Molecules of Life

Chapter 2

Part 1

2.1 Impacts/IssuesFear of Frying

All living things consist of the same kinds of molecules, but small differences in the ways they are put together have big effects on health

Artificial trans fats found in manufactured and fast foods raise cholesterol and increase risk of atherosclerosis, heart attack, and diabetes

Video: Fear of frying

Fear of Frying

Trans fats are made by adding hydrogen atoms to liquid vegetable oils

Fig. 2-1, p. 20

trans fatty acid

2.2 Start With Atoms

All substances consist of atoms

Atom• Fundamental building-block particle of matter

Life’s unique characteristics start with the properties of different atoms

Subatomic Particles and Their Charge

Atoms consist of electrons moving around a nucleus of protons and neutrons

Electron (e-) • Negatively charged subatomic particle that

occupies orbitals around the atomic nucleus

Charge• Electrical property of some subatomic particles• Opposite charges attract; like charges repel

Subatomic Particles in the Nucleus

Nucleus• Core of an atom, occupied by protons and

neutrons

Proton (p+)• Positively charged subatomic particle found in the

nucleus of all atoms

Neutron• Uncharged subatomic particle found in the atomic

nucleus

An Atom

Fig. 2-2a, p. 21

an atom

Elements: Different Types of Atoms

Atoms differ in numbers of subatomic particles

Element• A pure substance that consists only of atoms with

the same number of protons

Atomic number• Number of protons in the atomic nucleus• Determines the element

Elements in Living Things

The proportions of different elements differ between living and nonliving things

Some atoms, such as carbon, are found in greater proportions in molecules made only by living things – the molecules of life

Same Elements, Different Forms

Isotopes • Forms of an element that differ in the number of

neutrons their atoms carry• Changes the mass number, but not the charge

Mass number• Total number of protons and neutrons in the

nucleus of an element’s atoms

Radioactive Isotopes

Radioisotope• Isotope with an unstable nucleus, such as

carbon 14 (14C)

Radioactive decay• Process by which atoms of a radioisotope

spontaneously emit energy and subatomic particles when their nucleus disintegrates

Carbon 14: A Radioisotope

Most carbon atoms have 6 protons and 6 neutrons (12C)

Carbon 14 (14C) is a radioisotope with six protons and eight neutrons

When 14C decays, one neutron splits into a proton and an electron, and the atom becomes a different element – nitrogen 14 (14N)

Radioactive Tracers

Researchers introduce radioisotope tracers into living organisms to study the way they move through a system

Tracers• Molecules with a detectable substance attached,

often a radioisotope• Used in research and clinical testing

Why Electrons Matter

Electrons travel around the nucleus in different orbitals (shells) – atoms with vacancies in their outer shells tend to interact with other atoms• Atoms get rid of vacancies by gaining or losing

electrons, or sharing electrons with other atoms

Shell model• Model of electron distribution in an atom

Shell Models

Fig. 2-3 (top), p. 22

Fig. 2-3 (a-c), p. 22

Fig. 2-3 (a-c), p. 22

1 proton 1 21 electron

first shell hydrogen (H) helium (He)

6 8 10

second shell carbon (C) oxygen (O) neon (Ne)

11 17 18

third shell sodium (Na) chlorine (Cl) argon (Ar)

Fig. 2-3 (a-c), p. 22

Stepped Art

1 proton 1 21 electron

first shell hydrogen (H) helium (He)

A) The first shell corresponds to the first energy level, and it can hold up to 2 electrons. Hydrogen has one proton, so it has one vacancy. A helium atom has 2 protons, and no vacancies. The number of protons in each shell model is shown.

6 8 10

second shell carbon (C) oxygen (O) neon (Ne)

B) The second shell corresponds to the second energy level, and it can hold up to 8 electrons. Carbon has 6 protons, so its first shell is full. Its second shell has 4 electrons, and four vacancies.Oxygen has 8 protons and two vacancies. Neon has 10 protons and no vacancies.

