CHAPTER 2The Chemical Basis of Life

ATOMS AND MOLECULES The emergence of biological function starts

at the chemical level

• Everything an organism is and does depends on chemistry

• Chemistry is in turn dependent on the arrangement of atoms in molecules

Molecules and ecosystems are at opposite ends of the biological hierarchy

– Each level of organization in the biological hierarchy builds on the one below it

– At each level, new properties emerge

Elements• An element is a substance that

can’t be broken down into simpler chemical substances.

• About 25 different chemical elements are essential to life

• Carbon, hydrogen, oxygen, and nitrogen make up more that 96 percent of the mass of a human body.

Trace elements: such as iron and copper, play a vital role in maintaining healthy cells in all organisms.

2.2 CONNECTION: Trace elements are common additives to food and water

• Several chemicals are added to food for a variety of reasons– Help preserve it– Make it more nutritious– Make it look better

• Check out the “Nutrition Facts” label on foods and drinks you purchase

Copyright © 2009 Pearson Education, Inc.

Elements can combine to form compounds

• Chemical elements combine in fixed ratios to form compounds

• Example: sodium + chlorine sodium chloride

Atoms: The Building Blocks of Elements• An atom is the smallest particle of an element that

has the characteristics of that element.

• Atoms are the basic building blocks of all matter.

• Different elements have different types of atoms

Atoms consist of protons, neutrons, and electrons

• An atom is made up of protons ( + charge) and neutrons (no charge) located in a central nucleus

• The nucleus is surrounded by electrons ( - charge)

Isotopes of an Element• Atoms of each element are distinguished by a

specific number of protons– The number of neutrons may vary – Variant forms of an element are called isotopes– Some isotopes are radioactive

Connection: Radioactive isotopes can help or harm us

• Radioactive isotopes can be useful tracers for studying biological processes

• PET scanners use radioactive isotopes to create anatomical images

PET scan of a normal brainPET scan of an

alzheimer's patient

2.5 CONNECTION: Radioactive isotopes can help or harm us

• In addition to benefits, there are also dangers associated with using radioactive substances– Uncontrolled exposure can cause damage to some

molecules in a living cell, especially DNA– Chemical bonds are broken by the emitted energy,

which causes abnormal bonds to form

Copyright © 2009 Pearson Education, Inc.

Electron arrangement determines the chemical properties of an atom

• Electrons are arranged in shells– The outermost shell determines the chemical properties of an atom– In most atoms, a full outer shell holds eight electrons

• Electrons occur in energy levels called electron shells– Information about the distribution of electrons is found in the periodic

table of the elements• Atoms whose shells are not full tend to interact with other atoms

and gain, lose, or share electrons

2.6 Electron arrangement determines the chemical properties of an atom

• Atoms want to fill their outer electron shells– To accomplish this, the atom can share, donate, or

receive electrons– This results in attractions between atoms called

chemical bonds

Copyright © 2009 Pearson Education, Inc.

Ionic bonds are attractions between ions of opposite charge

• When atoms gain or lose electrons, charged atoms called ions are created– An electrical attraction between ions with opposite

charges results in an ionic bond• Sodium and chloride ions bond to form sodium

chloride, common table salt

Covalent bonds, the sharing of electrons, join atoms into molecules

• Some atoms share outer shell electrons with other atoms, forming covalent bonds– Atoms joined

together by covalent bonds form molecules

2.9 Unequal electron sharing creates polar molecules

• Atoms in a covalently bonded molecule continually compete for shared electrons– The attraction (pull) for shared electrons is

called electronegativity– More electronegative atoms pull harder

Copyright © 2009 Pearson Education, Inc.

covalent bonds

• Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule

• If electrons are shared unequally, a polar molecule is created

In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen

– This makes the oxygen end of the molecule slightly negatively charged

– The hydrogen end of the molecule is slightly positively charged

– Water is therefore a polar molecule

2.10 Hydrogen bonds are weak bonds important in the chemistry of life

• Some chemical bonds are weaker than covalent bonds

• Hydrogen, as part of a polar covalent bond, will share attractions with other electronegative atoms

– Examples are oxygen and nitrogen

• Water molecules are electrically attracted to oppositely charged regions on neighboring molecules

– Because the positively charged region is always a hydrogen atom, the bond is called a

hydrogen bondCopyright © 2009 Pearson Education, Inc.