Re: If in an experiment, the water around an amoeba

Dear Cherrie,

Freshwater amoebae use their contractile vacuoles to overcome a persistent 
problem: water seeps continually into their bodies and, if there were no 
way to pump it back out, would cause them to burst.  In order to 
understand why the contractile vacuoles disappear in salty water, we have 
to understand why water behaves this way.

To get an intuitive understanding, think about pouring salt into water.  
In time the salt dissolves, diffusing into the water so that the salt 
concentration is constant throughout. Now imagine if the salt was in a bag 
that prevented it from flowing out but allowed water to flow in.  The 
system “wants” to have the two substances mixed but now the salt can’t go 
anywhere.  What happens?  Water flows into the bag until the salt 
concentration inside the bag is the same as the salt concentration 
outside.  Since there is no salt outside the bag, this is impossible.  
Nature effects a compromise—the bag bursts, salt diffuses everywhere, and 
the system is satisfied.  This movement of water is termed osmosis.

Of course, the bag is not so happy.  The freshwater amoeba is very much 
like our bag of salt in water.  All of the proteins, lipids, nucleic 
acids, and carbohydrates that compose the creature are locked inside its 
plasma membrane.  There is a higher concentration of substances inside the 
membrane than outside, so water tends to move into the amoeba to try and 
cancel out the difference.  “Canceling the difference” means canceling the 
amoeba.

Let’s take an aside to lay down some terminology.   Let’s say area A has a 
higher concentration of substances than area B.  A is “hyperosmotic” to B, 
and B is “hypoosmotic” to A.  If  area C has the same concentration as A, 
then A and C are “isoosmotic.”  The amoeba is hyperosmotic to the 
surrounding water, and nature likes it when things are isoosmotic.

The amoeba manages this unfavorable circumstance by using its contractile 
vacuoles.  Each vacuole is a small bag within the larger bag of the amoeba 
itself.  And where the amoeba is hyperosmotic to the water, the 
contractile vacuole is hyperosmotic to the amoeba.  Water flows from the 
environment to the amoeba’s cytoplasm, and from the amoeba’s cytoplasm 
into the contractile vacuole.  When the amoeba feels like it’s had enough, 
it dumps the contractile vacuole into the environment.  This process is 
aided by cytoskeletal elements around the vacuole, which contract and 
squeeze the water out. 

When you add salt to an amoeba’s environment, you are managing the 
situation for the amoeba.  The more salt you add to the water, the less 
hyperosmotic the amoeba is and the less water enters it.  The vacuole does 
not need to contract as often.  When you have added enough salt to make 
the environment isoosmotic to the amoeba, there will be no net movement of 
water at all.  The contractile vacuoles will empty and then not fill 
anymore; looking through a light microscope, you will see them disappear.  

What will happen if you add even more salt?  Now the environment is 
hyperosmotic to the amoeba.  Water will flow OUT of the amoeba and cause 
it to shrink.  Unfortunately, our freshwater amoeba can’t use its 
contractile vacuole to suck water back in and, unless you dilute that salt 
away, our much-abused friend will be no more.

You can find most of the information in this answer at 
www.polenth.demon.co.uk/science/amoeba.html, a virtual temple to the 
amoeba.  Check out 
bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-02/Amoeba_Model/amoeba_model.htm
 for a great diagram of the amoeba and others of its ilk.