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Ap biology 3 water and life
- 1. 2016-12-20 1 BIOLOY À³Ë¼¹ú¼Ê½ÌÓýAPÏîÄ¿ ALBERT ÀÏʦ 4000-663-098 BIOLOGY À³Ë¼¹ú¼Ê½ÌÓýAPÏîÄ¿ ALBERT ÀÏʦ 4000-663-098 3.1 Polar covalent bonds in water molecules result in hydrogen bonding 3.2 Four emergent properties of water contribute to Earth¡¯s suitability for life 3.3 Acidic and basic conditions affect living organisms As astronomers study newly discovered planets orbiting distant stars, they hope to find evidence of water on thesefar- off celestial bodies, for water is the substance that makes possible life as we know it here on Earth. All organisms familiar to us are made mostly of water and live in an environment dominated by water. Water is the biological medium here on Earth, and possibly on other planets as well. The water molecule is deceptively simple. It is shaped like a wide V. These arepolar covalent bonds.This unequal sharing of electrons and water¡¯s V-like shape make it apolar molecule. The properties of water arise from attractions between oppositely charged atoms of different water molecules. The two molecules are thus held together by a hydrogen bond . When water is in its liquid form, itshydrogen bondsare very fragile, each about 1/20 as strong as a covalent bond. QQ201275621 QQ201275621 QQ201275621
- 2. 2016-12-20 2 Why is it unlikely that two neighboring water molecules would be arranged like this ? We will examine four emergent properties of water that contribute to Earth¡¯s suitability as an environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent. Water molecules stay close to each other as a result of hydrogen bonding. Although the arrangement of molecules in a sample of liquid water is constantly changing, at any given moment many of the molecules are linked by multiple hydrogen bonds. These linkages make water more structured than most other liquids. Collectively, the hydrogen bonds hold the substance together, a phenomenon calledcohesion. Evaporation from leaves pulls water upward from the roots through water-conducting cells. Because of the properties of cohesion and adhesion, the tallest trees can transport water more than 100 m upward. Adhesion, the clinging of one substance to another, also plays a role. Adhesion of water to cell walls by hydrogen bonds helps counter the downward pull of gravity . Related to cohesion issurface tension, a measure of how difficult it is to stretch or break the surface of a liquid. Anything that moves haskinetic energy, the energy of motion.Heatis a form of energy. For a given body of matter, the amount of heat is a measure of the matter¡¯s total kinetic energy due to motion of its molecules.Temperatureis a measure of heat intensity that represents the average kinetic energy of the molecules, regardless of volume.Thespecific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1¡ãC. QQ201275621 QQ201275621 QQ201275621
- 3. 2016-12-20 3 An Evaporative coolingof water contributes to the stability of temperature in lakes and ponds and also provides a mechanism that prevents terrestrial organisms from overheating. For example, evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. Floating of Ice on Liquid Waterdue to its lower density is an important factor in the suitability of the environment for life. If ice sank, then eventually all ponds, lakes, and even oceans would freeze solid, making life as we know it impossible on Earth. Besides insulating the water below, ice also provides solid habitat for some animals, such as polar bears and seals. A liquid that is a completely homogeneous mixture of two or more substances is called asolution. The dissolving agent of a solution is thesolvent, and the substance that is dissolved is the solute. In this case, water is the solvent and sugar is the solute. Anaqueous solutionis one in which water is the solvent. The sphere of water molecules around each dissolved ion is called a hydration shell.Working inward from the surface of each salt crystal, water eventually dissolves all the ions. The result is a solution of two solutes, sodium cations and chloride anions, homogeneously mixed with water, the solvent. Even molecules as large as proteins can dissolve in water if they have ionic and polar regions on their surface . Many different kinds of polar compounds are dissolved (along with ions) in the water of such biological fluids as blood, the sap of plants, and the liquid within all cells. Water is the solvent of life. QQ201275621 QQ201275621 QQ201275621
- 4. 2016-12-20 4 Any substance that has an affinity for water is said to be hydrophilic(from the Greek hydro, water, and philios, loving). In some cases, substances can be hydrophilic without actually dissolving. Such a mixture is an example of a colloid, a stable suspension of fine particles in a liquid. Substances that are nonionic and nonpolar (or otherwise cannot form hydrogen bonds) actually seem to repel water;these substances are said to be hydrophobic. We know the mass of each atomin a given molecule, so we can calculate themolecular mass, which is simply the sum of the masses of all the atoms in a molecule.Molarity¡ªthe number of moles of solute per liter of solution¡ªis the unit of concentration most often used by biologists for aqueous solutions. hydrogen ion (H+) hydroxideion(OH-) hydroniumion (H3O+) An acid is a substance that increases the hydrogen ion concentration of a solution. A substance that reduces the hydrogen ion concentration of a solution is called abase.In any aqueous solution at 25¡ãC, the product of theH+ and OH- concentrations is constant at 10-14 A bufferis a substance that minimizes changes in the concentrations ofH+ and OH-_x0002_ in a solution. It does so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions to the solution when they have been depleted. When CO2 dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH, a process known asocean acidification.Acid precipitationrefers to rain, snow, or fog with a pH lower (more acidic) than 5.2. QQ201275621 QQ201275621 QQ201275621