Chapter 3 Water: Supports All Life Water is the biological medium - - PDF document

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Chapter 3

Water and the Fitness of the Environment Slide 2 / 44

Water: Supports All Life

Water is the biological medium on Earth All living organisms require water more than any other substance Most cells are surrounded by water, and cells consist

• f about 70-95% water

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Three-quarters of the Earth’s surface is submerged in water

Water and Earth

The abundance of water is the main reason the Earth is habitable

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The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule due to uneven distribution of electrons. The opposite ends of a water molecule have opposite charges.

Hydrogen Bonding of water

+ +

• Slide 5 / 44

The polarity of water molecules allows them to form hydrogen bonds with each other and contributes to the various properties water exhibits

Polarity

click here for an animation about water

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A hydrogen bonds. B nonpolar covalent bonds. C polar covalent bonds. D ionic bonds.

E

van der Waals interactions.

1 In a single molecule of water, the two hydrogen atoms are bonded to a single oxygen atom by

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2 The slight negative charge at one end of one water molecule is attracted to the slight positive charge of another water molecule. What is this attraction called?

A a covalent bond B a hydrogen bond C an ionic bond D a hydrophilic bond

E

a hydrophobic bond

Slide 8 / 44 Four properties of water

Cohesive behavior Ability to moderate temperature Expansion upon freezing Versatility as a solvent All contribute to Earth’s fitness for life

Slide 9 / 44 Cohesion

Water molecules exhibit cohesion Cohesion is the bonding of a high percentage of the molecules to neighboring molecules Cohesion is due to hydrogen bonding

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Cohesion helps pull water up through the microscopic vessels of plants Adhesion of water to plant cell walls also helps counteract the force of gravity

100 mm

Water conducting cells

100 µm

Cohesion and Adhesion

click here for a video on cohesion and adhesion

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Surface tension is related to cohesion It is a measurement of how hard it is to break the surface of a liquid

Surface Tension Slide 12 / 44

3 What determines the cohesiveness of water molecules?

A

hydrophobic interactions

B

nonpolar covalent bonds C ionic bonds

D hydrogen bonds

E

both A and C

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4 Which of the following is possible due to the high surface tension of water?

A Lakes don't freeze solid in winter, despite low temperatures. B A water strider can walk across the surface of a small pond. C Organisms resist temperature changes, although they give off heat due to chemical reactions.

D Water can act as a solvent.

E

The pH of water remains exactly neutral.

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5 What do cohesion, surface tension, and adhesion have in common with reference to water?

A

All increase when temperature increases.

B

All are produced by ionic bonding. C All are properties related to hydrogen bonding. D All have to do with nonpolar covalent bonds. E C and D only

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Moderation of Temperature

Water moderates air temperature by absorbing heat from warmer air and releasing the stored heat to cooler air. Water can absorb or release a large amount to heat with

• nly a slight change in its own temperature.

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Review: Heat and Temperature

Kinetic energy is the energy of motion Heat is a measure of the total amount of kinetic energy due to molecular motion Temperature measures the intensity of heat due to average KE of molecules

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Water’s High Specific Heat

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

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Water has a high specific heat, which allows it to minimize temperature fluctuations to within limits that permit life

Hydrogen Bonding and Specific Heat

Heat must be absorbed in order to break hydrogen bonds Heat is released when hydrogen bonds form

Slide 19 / 44 Evaporation

Evaporation is the transformation of a substance from a liquid to a gas Heat of vaporization is the quantity of heat a liquid must absorb for 1 gram of it to be converted from a liquid to a gas

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Evaporative cooling is a process in which as liquid evaporates, its remaining surface cools Evaporative cooling is due to water’s high heat of vaporization Evaporative cooling of water helps stabilize temperatures in living things and in bodies of water

Evaporative Cooling Slide 21 / 44

6 Water's high specific heat is mainly a consequence of the

A

small size of the water molecules. B high specific heat of oxygen and hydrogen atoms. C absorption and release of heat when hydrogen bonds break and form.

D fact that water is a poor heat conductor. E inability of water to dissipate heat into dry air.

Slide 22 / 44 Floating Ice

The hydrogen bonds in ice are more “ordered” than in liquid water, making ice less dense, and able to float.

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Water can also interact with large polar molecules such as proteins.

