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ลำดับตอนที่ #3 : [[R.S.]]AP Biology Test::Chapter.3::4.9.09

review sheet of AP Biology for

Chapter 3* 4.Sep.09

 

Chapter.3 :: Water and the Fitness of the Environment  [[Page.47]]

Overview ‘The Molecule That Supports All of Life [[Page.47]]

-          Because water is the substance that makes life possible on Earth, astronomers hope to find evidence of water on newly discovered planet orbiting distant stars.

-          All organisms familiar to human are made mostly of water and live in an environment dominated by water.

-          Water is the biological medium on Earth, and possibly on other planets as well.

-          Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Even terrestrial (land-dwelling) organisms are tied to water because….

·           All living organisms require water more than any substances.

·           Molecules of water participate in many chemical reactions necessary to sustain life.

·           Most cells are surrounded by water (cells themselves are 70-95% water)

-          ¾ of Earth’s surface is submerged in water.

-          Water is the only common substance to exist in the natural environment in all three physical states of matter: solid, liquid, and gas.

-          The abundance of water is a major reason Earth is habitable.

Concept 3.1 ‘The polarity of water molecules results in hydrogen bonding’ [[Page.47]]

-          Water’s two hydrogen atoms are joined to the oxygen atom by single covalent bonds.

-          Because oxygen is more electronegative (more strongly pulls shared electrons toward itself) than hydrogen, the region around the oxygen atom has a partial negative charge and the region near the two hydrogen atoms have a partial positive charges.

-          The water molecule, shaped like a wide V, is a polar molecule, meaning that opposite ends of the molecule have opposite charges.

(oxygen = negative; hydrogen = positive)

•Polar covalent bond = electrons of the bond are not shared equally

-          Water has a variety of unusual properties becuz of the attraction between polar water molecules

•The slightly negative regions of one water molecule are attracted to the slightly positive regions of nearby water molecules, forming hydrogen bonds.

•Each water molecule can form hydrogen bonds with up to four neighbors.

-          The arrangement of molecules in liquid water is constantly changing, but still at any moment they are linked by multiple hydrogen bonds.

 

Concept 3.2 ‘Four emergent properties of water contribute to Earth’s fitness of life’ [[Page.48]]

-          The four water’s properties that contribute to the suitability of Earth as an environmental for life are water’s cohesive behavior, its ability to moderate temperature, its expansion upon freezing, and its versatility as a solvent.

1.       Cohesion

-           The hydrogen bonds that join water molecules together are very fragile (weak), about 1/20 as strong as covalent bonds.

•          They form, break, and re-form with great frequency.

•          Each hydrogen bond lasts only a few trillionths of a second but at any instant, a substantial (significant) percentage of water molecules are bonded to their neighbors, creating a high level of structure. 

•          Collectively, the hydrogen bonds hold the substance together, a phenomenon called cohesion. (แรงดึงระหว่างน้ำกับน้ำ)

-          Cohesion among water molecules plays a key role in the transport of water and dissolved nutrients against gravity in plants.

•          Water from roots reaches the leaves through a network of water-conducting cells (which are Xylem and Phloem)

•          As water evaporates from a leaf, hydrogen bonds cause water molecules that are leaving the vessels in the leaf to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way down to the roots.

•          Adhesion(แรงดึงระหว่างน้ำกับสิ่งอื่น), the clinging of one substance to another, also plays a role as water adheres (attaches) to the wall of the vessels, helping to resist the downward pull of gravity. 

-          Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion.

•          Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.

•          Water behaves as if covered with an invisible film that’s why some animals can stand, walk, or run on water without breaking the surface.

2.       Moderation of Temperature

-          Water stabilizes air temperature by absorbing heat from warmer air and releasing heat to cooler air.

-          Water is effective as a heat bank because it can absorb or release a relatively large amount of heat with only a slight change in its own temperature.

-          Atoms and molecules have kinetic energy, the energy of motion, because they are always moving. (The faster a molecule moves, the more kinetic energy it has)

-          Heat is a measure of the total amount of kinetic energy due to molecular motion in a body of matter.

