Human Physiology, 14th edition (2016)

100 Chapter 4

a particular atom or molecule can play both roles; it may func-

tion as an oxidizing agent in one reaction and as a reducing agent

in another reaction. When atoms or molecules play both roles,

they gain electrons in one reaction and pass them on in another

reaction to produce a series of coupled oxidation-reduction

reactions—like a bucket brigade, with electrons in the buckets.

Notice that the term oxidation does not imply that oxygen

participates in the reaction. This term is derived from the fact

that oxygen has a great tendency to accept electrons; that is,

to act as a strong oxidizing agent. This property of oxygen is

exploited by cells; oxygen acts as the final electron acceptor in

a chain of oxidation-reduction reactions that provides energy

for ATP production (chapter 5, section 5.2).

Oxidation-reduction reactions in cells often involve the

transfer of hydrogen atoms rather than free electrons. Because a

hydrogen atom contains 1 electron (and 1 proton in the nucleus),

a molecule that loses hydrogen becomes oxidized, and one that

gains hydrogen becomes reduced. In many oxidation-reduction

reactions, pairs of electrons—either as free electrons or as a pair

of hydrogen atoms—are transferred from the reducing agent to

the oxidizing agent.

Two molecules that serve important roles in the transfer

of hydrogens are nicotinamide adenine dinucleotide (NAD),

which is derived from the vitamin niacin (vitamin B 3 ), and flavin

adenine dinucleotide (FAD), which is derived from the vitamin

riboflavin (vitamin B 2 ). These molecules ( fig.  4.17 ) are coen-

zymes that function as hydrogen carriers because they accept

hydrogens (becoming reduced) in one enzyme reaction and

donate hydrogens (becoming oxidized) in a different enzyme

reaction ( fig. 4.18 ). The oxidized forms of these molecules are

written simply as NAD (or NAD^1 ) and FAD.

Figure 4.16 A model of ATP as the universal energy carrier of the cell. Exergonic reactions are shown as blue gears

with arrows going down (these reactions produce a decrease in free energy); endergonic reactions are shown as green gears with

arrows going up (these reactions produce an increase in free energy).

ATP produced ATP used for cell work

Food

AT P

ADP + Pi AT P

ADP + Pi

AT P

ADP + Pi

AT P

ADP + Pi

AT P

ADP + Pi

AT P

ADP + Pi

CO 2 + H 2 O

Figure 4.15 The formation and structure of
adenosine triphosphate (ATP). ATP is the universal energy
carrier of the cell. High-energy bonds are indicated by a squiggle
(~). When the bond to the last phosphate is broken, energy is
released and ATP is converted back into ADP and P i.

Adenosine diphosphate (ADP)

Inorganic phosphate (Pi)

NH 2

CH 2

C N

C

C

CH

HC

N

N

N

H H

H

OH

Adenosine triphosphate (ATP)

+

H

OOP~ O

OH

_

O

_

O

P~O

O

_

O

OP

O

_

O

donates electrons to another, which therefore becomes reduced.

The atom or molecule that donates electrons to another

is a reducing agent, and the one that accepts electrons from

another is an oxidizing agent. It is important to understand that