Imagine that you’re stranded on a raft in the middle of the ocean.
You don’t have any emergency food or water, but you also know that humans can only survive
about 3 days without fluids.
Should you drink sea water or not?
Stick with us, we’re going to find out and also review hypertonic, hypotonic and isotonic
solutions along the way.
Cells have a very finite range of conditions in which they can survive.
If the conditions are too hot, too cold, too watery, too salt, too acidic or too basic,
they can’t function properly.
So, we’re going to explore what happens when you place a cell in a solution that is
much more salty (hypertonic), much more watery (hypotonic), or has an equal salt concentration
(isotonic).
Remember that cell membranes are selectively permeable; they allow certain particles to
pass through, but not others.
Usually, larger molecules can’t fit through the membrane without special channels.
The cell membrane is fitted with special protein channels called Aquaporins, which allow water
molecules to pass through without expending energy.
However, the ease with which the water molecules can cross is going to depend on something
called the concentration.
Concentration is a measure of how much solute there is per volume of solvent.
In other words, whether the liquid is more salty, or more watery.
We’d usually expect that the molecules would follow a process called diffusion.
They would flow from areas of higher to lower concentration.
Eventually, they will reach equilibrium, where there will be equal concentrations on both
sides of the membrane.
However, in the case of a liquid like salt water, the salt molecules are much, much too
big to fit through the cell membrane.
So the only particle that can move is water.
Let’s look a little more closely.
Water molecules are highly attracted to salt molecules.
They cluster around salt and really, really don’t want to move.
The positive parts of the water molecule stick to the negative parts of the sodium chloride,
and vice versa.
The interactions between oxygen and sodium, and between hydrogen and chlorine are called
ion-dipole interactions, which you might remember from chemistry.
Water molecules that are not stuck to a molecule of salt are far more likely to relocate.
Today, we’re going to immerse a cell in three kinds of liquids; a very solute-rich
liquid, a very watery liquid, and a liquid that’s in between.
Then we’ll see what happens to the water molecules.
In this first scenario, the liquid surrounding the cell has many molecules of solute in it.
This liquid is hyper-tonic compared to the cell.
It has a much higher concentration of solute particles, and a much lower concentration
of water.
Unfortunately, the solute molecules can’t pass through the membrane to reach equilibrium,
but water molecules can.
The molecules of water on the outside of the cell are going to be obstructed from passing
through by the many molecules of solvent.
However, the molecules on the inside have much less solvent getting in the way.
Water will begin rushing out of the cell.
Some water will come in, but the net movement, the overall movement, of water molecules will
be outwards, and the cell will shrink.
We call this cell contraction plasmolysis.
This is also how pickles are made.
Now let’s immerse a cell in a much less concentrated solution; a hypotonic solution.
Compared to the cell, it’s much less salty.
Some water molecules will exit the cell, but even more will rush in because they aren’t
obstructed by solute particles.
The cell grows in size, and it may even undergo cytolysis and burst!
Placing a cell in an isotonic solution, where the concentrations inside and outside the
cell are equal, is much more pleasant.
Equal amounts of water molecules continue to pass in and out of the cell.
The system is in dynamic equilibrium; particles are continuing to move, but the net movement
is zero.
The cell neither grows, nor shrinks, and is much more likely to survive.
Now let’s return to our sea water question.
Sea water is incredibly salty, much more salty than our body’s cells.
Since it’s such a hypertonic solution, if you immersed your cells in it, they would
shrivel up and die.
The real danger, however, lies in how your kidneys would react to sea water.
The kidneys would attempt to remove the toxic levels of salt by using water pulled from
your cells.
Your body would actually use more water in removing the salt than was originally contained
in the seawater itself.
In summary, drinking sea water is a lousy idea, even in extreme circumstances.
You’re better off drinking your own pee!
So, there we have it!
Hypertonic, hypotonic and isotonic solutions, as well as a couple of practical examples!
Thanks again for watching! Take care.