Water and pH: importance of water. Most cells are surrounded by water, and cells themselves are...

Water and pH:importance of water

• Most cells are surrounded by water, and cells themselves are about 70–95% water

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 3.2: Water properties

• Collectively, hydrogen bonds hold water molecules together, a phenomenon called cohesion

• Cohesion helps the transport of water against gravity in plants

• Adhesion is an attraction between different substances, for example, between water and plant cell walls

Animation: Water TransportAnimation: Water Transport


Fig. 3-2

Hydrogenbond

–H

+

H

O

——

——

+ +

+

–

–

–

Cohesion

Fig. 3-3

Water-conductingcells

Adhesion

Cohesion

150 µm

Directionof watermovement

Surface Tension

• Surface tension is a measure of how hard it is to break the surface of a liquid

• Surface tension

is related to cohesion


Regulation of Temperature• Water absorbs heat from warmer air and releases

stored heat to cooler air

• Water can absorb or release a large amount of heat with only a slight change in its own temperature because it has a high specific heat.


San Diego 72°

40 miles

Pacific Ocean

70s (°F)

80s

90s

100s

Santa Barbara 73°

Los Angeles (Airport) 75°

Burbank90°

San Bernardino100°

Riverside 96°Santa Ana 84° Palm Springs

106°

Evaporative Cooling• As a liquid evaporates, its remaining surface cools, a

process called evaporative cooling

• Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water


Water- Universal Solvent• A solution is a liquid that is a homogeneous

mixture of substances (Most biological chemical reactions take place in water)

• A solvent is what things dissolve in

• The solute is the substance that is dissolved


Cl–

Na

Cl–

++

+

++

+++

––

––

––

––

Na+

––

–

+

• Water is a versatile solvent due to its polarity, it dissolves many things for the body (vitamins, hormones, waste)

• When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell


Hydrophilic vs. Hydrophobic• A hydrophilic substance is one that has an affinity for

water

• A hydrophobic substance is one that does not have an affinity for water

• Oil molecules are hydrophobic because they have relatively nonpolar bonds


Insulation by Ice• Ice floats in liquid water because hydrogen

bonds in ice create a crystal like structure. So, ice is less dense than water.

• If ice sank, all bodies of water would eventually freeze solid, making life impossible on Earth


Concept 3.3: Acids, Bases and pH– Water is in a state of dynamic equilibrium in which

water molecules can break up and reform.

– The molecule with the extra hydrogen is now a hydronium ion (H3O+), though it is often represented as H+ (Acids donate H+)

– The molecule that lost the hydrogen is now a hydroxide ion (OH–) (Bases accept H+)


Hydroniumion (H3O+)

Hydroxideion (OH–)

2H2O

H

H

H

H

H

H

H

H

OOOO

• pH measures the H+ concentration in a solution.

• Acidic solutions have pH values less than 7 (lots of H+)

• Basic solutions have pH values greater than 7 (little H+ but lots of OH-)

• Most biological fluids have pH values in the range of 6 to 8

• Water has a pH of 7


The pH Scale

Fig. 3-9

Neutral solution

Acidic solution

Basic solution

OH–

OH–

OH–

OH–

OH–OH–

OH–

H+

H+

H+

OH–

H+ H+

H+ H+

OH–

OH–

OH–OH–

H+

OH–

H+

H+

H+

H+

H+

H+

H+

OH–

Neutral [H+] = [OH–]

Incr

easi

ng

ly A

cid

ic [

H+]

> [

OH

–]

Incr

easi

ng

ly B

asic

[H

+]

< [

OH

–]

pH Scale0

1

2

3

4

5

6

7

8

Battery acid

Gastric juice,lemon juice

Vinegar, beer,wine, cola

Tomato juice

Black coffee

Rainwater

Urine

SalivaPure water

Human blood, tears

Seawater

9

10

Milk of magnesia

Household ammonia

Householdbleach

Oven cleaner

11

12

13

14

pH in the Body

• The internal pH of most living cells must remain close to pH 7

• Water helps to regulate pH level in the body (Kidneys)


Threats to Water Quality on Earth

• Acid precipitation refers to rain, snow, or fog with a pH lower than 5.6 (from fossil fuels)

• Acid precipitation can damage life in forests, lakes, streams, and oceans (coral reefs have lack of calcification)

• Effects of acid precipitation on soil chemistry are contributing to the decline of some forests


Fig. 3-10

Moreacidic

0

AcidrainAcidrain

Normalrain

Morebasic

123456789

1011121314

Fig. 3-11a

You should now be able to:

1. List and explain the four properties of water that emerge as a result of its ability to form hydrogen bonds

2. Distinguish between the following sets of terms: hydrophobic and hydrophilic substances; a solute, a solvent, and a solution

3. Define acid, base, and pH


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