-          Temperaturemeasures intensity of heat due to average kinetic energy of molecules.

•          As the average speed of molecules increases, a thermometer will record an increase in temperature.

-          Heat and temperature are related, but they are not the same.

•          When two objects with different temperatures come together, heat passes from the warmer object to the cooler object until the two are the same temperature.

ðMolecules in cooler object speed up at expense of kinetic energy of warmer object.

ðAn ice cube cools a drink not by adding coldness to the liquid, but by absorbing heat from the liquid as the ice itself melts.

-          There are basically three scale to indicate temperature

1.Celsius scale (^oC)

ð      At sea level, water freezes at 0^oC and boils at 100^oC.

ð      Human body temperature is typically 37^oC.

2.Fahrenheit scale (^oF)

3.Kelvin scale (k)

-          There are also several ways to measure heat energy …

•             One convenient unit is the calorie (cal).

ð      One calorie is the amount of heat energy necessary to raise the temperature of one g of water by 1^oC

ð      A calorie is released when 1 g of water cools by 1^oC.

•             In many biological processes, the kilocalorie (kcal), is more convenient.

ð      A kilocalorie is the amount of heat energy necessary to raise the temperature of 1 kg (1000 g) of water by 1^oC. (1 kilocalorie = 1,000 calories)

•             Another common energy unit, the joule (J), is equivalent to 0.239 cal.

-          Water stabilizes temperature because it has a high specific heat.

•          The specific heat of a substance is the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1^oC.

ð      By definition, the specific heat of water is 1 cal per gram per degree Celsius or 1cal/g/^oC.

•          Water has a high specific heat compared to other substances.

ð      Eg. Ethyl alcohol has a specific heat of 0.6 cal/g/^oC

       Iron has a specific heat of 1/10 of water

•          Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat.

ð      Water absorbs/releases a relatively large quantity of heat for each degree of temperature change.

•          Water’s high specific heat is due to hydrogen bonding.

ð      Heat must be absorbed to break hydrogen bonds, and heat is released when hydrogen bonds form.

ð      A calorie of heat causes a relatively small change in the temperature of water because much of the heat is used to disrupt (disturb) hydrogen bonds, not speed up the movement of water molecule.

(Recall that the molecules must move faster in order to change state to gas)

-          Water’s high specific heat has effects that range from the level of the whole Earth to the level of individual organisms.

•          A large body of water can absorb a large amount of heat from the sun in daytime during the summer and yet warm only a few degrees.

•          At night and during winter, the warm water will warm cooler air. 

•          Therefore, ocean temperature & coastal areas have more stable temperature than inland areas.

•          Living things are made primarily of water. Consequently, they resist changes in temperature better than they would if composed of a liquid with a lower specific heat.

-          The transformation of a molecule from a liquid to a gas is called vaporization or evaporation.

•          This occurs when the molecule moves fast enough to overcome the attraction of other molecules in the liquid.

•          Heating a liquid increases average kinetic energy & increases rate of evaporation.

-          Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.

•          Water has a relatively high heat of evaporation, requiring about 580 cal of heat to evaporate 1 g of water at room temperature.

•          This is double the heat that required to vaporize the same quantity of alcohol or ammonia.

•          This is because hydrogen bonds must be broken before a water molecule can evaporate from the liquid.

•          Water’s high heat of vaporization helps moderate Earth’s climate.

ð       Much of the sun’s heat absorbed by tropical oceans is used for evaporation of surface water.

ð       As moist tropical air moves to the poles, water vapor condenses to form rain, releasing heat.

-          As a liquid evaporates, the surface of liquid that remains behind cools. This evaporative cooling occurs because the ‘hottest’ molecules, those with the greatest kinetic energy, are the most likely to leave as gas ... leaving the lower-kinetic energy molecules behind. 

-          Evaporative cooling of water contributes to the stability of temperature in lakes & ponds and also provides a mechanism that prevents terrestrial organisms from overheating.

•          Evaporation of water from the leaves of plants or the skin of humans removes excess heat.

