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: 14 C → 14 N – 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 b onds? 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 + H 2 O

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