Osmosis

A teaching demonstration by Dr. Andrew Joseph Ippolito

For the faculty of Montgomery County Community College

May 3rd, 2007

Osmosis in 7 Parts

I: Introduction to Osmosis

II: How Osmosis Works

III: Who Pulls, Who Pushes?

IV: Osmotic Pressure Revealed

V: Keeping it All Straight

VI: Osmosis “IRL” (or, How Osmosis Saved the Children)

VII: Segue

When we think about diffusion, we don’t concern ourselves about the liquid (the solvent, e.g. water) in which something is dissolved.

A solute particle moving randomly in a solution of water (Brownian motion)

(Check out www.dhmo.org)

OOHHHHA water molecule

Brownian Motion of liquid water Molecules

But “water” is just a molecule,

bouncing around like any other.

To distinguish between diffusion of dissolved particles (the “solute”), and the surrounding liquid

(the “solvent”), we use the term “osmosis”

Why do we care about osmosis in biology?

$.02 Answer: Because it is essential for bringing water in and out of cells.

What do a Carthusian abbot, a French

physician/naturalist, a German Botanist, and a Dutch Chemist have in

common?1748

1837

18671901

Introduction to Osmosis

Historic Perspective

• 1748: French priest/scientist Physicist Abbe’ Jean Antoine Nollet discovers the phenomenon: he submerges a pig bladder filled with concentrated wine into water, and observes water move from outside the bladder to inside, causing the bladder to rupture (likely to his dismay).

• 1837: René Henri Dutrochet watched water pass through the membrane of plant and animal cells. This process of taking up liquid was termed “endosmosis” (endo, “inside”, and -osis, that relating to a condition or process), with the reverse process called, “exosmosis” The term was later shortened to “osmosis”. – Why wasn’t it “Endosis” and “Exosis”?

• 1867: Wilhelm Pfeffer – a co-father of modern plant physiology – constructed a device to measure osmotic pressure (the Pfeffer cell).

• 1901: Jacobus Henricus van't Hoff won the Nobel Prize for his work in deciphering the underlying mathematics of osmosis. He also coined the phrase, “semi-permeable”.

How Osmosis Works

Fill a divided chamber with water.

The constantly moving water molecules can pass freely through the barrier.

Add a solute to the Red side (such as salt)

The solute molecules act as a sort of molecular “sponge”, strongly binding up the water molecules, thereby removing them from the surrounding volume.

This results in an effective decrease in water concentration in the Red chamber.

The solute molecules are too big to pass through the barrier

The constantly moving water molecules, just like any other molecule, will diffuse from the area of HIGH concentration to the area of LOW concentration

(REDBLUE)

There will be some movement of water the other way, too, but the average will be a net increase in the red side.

The result is a net movement of water from the Blue side, where there are more “free” water molecules to the Red side.

Movement will continue until a Dynamic Equilibrium is achieved.

Net Movement of Water

Who Pulls, Who Pushes?

The solution with the higher concentration of solute is said to be “hypertonic” with respect to the solution of lower concentration.

The solution with lower concentration, in turn, is referred to as “hypotonic”

Hypertonic

More Concentrated

Hypotonic

Less Concentrated

Key Concept: Water always towards the hypertonic (more concentrated) solution (assuming there’s a semi-permeable membrane)

Hypertonic

More Concentrated

Hypotonic

Less Concentrated

Isotonic

Equal Concentration

When two solutions have the exact same concentration, they are said to be “isotonic”. (iso- ‘the same’)

Hypertonic, hypotonic, and isotonic, describe relationships between solutions. They are relative terms.

If you ever see the word describing a solution itself, you can usually figure out the context.

An isotonic saline solution for eyes, would be referring to its concentration relative to the liquid of your eye.

It answers the question, “which solution will pull the water away, and which will have water taken from it?”

A solution is never just “hypertonic”. It is always hypertonic with respect to another solution.

Hyper: “more” or “higher”

(hyperactive)

Hypo: “less” or “lower”

(hypothermia)

Osmotic Pressure Revealed

Lets give the solutions some breathing room.

What happens to the volume of each side?

Since we’re losing water on the blue side, and gaining it on the red, the blue side will decrease in volume – while the red increases.

On this side, water is pushing against gravity.

It’s doing work.

How much work?

That depends on the concentration of the solute – (or the differences between them, if both solutions have solutes) and the amount of pressure acting on each.

Question: Does the amount of work depend on

the type of dissolved substance?

NO. This is an example of a “colligative property” of a solution – all that matters is how much stuff is there, not what it is.

If you needed to weigh down the back of a pickup truck, all you really care about is how heavy (and handy) the stuff is – cement blocks, gravel, sand, or even filling it with water and letting it freeze - they all do the same thing.

The original science here was based on the theory that molecules in solution acted in the same way as noble gases – they didn’t react with anything else (including each other), but they bumped around and exerted pressures on their constraints.

How could we measure this pressure? Push back!

With the right amount of pressure, we can return both sides to the same

volume

This value is the “Osmotic Pressure” of the solution.

Another word for this pressure is “Turgor”

The value of osmotic pressure is always negative.Why?

Like any other physical force, it’s a vector, and therefore has both value and direction. It’s negative by tradition, probably because it was a measure of water being pulled away from its starting point – like a vacuum cleaner.

