Campbell Biology ninth edition Chapter 3 Water and Life Campbell Biology ninth edition Chapter 3

The molecule that supports all of life Water is the substance that makes life possible. Water is the only common substance that exists in the environment in all three physical states of mater. Solid (ice) and gas (water vapor) Life on earth began in water and evolved onto land. All living organisms require water more than any other organism. Most cells are surrounded by water and cells are about 70-95% water

Polar covalent bonds in water molecules result in hydrogen bonding Shaped like a v Oxygen and hydrogen have polar covalent bonds (electrons spend more time closer to oxygen because oxygen is more electronegative) Water is a polar molecule, meaning its overall charge is unevenly distributed. Water molecules are held together by hydrogen bonds.

The emergent properties of water contribute to earth’s suitability for life Four emergent properties of water Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent

Cohesion of water molecules Many hydrogen bonds hold water together, called cohesion. Cohesion contributes to the transport of water in plants. The clinging of one substance to another, called adhesion, also plays a role. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than most liquids.

Moderation of temperature by water Water moderates temperature by releasing heat into cooler air and absorbing heat from warmer air. Water can release or absorb heat with only a slight change in its own temperature.

Heat and temperature Anything that moves has kinetic energy, the energy of motion. Atoms and molecules are always moving and therefore have kinetic energy. The faster a molecule moves, the greater its kinetic energy. Heat is a form of energy. Heat and temperature are different. Temperature is a measure of heat intensity that represents the average kinetic energy of the molecules, regardless of volume. However, heat depends on volume. When two objects of different temperatures are brought together, heat is exchanged until the objects reach the same temperature. (Ex: ice in a drink) Temperature is measured with the Celsius scale. A calorie (cal) is a unit of MEASUREMENT FOR HEAT. A CALORIE IS THE AMOUNT OF HEAT IT TAKES TO RAISE THE TEMP OF 1G OF water by 1˚C. another energy unit is the joule (j).

Water’s high specific heat The specific heat of a substance is the amount of heat that must be absorbed or lost for 1g of that substance to change its temperature by 1˚C. Water’s specific heat is 1 calorie per gram and per degree. 1 cal/g∙˚C Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water’s high specific heat helps to stabilize ocean temperature creating a favorable environment for marine life. Also helps to keep temperature fluctuations on land within limits that permit life.

Evaporative cooling Molecules of any liquid stay close together because they are attracted to one another. When molecules can move fast enough to overcome the attraction, they can exit as a gas. This transformation is called vaporization, or evaporation. Evaporation can occur at any temperature. If a liquid is heated, the kinetic energy will increase and it will evaporate more quickly. Heat of vaporization is the quantity of heat a liquid must absorb for 1g of it to be converted from a liquid to a gas. Water has a high heat of vaporization due to hydrogen bonds which helps regulate Earth’s climate. As a liquid evaporates the liquid that remains behind cools down. Evaporative cooling occurs because the hottest molecules are most likely to leave as a gas. Evaporative cooling helps moderate temperatures of ponds and lakes and prevents overheating.

Floating of ice on liquid water Water is less dense as a solid than as a liquid because of hydrogen bonding. If ice sank, ponds, lakes, and oceans would eventually freeze. If water did not form hydrogen bonds, what would happen to the shrimp’s environment?

Water: the solvent of Life A sugar cube placed in water will dissolve A liquid that is a completely homogeneous mixture of 2 or more substances is called a solution. The dissolving agent is the solvent (water) and the substance that is dissolved is the solute (sugar). In an aqueous solution, water is the solvent. The sphere of water molecules around each dissolved ion is called a hydration shell. Water is NOT an universal solvent. Why?

Hydrophilic and Hydrophobic substances Any substance that has an affinity for water is hydrophilic (water loving). Some substances may be hydrophilic but not dissolve in water because the molecules are too large. This is an example of a colloid, a stable suspension of fine particles in a liquid. (ex: cotton has cellulose fibers) Substances that are nonionic and nonpolar (can’t form hydrogen bonds) repel water and are hydrophobic (water fearing). (ex: vegetable oil)

Possible evolution of life on other planets with water Why are scientists trying to learn if other planets have water? It has been discovered that mars has an ice cap at both poles and ice is present underneath the surface. There is also enough water vapor for frost to form.

Acids, bases, and buffers An acid is a substance that increases the hydrogen ion concentration of a solution. pH less than 7 A substance that reduces the hydrogen ion concentration of a solution is a base. pH greater than 7 The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration. Water has a ph of 7 (neutral) The internal ph of most living cells is close to 7. The presence of buffers allows biological fluids to maintain a constant ph. Carbonic acid is an example of a buffer in human blood.

Acidification: a threat to water quality Gaseous compounds released from the Burning of fossil fuels cause water to become more acidic. About 25% of human generated co2 is absorbed by the oceans which harms marine ecosystems. When co2 dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean ph, a process known as ocean acidification. Acid rain also has a lower ph due to the burning of fossil fuels. This causes harm to lakes, streams, plants etc.