11 17 18

third shell sodium (Na) chlorine (Cl) argon (Ar)

C) The third shell, which corresponds to the third energy level, can hold up to 8 electrons, for a total of 18. A sodium atom has 11 protons, so its first twoshells are full; the third shell has one electron. Thus, sodium has seven vacancies. Chlorine has 17 protons and one vacancy. Argon has 18 protons and no vacancies.

Animation: Shell models of common elements

Ions

The negative charge of an electron balances the positive charge of a proton in the nucleus

Changing the number of electrons may fill its outer shell, but changes the charge of the atom

Ion• Atom that carries a charge because it has an

unequal number of protons and electrons

Ion Formation

Fig. 2-4, p. 23

electron loss

Sodium atom

1111p+

11e–

charge: 0

Sodium ion

11p+11

charge: +110e–

electron gain

Chlorine atom

17 17p+

17e–

charge: 0

Chloride ion

18e–

17 17p+

charge: –1

Fig. 2-4a, p. 23

Fig. 2-4a, p. 23

electron gain Chlorine

atom

17 17p+

17e–

charge: 0

Chloride ion

18e–1717p+

charge: –1

Fig. 2-4b, p. 23

Fig. 2-4b, p. 23

electron loss Sodium

atom

11 11p+

11e–

charge: 0

Sodium ion

11p+11

charge: +1

10e–

Fig. 2-4, p. 23

Sodium atom

1111p+

11e–

charge: 0

electron loss

Sodium ion

11p+11

charge: +110e–

Stepped Art

Chlorine atom

17 17p+

17e–

charge: 0

electron gain

Chloride ion

18e–

17 17p+

charge: –1

Animation: How atoms bond

Animation: PET scan

Animation: The shell model of electron distribution

Animation: Subatomic particles

Animation: Atomic number, mass number

Animation: Electron arrangements in atoms

Animation: Isotopes of hydrogen

Video: ABC News: Nuclear Energy

Animation: Electron distribution

2.3 From Atoms to Molecules

Atoms can also fill their vacancies by sharing electrons with other atoms

A chemical bond forms when the electrons of two atoms interact

Chemical bond• An attractive force that arises between two atoms

when their electrons interact

From Atoms to Molecules

Molecule• Group of two or more atoms joined by chemical

bonds

Compound• Type of molecule that has atoms of more than

one element

Referring to a Molecule

Same Materials, Different Results

Animation: Building blocks of life

Ionic Bonds and Covalent Bonds

Depending on the atoms, a chemical bond may be ionic or covalent

Ionic bond• A strong mutual attraction formed between ions of

opposite charge

Covalent bond• Two atoms sharing a pair of electrons

An Ionic Bond: Sodium Chloride

p. 24

ionic bond

11 17

sodium ion (Na+) chloride ion (Cl–)

Covalent Bonds

Molecular hydrogen (H—H) and molecular oxygen (O=O)

p. 24

1 1

molecular hydrogen (H2)

8 8

molecular oxygen (O2)

Polarity

A covalent bond is nonpolar if electrons are shared equally, and polar if the sharing is unequal

Polarity• Any separation of charge into distinct positive and

negative regions

Polar and Nonpolar Covalent Bonds

Nonpolar• Having an even distribution of charge• When atoms in a covalent bond share electrons

equally, the bond is nonpolar

Polar• Having an uneven distribution of charge• When the atoms share electrons unequally, the

bond is polar

Importance of Polar Molecules

A water molecule (H-O-H) has two polar covalent bonds – the oxygen is slightly negative and the hydrogens are slightly positive – which allows water to form hydrogen bonds

p. 25

p. 25

1 8 1

water (H2O)

Hydrogen Bonds

Hydrogen bond• Attraction that forms between a covalently

bonded hydrogen atom and another atom taking part in a separate covalent bond

p. 25

hydrogen bond

Importance of Hydrogen Bonds

Hydrogen bonds form and break more easily than covalent or ionic bonds – they do not form molecules