3.       Insulation of Bodies of Water by Floating Ice

-          Water is unusual because it’s less dense as a solid than as a cold liquid (ice floats in liquid water)

-          While other materials contract when they solidify, water expands becuz of hydrogen bonding

-          At temperature above 4^oC, water behaves like other liquid, expanding as it warms and contracting as it cools. Water begins to freeze when its molecules are no longer moving vigorously enough to break their hydrogen bonds.

-          When water reaches 0 ^oC, water becomes locked into a crystalline lattice, with each water molecule bonded to a maximum of four partners. Because the crystal is spacious, ice has fewer molecules than an equal volume of liquid water. 

-          As ice starts to melt, some of the hydrogen bonds break & the crystal collapse, so water molecules can slip closer together than they can while in the ice state.

-          Ice is about 10% less dense than water at 4^oC(water reaches its greatest density at 4 ^oC)

This oddity has important consequences for life.

•          If ice sank, eventually all ponds, lakes, and even the ocean would freeze solid.

•          During the summer, only the upper few centimeters of the ocean would thaw.

•          Instead, the surface layer of ice insulates liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

4.       The Solvent of Life

-          A liquid that is a completely homogeneous mixture of two or more substances is called a solution.

•          The dissolving agent is the solvent, and the substance that is dissolved is the solute.

-          In an aqueous solution, water is solvent.

-          Water is not a universal solvent, but it’s very versatile because of the polarity of water molecules.

•          Water is an effective solvent because it readily forms hydrogen bonds with charged and polar covalent molecules.

•          For example :: When a crustal of salt (NaCl) is placed in water …

ð      The Na⁺ cations interact with the partial negative charges of oxygen regions of water molecules.

ð      The Cl^- anions interact with the partial positive charges of the hydrogen regions of water molecules.

ð      As a result, water molecules surround the individual sodium and chloride ions, separating and shielding them from one another.  A sphere of water molecules form a hydration shell, surrounding each dissolved ion.

ð      Eventually, water dissolves all the ions, resulting in a solution with two solutes: sodium and chloride ions.

-            Polar molecules are also soluble in water because they form hydrogen bonds with water.

-            Even large molecules, like proteins, can dissolve in water if they have ionic and polar regions.

-          Whether ionic or polar, any substance that has an affinity for water is said to be hydrophilic. (water-loving)

•          Some hydrophilic substances do not dissolve because their molecules are too large

•          For example, some components in cells do not dissolve but they remain suspended in the aqueous liquid of the cell. Such a mixture is an example of a colloid, a stable suspension of fine particles in a liquid.

•          Another example is cotton; it has numerous polar covalent bonds due to cellulose. However, its giant cellulose molecules are too large to dissolve in water.

ð       Water molecules form hydrogen bonds with the cellulose fibers of cotton, allowing you to dry yourself with your cotton towel as the water is pulled into the towel.

-          Substances that are nonionic and nonpolar actually seem to repel water; these substances are said to be hydrophobic. (water-fearing); they don’t have an affinity for water.

•       Because there are no consistent regions with partial or full charges, water molecules cannot form hydrogen bonds with hydrophobic molecules.

•       For example, vegetable oil; they are hydrophobic because the dominant bonds, carbon-carbon and carbon-hydrogen, share electrons equally.

•       Hydrophobic molecules are major ingredients of cell membranes (or else the it’d dissolved inside body !)

-          Biological chemistry is ‘wet’ chemistry with most reactions involving solutes dissolved in water.

-          Chemical reactions depend on collisions of molecules and therefore on the concentration of solutes in aqueous solution.

•       To calculate number of molecules, we must know the mass of each atom in a given molecule, so we can calculate its molecular mass, which is simply the sum of the masses of all the atoms in a molecule.

•       We measure the number of molecules in units called moles (mol).

ð       The actual number of molecules in a mole is called Avogadro’s number, 6.02 x 10²³

ð       A mole is equal to the molecular weight of a substance but scaled up from Daltons to grams.

