C e llu la r r e s p i r a t i o n • F ig u r e 1 4 . 1 2
During cellular respiration, glucose molecules are broken down into carbon dioxide, ATP is
made, oxygen is consumed, and water is made. The process involves glycolysis, formation
of acetyl coenzyme A, the Krebs cycle, and oxidative phosphorylation. Glycolysis occurs in
the cytosol and requires no oxygen; the subsequent processes occur in the mitochondrion,
and use oxygen.
Glycolysis. In the
cytosol, glucose (6
carbons) is broken
down into 2 pyruvic
acid molecules (3
carbons each) in a
10-step pathway. ATP
in cytosol
must be used to begin
the process.
Formation of acetyl
coenzym e A (2
carbons). Each pyruvic
acid from glycolysis
enters the mitochon-
drion, loses a carbon
dioxide, and is
attached to
coenzyme A.
Electrons are
transferred to
Krebs cycle.
In the matrix of
the mitochon-
drion, the acetyl
groups are
broken down. In
• More CO2 is
• A small amount
of ATP is
• Many electrons
are transferred
to NADH and
FADH2 (electron
Water and heat are created
as by-products of cellular
This seemingly simple equation involves four complex
biochemical pathways: | l
glycolysis, p
the formation
of acetyl coenzyme A (acetyl CoA), 0
the Krebs cy-
cle, and
oxidative phosphorylation (or the electron
transport chain [ETC]).
As you can see from Figure 14.12 and the formula just
shown, each glucose molecule that goes through cellular
respiration produces 36 ATP molecules (2 from glycolysis,
2 from the Krebs cycle, and 32 from oxidative phosphoryla-
tion). Glycolysis is an
process—that is, it requires
no oxygen. (If there is no oxygen available after the process
finishes, however, pyruvic acid will be converted into lactic
acid and very little ATP will be made from the glucose.) Be-
cause the formation of acetyl CoA, the Krebs cycle, and oxi-
dative phosphorylation use oxygen, this part of the process
of cellular respiration is also known as aerobic respiration.
Nutrients Are Metabolized in a Number of Ways 419
p r o c e s s
d ia g ra m
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