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on 4 December 2013

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Transcript of Tonicity

A measure of the osmotic pressure gradient of two solutions separated by a semipermeable membrane.
This means the osmotic pressure or tension of a solution, as in the cells would swell or shrink depending on the
of the environment.
Definition of Tonicity
It is commonly used when describing the response of cells when immersed in a solution.
Tonicity is influenced only by solutes that
cross the membrane, as only these exert an osmotic pressure. Solutes that are
able to
freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane.
Tonicity inside the cell
Types of Tonicity
There are different types of tonicity based on the different concentrations of solutes inside and outside of the cell.
There are three different concentrations a solution can be. It must be either hypertonic, hypotonic, or isotonic.
The stem “Hyper” means “above”.

The stem “Hypo” means “under”.

The stem “Iso” means “equal”.

A hypertonic solution is one with a higher concentration of solutes outside the cell than inside the cell.
A hypotonic solution has a lower concentration of solutes outside the cell than inside the cell.
An isotonic solution is one in which its effective osmole concentration is the same as the solute concentration of a cell.
When a cell is immersed into a hypertonic solution, the tendency is for water to flow out of the cell in order to balance the concentration of the solutes. This is when a cell will shrink and shrivel.
In an attempt to balance the concentrations of solutes inside and outside the cell, water will rush into the cell, causing it to swell or possibly burst.
In this case the cell neither swells or shrinks because there is no concentration gradient or osmotic pressure for water across the cell membrane. Water molecule diffuse through the plasma membrane in both directions, and as the rate of water diffusion is the same in each direction that the cell will neither gain nor lose water.
An Example:
When plant cells are in a hypertonic solution, the flexible cell membrane pulls away from the rigid cell wall, but remains joined to the cell wall at points called plasmodesmata. The cell takes on the appearance of a pincushion, and the plasmodesmata almost cease to function because they become constricted: a condition known as plasmolysis. In plant cells the terms isotonic, hypotonic and hypertonic cannot strictly be used accurately because the pressure exerted by the cell wall significantly affects the osmotic equilibrium point.

Applied to a real-life situation:
A hypertonic solution is used in osmotherapy to treat cerebral hemorrhage.
An Example:
Some organisms have evolved intricate methods of circumventing hypotonicity. For example, saltwater is hypertonic to the fish that live in it. They need a large surface area in their gills in contact with seawater for gas exchange, thus they lose water osmotically to the sea from gill cells. They respond to the loss by drinking large amounts of saltwater, and actively excreting the excess salt. This process is called osmoregulation.

An iso-osmolar solution can be hypotonic if the solute is able to penetrate the cell membrane. For example an iso-osmolar urea solution is hypotonic to red blood cells causing their lysis. This is due to urea entering the cell down its concentration gradient followed by water. For example, the osmolarity of Normal saline, 9 grams NaCl dissolved in water to a total volume of one litre, is a close approximation to the osmolarity of NaCl in blood, i.e. Normal saline is almost isotonic to blood plasma. Both sodium and chloride ions cannot freely pass through the plasma membrane as opposed to urea.
Osmotic pressure is the pressure of a solution against a semipermeable membrane.
Tonicity is the measure of this pressure.
What Tonicity influences:

Diffusion is the movement of particles from an region of higher concentration to one of lower concentration.
Diffusion and Osmosis are both types of PASSIVE TRANSPORT - that is, no energy is required for the molecules to move into or out of the cell.
Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by carrier proteins - this process is called facilitated diffusion.
When the molecules are even throughout a space it is called EQUILIBRIUM.
Depending on the tonicity of a solution, certain things happen to cells in order to compensate for the imbalanced tonicity on either side of the semipermable membrane. The first is diffusion and then osmosis.
Osmosis is the movement of molecules through a semipermeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides.
So say a cell has a higher concentration of molecules than the outside of the cell. Diffusion would occur in order to move the molecules to the outside of the cell until equilibrium is reached.
Quiz Time
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