MadSci Network: Anatomy |
Hi Amanda, You asked why our cells don’t have contractile vacuoles. That’s a really interesting question! I’ll do my best to answer it. Contractile vacuoles are found in single celled protozoans, like amoebas, where they function to maintain normal cell volume by extruding water. You can find a definition of contractile vacuoles online: “A membrane-bound organelle found in certain protists that pumps fluid in a cyclical manner from within the cell to the outside by alternately filling and then contracting to release its contents at various points on the surface of the cell. It functions in maintaining osmotic equilibrium.” From Answers.com This is just one strategy that organisms take to regulate their volume. Our cells have different strategies that are more complex. This is in part because we are not single celled organisms, so water cannot just be taken from or extruded into the extracellular space without consequences for neighboring cells. From the textbook Molecular Cell Biology (Fourth Edition) ed. Harvey Lodish • Most biological membranes are more permeable to water than to ions or other solutes, and water moves across them by osmosis from a solution of lower solute concentration to one of higher solute concentration. • Animal cells swell or shrink when placed in hypotonic or hypertonic solutions, respectively. To maintain their normal cytosolic osmolarity and hence cell volume, animal cells must export Na+ and other ions that leak or are transported from the extracellular space into the cytosol. • The rigid cell wall surrounding plant cells prevents their swelling and leads to generation of turgor pressure in response to the osmotic influx of water. • In response to the entry of water, protozoans maintain their normal cell volume by extruding water from contractile vacuoles. • Aquaporin in the erythrocyte plasma membrane and other water-channel proteins increase the water permeability of biomembranes. • Opening and closing of K+ and Cl− channels and the resulting changes in cytosolic solute concentrations of guard cells cause stomata in leaves to open and close. As you can see, there are many ways that cells can move water in and out. I hope this answers your question! Thanks, Liz
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