The Chemical Basis of Life

Element Functions

Carbon:

………….backbone of organic molecules Hydrogen

+ Oxygen………components of water Nitrogen

………………..... Component of proteins and nucleic acids

IsotopesIsotopes are two atoms of an element that differ in number of neutronsRadioactive isotopes decay spontaneously, giving off particles and energyAll atoms of an element have the same number of protons but may differ in number of neutrons

Some applications of radioactive isotopes in biological research are:

Dating fossilsTracing atoms through metabolic processesDiagnosing medical disorders

Radioactive Iodine is Used in cancer treatment.It binds with thyroid tissue and destroys the cells in this region duringRadioactive decay.

Chemical properties of an atom are determined by its highest-energy (valence) electrons.

IMPORTANT!

Valence Electrons

Electron in the outer shellMost energetic electronsNumber and arrangement of an

atom’s valence electrons determine its chemical properties

Valence Electrons

An atom tends to lose, gain or share electrons to fill its valence shell

Electrons needed to fill valence shell:• Most atoms is 8 electrons• Hydrogen or helium= 2 electrons

Ionic Bonds

Form between a posititively charged cation and a negatively charged anion.

Are strong in the absence of water, but relatively weak in aqueous solution.

• dissassociation

Sodium atom Chlorine atom

Sodium ion (a cation)

Chloride ion (an anion)

Na+Cl–

Covalent Bonds

Strong, stable bonds

Formed when atoms share valence electrons.

Form molecules

May rearrange the orbitals of valence electrons (orbital hybridization)

Molecular Shape and

FunctionA molecule’s shape is usually very important to its functionA molecule’s shape is determined by the positions of its atoms’ valence orbitalsIn a covalent bond, the s and p orbitals may hybridize, creating specific molecular forms

Tetrahedron

s orbitalz

x

y

Three porbitals

Hybridization of orbitals

Four hybrid orbitals

Tetrahedron(a)

Biological molecules recognize and interact with each other with a specificity based on molecular shapeMolecules with similar shapes can have similar biological effects

(a) Structures of endorphin and morphine

Naturalendorphin

Endorphinreceptors

Morphine

Brain cell

Morphine

Natural endorphin

CarbonHydrogen

A molecule consists of two or more atoms held together by covalent bondsA single covalent bond, or single bond, is the sharing of one pair of valence electronsA double covalent bond, or double bond, is the sharing of two pairs of valence electrons

Electronegativity is an atom’s attraction for the electrons in a covalent bondThe more electronegative an atom, the more strongly it pulls shared electrons toward itself

In a nonpolar covalent bond, the atoms share the electron equallyIn a polar covalent bond, one atom is more electronegative, and the atoms do not share the electron equallyUnequal sharing of electrons causes a partial positive or negative charge for each atom or molecule

Chemical reactions are the making and breaking of chemical bondsThe starting molecules of a chemical reaction are called reactantsThe final molecules of a chemical reaction are called products

Reactants Reaction Products

2 H2 O2 2 H2O

Photosynthesis is an important chemical reaction Sunlight powers the conversion of carbon dioxide and water to glucose and oxygen

6 CO2 + 6 H20 → C6H12O6 + 6 O2

Some chemical reactions go to completion: all reactants are converted to productsAll chemical reactions are reversible: products of the forward reaction become reactants for the reverse reactionChemical equilibrium is reached when the forward and reverse reaction rates are equal

Hydrogen Bonds

Relatively weak bondsForm when:

• A hydrogen atom with a partial positive charge is attracted to an atom (usually O or N) with a partial negative charge

• Can be already bonded to another molecule or part of the same molecule

van der Waals Interactions

If electrons are distributed asymmetrically in molecules or atoms, they can result in “hot spots” of positive or negative chargeVan der Waals interactions are attractions between molecules that are close together as a result of these charges

•Collectively, such interactions can be strong, as between molecules of a gecko’s toe hairs and a wall surface