Lectures by Tom Chen

The Chemical Basis of LifeThe Chemical Basis of Life

Atoms, elements, compounds, & molecules

How molecules interact

Water is essential for life

Acids, bases and salts

Chemicals play many more roles in life

– Making up our bodies, those of other organisms, and the physical environment

ELEMENTS, ATOMS, AND MOLECULES

2.1 Living organisms are composed of about 25 chemical elements

• About 25 different chemical elements

– Are essential to life

• Trace elements

– Are essential to life, but occur in minute amounts

Atom: A unit of matter, the smallest unit of an element.

Element: A substance composed of atoms having an identical number of protons in each nucleus. Elements cannot be reduced to simpler substances by normal chemical means.

Carbon, hydrogen, oxygen, and nitrogen Make up the bulk of living matter

Table 2.1

2.2: Trace elements are common additives to food and water

• Dietary deficiencies in trace elements– Can cause various physiological

conditions

Figure 2.2A

Figure 2.2B

2.3 Elements can combine to form compounds• Chemical elements

– Combine in fixed ratios to form compounds

Sodium Chlorine Sodium Chloride

Figure 2.3

Subatomic Particles

• An atom is made up of protons and neutrons

Located in a central nucleus

• The nucleus is surrounded by electrons

• Arranged in electron shells

Figure 2.4A

+

+

– –

+

–

2

2

2

Protons

Neutrons

Electrons

Massnumber = 4

++

2e–

Electroncloud

Nucleus

2.4 Atoms consist of protons, neutrons, and electrons

• Atom: The smallest particle of matter that still retains the properties of an element

Differences in Elements

• Atoms of each element

– Are distinguished by a specific number of protons

Figure 2.4B

+

–

6

6

6

Protons

Neutrons

Electrons

Massnumber = 12

+ +

6e–

Nucleus

Electroncloud

Isotopes The number of neutrons in an atom may vary

• Variant forms of an element are called isotopes • Some isotopes are radioactive

Table 2.4

2.5 Radioactive isotopes can help or harm us

• Radioactive isotopes are useful as tracers

- For monitoring the fate of atoms in living organisms

Medical Diagnosis

• Radioactive tracers are often used for diagnosis

- In combination with sophisticated imaging instruments

Figure 2.5A Figure 2.5B

Basic Research

• Biologists often use radioactive tracers

- To follow molecules as they undergo chemical changes in an organism

How brain function is studied now?PET, CAT, MRI, etc.

Music Appreciation

Hearing, speaking, seeing, thinking about words

2.6 Electron arrangement determines the chemical properties of an atom

• Electrons in an atom

– Are arranged in shells, which may contain different numbers of electrons

Hydrogen (H)Atomic number = 1

Electron

Carbon (C)Atomic number = 6

Nitrogen (N)Atomic number = 7

Oxygen (O)Atomic number = 8

Outermost electron shell (can hold 8 electrons)

First electron shell (can hold 2 electrons)

Figure 2.6

Atoms whose shells are not full, tend to interact with other atoms and gain, lose, or share electrons. These interactions form chemical bonds

Types of chemical bonds:

1. Ionic bond

2. Covalent bond

3. Hydrogen bond

Ionic bonds are attractions between ions of opposite charge

• When atoms gain or lose electrons

– Charged atoms called ions are created

Transfer of electron

NaSodium atom

ClChlorine atom Na+

Sodium ion

Cl–

Chloride ion

Sodium chloride (NaCl)

Na Cl ClNa

+ ––

–

Covalent bonds join atoms into molecules through electron sharing

• In covalent bonds, two atoms share one or more pairs of outer shell electrons, forming molecules

Sharing of electrons may be EQUAL or UNEQUAL

• A molecule is nonpolar

– When its covalently bonded atoms share electrons equally

• In a polar molecule

– Electrons are shared unequally between atoms, creating a polar covalent bond

(–) (–)

(+) (+)

O

HH

Figure 2.9

Hydrogen

bond (+)

(+)

H

H(+)

(+)

(–)

(–)

(–)(–)

O

Hydrogen bonds are weak bonds important in the chemistry of life

• The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules

Hydrogen bonding occurs in other biologically important compounds such as proteins and DNA.

Used in food preparation

Used for hygienic purposes

Can be used for recreation

Supplies body with essential

minerals

Helps in maintaining homeostasis

Source of oxygen

Surrounds ¾ of the Earth’s surface

Serves as habitat for

aquatic organisms

Makes up almost 70% of human’s

body

Source of energy e.g

hydroelectric power plants

Essential Minerals

•are inorganic substances that exist naturally on and in the earth

•14 minerals that have been shown by research to be essential to human health are: calcium, chromium, copper, fluorine, iodine, iron, magnesium, manganese, molybdenum, phosphorus, potassium, selenium, sodium and zinc

• “Essential” because they comprise human tissue

•Good sources of essential minerals include fruits, vegetables, meats, nuts, beans and dairy products.

