable membrane and exert the water potential.

What is the equation to calculate water potential

water potential ψ = solute potential ψs + pressure potential ψp

How does water cross membranes

It is small enough to fit in between the phospholipids although this is slow as it is polar. They may cross the membrane via a channel protein (aquaporin).

Hypertonic solution

Solute concentration of the external solution is lower than the solute concentration in the cell.

What is the effect of hypertonic solutions on plant and animal cells

In animal, this may result in cytolysis (cell bursting) as the plasma membrane cannot withstand the pressure. In plant, the cells will be turgid.

Isotonic solution

Solute concentration of the external solution is the same as the solute concentration in the cell.

What is the effect of isotonic solutions on plant and animal cells

In animal , the cells will be normal and healthy. In plants, the cells may become flaccid and reach a point of incipient plasmolysis.

Hypotonic solution

solute concentration of the external solution is higher than the solute concentration in the cell.

What is the effect of hypotonic solutions on plant and animal cells

In animals, the cells will become crenated (shriveled). In plants, the cells will become plasmolysed as the plasma membrane pulls away from the cell wall.

Why do plant cells not burst in hypertonic solutions

Because the cell wall is strong and can withstand the internal pressure of the protoplast increasing in volume and pushing against it.

How does the cell wall prevent any further water from entering plant cells

By exerting a pressure potential.

Incipient plasmolysis

The point at which the pressure potential that the cell wall exerts is 0. If the cell loses anymore water than it will become plasmolysed.