Osmosis

Front 1

Give another name for tonicity.

Back 1

Tonicity is also referred to as effective osmolality.

Front 2

What is effective osmolality?

Back 2

Effective osmolality is another term for tonicity.

Effective osmolality/tonicity is contrasted with osmolality in general. Effective osmolality/tonicity is an expression of the concentration of effective osmoles in a solution while osmolality refers to the total number of osmoles, both effective (e.g. Na+, glucose) and ineffective (urea).

Front 3

Name the three principle osmoles present in the ECF.

Back 3

Na+ (effective osmole, contributes to tonicity)

Glucose (effective osmole, contributes to tonicity)

BUN (ineffective osmole, does not contribute to tonicity)

Front 4

Na+ diffuses passively across the cell membrane. Why then is it considered an effective osmole?

Back 4

Na+ is FUNCTIONALLY impermeant owing to its extrusion by the Na-K pump.

Front 5

What is an isotonic solution?

Back 5

A solution is isotonic when its contains the same concentration of effective osmoles as the solution it is being compared to (the "reference solution").

There is no net movement of water between an isotonic solution and the reference solution across a semi-permeable membrane.

Under normal conditions, ECF is isotonic to ICF. As a result, there is a kind of equilibrium between the two compartments, with water entering and exiting the cell at the same rate.

This follows necessarily from the fact that the cell size (again, under normal conditions) stays relatively constant and does not shrink or swell.

Front 6

What is a hypertonic solution?

Back 6

A hypertonic solution is one with a greater concentration of EFFECTIVE osmoles than the solution it is being compared to.

Water flows into hypertonic regions across semi-permeable membranes.

Front 7

What is a hypotonic solution?

Back 7

A hypotonic solution is one with a lower concentration of effective osmoles than the solution it is being compared to.

There is a net flow of water out of hypotonic regions across semi-permeable membranes.

Front 8

What is the difference between effective and ineffective osmoles?

Back 8

Effective osmoles are those that have the capacity to exert osmotic force across a semi-permeable membrane.

Remember that particles don't really "exert" an osmotic force. Rather, osmosis simply occurs naturally when there is a difference in solute concentration between two compartments separated by a semi-permeable membrane.

If an osmole can diffuse across the membrane, the difference in solute concentration across the membrane won't last for long as the osmoles simply diffuse down their own gradient until equilibrium occurs.

These osmoles are INEFFECTIVE in that they allow equal solute concentrations to be reached without the movement of water.

Without a difference in solute concentration, osmosis doesn't occur, plain in simple.

In order to be "effective" at contributing to the motion of water from one side of the membrane to the other, therefore, an osmole must be itself unable to cross the membrane.

Effective osmoles, then, are those osmoles that are confined to the intracellular or extracellular space and cannot cross the semi-permeable membrane.

Ineffective osmoles are those that diffuse easily across the semi-permeable water, equalizing solute concentration and thereby eliminating the driving force between osmosis.