The Chemical Level of Organization

Introduction

Since chemicals compose your body and all body activities are chemical in nature, it is important to become familiar with the language and fundamental concepts of chemistry.

How Matteris Organized

Chemical Elements • All forms of matter are composed of chemical

elements which are substances that cannot be split into simpler substances by ordinary chemical means.

• Elements are given letter abbreviations called chemical symbols.

• Trace elements are present in tiny amounts

Structure of Atoms

• Units of matter of all chemical elements are called atoms. An element is a quantity of matter composed of atoms of the same type.

• Atoms contain:

– Nucleus: protons (p+) & neutrons (neutral charge) – Electrons (e-) surround the nucleus as a cloud

(electron shells are designated regions of the cloud)

Representations of the Structure of an Atom

Protons (p+) J Neutrons (n°) Electrons (e-)

(a) Electron cloud model (b) Electron shell model

Atomic Number and Mass Number

• Atomic number is number of protons in the nucleus.

• Mass number is the sum of its protons and neutrons.

Atomic Mass

• Mass is measured as a dalton (atomic massunit) – Neutron has mass of 1.008 daltons

– proton has mass of 1.007 daltons

– electron has mass of 0.0005 dalton

• Atomic mass (atomic weight) is close to the mass

number of its most abundant isotope.

Ions, Molecules, & Compounds

• Ions – an atom that gave up or gained an electron – written with its chemical symbol and (+) or (-)

• Molecule

– atoms share electrons – written as molecular formula showing the number

of atoms of each element (H2O)

Free Radicals

• A free radical is an electrically charged atom or group of atoms with an unpaired electron in its outermost shell

• Unstable and highly reactive; can become stable

– by giving up an electron – taking an electron from another molecule – Antioxidants are substances that inactivate oxygen-

derived free radicals

Chemical Bonds

• The atoms of a molecule are held together by forces of attraction called chemical bonds.

• The likelihood that an atom will form a chemical bond with another atom depends on the number of electrons in its outermost shell, also called the valence shell.

Ionic Bonds

• When an atom loses or gains a valence electron, ions are formed. – Positively and negatively charged ions are

attracted to one another. – Cations are positively charged ions that have given

up one or more electrons (they are electron donors).

– Anions are negatively charged ions that have picked up one or more electrons that another atom has lost (they are electron acceptors).

Covalent Bonds • Covalent bonds are formed by the atoms of molecules

sharing one, two, or three pairs of their valence electrons. – Covalent bonds are common and are the strongest

chemical bonds in the body. – Single, double, or triple covalent bonds are formed by

sharing one, two, or three pairs of electrons, respectively.

• Covalent bonds may be nonpolar or polar. – In a nonpolar covalent bond, atoms share the electrons

equally; one atom does not attract the shared electrons more strongly than the other atom

Polar Covalent Bonds

• Unequal sharing of electrons between atoms. In a water molecule, oxygen attracts the hydrogen electrons more strongly

Hydrogen Bonds

• Approximately 5% as strong as covalent bonds • Useful in establishing links between molecules

or between distant parts of a very large molecule

• Large 3-D molecules are often held together by a large number of hydrogen bonds.

• are weak intermolecular bonds; they serve as links between molecules.

Chemical Reactions

• New bonds form and/or old bonds are broken. • Metabolism is “the sum of all the chemical

reactions in the body.” • Law of conservation of energy

– The total mass of reactants equals the total mass of the products.

Types of Chemical Reactions

• Synthesis reactions -- Anabolism • Decomposition reactions-- Catabolism • Exchange reactions • Reversible reactions

Inorganic Compounds and Solutes

• Inorganic compounds usually lack carbon and are simple molecules

• whereas organic compounds always contain carbon and hydrogen, usually contain oxygen, and always have covalent bonds.

Concept of pH • pH scale runs from 0 to 14 (concentration of H+ in

moles/liter) • pH of 7 is neutral (distilled water -- concentration of

OH- and H+ are equal) • pH below 7 is acidic ([H+] > [OH-]). • pH above 7 is alkaline ([H+] < [OH-]). • pH is a logarithmic scale

• Example: a change of two or three pH units

– pH of 1 contains 10x10=100 more H+ than pH of 3 – pH of 8 contains 10x10x10=1000 more H+ than pH of 11

The pH Scale

Maintaining pH: Buffer Systems

• The pH values of different parts of the body are maintained fairly constant by buffer systems, which usually consist of a weak acid and a weak base. – convert strong acids or bases into weak acids or

bases.