3-WaterandLife-Fall-2005 - Intro. Biol. Course Guide...

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Intro. Biol. Course Guide Sathasivan, 2005 Chap. 3. Water and aqueous solutions 1 3. WATER AND AQUEOUS SOLUTIONS I. Water and its Properties II. Aqueous Solutions Water is essential for life as living organisms are made of up to 95 % water. In addition, it is the medium and an important ingredient for many biochemical reactions. In this chapter, we will look at the properties of water and water based (aqueous) solutions. I Water and H-bonding Hydrogen and oxygen share the electrons unequally resulting in partial positive and partial negative charges on hydrogen and oxygen, respectively. A single water molecule is tetrahedral in shape. The two electron orbitals of oxygen and two of hydrogen make the four corners of the tetrahedral structure of the water molecule. Water molecule (+) (+) (-) (-) O H H O H H O H H O H H O H H O H H Hydrogen bond Because of the two partial positive and negative charges, water molecules can H-bond with up to 4 other water molecules. The relatively strong H-bonding in water is the main reason for its unique properties. Properties of Water 1. Cohesiveness due to constant forming and breaking of H- bonds in liquid water. This is important for water uptake (transport from roots to leaves and other parts of the plant) and imbibition (water intake) by seeds by absorption. Adherence of water to the surface of other objects is referred to adhesiveness. This is also important for the attachment of water (adsorption) which may result in absorption. 2. High specific heat. The specific heat of water is 1 cal/g/ o C compared to 0.1 cal/g/ o C for iron and 0.6 cal/g/ o C for ethanol. Water can absorb and release heat to stabilize the
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Intro. Biol. Course Guide Sathasivan, 2005 Chap. 3. Water and aqueous solutions 2 temperature in the surrounding area to make it habitable. 3. High heat of vaporization. It takes 540 cal to evaporate 1 g of water compared to 237 cal/g for ethanol and 59 cal/g for chloroform. This helps in evaporative cooling for both animals and plants during the hot weather. 4. Freezing and expansion. Water is densest at 4 o C and it expands during freezing. The H-bonds between water molecules in ice keep each molecule farther apart from one other compared to liquid water. Since ice is lighter than water, it floats on the surface to keep the water below
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This note was uploaded on 08/31/2010 for the course BIO 49810 taught by Professor Satasivan during the Spring '10 term at University of Texas.

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3-WaterandLife-Fall-2005 - Intro. Biol. Course Guide...

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