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.
