Water:The Molecule of Life

The Molecule That Supports All of 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 themselves are about 70-95% water.• The abundance of water is the main reason

the Earth is habitable.

The polarity of water molecules results in hydrogen bonding.

• The water molecule is a polar molecule: The opposite ends have opposite charges

• Polarity allows water molecules to form hydrogen bonds with each other.

Animation: Water Structure

Question #1:Based on your knowledge of the polarity of water, this solute molecule is most likely

a) positively charged.b) negatively charged.c) hydrophobic.d) nonpolar.

Four properties of water thatfacilitate an environment for life:

1. Cohesive behavior2. Ability to moderate

temperature3. Expansion upon freezing4. Versatility as a solvent

Cohesion• Hydrogen bonds hold water molecules

together.• Cohesion and adhesion help in the transport

of water against gravity in plants.

Animation: Water Transport

Water’s High Surface Tension

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

Surface Tension

Moderation of Temperature

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

• Water can moderate air temperatures because it absorbs heat from warmer air and releases stored heat to cooler air.

Heat vs. 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 the average kinetic energy of molecules

Water’s High Specific Heat

Water’s high specific heat minimizes temperature fluctuations to within limits that permit life.

Heat is absorbed when hydrogen bonds break.

Heat is released when hydrogen bonds form.

Specific Heat•the amount of heat that must be absorbed or lost for 1 gram of a substance to change its temperature by 1ºC

Water’s High Heat of Vaporization

• 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.

Heat of Vaporization•Amount of heat a liquid must absorb for 1 gram to evaporate•Hydrogen bonds must be broken for water molecules to evaporate.

Insulation of Bodies of Water by Floating Ice• Ice floats in liquid water because hydrogen bonds

in ice are more “ordered,” making ice less dense.• If ice sank, all bodies of water would eventually

freeze solid, making life impossible on Earth.

Hydrophilic vs. Hydrophobic•Hydrophilic: substance with an affinity for water•Hydrophobic: substance without an affinity for water

The Solvent of Life• Solution, solvent, solute• Water is a versatile solvent due to its

polarity and because it readily forms hydrogen bonds.

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

• Water can also dissolve compounds made of nonionic polar molecules.

• Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions.

Lysozyme moleculein a nonaqueous environment.

Lysozyme moleculein an aqueous environment.

The Solvent of Life

Biological Chemistry is “Wet” Chemistry

• Most biochemical reactions occur in water.• Chemical reactions depend on collisions of

molecules and therefore on the concentration of solutes in an aqueous solution.

Determining Solute Concentration

Numbers of molecules are measured in moles.

1 mole = 6.02 x 1023 molecules1 gram = 6.02 x 1023 daltons

1 mole of substance = molecular mass in grams

Molarity is the number of moles of solute per liter of solution.

Molecular Mass•sum of all masses of all atoms in a molecule•measured in daltons•Example: sucrose (C12H22O11) (C=12, H=1, O=16)

12(12) + 22(1) + 11(16) = 342 daltons

a) 8b) 12c) 24d) 60

Question #2: How many grams of the molecule in this figure would be required to make 1 L of a 0.2 M solution of the molecule?(Carbon=12, Oxygen=16, Hydrogen=1)

2(12) + 4(1) + 2(16) = 60 g/mol

60 g/mol x 0.2 mol/L = 12 g/L

C2H4O2

Dissociation of water molecules leads to acidic and basic conditions that affect

living organisms• A hydrogen atom in a hydrogen bond between two water molecules can shift from one to the other

• The hydrogen atom leaves its electron behind and is transferred as a proton, or hydrogen ion (H+)

Hydroniumion (H3O+)

Hydroxideion (OH–)

Though statistically rare, the dissociation of water molecules has a great effect on organisms because changes in concentrations of H+ and OH- can drastically affect the chemistry of a cell.

Effects of changes in pH

• Biologists use the pH scale to describe how acidic or basic a solution is.

Pure water[H+] = [OH-]

Acids and Bases•Acid: any substance that increases the H+ concentration

of a solution•Base: is any substance that reduces the H+ concentration

of a solution

The pH Scale

• The pH of a solution is determined by the relative concentration of hydrogen ions.

• Acidic solutions have pH values less than 7.

• Basic solutions have pH values greater than 7.

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

pH Scale0

1

2

3

4

5

6

7

8

9

10

11

12

13

14Oven cleaner

Household bleach

Household ammonia

Milk of magnesia

Seawater

Pure waterHuman blood

UrineRainwater

Black coffee

Tomato juice

Vinegar, beer, wine,cola

Digestive (stomach)juice, lemon juice

Battery acid

Neutral[H+] = [OH–]

Inc

rea

sin

gly

Ac

idic

[H+]

> [

OH

– ]In

cre

as

ing

ly B

as

ic[H

+]

< [

OH

– ]

The pH Scale• Compresses the range

of H+ and OH− concentrations by employing logarithms.

• The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration

• pH = −log[H+]• [H+][OH−] = 10−14

[10−7][10−6] = 10−14

[10−8][10−7] = 10−14

[10−9][10−5] = 10−14

[10−10][10−4] = 10−14

[10−11][10−3] = 10−14

[10−12][10−2] = 10−14

[10−13][10−1] = 10−14

[10−14][10−0] = 10−14

[10−6][10−8] = 10−14

[10−5][10−9] = 10−14

[10−4][10−10] = 10−14

[10−3][10−11] = 10−14

[10−2][10−12] = 10−14

[10−1][10−13] = 10−14

[10−0][10−14] = 10−14

Question 3: The following are pH values: cola-2; orange juice-3; coffee-5; human blood-7.4.

Which of these liquids has the highest concentration of OH-? a) cola

b) orange juicec) coffeed) human blood

Question 4:

If the pH of a solution is increased from pH 8 to pH 9, it means that the

a) concentration of H+ is 10 times greater than what it was at pH 8.

b) concentration of H+ is 100 times less than what it was at pH 8.

c) concentration of OH- is 10 times greater than what it was at pH 8.

d) concentration of OH- is 100 times less than what it was at pH 8.

Question 5:

Acid precipitation has lowered the pH of a particular lake to 4.0. What is the hydroxide ion concentration of the lake?

a) 10-7 Mb) 10-4 Mc) 10-10 Md) 10-14

M

[10−7][10−6] = 10−14

[10−8][10−7] = 10−14

[10−9][10−5] = 10−14

[10−10][10−4] = 10−14

[10−11][10−3] = 10−14

[10−12][10−2] = 10−14

[10−13][10−1] = 10−14

[10−14][10−0] = 10−14

[10−6][10−8] = 10−14

[10−5][10−9] = 10−14

[10−4][10−10] = 10−14

[10−3][10−11] = 10−14

[10−2][10−12] = 10−14

[10−1][10−13] = 10−14

[10−0][10−14] = 10−14

Buffers• Buffers are substances

that minimize changes in concentrations of H+ and OH- in a solution

• Most buffers consist of an acid-base pair that reversibly combines with H+