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

HOW CELLS OBTAIN ENERGY FROM FOOD
75
Two molecules of NADH are formed per molecule of glucose in the course of
glycolysis. In aerobic organisms, these NADH molecules donate their electrons to
the electron-transport chain described in Chapter 14, and the NAD + formed from
the NADH is used again for glycolysis (see step 6 in Panel 2–8, pp. 104–105).
Fermentations Produce ATP in the Absence of Oxygen
For most animal and plant cells, glycolysis is only a prelude to the final stage of
the breakdown of food molecules. In these cells, the pyruvate formed by glycolysis
is rapidly transported into the mitochondria, where it is converted into CO 2 plus
acetyl CoA, whose acetyl group is then completely oxidized to CO 2 and H 2 O.
In contrast, for many anaerobic organisms—which do not utilize molecular
oxygen and can grow and divide without it—glycolysis is the principal source of
the cell’s ATP. Certain animal tissues, such as skeletal muscle, can also continue
to function when molecular oxygen is limited. In these anaerobic conditions, the
pyruvate and the NADH electrons stay in the cytosol. The pyruvate is converted
into products excreted from the cell—for example, into ethanol and CO 2 in the
yeasts used in brewing and breadmaking, or into lactate in muscle. In this process,
the NADH gives up its electrons and is converted back into NAD + . This regeneration
of NAD + is required to maintain the reactions of glycolysis (Figure 2–47).
Energy-yielding pathways like these, in which organic molecules both donate
and accept electrons (and which are often, as in these cases, anaerobic), are called
CH 2 OH
O
one molecule
of glucose
HO
ATP
OH
OH
OH
STEP 1
energy
investment
to be
recouped
later
STEP 2
ATP
STEP 3
fructose 1,6-
bisphosphate
P OH 2 C
O
CH 2 O
HO
OH
OH
STEP 4
P
STEP 5
cleavage of
six-carbon
sugar to two
three-carbon
sugars
two molecules of
glyceraldehyde
3-phosphate
CHO
CHOH
CH 2 O
P
CHO
CHOH
CH 2 O
P
NADH
two molecules
of pyruvate
ATP
ATP
STEP 6
STEP 7
STEP 8
STEP 9
STEP 10
COO –
C O
CH 3
COO –
C
CH 3
O
NADH
ATP
ATP
energy
generation
Figure 2–46 An outline of glycolysis.
Each of the 10 steps shown is catalyzed
by a different enzyme. Note that step 4
cleaves a six-carbon sugar into two threecarbon
sugars, so that the number of
molecules at every stage after this doubles.
As indicated, step 6 begins the energygeneration
phase of glycolysis. Because
two molecules of ATP are hydrolyzed in the
early, energy-investment phase, glycolysis
results in the net synthesis of 2 ATP and 2
NADH molecules per molecule of glucose
(see also Panel 2–8).