BIOWEB
nverdae@nvcc.edu
Emma Erdahl, Associate Professor of Biology
Northern Virginia Community College
Revised 8/23/00
|
16. Describe the type of bonding within a water molecule and between water molecules. Give one property that characterizes water that is due to the bonds between water molecules. Why is this important to life? The bonds between oxygen and the hydrogen atoms within the water molecule are polar covalent bonds,i.e., the electrons are not shared equally between oxygen and hydrogen. Oxygen has a higher affinity for electrons than does hydrogen. However, the difference is not great enough for oxygen to completely pull away either hydrogen atom's electron. Instead, the electron pairs spend more time around the nucleus of the oxygen atom than the hydrogen atoms. Because of the way these polar covalent bonds form between oxygen and hydrogen, there is an "oxygen" end in the molecule and a "hydrogen" end (check out the diagram on page 27 of your text). The oxygen end, where the electrons spend more of their time, has a partial negative charge. The hydrogen end of the molecule, which doesn't have as much access to these electrons, has a partial positive charge. Thus , the water molecule exhibits polarity (a positive end and a negative end). Because the molecule exhibits polarity, water molecules are attracted to each other (opposite charges attract). This weak attraction between molecules or parts of large molecules due to areas of charge on the molecule results in bonds called hydrogen bonds. Look at the diagram on page 2-13 in your study guide. This diagram shows the polarity of the water molecule as well as the hydrogen bond between two water molecules. The polarity of the water molecule is significant for life since cells are primarily water! The fact that water has a high specific heat (a reflection of hydrogen bonds between water molecules) means that organisms are resistant to changes in temperature. Why would this be important? Another important characteristic of water, again a reflection of the polarity of the water molecule, is its ability to act as a solvent. Hydrophilic substances dissolve readily in water. Read about water in your text on pages 26 and 27. Can you explain why salt and sugar dissolve in water and fats do not dissolve? When we study the cell membrane in the next unit, we'll come back to this point. |
|