upload4 - Chapter 3 Water and the Fitness of the...

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Chapter 3: Water and the Fitness of the Environment: Because water is the substance that makes life possible on Earth, astronomers hope to find evidence of water on newly discovered planets orbiting distant stars. Life on Earth began in water and evolved there for 3 billion years before colonizing the land. Even terrestrial organisms are tied to water. ° Most cells are surrounded by water. ° Cells are about 70–95% water. ° Water is a reactant in many of the chemical reactions of life. Water is the only common substance that exists in the natural world in all three physical states of matter: solid ice, liquid water, and water vapor. A. The Effects of Water’s Polarity 1. The polarity of water molecules results in hydrogen bonding. In a water molecule, two hydrogen atoms form single polar covalent bonds with an oxygen atom. ° Because oxygen is more electronegative than hydrogen, the region around the oxygen atom has a partial negative charge. ° The regions near the two hydrogen atoms have a partial positive charge. A water molecule is a polar molecule in which opposite ends of the molecule have opposite charges, like opposite poles on a magnet. Water has a variety of unusual properties because of the attraction between polar water molecules. ° The slightly negative regions of one water molecule are attracted to the slightly positive regions of nearby water molecules, forming hydrogen bonds . ° Each water molecule can form hydrogen bonds with up to four neighbors. 2. Organisms depend on the cohesion of water molecules. The hydrogen bonds joining water molecules are weak, about 1/20 as strong as covalent bonds. The more hydrogen bonds, the greater the hold of bonds. Each single bond is week, but many bonds confer strength. They form, break, and reform with great frequency. Each hydrogen bond lasts only a few trillionths of a second. At any instant, a substantial percentage of all water molecules are bonded to their
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neighbors, creating a high level of structure. Collectively, hydrogen bonds hold water together, a phenomenon called cohesion. Cohesion among water molecules plays a key role in the transport of water and dissolved nutrients against gravity in plants. ° Water molecules move from the roots to the leaves of a plant through water-conducting vessels. ° As water molecules evaporate from a leaf, other water molecules from vessels in the leaf replace them. ° Hydrogen bonds cause water molecules leaving the vessels to tug on molecules farther down. ° This upward pull is transmitted down to the roots. ° Adhesion, clinging of one substance to another , contributes too, as water adheres to the wall of the vessels. Surface tension, a measure of the force necessary to stretch or break the surface of a liquid , is related to cohesion.
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