SPRING 2015

Chemical Principles:

Water and Polarity

Overview: A Chemical Connection to Biology(Chapter 2 and Chapter 3)

• Living organisms are subject to basic laws of physics and chemistry

• To understand living organisms you have to understand their physics and chemistry

• “Reductionist” approach

• Life is an “emergent property” of atoms and molecules

• In a nonpolar covalent bond, the atoms share the electron equally

• In a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equally

• Unequal sharing of electrons causes a partial positive or negative charge for each atom or molecule

All covalent bonds are not the same

Water is an excellent example of a compound with polar covalent bonds

–

+ +H H

O

H2O

The oxygen end is slightly negative and the hydrogen ends are slightly positive

Oxygen’s electron distribution causes the bent shape

Methane is an example of a compound with non-polar covalent bonds

• Roughly equal sharing of electrons between C and H

• Electron distribution is even

Weak Chemical Bonds

• Strong chemical bonds such as covalent bonds are not the only important bonds in living systems.

• Weak chemical bonds are also important

• Weak chemical bonds reinforce shapes of large molecules, help molecules attach to each other and provide on/off switches

• Living systems are smart! They don’t waste energy making things stronger than they need to be.

Hydrogen Bonds a key type of weak bond

• A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom

• The hydrogen atom is shared between two more electronegative atoms

• In living cells, the electronegative partners are usually oxygen or nitrogen atoms

Hydrogen bonding between water and ammonia-a simple and non-biological example

+

+

+

+

+

Water (H2O)

Ammonia (NH3)

Hydrogen bond

H-bonds are

weak, short-range,temporary,directional

Van der Waals Interactions-a less important weak bond

• If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative charge

• Van der Waals interactions are attractions between molecules that are very close together as a result of these charges

• Individually very weak but collectively strong

The polarity of water molecules results in hydrogen bonding in water

• The H bonds make water molecules stick together

• They will also stick to other polar molecules (but not nonpolar ones)

Five emergent properties of water result from H bonding and contribute to

water’s fitness for life

–Cohesion/adhesion/surface tension–Ability to moderate temperature•High specific heat•Evaporative cooling

–Expansion upon freezing–Properties as a solvent–Dissociation

Cohesion, Adhesion, Surface Tension

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

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

• Both help water flow inside organisms for example the transport of water against gravity in plants

Water-conductingcells

Adhesion

Cohesion

Directionof watermovement

• Cohesion of water molecules leads to a property called surface tension

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

Ability to Moderate Temperature-Important

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

• Water absorbs heat from warmer air and releases stored heat to cooler air-it moderates temperature changes

• It protects against large changes in temperature

Water’s 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 1ºC

• The specific heat of water is 1 cal/g/ºC

• Water resists changing its temperature because of its high specific heat

• Note: The “calories” on food packages are actually kilocalories (kcal), where 1 kcal = 1,000 cal

• Water’s high specific heat can be traced to hydrogen bonding

– Heat is absorbed when hydrogen bonds break

– Heat is released when hydrogen bonds form

• Added heat breaks hydrogen bonds in water without raising water’s temperature very much.

• high specific heat of water minimizes temperature fluctuations to within limits that permit life

Water acts as a temperature moderator-global example

• The Gulf Stream is an ocean current that carries heat from the equator to the northern latitudes of Europe

It always heats up faster than water. Why?

Efficient evaporative cooling by water is also a consequence of hydrogen bonding

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

Evaporative cooling in action

• This is the most effective way for many animals to maintain body temperature because turning water into steam gets rid of so much excess heat.

Expansion Upon Freezing-Insulation of Bodies of Water by Floating Ice• When water solidifies, each molecule is H-

bonded to four others in a rigid array: ice

• All the bonding holds the molecules relatively motionless at a certain distance apart

• In liquid water molecules move more freely and get closer together

• Water reaches its greatest density as a liquid at 4°C not as a solid at 0°C

H Bonds in Ice and Liquid Water

Hydrogenbond

Hydrogen bonds break and re-form

Molecules can approach more closely

Hydrogen bonds are stable

Molecules do not move easily

Solvent properties-The Solvent of Life

• Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily with many solutes.

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

• Non-ionic or non-polar substances cannot form a hydration shell with water.

• Water can dissolve compounds made of polar or ionic molecules: like dissolves like.

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

Hydrophilic and Hydrophobic Substances

• A hydrophilic substance is one that has an affinity for water (water-loving)

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

• Oil molecules are hydrophobic because they have relatively nonpolar bonds; carbohydrates and proteins tend to be hydrophilic because of polar covalent bonds.

IMPORTANT!!!!!

Water dissociates or ionizes

• 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+)

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

– The molecule that lost the proton is now a hydroxide ion (OH–)

• This reaction is called the dissociation of water.

• Water is in a state of dynamic equilibrium in which water molecules dissociate at the same rate at which they are being reformed

• But even pure water has some hydronium and hydroxide in it.

Hydroniumion (H3O+)

Hydroxideion (OH–)

• Though statistically rare the dissociation of water molecules has a great effect on organisms

• Changes in concentrations of H+ and OH– can drastically affect the chemistry of a cell

• In pure water hydronium=hydroxide

• Some substances shift the balance so there is more hydronium = acids.

• Others shift the balance so there is more hydroxide = bases.

• Biologists use something called the pH scale to describe whether a solution is acidic or basic

• Pure water has a pH of 7.0

• Acidic solutions have pH values less than 7

• Basic solutions have pH values greater than 7

Most liologicalfluids have pH values in the range of 6 to 8

Life unctionsbest near neutral pH 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

Therefore living

systems must maintain their

internal pH close to pH 7

Buffers

• Buffers are substances that minimize changes in concentrations of H+ and OH– in a solution

• Living systems have to contain buffers

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