What is life’s chemical basis?

• Atoms – Fundamental building blocks of matter

• Nucleus– Positively charged protons– Uncharged neutrons (except for

hydrogen)

• Electrons move around the nucleus – Negatively charged

What are elements?

• Element – pure substance

consisting of atoms

– same number of protons (atomic number)

• Isotopes – atoms of same

element– differ in number

of neutrons (atomic weight)

– Living cells have ~– H- 62%– O- 24%%– C – 12%– N – 2%

Isotopes emit radiation!

• Radioisotopes – are not stable– emit particles and energy as they decay

spontaneously into other elements– Example: 14C → 14N – Tracer• Molecule with a detectable substance

attached• PET scans

How electrons are arranged?

• Electrons occupy orbitals – space around nucleus – two electrons

• Shell model – energy levels as successively larger

circles, or shells– view an atom’s electron structure

unpaired electrons • outermost shell interact

with other atoms• donate, accept, or share

electrons

Atoms are neutral!• atom with equal numbers of protons

and electrons has no net charge• Ions (positive or negative)– Atoms that have gained or lost electrons

Difference between molecules and mixtures

• Chemical bond – Attractive force that

unites atoms into a molecule

• Compounds – Molecules

consisting of two or more elements

• Mixture– Substances

intermingle but don’t bond

What happens when atoms interact?

Whether one atom will bond with others depends on number and arrangement of its

electrons

• Common interactions in biological molecules:– Ionic bond– Covalent bond– Hydrogen bond

What are ionic bonds?

• Strong association between a positive ion and a negative ion (attraction of opposite charges)

What are covalent bonds?

• Two atoms share a pair of electrons• Nonpolar covalent bond– Atoms share electrons equally

• Polar covalent bond– Electrons are shared unequally– One end slightly negative, other slightly

positive– Polar molecule has a separation of

charge

What are hydrogen bonds?

• Form between a hydrogen atom and an electronegative atom– Each with separate

polar covalent bonds

• Are not chemical bonds– Do not make atoms into

molecules – Individually weak– Collectively stabilize

structures of large molecules

How is water molecule formed?

• Water molecules are polar– Form hydrogen

bonds with other polar molecules

– Hydrophilic substances (water-loving)

– Hydrophobic substances (water-dreading)

Why is water important for life?

• Polarity gives liquid water unique properties that make life possible: – Resistance to

temperature changes

– Internal cohesion– Dissolves polar and

ionic substances

What are acids and bases?

• pH scale – hydrogen ion (H+) concentration– 0 (most acidic) to 14 (most basic or alkaline)

• pH 7 (neutral) – H+ & OH– concentrations equal

• Acids donate H+ in water– More H+ than OH-

• Bases accept H+ in water – More OH- than H+

How are salts formed?

• Compounds that dissolve easily in water, and release ions other than H+ and OH-

– Form when an acid interacts with a base– Example: NaCl

HCl + NaOH ↔ NaCl + H2O

What are buffer systems?

• A set of chemicals (a weak acid or base and its salt) that keeps the pH of a solution stable

• buffers help maintain homeostasis • processes proceed only within a

narrow pH range, usually near neutrality– Acidosis– Alkalosis– One donates ions, the other accepts

them