• Varying amounts of minerals can be obtained from WATER

Importance of minerals in body

• crucial to the growth and production of bones, teeth, hair, blood, nerves, skin, vitamins, enzymes and hormones

•For healthy functioning of nerve transmission

• blood circulation

•cellular integrity

• energy production and muscle contraction

Water is the solvent of life

• Polar or charged solutes dissolve when water molecules surround them, forming aqueous solutions

+

–

–

–

–

–

–

–

–

––

++

+

++

Na+

+

Na+

Cl–

Ion insolution Salt

crystal

Cl–

WATER’S LIFE-SUPPORTING PROPERTIES

WATER’S LIFE-SUPPORTING PROPERTIES

Hydrogen bonds make liquid water cohesive

• Due to hydrogen bonding

– Water molecules can move from a plant’s roots to its leaves by adhesion

• Insects can walk on water due to surface tension

– Created by cohesive water molecules

Figure 2.11

Water’s hydrogen bonds regulate temperature

• Water’s ability to store heat (high heat capacity)

– Moderates body temperature and climate

• It takes a lot of energy to disrupt hydrogen bonds

- So water is able to absorb a great deal of heat energy without a large increase in temperature

• As water cools

- A slight drop in temperature releases a large amount of heat

WATER’S LIFE-SUPPORTING PROPERTIES

Figure 2.12

Water has high heat of vaporization

A water molecule takes energy with it when it evaporates

• Leading to evaporative cooling

WATER’S LIFE-SUPPORTING PROPERTIES

Storm Turns Focus to Global Warming

Is the rash of powerful Atlantic storms in recent years a symptom of global warming?some scientists have maintained that the rise in mean global temperatures over the last half a century — a well-documented trend widely linked to human activities such as the burning of fossil fuels — will inevitably have an effect on storms.

Liquid waterHydrogen bonds

constantly break and re-form

IceHydrogen bonds are stable

Hydrogen bond

Ice is less dense than liquid water

• Hydrogen bonds hold molecules in ice farther apart than in liquid water

• Ice is therefore less dense than liquid water which causes it to float

• Floating ice protects lakes and oceans from freezing solid

Living organisms are sensitive to acidic and basic conditions

Acid = An ionic compound that releases H+ ions in solution

Base = An ionic compound that accepts H+ ions or produces OH- ions in solution

• Acidity is measured on the pH scale from 0 (most acidic) to 14 (most basic or alkaline)

The pH scale

Basic solution

Oven cleaner

Acidic solution

Neutral solution

pH scale

0

1

2

3

4

5

6

7

8

9

10

11

12

Lemon juice, gastric juice

Grapefruit juice, soft drink

Tomato juice

Human urine

Pure waterHuman blood

Seawater

Milk of magnesia

Household ammonia

Household bleach

Incr

easi

ngly

AC

IDIC

(Hig

her

conc

entr

atio

n of

H+)

NEUTRAL[H+]=[OH–]

H+ H+

H+OH– H+

H+ H+

OH– H+ H+

OH–

OH–

H+ H+OH–

OH– OH–

H+ H+H+

OH–

OH–

OH– OH–

OH–OH– H+

Incr

easi

ngly

BA

SIC

(Low

er c

once

ntra

tion

of H

+)

OH–

H+

14

13

Figure 2.15

• The pH of most cells

Is kept close to 7 (neutral) by buffers

• Buffers are substances that resist pH change

Acid precipitation threatens the environment

• Some ecosystems are threatened by acid precipitation

• Acid precipitation is formed when air pollutants from burning fossil fuels

– Combine with water vapor in the air to form sulfuric and nitric acids

– Can kill trees and damage buildings

Figure AFigure B

Salts

Are formed when an acid and a base react with one another neutralizing the concentration of H+ ions

the process is called neutralization

Acid Base Salt

1. Life processes occur in neutral pH.

2. Acids, bases and salts when dissolved in water serve as electrolytes.

3. Salts are abundant in our body in the form of different body fluids like sweat and tears.

Importance of acids, bases and salts in living organisms

Electrolytes

•are the elements necessary for electrochemical activity in our body.

•Water (H20) – and the three minerals sodium, potassium and chloride are all necessary for the transmission of electrical impulses between cells

CHEMICAL REACTIONS

2.17 Chemical reactions change the composition of matter• In a chemical reaction

- Reactants interact, atoms rearrange, and products result

2 H2O2 2 H2O

Figure 2.17A

This is the general equation for photosynthesis—the process of capturing sunlight energy and converting it to chemical energy. Which of the following are the reactants of this reaction?

A. C6H12O6 and O2.

B. CO2 and H2O.