The Chemical Context of Life Campbell and Reece Chapter 2 The Chemical Context of Life

Elements & Compounds Element: substance that cannot be broken down to other substances by chemical reactions 92 natural-occurring elements

Compounds 2 or more different elements combined in a fixed proportion compound has different properties than properties of individual elements that make up the compound

Compounds are pure substances

Essential Elements of life those of the 92 that organisms must have to live & reproduce species specific; humans need 25 elements plants need 17

Essential elements 96% of living matter (by mass) made up of: Oxygen Carbon Hydrogen Nitrogen

4% of living matter (by mass) made up of: Calcium Phosphorus Potassium Sulfur Chlorine Magnesium

Trace Elements make up <1% organisms require these in minute amounts to name only a few: Iron Iodine (in vertebrates only) Zinc Selenium Manganese

Toxic elements Arsenic: Mercury

Properties of elements

Atomic mass 1 proton or 1 neutron = 1 amu 1 amu = 1.7 x 10² g = 1 dalton mass of e- ignored

Mass number #protons + #neutrons

isotopes different forms of same element: same # protons different # of neutrons occurs naturally Atomic Mass see on periodic table is weighted mass average of all isotopes

Radioactive Isotopes nucleus spontaneously decays giving off particles & energy used medically to measure an organ’s function, monitor tumor growth

Radioactive isotopes decay hazardous to living organisms by damaging cellular molecules

Energy levels of electrons electrons only subatomic particle involved in chemical reactions e- have PE due to their position around nucleus of atom electron’s (-) charge attracted to (+) proton the farther out e- is from nucleus, the more PE that e- has

Electron energy levels

Valence electrons electrons in outermost shell if valence shell completely filled: atom is nonreactive (inert like noble gases)

Electron orbitals

Chemical bonds formed by interactions between atoms with incomplete valence orbitals

Covalent bonds sharing of a pair of valence electrons by 2 atoms 2 or more atoms held together by covalent bonds = molecule

Types of covalent bonds 1 pair e- shared = single covalent bond 2 pair e- shared = double covalent bond 3 pair e- shared = triple covalent bond hydrogen gas

Nonpolar covalent bond shared pair of electron are shared evenly between the 2 atoms

electronegativity attraction of a particular atom for the electrons in covalent bond greater the electronegativity on 1 atom over the other in the covalent bond, the more likely the bond will be a polar covalent bond

Polar covalent bonds electrons of the bond are not shared equally bonds vary in their polarity: depends on the relative electronegativity of the 2 atoms

Ionic bonds transfer of e- from metal  nonmetal creates cations (+) & anions (-) opposite charges attract forming ionic bond compounds formed by ionic bonds called ionic compounds or salts as solids form crystals, very strong bonds as liquids completely dissociate

Weak chemical bonds in living organisms: most of the strongest chemical bonds are covalent weak bonds also important: hydrogen bonds, van der Waals, ionic bonds in water maintain shape of proteins, DNA reversibility key to their importance

Hydrogen bonds water: H held to O in polar covalent bonds partial (+) charge on H makes it attract the partial (-) charge on O from another water molecule

Van der Waals forces nonpolar covalent bonds may not have electrons symmetrically distributed @ any given instant in time a region of the molecule may be slightly (-) or (+)  ever-changing regions of (+) or (-) charge occurs only when atoms or molecules are very close together

Molecular shape & function shape of a molecule key to its function in cell shape of a molecule with >2 atoms determined by positions of atom’s orbitals

Molecular shape determines how biological molecules recognize & respond to each other with specificity 2 molecules temporarily forming weak bond can only happen if their shapes are complimentary to each other

Molecular shape because morphine and endorphins have same shape that fits into endorphin receptors morphine is able to create same response as endorphins: pain relief & euphoria during stress

Endorphins & morphine

Chemical reactions chemical bonds of reactants broken, chemical bonds of products formed in a chemical reaction

Chemical equilibrium