16. Describe the type of bonding within a water molecule and betweenwater molecules. Give one property thatcharacterizes water that is due to the bonds between water molecules. Why is this important to life?

The bonds between oxygen and the hydrogen atoms withinthe water molecule are polar covalent bonds,i.e., the electrons are notshared equally between oxygen and hydrogen. Oxygen has a higher affinity forelectrons than does hydrogen. However, the difference is not great enough for oxygento completely pull away either hydrogen atom"s electron. Instead, the electron pairsspend 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 isan "oxygen" end in the molecule and a "hydrogen" end (check out thediagram on page 27 of your text). The oxygen end, where the electrons spend more oftheir time, has a partial negative charge. The hydrogen end of the molecule, whichdoesn"t have as much access to these electrons, has a partial positive charge. Thus, the water molecule exhibits polarity (a positive end and a negativeend). Because the molecule exhibits polarity, water molecules are attracted to eachother (opposite charges attract). This weak attraction between molecules or partsof large molecules due to areas of charge on the molecule results in bonds called hydrogen bonds. Look at the diagram on page2-13 in your study guide. This diagram shows the polarity of the water molecule aswell as the hydrogen bond between two water molecules.

The polarity of the water molecule is significant for life since cells are primarilywater! The fact that water has a high specific heat (a reflectionof hydrogen bonds between water molecules) means that organisms are resistant to changesin temperature. Why would this be important? Another important characteristic ofwater, again a reflection of the polarity of the water molecule, is its ability to act asa solvent. Hydrophilic substances dissolve readily inwater. Read about water in your text on pages 26 and 27.

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Can you explainwhy salt and sugar dissolve in water and fats do not dissolve? When we study thecell membrane in the next unit, we"ll come back to this point.

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