BIOL 1020 3

BIOL 1020 - CHAPTER 3 LECTURE NOTES 1 of 3 Chapter 3: Water and life Overview: Life as we know it requires water. All organisms that we know of are made mostly of liquid water, and most of their metabolism requires an aqueous medium. In addition, many organisms live in liquid water or in an environment dominated by water in its various states (solid, liquid, or gas). Some numbers: •  cells are typically 70% or more water by mass •  about 75% of the Earth’s surface is covered by liquid water But then, just being common on the Earth doesn’t make something essential for life.Alarge percentage of the Earth’s crust is sand, but we don’t consider sand a requirement for life. What is it about water that makes it so special? I. The chemistry of water is dominated by the polar nature of water molecules. A. oxygen atoms are electron seeking (electronegative), especially compared to hydrogen; thus for an oxygen-hydrogen bond: 1. the oxygen atom has a partial (-) charge 2. the hydrogen atoms have a partial (+) charge B. the polar character of water allows water molecules to form many (up to 4) hydrogen bonds. II. What properties of water are important for life? A. four properties of water are critical for life as we know it, and all of them come in some way from water’s polar nature and the resulting tendency of water to form hydrogen bonds and similar interactions 1. water is the principal solvent in living things 2. water exhibits both cohesive and adhesive forces 3. water helps maintain a stable temperature 4. ice (solid water) floats in liquid water B. water is the principal solvent in living things 1. the highly polar character of water makes it an excellent solvent for other polar substances, and forionic compounds 2. hydrophilic substances – interact readily with water 3. water does not readily dissolve nonpolar (hydrophobic) substances – thus, hydrophobic substances are good components for membranes C. water exhibits both cohesive and adhesive forces (due to hydrogen bonding) 1. cohesive forces are caused by the attraction of water molecules to other water molecules, and give water a high surface tension (the ability of a water surface to withstand some stress) 2. adhesive forces cause water molecules to be attracted to other kinds of molecules; it is how things are made wet 3. capillary action, the tendency of water to move up narrow tubes even against gravity, results from cohesion and adhesion; living organisms take advantage of this D. water helps maintain a stable temperature 1. the unusual specific heat of water leads to temperature stability •  specific heat - the amount of energy required to raise the temperature of a specific amount of a substance one degree Celsius (for water, 1 calorie = heat needed for 1 g of water to raise by one degree Celsius) •  the specific heat of water is much higher than most other substances, due to hydrogen bonding (as a comparison, the specific heat of sand is about 0.2 calories for 1 g) 540 calories to convert one gram of water to vapor • thus, it requires the gain or loss of more energy (heat) to change the temperature of water than it does other substances • since much of the ecosphere is water, and most biological organisms are more than 70% water, this property of water leads to temperature stability (which is critical for most living organisms) 2. the high heat of vaporization of water helps cool the ecosphere and biological organisms • heat of vaporization is the amount of heat energy required to convert one gram of liquid into the gaseous state • because of hydrogen bonds in liquid water, water has an extremely high heat of vaporization a commonly used unit for measuring energy is the calorie, the amount of heat (energy) required to cause the temperature of one gram of pure water to rise one degree Celsius 540 calories are required to convert one gram of liquid water into water vapor • biological organisms take advantage of this property of water to cool themselves, examples sweating and cooling a leaf E. ice floats (ice is less dense than liquid water) 1. liquid water, like most substances, becomes denser as it cools – but only up to a point 2. at 4oC (under standard atmospheric pressure), water begins to expand as it cools furthe