BIOS 208. Chapter 3. Water and Life

BIOS 208/210: Fundamentals of Cell Biology

Chapter 3: Water and Life

Key Concepts

3.1) Polar covalent bonds in water molecules result in hydrogen bonding

·         A hydrogen bond forms when the slightly negatively charged oxygen of one water molecule is attracted to the slightly positively charged hydrogen of a nearby water molecule. Hydrogen bonding between water molecules is the basis for water’s properties.

3.2) Four emergent properties of water contribute to Earth’s suitability for life

·         Hydrogen bonding keeps water molecules close to each other, and this cohesion helps pull water upward in the microscopic water-conducting cells of plants. Hydrogen bonding is also responsible for water’s surface tension.

·         Water has a high specific heat: Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form. This helps keep temperatures relatively steady, within limits that permit life. Evaporative cooling is based on water’s high heat of vaporization. The evaporative loss of most energetic water molecules cools a surface.

·         Ice floats because it is less dense than liquid water. This allows life to exist under the frozen surfaces of lakes and polar seas.

·         Water is an unusually versatile solvent because its polar molecules are attracted to charged and polar substances capable of forming hydrogen bonds. Hydrophilic substances have an affinity for water; hydrophobic substances do not. Molarity, the number of moles of solute per liter of solution, is used as a measure of solute concentration in solutions. A mole is a certain number of molecules of a substance. The mass of a mole of a substance in grams is the same as the molecular mass in daltons.

·         The emergent properties of water support life on Earth and may contribute to the potential for life to have evolved on other planets.

3.3) Acidic and basic conditions affect living organisms

·         A water molecule can transfer an H+ to another water molecule to form H3O+.

·         The concentration of H+ is expressed at pH. Buffers in biological fluids resist changes in pH. A buffer consists of an acid-base pair that combines reversibly with hydrogen ions.

·         The burning of fossil fuels increases the amount of CO2 in the atmosphere. Some CO2 dissolves in the oceans, causing ocean acidification, which has potentially grave consequences for coral reefs. The burning for fossil fuels also releases oxides of sulfur and nitrogen, leading to acid precipitation.