Ionic and polar regions on the protein’s surface attract water molecules.

The Solvent of Life

Lysozyme molecule in a nonaqueous environment. Lysozyme molecule in an aqueous environment such as tears or saliva.

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7 Ice is lighter and floats in water because it is a crystalline structure in which each water molecule is bonded to a maximum of four other water molecules by which kind of bond?

A ionic B

hydrogen

C

covalent D A and C only

E

A, B, and C

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Hydrophilic and Hydrophobic Substances

A hydrophilic substance has an affinity for water A hydrophobic substance does not have an affinity for water

Slide 27 / 44 Solute Concentration in Aqueous Solutions

Most biochemical reactions occur in water Therefore, it is important to learn to calculate the concentration of solutes in an aqueous solution

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A mole represents an exact number of molecules of a substance, 6.02 x 1023 Molarity is the number of moles of solute per liter of solution Concentrations of aqueous solutions may also be measured in mass percent, e.g. 10% salt solution

Aqueous Solutions Calculations Slide 29 / 44

Dissociation of water molecules leads to acidic and basic conditions that affect living organisms Water can dissociate into hydronium ions and hydroxide ions Changes in the concentration of these ions can have a great affect on living organisms

Dissociation of water Slide 30 / 44

H Hydronium ion (H3O+)

O

Hydroxide ion (OH

–)

H H H H H

+

–

O

click here for an animation about hydrogen bonding

Slide 31 / 44 Effects of Changes in pH

Concentrations of H+ and OH- are equal in pure water Adding certain solutes, called acids and bases, modifies the concentrations of H+ and OH- Biologists use the pH scale to describe how acidic or basic (or alkaline) a solution is

Slide 32 / 44 Acids and Bases

A base is any substance that reduces the hydrogen ion concentration of a solution

H+

An acid is any substance that increases the hydrogen ion concentration of a solution.

OH-

Slide 33 / 44 The pH Scale

The pH of a solution is determined by the relative concentration of hydrogen ions The pH of acids are low, below 7 The pH of bases are above 7 up to 14 Most biological fluids have pH values around 6-8

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The pH scale and pH values

• f various aqueous solutions

Increasingly Acidic [H+] > [OH–] Increasingly Basic [H+] < [OH–] Neutral [H+] = [OH–]

The pH Scale Slide 35 / 44 Buffers

The internal pH of most living cells must remain close to pH 7 Buffers are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution They consist of an acid-base pair that reversibly combines with hydrogen ions

Slide 36 / 44 Acid Precipitation

Acid precipitation refers to rain, snow, or fog with a pH lower than pH 5.6 Caused primarily by the mixing of different pollutants with water in the air

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Acid precipitation can damage life in Earth’s water and soil ecosystems, such as lakes, streams and forests

1 2 3 4 5 6 7 8 9 10 11 12 13 14

More acidic Acid rain Normal rain More basic

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8 A given solution contains 0.0001(10-4) moles of hydrogen ions [H+] per liter. Which of the following best describes this solution?

A acidic: H+ acceptor B basic: H+ acceptor

C acidic: H+ donor D basic: H+ donor

E neutral

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9 What would be the pH of a solution with a hydrogen ion [H+] concentration of 10-8 M? pH = - log [H+]

A pH 2 B pH 4 C pH 6 D pH 8 E pH 10

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10 What would be the pH of a solution with a hydroxide ion [OH-] concentration of 10-12 M? Kw = 10-14

A pH 2 B pH 4 C pH 10 D pH 12 E pH 14

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11 Which of the following solutions has the greatest concentration of hydrogen ions [H+]?

A gastric juice at pH 2 B vinegar at pH 3 C tomato juice at pH 4 D black coffee at pH 5 E household bleach at pH 12

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12 Which of the following statements is true about buffer solutions?

A They maintain a constant pH when bases are added to them but not when acids are added to them. B They maintain a constant pH when acids are added to them but not when bases are added to them. C They maintain a constant pH of exactly 7 in all living cells and biological fluids. D They maintain a relatively constant pH. E They are found only in living systems and biological fluids.

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13 Buffers are substances that help resist shifts in pH by

A releasing H+ in acidic solutions. B donating H+ to a solution when they have been depleted. C releasing OH- in basic solutions. D accepting H+ when they are in excess.

E both B and D

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