|MadSci Network: Cell Biology|
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.
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