NOTES: Ch 9, part 4 9.5 & 9.6 - Fermentation &
Regulation of Cellular Respiration
9.5 - Fermentation enables some cells
to produce ATP without the use of
oxygen
● Cellular respiration requires O2 to produce
ATP
● Glycolysis can produce ATP with or without O2
(in aerobic or anaerobic conditions)
● In the absence of O2, glycolysis couples with
fermentation to produce ATP
Alternative Metabolic
Pathways - Vocabulary:
● aerobic: existing in presence of oxygen
● anaerobic: existing in absence of oxygen
● FERMENTATION = anaerobic catabolism of
organic nutrients
Types of Fermentation
● Fermentation consists of glycolysis plus
reactions that regenerate NAD+, which can
be reused by glycolysis
● Two common types are alcohol
fermentation and lactic acid fermentation
Alcohol
Fermentation
Pyruvate + NADH  ethanol + CO2 + NAD+
● pyruvate is converted to ethanol
● NADH is oxidized to NAD+ (recycled)
● performed by yeast and some bacteria
Alcohol Fermentation
● In alcohol fermentation, pyruvate is converted
to ethanol in two steps, with the first releasing
CO2
● Alcohol fermentation by yeast is used in
brewing, winemaking, and baking
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 Pyruvate
2 NAD+
2 Ethanol
Alcohol fermentation
2 NADH
+ 2 H+
2 CO2
2 Acetaldehyde
Lactic Acid Fermentation
Pyruvate + NADH  lactic acid + NAD+
● pyruvate is reduced to lactic acid (3-C
compound); no CO2 produced
● NADH is oxidized to NAD+ (recycling of
NAD+)
Lactic Acid Fermentation
● Lactic acid fermentation by some fungi
and bacteria is used to make cheese and
yogurt
● Human muscle cells use lactic acid
fermentation to generate ATP when O2 is
scarce
2 ADP + 2 P i
Glucose
2 ATP
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
Lactic acid fermentation
Fermentation and Cellular
Respiration Compared:
● Both processes use glycolysis to oxidize
glucose and other organic fuels to
pyruvate
● in fermentation, NADH is recycled back to
NAD+
● in fermentation, final electron acceptor is
pyruvate, not O2
Fermentation and Cellular
Respiration Compared:
● amount of energy harvested:
Fermentation = 2 ATP
Cellular respiration = 36-38 ATP
● oxygen NOT required for fermentation
● Obligate anaerobes:
only grow in absence of
oxygen (e.g. clostridium
botulinum)
● Obligate aerobes: only
grow in presence of
oxygen
Micrococcus luteus
● Facultative anaerobes: can grow in either
presence or absence of oxygen (e.g. yeast or
bacteria that make yogurt, cheese; our
muscle cells at the cellular level)
*in a faculatative anaerobe,
pyruvate is a “fork” in the
metabolic road which
leads to 2 alternate
catabolic routes:
-if O2 is present: Krebs
and E.T.C.
-if no O2 is present:
Fermentation
Glucose
CYTOSOL
Pyruvate
No O2 present
Fermentation
O2 present
Cellular respiration
MITOCHONDRION
Ethanol
or
lactate
Acetyl CoA
Citric
acid
cycle
The Evolutionary Significance
of Glycolysis
● Glycolysis occurs in nearly all organisms
● Glycolysis probably evolved in ancient
prokaryotes before there was oxygen in the
atmosphere
9.6 - Glycolysis and the Krebs cycle
connect to many other metabolic
pathways
● Gycolysis and
the Krebs cycle
are major
intersections to
various
catabolic and
anabolic
pathways
The Versatility of Catabolism
● Catabolic pathways funnel electrons from many
kinds of organic molecules into cellular respiration
● Glycolysis accepts a wide range of carbohydrates
● Proteins must be digested to amino acids; amino
groups can feed glycolysis or the Krebs cycle
● Fats are digested to glycerol (used in glycolysis)
and fatty acids (used in generating acetyl CoA)
● An oxidized gram of fat produces more than twice
as much ATP as an oxidized gram of carbohydrate
Proteins
Carbohydrates
Amino
acids
Sugars
Glycerol Fatty
acids
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Fats
Pyruvate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation
Biosynthesis
(Anabolic Pathways)
● The body uses small molecules to build other
substances
● These small molecules may come directly
from food, from glycolysis, or from the Krebs
cycle
Regulation of Cellular Respiration via
Feedback Mechanisms
● FEEDBACK INHIBITION is the most common
mechanism for control
● If ATP concentration begins to drop, respiration
speeds up;
● when there is plenty of ATP, respiration slows
down
● Control of catabolism is based mainly on
regulating the activity of enzymes at strategic
points in the catabolic pathway
Glucose
AMP
Glycolysis
Fructose-6-phosphate
–
Stimulates
+
Phosphofructokinase
–
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Citric
acid
cycle
Oxidative
phosphorylation