Hydrogen bonds impart unique properties to substances such as water, and hold molecules such as DNA in their characteristic shapes

Animation: Ionic bonding

Animation: Examples of hydrogen bonds

Video: ABC News: Fuel Cell Vehicles

Animation: Sucrose synthesis

Animation: Covalent bonds

2.4 Water

All living organisms are mostly water, and all chemical reactions of life are carried out in water

Hydrogen bonds between water molecules give water unique properties that make life possible• Capacity to dissolve many substances• Cohesion (surface tension)• Temperature stability

Polarity and theUnique Properties of Water

Fig. 2-7a, p. 26

Fig. 2-7a, p. 26

slight negative charge

slight positive charge slight positive charge

Fig. 2-7b, p. 26

Fig. 2-7c, p. 26

Animation: Structure of water

Water and Solutions

Polar water molecules hydrogen-bond to other polar (hydrophilic) substances, and repel nonpolar (hydrophobic) substances

Hydrophilic (water-loving)• A substance that dissolves easily in water

Hydrophobic (water-dreading)• A substance that resists dissolving in water

Water and Solutions

Water is an excellent solvent

Solvent• Liquid that can dissolve other substances

Solute• A dissolved substance

Water and Solutions

Salts, sugars, and many polar molecules dissolve easily in water

Salt• Compound that dissolves easily in water and

releases ions other than H+ and OH-

• Example: sodium chloride (NaCl)

Water and Solutions

Water molecules surround the atoms of an ionic solid and pull them apart, dissolving it

Animation: Spheres of hydration

Temperature Stability

Temperature stability is an important part of homeostasis• Water absorbs more heat than other liquids

before temperature rises• Hydrogen bonds hold ice together in a rigid

pattern that makes ice float

Temperature• Measure of molecular motion

Cohesion

Cohesion helps sustain multicelled bodies and resists evaporation

Cohesion• Tendency of water molecules to stick together

Evaporation • Transition of liquid to gas• Absorbs heat energy (cooling effect)

2.5 Acids and Bases

Water molecules separate into hydrogen ions (H+) and hydroxide ions (OH-)

pH• A measure of the number of hydrogen ions (H+) in

a solution• The more hydrogen ions, the lower the pH

Pure water has neutral pH (pH=7)• Number of H+ ions = OH- ions

Acids and Bases

Acid• Substance that releases hydrogen ions in water• pH less than 7

Base• Substance that releases hydroxide ions (accepts

hydrogen ions) in water• pH greater than 7

A pH Scale

Fig. 2-9, p. 27

— 0 battery acid

— 1 gastric fluid

lemon juice— 2 acid rain

cola

— 3vinegar

mo

re a

cid

ic

tomatoes, wine

orange juice

— 4 bananasbeer

— 5 black coffeebread

urine, tea, typical rain

butter— 6 corn

milk— 7 pure water

— 8 seawateregg whiteblood, tears

— 9 detergentsbaking soda

Tums

— 10 hand soaptoothpaste

milk of magnesiahousehold ammonia

— 11

mo

re b

asic

— 12 hair removerbleach

— 13

— 14 drain cleaner

oven cleaner

Animation: The pH scale

Acid Rain

Sulfur dioxide and other airborne pollutants dissolve in water vapor to form acid rain

Buffer Systems

Most molecules of life work only within a narrow range of pH – essential for homeostasis

Buffers keep solutions in cells and tissues within a consistent range of pH

Buffer• Set of chemicals that can keep the pH of a

solution stable by alternately donating and accepting ions that contribute to pH

CO2 and the Bicarbonate Buffer System

CO2 forms carbonic acid in water

• CO2 + H2O → H2CO3 (carbonic acid)

Bicarbonate buffer system• Excess H+ combines with bicarbonate

• H+ + HCO3- (bicarbonate) ↔ H2CO3

Video: ABC News: Bottle Backlash

Video: ABC News: Water Use

Video: ABC News: Water Wars

3D Animation: Dissolution