Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

56 Chapter 2: Cell Chemistry and Bioenergetics
Oxidation refers to more than the addition of oxygen atoms; the term applies
more generally to any reaction in which electrons are transferred from one atom
to another. Oxidation in this sense refers to the removal of electrons, and reduction—the
converse of oxidation—means the addition of electrons. Thus, Fe 2+ is
oxidized if it loses an electron to become Fe 3+ , and a chlorine atom is reduced
if it gains an electron to become Cl – . Since the number of electrons is conserved
(no loss or gain) in a chemical reaction, oxidation and reduction always occur
simultaneously: that is, if one molecule gains an electron in a reaction (reduction),
a second molecule loses the electron (oxidation). When a sugar molecule is
oxidized to CO 2 and H 2 O, for example, the O 2 molecules involved in forming H 2 O
gain electrons and thus are said to have been reduced.
The terms “oxidation” and “reduction” apply even when there is only a partial
shift of electrons between atoms linked by a covalent bond (Figure 2–20). When
a carbon atom becomes covalently bonded to an atom with a strong affinity for
electrons, such as oxygen, chlorine, or sulfur, for example, it gives up more than
its equal share of electrons and forms a polar covalent bond. Because the positive
charge of the carbon nucleus is now somewhat greater than the negative charge of
its electrons, the atom acquires a partial positive charge and is said to be oxidized.
Conversely, a carbon atom in a C–H linkage has slightly more than its share of
electrons, and so it is said to be reduced.
When a molecule in a cell picks up an electron (e – ), it often picks up a proton
(H + ) at the same time (protons being freely available in water). The net effect in
this case is to add a hydrogen atom to the molecule.
A + e – + H + → AH
Even though a proton plus an electron is involved (instead of just an electron),
such hydrogenation reactions are reductions, and the reverse, dehydrogenation
reactions are oxidations. It is especially easy to tell whether an organic molecule
is being oxidized or reduced: reduction is occurring if its number of C–H bonds
increases, whereas oxidation is occurring if its number of C–H bonds decreases
(see Figure 2–20B).
Cells use enzymes to catalyze the oxidation of organic molecules in small
steps, through a sequence of reactions that allows useful energy to be harvested.
We now need to explain how enzymes work and some of the constraints under
which they operate.
(A)
Figure 2–20 Oxidation and reduction. (A) When two atoms form a polar
covalent bond, the atom ending up with a greater share of electrons is said
to be reduced, while the other atom acquires a lesser share of electrons and
is said to be oxidized. The reduced atom has acquired a partial negative
charge (δ – ) as the positive charge on the atomic nucleus is now more than
equaled by the total charge of the electrons surrounding it, and conversely,
the oxidized atom has acquired a partial positive charge (δ + ). (B) The single
carbon atom of methane can be converted to that of carbon dioxide by
the successive replacement of its covalently bonded hydrogen atoms
with oxygen atoms. With each step, electrons are shifted away from the
carbon (as indicated by the blue shading), and the carbon atom becomes
progressively more oxidized. Each of these steps is energetically favorable
under the conditions present inside a cell.
FORMATION OF
A POLAR
e _
e _
COVALENT
e _
BOND
e _
+ +
e _ +
e _
+
e _
partial
+
positive
ATOM 1
ATOM 2
charge (δ + )
oxidized
MOLECULE
partial
negative
charge (δ – )
reduced
(B)
O
X
I
D
A
T
I
O
N
H
H
C
methane
H
H
H
H
C
methanol
OH
H
H
C
formaldehyde
O
H
H
C
formic acid
O
HO
O
C O
carbon dioxide
R
E
D
U
C
T
I
O
N