Chapter 3 - CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT Introduction Because water is the substance that makes possible life as we know it

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CHAPTER 3 - WATER AND THE FITNESS OF THE ENVIRONMENT Introduction Because water is the substance that makes possible life as we know it 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 spreading onto land. Even terrestrial organisms are tied to water. Most cells are surrounded by water and cells are about 70-95% water. Water exists in three possible states: ice, liquid, and 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, the region around oxygen has a partial negative charge. The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges. Water has a variety of unusual properties because of attractions between these polar molecules. The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. 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 1/20th as strong as covalent bonds. They form, break, and reform with great frequency. At any instant, a substantial percentage of all water molecules are bonded to their neighbors, creating a high level of structure. Hydrogen bonds hold the substance together, a phenomenon called cohesion . Cohesion among water molecules plays a key role in the transport of water against gravity in plants. Water that evaporates from a leaf is replaced by water from vessels in the leaf. Hydrogen bonds cause water molecules leaving the veins to tug on molecules further down. This upward pull is transmitted 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. Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. Water behaves as if covered by an invisible film. Some animals can stand, walk, or run on water without breaking the surface. 3. Water moderates temperatures on Earth Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature.
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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Chapter 3 - CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT Introduction Because water is the substance that makes possible life as we know it

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