BIOCHEMISTRY The Chemical Context of Life


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BIOCHEMISTRY The Chemical Context of Life

The Elements of Life C. HOPKINS CaFe
• About 20–25% of the 92 elements are essential to life
• Carbon, hydrogen, oxygen, and nitrogen make up 96% of living matter
• Most of the remaining 4% consists of calcium, phosphorus, potassium, and sulfur
• Trace elements are those required by an organism in minute quantities
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Covalent Bond Review
• A covalent bond is the sharing of a pair of valence electrons by two atoms
• In a covalent bond, the shared electrons count as part of each atom’s valence shell
• A covalent bond is formed between shared pairs of electrons:
 1 pair—a single bond
 2 pairs—a double bond
 3 pairs—a triple bond
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Not all sharing is created EQUAL!

• In a nonpolar covalent bond,

the atoms share the electron

equally

–

• In a polar covalent bond, one

atom is more electronegative,

O

and the atoms do not share

the electron equally
• Unequal sharing of electrons

H +

H +

H2O

causes a slightly positive and

negative pole(s)

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Question
Why are covalent bonds more prevalent among biological molecules than ionic bonds? A. Ionic bonds only occur between metals and non-metals,
and therefore aren't usually present in biological systems. B. You can have double covalent bonds, but not double ionic bonds, so covalent bonds provide more variety consistent with the structural demands required in biological systems. C. Biological conditions are often aqueous, and the water would cause ionic bonds to dissociate. D. Ions only form under extreme conditions not compatible with the cell's environment.

Question
Why are covalent bonds more prevalent among biological molecules than ionic bonds? A. Ionic bonds only occur between metals and non-metals,
and therefore aren't usually present in biological systems. B. You can have double covalent bonds, but not double ionic bonds, so covalent bonds provide more variety consistent with the structural demands required in biological systems. C. Biological conditions are often aqueous, and the water would cause ionic bonds to dissociate. D. Ions only form under extreme conditions not compatible with the cell's environment.

IMFs
• There is a rift between biology and chemistry text books.
• The biology book often speaks of “weak bonds” which are really intermolecular forces (IMFs)
• IMFs are intermolecular whereas chemical bonds are intramolecular
• Inter—means between molecules (molecule to molecule, like magnets)
• Intra—means within the molecule (atom to atom, a real bond)
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What’s the big deal?
• With IMFs, molecules become “sticky” or adhere to one another.
– Hydrogen bonds are IMFs; aka not REAL bonds; aka WEAK bonds
• Collectively, IMFs can be pretty strong, as between the molecules of a gecko’s toe hairs and the surface of a wall. He’s not really defying gravity!
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Hydrogen Bonding: A Special Case of IMFs

• A hydrogen bond is not the same as a bonded hydrogen!
• It’s a special case of IMFs
• A bonded hydrogen is within a water molecule
• A hydrogen bond is between molecules!

Bonded Hydrogens
(actual chemical bonds consisting of a shared
pair of electrons)
Hydrogen bonds (IMFs)

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Hydrogen Bonding: A Special Case of Dipole-Dipole IMFs

• Hydrogen bonding in water occurs when the positive H of one water molecule is attracted to the negative O on an adjacent molecule. (It happens in any other polar molecules too.)
(Draw one in your notes…)

Bonded Hydrogens
(actual chemical bonds consisting of a shared
pair of electrons)
Hydrogen “bonds” (IMFs)

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BIOCHEMISTRY The Chemical Context of Life