And since we usually ascribe the force asserted by a vacuum to be negative – well, there you have it.

Going back to this earlier image: what if we filled the blue side with more water? Would the red keep rising?

To a point. Eventually, the pressure of the water being pulled down by gravity would be stronger than the Turgor pressure.

This is hydrostatic pressure (static, because the water isn’t moving anywhere).

G

Keeping it AllStraight

Diffusion: Moving from more concentrated areas to less concentrated areas.

Osmosis: Moving from less concentrated areas to more concentrated areas.

Just keep in mind that the liquid in which something is dissolved acts just like any other molecule bumping around. It will diffuse from regions of low concentration to high.

Key Concept: Solutions with the higher solvent concentration will have the lower solute concentration. Solutions with the lower solvent concentration will have the higher solute concentration.

Because we’re generally more interested in what’s dissolved in a liquid, the language that’s normally used revolves around the solute. (When you think about salt water, you are thinking about how ‘salty’ it is – not how ‘watery’ it is!)

In fact, when we dive into the underlying chemistry and mathematics of osmosis, the concentration of water isn’t even used. Instead of talking in terms of concentrations, we talk in terms of how much the water will potentially move against the concentration gradient of the solute. This is called the “water potential” of the solution.

Summary

Osmosis…

…requires a semi-permeable membrane to occur

…is the net movement of solvent through a semi-permeable membrane, from regions of low solute (high solvent) concentration to high solute (low solvent) concentration.

...is a process that occurs between two solutions of different concentrations

…depends upon the concentration (but not type) of solute.

Summary

Osmotic Pressure…

… is the force exerted by a hypertonic solution on a hypotonic solution

…of a solution is always in relation to another solution.

hypertonic solutions draw water away from a hypotonic solution; Isotonic solutions have the same concentration – and therefore an osmotic pressure of 0.

… is always a negative number

Osmosis “IRL”

• When cells are placed in a hypotonic solution, what will happen?

– Red blood cells will swell, and lyse, or rupture. This is called osmotic lysis

– Plant cells, with sturdy cell walls, do not lyse in hypotonic solution. Instead of increasing volume, they become more rigid.

• This rigidity is a result of the osmotic (turogr) pressure discussed earlier – the water molecules trapped in a small space push against the inside of the cell wall.

A Practical Example of Osmosis:

Or, How Understanding Osmosis Helped Save the Children

Oral Rehydration Therapy (ORT)

Measuring spoons for oral rehydration therapy, 1981. Made by the company Teaching Aids at Low Cost, each spoon has instructions in a different language.

Cholera: Easily Fatal, Easily Treatable• Diarrheal illness caused by a bacterial infection in the

intestines (Vibrio cholerae)

• Left untreated, Cholera will cause massive, fatal dehydration from diarrhea and vomiting.– Rapid loss in blood volume (extreme cases: as short as an hour

to a hypotensive state, with death being 2-3 hours later if left untreated).

• Without treatment the death rate is as high as 50%; with treatment the death rate can be well below 1%

• It is still a major cause of death amongst third world children.

Blood Epithelial Cells Gut

High Low High

Low High

Na+

K+

WaterLow HighHigh

Sugar and salt are co-transported from the gut into the epithelial cells.

This causes a net increase in solute concentration inside the cells, causing

the cells to become hypertonic

Water from the hypotonic gut flows into the epithelial cells

Salt is also secreted into the blood, causing it to be hypertonic with

respect to the epithelial cells

Water from the hyptonic epithelial cells flows into the blood, increasing

blood volume.

Basis of Cholera Toxcity

Blood Epithelial Cells Gut

Low Low HighNa+

Low LowK+

Water LowerHigh

Cholera uses a protein (the eponymous Cholera Toxin), which activates cellular proteins that, in the process of their normal activities, causes the excretion of salts into the gut.

Due to the new osmotic pressure of the now-hypertonic gut, water moves from the blood, through the epithelial cells, and into the gut, causing massive diarrhea.

The massive drop in blood loss results in a sharp increase in blood pressure, which can result in death if not treated.

HighHigh

Drinking water alone won’t help – it will just pass right through the hypertonic gut. Salt water won’t help, either – it will simply increase the hypertonicity of the gut.

But, we can take advantage of the fact that glucose and Na+ are co-transported into the epithelium,If we mix sugar into the salt water, this will drive the salt into the epithelium – and with it, the life-saving water.

Segue

Even in an isotonic environment, cells are continually struggling to maintain their volume.

Cells experience a lot of leakage – ions such as Na+ and Cl- diffuse into the cell (down their concentration gradients).

Without an active mechanism to counter this, the cell would increase in solutes, become hypertonic, and therefore exert an

osmotic pressure on the hypotonic environment, drawing in water and eventually lysing.

So why don’t cells lyse?

Think of a rowboat with a leak. It’s not going to sink immediately, and as long as you can bale the water over the side quicker than the boat fills, you can prevent the boat from sinking. But if you stop putting in the effort, over

time, the boat will sink.

This leads into the next section of Active Transport; where diffusion and osmosis are both spontaneous processes, Active Transport requires the cell to expend energy, pumping molecules against their concentration gradient.

Internet Links

Fun• The dangers of dihydrogen monoxide can be

found at www.dhmo.org. • Try this fun simulation here.

Technical• Lab Excerise Link• Webster’s

various definitions and underlying mathematics.

Done!