Essential Cell Biology 5th edition

The Breakdown and Utilization of Sugars and Fats
429
use them—either as a source of energy or as building blocks for making
other organic molecules. This breakdown process—in which enzymes
degrade complex organic molecules into simpler ones—is called
catabolism. The process takes place in three stages, as illustrated in
Figure 13–3.
In stage 1 of catabolism, enzymes convert the large polymeric molecules
in food into simpler monomeric subunits: proteins into amino acids,
polysaccharides into sugars, and fats into fatty acids and glycerol. This
(A)
STAGE 1:
BREAKDOWN
OF LARGE FOOD
MOLECULES
TO SIMPLE
SUBUNITS
proteins
amino acids
polysaccharides
simple sugars
fats
fatty acids
and glycerol
Figure 13–3 In animals, the breakdown
of food molecules occurs in three stages.
(A) Stage 1 mostly occurs outside cells, with
the breakdown of large food molecules
in the mouth and the gut—although
intracellular lysosomes can also digest such
large molecules. Stage 2 starts intracellularly
with glycolysis in the cytosol, and ends with
the conversion of pyruvate to acetyl groups
on acetyl CoA in the mitochondrial matrix.
Stage 3 begins with the citric acid cycle in
the mitochondrial matrix and concludes
with oxidative phosphorylation on the
mitochondrial inner membrane. The NADH
generated in stage 2 adds to the NADH
produced by the citric acid cycle to drive
the production of large amounts of ATP by
oxidative phosphorylation.
(B) The net products of the complete
oxidation of food include ATP, NADH, CO 2 ,
and H 2 O. The ATP and NADH provide
the energy and electrons needed for
biosynthesis; the CO 2 and H 2 O are waste
products.
glucose
CYTOSOL
STAGE 2:
BREAKDOWN OF
SIMPLE SUBUNITS
TO ACETYL CoA;
LIMITED AMOUNTS
OF ATP AND NADH
PRODUCED
GLYCOLYSIS
pyruvate
NADH
ATP
NADH
plasma
membrane
of eukaryotic
cell
acetyl CoA
CO 2
CO 2
CO 2
CITRIC
ACID
CYCLE
mitochondrial
matrix
STAGE 3:
COMPLETE
OXIDATION OF THE
ACETYL GROUP IN
ACETYL CoA TO H 2O
AND CO 2 ; LARGE
AMOUNTS OF
ATP PRODUCED
ON THE INNER
MITOCHONDRIAL
MEMBRANE
NADH
OXIDATIVE
PHOSPHORYLATION
ATP
ATP
ATP
CO 2
outer mitochondrial
membrane
inner mitochondrial
membrane
O 2
H 2O
O 2
(B)
NET RESULT:
FOOD + O 2 ATP + NADH + CO 2 +
H 2 O