ð       For example, if we want to measure out a mole of table sugar – sucrose (C₁₂H₂₂O₁₁). A carbon atom weights 12 Daltons, hydrogen 1 Daltons, and oxygen 16 Daltons, so the molecular mass of one sucrose molecule would be 342 Daltons [[12(12) + 22(1) + 11(12) = 342]]

To get one mole of sucrose, we would weigh out 342 g.

•       The advantage of using moles as a measurement is that a mole of one substance has the same number of molecules as a mole of any other substances.

ð       If substance A has a molecular weight of 10 Daltons and substance B has a molecular weight of 100 Daltons, then we know that 10 g of substance A has the same number of molecules as 100 g of substance B.

•       Measuring in moles allows scientists to combine substances in fixed ratios of molecules.

-          In ‘wet’ chemistry, we are typically combining solutions or measuring the quantities of materials in aqueous solutions.

•          The concentration of a material in solution, or the number of moles of solute per liter of solution, is called its molarity.

•          A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water.

•          To make a 1 molar (1 M) solution of sucrose, we’d slowly add water to 342 g of sucrose until the total volume was 1 liter and all the sugar was dissolved.

Concept 3.3 ‘Dissociation of water molecules leads to acidic and basic conditions

that affect living organisms’ [[Page.53]]

-            Occasionally, a hydrogen atom participating in a hydrogen bond between two water molecules shifts from one molecule to the other. When this happens …

•        The hydrogen atom leaves its electron behind and is transferred as a single proton – a hydrogen ion (H⁺) with the charge of 1+

•        The water molecule that lost the proton is now a hydroxide ion (OH^-) with the charge of 1-

•        The water molecule with the  extra proton is now a hydronium ion (H₃O⁺)

-         A simplified way to view this process is to say that a water molecule dissociates (separate) into a hydrogen ion and a hydroxide ion:    H₂O <==> H⁺ + OH^-

      (The double arrows indicate that this is reversible reaction that will reach a state of dynamic equilibrium when water dissociates at the same rate that it’s being re-formed from H⁺ & OH^-) 

•         At this equilibrium point, the concentration of water molecules greatly exceeds the concentrations of H⁺ and OH^-

-         In pure water, only one water molecule in every 554 million is dissociated.

•         At equilibrium, the concentration of H⁺ or OH^- is 10^-7 M (at 20^oC)

 -         Althought the dissociation of water is reversible and statistically rare, it’s very important in the chemistry of life.

-         Because hydrogen and hydroxide ions are very reactive, changes in their concentrations can drastically affect a cell’s proteins and other complex molecules.

-        Adding certain solutes, called acids and bases, disrupts the equilibrium and modifies the concentrations of hydrogen and hydroxide ions.

•          The pH scale (power Hydrogen) is used to describe how acidic or basic a solution is.

-         An acid is a substance that increases the hydrogen ion (H⁺) concentration of a solution.

•          Eg. When hydrochloric acid is added to water, hydrogen ions dissociate from chloride ions: HCl => H⁺ + Cl^-. Addition of an acid makes a solution known as an acidic solution.

-         A substance that reduces the hydrogen ion (H⁺) concentration of a solution is called a base.

•          Some bases reduce the H⁺ concentration directly by accepting hydrogen ions.

ð      Eg. Ammonia (NH₃) acts as a base when the nitrogen’s unshared electron pair attracts a hydrogen ion from the solution, creating an ammonium ion (NH₄⁺):

NH₃ + H⁺ <==> NH₄⁺

•          Other bases reduce H⁺ indirectly by dissociating to OH^-, which then combines with H⁺ to form water.

ð   Eg. Sodium hydroxide(NaOH) dissociates into its ion in water: NaOH => Na⁺ + OH^-

OH^- + H⁺ => H₂O

ð      Solutions with a higher concentration of OH^- than H⁺ are known as basic solution.

-          A solution in which the H⁺ and OH^- concentrations are equal is said to be neutral solutions.

-          Some acids and bases (eg. HCl and NaOH) are strong acids or bases because these molecules dissociate completely in water.

