120
UNIT ONE
to thirty-eight molecules of ATP can be produced. All but
two ATP molecules are formed by the aerobic reactions.
Glycolysis
Both aerobic and anaerobic pathways begin with glycolysis.
Literally “the breaking of glucose,” glycolysis is a series of ten
enzyme-catalyzed reactions that break down the 6-carbon
glucose molecule into two 3-carbon pyruvic acid molecules.
Glycolysis occurs in the cytosol (see F g. 4.9), and because it
does not require oxygen, it is sometimes referred to as the
anaerobic phase of cellular respiration.
Three main events occur during glycolysis
(f g. 4.10)
:
1. ±irst, glucose is phosphorylated by the addition of two
phosphate groups, one at each end of the molecule.
Although this step requires ATP, it “primes” the
molecule for some of the energy-releasing reactions that
occur later.
2. Second, the 6-carbon glucose molecule is split into two
3-carbon molecules.
3. Third, the electron carrier NADH is produced, ATP is
synthesized, and two 3-carbon pyruvic acid molecules
result. Some of the reactions of glycolysis release
hydrogen atoms. The electrons of these hydrogen
atoms contain much of the energy associated with
the chemical bonds of the original glucose molecule.
To keep this energy in a form the cell can use, these
hydrogen atoms are passed in pairs to molecules of
the hydrogen carrier NAD
+
(nicotinamide adenine
dinucleotide). In this reaction, two of the electrons and
one hydrogen nucleus bind to NAD
+
to form NADH.
The remaining hydrogen nucleus (a hydrogen ion) is
released as follows:
NAD
+
+ 2H
NADH + H
+
NADH delivers these high-energy electrons to the
electron transport chain elsewhere in the mitochondria,
where most of the ATP will be synthesized.
ATP is also synthesized directly in glycolysis. After sub-
tracting the two ATP used in the priming step, this gives a
net yield of two ATP per molecule of glucose.
Release of Chemical Energy
Most metabolic processes require chemical energy stored
in ATP. This form of energy is initially held in the chemi-
cal bonds that link atoms into molecules and is released
when these bonds break. Burning a marshmallow over a
campF re releases the chemical energy held in the bonds of
the molecules that make up the marshmallow as heat and
light. Similarly, when a marshmallow is eaten, digested, and
absorbed, cells “burn” glucose molecules from that marsh-
mallow in a process called
oxidation
(ok
sı˘-da
shun). The
energy released by oxidation of glucose is harnessed to pro-
mote cellular metabolism.
Oxidation of substances inside cells and the burning of
substances outside them have important differences. Burning
in nonliving systems (such as starting a F re in a F replace)
usually requires a great deal of energy to begin, and most of
the energy released escapes as heat or light. In cells, enzymes
initiate oxidation by lowering the activation energy. Also, by
transferring energy to ATP, cells are able to capture almost
half of the energy released in the form of chemical energy. The
rest escapes as heat, which helps maintain body temperature.
PRACTICE
9
What is energy?
10
Def
ne cellular respiration.
11
How does cellular oxidation diF
er ±rom burning?
4.5
CELLULAR RESPIRATION
Cellular respiration occurs in three distinct, yet intercon-
nected, series of reactions:
glycolysis
(gli-kol
ı˘-sis), the
citric acid cycle,
and the
electron transport chain
(oxi-
dative phosphorylation)
(f
g. 4.9)
. The products of these
reactions include carbon dioxide (CO
2
), water, and energy.
Although most of the energy is lost as heat, almost half is
captured as ATP.
Cellular respiration includes
aerobic
(a
er-o
¯b
ik), reac-
tions which require oxygen, and
anaerobic
(an-a
er-o
¯b
ik)
reactions, which do not require oxygen. ±or each glucose
molecule decomposed completely by cellular respiration, up
P
P
ADP
ATP
Energy transferred
from cellular
respiration used
to reattach
phosphate
Energy transferred
and utilized by
metabolic
reactions when
phosphate bond
is broken
PPP
P
P
FIGURE 4.8
ATP provides energy ±or
metabolic reactions in cells. Cellular respiration
generates ATP.
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