Human Physiology, 14th edition (2016)

108 Chapter 5

Each pyruvic acid molecule contains 3 carbons, 3 oxygens,
and 4 hydrogens (see fig. 5.4 ). The number of carbon and oxygen
atoms in 1 molecule of glucose—C 6 H 12 O 6 —can thus be accounted
for in the 2 pyruvic acid molecules. Because the 2 pyruvic acids
together account for only 8 hydrogens, however, it is clear that
4 hydrogen atoms are removed from the intermediates in glycoly-
sis. Each pair of these hydrogen atoms is used to reduce a molecule
of NAD. In this process, each pair of hydrogen atoms donates
2 electrons to NAD, thereby reducing it. The reduced NAD binds
1 proton from the hydrogen atoms, leaving 1 proton unbound as H^1
(see chapter 4, fig. 4.17). Starting from 1 glucose molecule, there-
fore, glycolysis results in the production of 2 molecules of NADH
and 2 H^1. The H^1 will follow the NADH in subsequent reactions,
so for simplicity we can refer to reduced NAD simply as NADH.
Glycolysis is exergonic, and a portion of the energy
that is released is used to drive the endergonic reaction
ADP 1 P i → ATP. At the end of the glycolytic pathway, there
is a net gain of 2 ATP molecules per glucose molecule, as indi-
cated in the overall equation for glycolysis:

Glucose 1 2 NAD 1 2 ADP 1 2 Pi →

2 pyruvic acid 1 2 NADH 1 2 ATP

Although the overall equation for glycolysis is exergonic,

glucose must be “activated” at the beginning of the pathway

before energy can be obtained. This activation requires the

addition of two phosphate groups derived from 2 molecules of

ATP. Energy from the reaction ATP → ADP 1 P i is therefore

consumed at the beginning of glycolysis. This is shown as an

“up-staircase” in figure 5.2. Notice that the P i is not shown in

these reactions in figure 5.2 ; this is because the phosphate is

not released, but instead is added to the intermediate molecules

of glycolysis. The addition of a phosphate group is known as

phosphorylation. Besides being essential for glycolysis, the

phosphorylation of glucose (to glucose 6-phosphate) has an

important side benefit: it traps the glucose within the cell. This

is because phosphorylated organic molecules cannot cross

plasma membranes.

At later steps in glycolysis, 4 molecules of ATP are pro-

duced (and 2 molecules of NAD are reduced) as energy is

liberated (the “down-staircase” in fig. 5.2 ). The 2 molecules

of ATP used in the beginning, therefore, represent an energy

investment; the net gain of 2 ATP and 2 NADH molecules by

the end of the pathway represents an energy profit. The overall

equation for glycolysis obscures the fact that this is a metabolic

Figure 5.1 overview of energy metabolism using blood glucose. The blood glucose may be obtained from food via the
digestive tract, or the liver may produce it from stored glycogen. Plasma glucose enters the cytoplasm of cells, where it can be used
for energy by either anaerobic metabolism or aerobic cell respiration. In this schematic diagram, the size of the plasma membrane is
greatly exaggerated compared to the size of the other structures and the interstitial (extracellular tissue) fluid.

Citric acid

cycle

Electron

transport

CO 2 + H 2 O

Lactic

acid

Capillary

Cytoplasm

Plasma membrane

Interstitial fluid

Mitochondrion

Glycogen

in liver

Glucose

from digestive tract

Glucose

in blood plasma

Glucose

in cell cytoplasm

Pyruvic acid

Aerobic

in skeletal muscle

into mitochondrion

Respiration

Anaerobic

Metabolism

Glycolysis

Glucose

from liver