Chapter 2 The Chemical Basis Of Life

Chapter 2 The Chemical Basis Of Life - (BIO201 MATERIAL...

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C HAPTER  2  T HE  C HEMICAL  B ASIS  O F  L IFE  (BIO201 M ATERIAL   IN  B LUE ,   BIO211  MATERIAL   IN  R ED MATERIAL   REQUIRED   FOR   NEITHER   COURSE   IN   BLACK I   CAN T   CONTROL   THE   COLOR   OF   ILLUSTRATIONS ).  T HIS   OUTLINE   WAS   MADE   FROM   AN   EARLIER   VERSION   OF   YOUR   TEXT   AND   MAY   NOT   CORRESPOND   EXACTLY . Covalent Bonds I. Molecular atoms are joined together by covalent bonds in which electron pairs are shared between atoms A. Formation of a covalent bond is governed by the basic principle that atoms are most stable with a full outer electron shell 1. Number of bonds an atom forms determined by how many electrons are needed to fill outer shell 2. Outer & only shell of hydrogen & helium atoms is filled when it contains 2 electrons; outer shells of other atoms are filled when they contain 8 electrons 3. Example: oxygen with 6 outer-shell electrons can fill its outer shell by combining with 2 H atoms, forming a molecule water; oxygen atom linked to each H by a single covalent bond B. Bond formation is accompanied by energy release 1. Later reabsorption of energy by bond breaks it; C—C, C—H or C—O covalent bonds require 80 - 100 kcal/mole to break 2. This energy is quite large so these bonds are stable under most conditions a. 1 calorie = the amount of thermal energy required to raise the temperature of 1 gram of water 1°C; 1 kilocalorie (kcal; or 1 large Calorie) = 1000 calories b. Energy also expressed in Joules (measure of energy in terms of work); 1 kcal = 4186 Joules c. 1 mole = Avogadro's number (6.023 x 10 23 ) of molecules; a mole of a substance is its molecular weight expressed in grams C. Atoms can be joined by bonds in which >1 pair of electrons are shared: if 2 pairs are shared -> double bond (O 2 ); if 3 pairs shared -> triple bond (N 2 ); no quadruple bonds are known D. Type of bond can determine molecular shape - atoms joined by single bond can rotate relative to II. Electronegativity and unequal or equal sharing of electrons A. When atoms sharing electrons are the same, electrons are shared equally between the 2 atoms B. If 2 unlike atoms share electrons, positively charged nucleus of 1 atom (the more electronegative atom) exerts a greater attractive force on the outer electrons than the other 1. Thus, the outer electrons are located closer to the more electronegative atom 2. Of atoms most often seen in biological molecules, nitrogen, oxygen - highly electronegative III. Polar and non-polar molecules A. Water - O-H bonds in H 2 O polarized; O atom is partially negative; the other [H] partially positive 1. It is a polar molecule – such molecules have an asymmetric charge distribution or dipole 1. O atom attracts electrons much more forcefully than does either of its H atoms B. Biologically important polar molecules have one or more electronegative atoms - usually O, N and/or S)
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Chapter 2 The Chemical Basis Of Life - (BIO201 MATERIAL...

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