-          Other acids and bases (eg. NH₃) are weak acids or bases because for these molecules, the binding and release of hydrogen ions are reversible. At equilibrium, there will be a fixed ratio of products to reactants.

•           Another example of weak acid is carbonic acid: H₂CO₃ <==> HCO₃^- + H⁺. At equilibrium, 1% of the H₂CO₃ molecules will be dissociated.

-         In any solution, the product of the H⁺ and OH^- concentrations is constant at 10^-14.

-         Brackets ([H⁺] and [OH^-]) indicate the molar concentration of the enclosed substance.

•          [H⁺] [OH^-] = 10^-14

•          In a neutral solution, [H⁺] = 10^-7 and [OH^-] = 10^-7 M (10^-14 is the product of 10^-7 x 10^-7)

-                         Adding acid to a solution shifts the balance between H+ and OH^- toward H⁺ and leads to a decline in OH^-. (eg. If [H⁺] = 10^-5 M, then [OH^-] = 10^-9 M)

•          Hydroxide concentrations decline because some of the additional acid combines with hydroxide to form water.

-         Adding a base does the opposite, increasing OH^- concentration& lowering H⁺ concentration.

-         The H+ & OH^- concentrations of solutions can vary by a factor of 100 trillion or more

-         To express this variation more conveniently, the H+ and OH^- concentrations are typically expressed through the pH scale.

•          The pH scale, ranging from 1 to 14, compresses the range of concentrations by employing logarithms.

•          pH = -log [H⁺] or [H⁺] = 10^-pH

•          In a neutral solution, [H⁺] = 10-7 M, and the pH = 7.

-         The number (value) of pH decline as [H⁺] increase.

-         Although the pH scale is based on H⁺ concentration, it also implies OH^- concentration, which can be easily calculated from the product relationship.

-         A pH value less than 7 denotes an acidic solution while the pH for basic solution is above 7.

-         Most biological fluids have pH values in the range f 6 to 8 except the human stomach that has strongly acidic digestive juice with a pH of about 2.

-         Each pH unit represents a tenfold different in H+ and OH^- concentrations.

•          A small change in pH actually indicates a substantial change in H+ & OH^- concentrations

-          The chemical processes in the cell can be disrupted by slightly changes to the H+ and OH^- concentrations. (internal pH of most living cells is close to 7)

•          The presence of buffers in biological fluids allows for a relatively constant pH despite the addition of acids or bases.

•          Buffers are substances that minimize changes in the concentrations of H+ and OH^- in a solution. It works by accepting hydrogen ions from the solution when they’re in excess and donating hydrogen ions to the solution when they have been depleted.

ð      Most buffer solutions contain a weak acid & its corresponding base, which combine reversibly with hydrogen ions.

ð       One important buffer in human blood and other biological solutions is carbonic acid (H₂CO₃), which dissociates to yield bicarbonate ion (HCO₃^-) & hydrogen ion (H⁺)

ð      The chemical equilibrium between carbonic acid and bicarbonate acts as a pH regulator. The equilibrium shifts left or right as other metabolic processes add or remove H⁺ from the solution.

ð      Most other buffers are also acid-base pairs.

-         One of the most serious assaults on water quality is acid precipitation which refers to rain, snow, or fog with pH lower or more acidic than pH 5.6.

•          The acid is a product of the formation of carbonic acid from carbon dioxide and water.

•          Acid precipitation is caused primarily by the presence in the atmosphere of sulfur oxides and nitrogen oxides, gaseous compounds that react with water in the air to form strong acids, which fall to earth with rain or snow.

ð       The major source of these oxides is the burning of fossil fuels (coal, oil, and gas) in factories and automobiles. Winds carry those oxides (pollutants) away, allows it to spread from its site of origin to contaminate other areas thousands of kilometers away.

•             Acids precipitation affects and damages lakes, streams, soils, and forests.

ð       It washes away key soil buffers & plant nutrients such as calcium & magnesium ions.

ð      It increases the concentrations of compounds such as aluminum to toxic levels.

 

 

Ps. Sorry for any mistakes I made ,,

and sorry for any misunderstandings or misspell words =)



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