How Cells Release
Chemical Energy
Chapter 8
Cellular or Aerobic Respiration
 Cellular respiration evolved to enable organisms
to utilize energy stored in glucose.
 Taps the energy found in the bonds of organic
compounds (carbohydrates, lipids, proteins)
Comparison of the Main Pathways
 Aerobic respiration
• Aerobic metabolic pathways (using oxygen) are used
by most eukaryotic cells
• Ends in the mitochondria
• Aerobes use oxygen as the final electron acceptor
 Fermentation
• Anaerobic metabolic pathways (occur in the absence
of oxygen) are used by prokaryotes and protists in
anaerobic habitats
• Anaerobes use nitrate or sulfate as the final eacceptor
Comparison of the Main Pathways
 Aerobic respiration and fermentation both begin
with glycolysis, which converts one molecule of
glucose into two molecules of pyruvate
 After glycolysis, the two pathways diverge
• Fermentation is completed in the cytoplasm,
yielding 2 ATP per glucose molecule
• Aerobic respiration is completed in mitochondria,
yielding 36 ATP per glucose molecule (liberates
the most energy)
Overview of Aerobic Respiration
 Three sequential stages
• Glycolysis
• Acetyl-CoA formation and Krebs cycle
• Electron transfer phosphorylation (ATP formation)
C6H12O6 (glucose) + O2 (oxygen) →
CO2 (carbon dioxide) + H2O (water)
• Coenzymes NADH and FADH2
Fig. 8-3b, p. 125
Animation: Overview of aerobic
respiration
8.2 Glycolysis –
Glucose Breakdown Starts
 Glycolysis starts and ends in the cytoplasm of all
prokaryotic and eukaryotic cells
 An energy investment/input of ATP (2) starts
glycolysis
 Requires a continous supply of glucose and
NAD+ (reduced in both glycolysis and the kreb
cycle)
Glycolysis
 Two ATP’s are used to split glucose (6 carbon
compound) and form 2 PGAL (3 carbon
compound)
 Enzymes convert 2 PGAL to 2 PGA, forming 2
NADH
 Four ATP are formed by substrate-level
phosphorylation (net yield of 2 ATP)
Fig. 8-4a (2), p. 126
ATP-Requiring Steps
Glycolysis
A An enzyme transfers a
phosphate group from ATP
to glucose, forming glucose6-phosphate.
glucose
ATP
ADP
glucose-6-phosphate
ATP
ADP
fructose-1,6-bisphosphate
(intermediate)
B A phosphate group
from a second ATP is
transferred to the glucose6- phosphate. The resulting
molecule is unstable, and it
splits into two three- carbon
molecules. The molecules
are interconvertible, so we
will call them both PGAL
(phosphoglyceraldehyde).
So far, two ATP have been
invested in the reactions.
Fig. 8-4b (1), p. 127
2 PGAL
2
NAD+
+ 2 Pi
NADH
2 reduced coenzymes
2 PGA
2 ADP
ATP
2 ATP produced
by substrate-level
phosphorylation
2 PEP
2 ADP
ATP
2 pyruvate
2 ATP produced
by substrate-level
phosphorylation
Net 2 ATP + 2 NADH
to second stage
ATP-Generating Steps
C Enzymes attach a
phosphate to the two PGAL,
and transfer two electrons
and a hydrogen ion from each
PGAL to NAD+. Two PGA
(phosphoglycerate) and two
NADH are the result.
D Enzymes transfer a
phosphate group from each
PGA to ADP. Thus, two ATP
have formed by substratelevel phosphorylation.
The original energy
investment of two ATP has
now been recovered.
E Enzymes transfer a
phosphate group from each
of two intermediates to ADP.
Two more ATP have formed
by substrate-level
phosphorylation.
Two molecules of pyruvate
form at this last reaction
step.
F Summing up, glycolysis
yields two NADH, two ATP
(net), and two pyruvate for
each glucose molecule.
Fig. 8-4b (2), p. 127
8.3 Second Stage of Aerobic Respiration
 The second stage of aerobic respiration finishes
breakdown of glucose that began in glycolysis
 Occurs in mitochondria (inner compartment)
begans with the chemical pyruvate to continue
respiration
 Includes two stages: acetyl CoA formation and
the Krebs cycle (each occurs twice in the
breakdown of one glucose molecule)
The Krebs Cycle
 Krebs cycle
• A sequence of enzyme-mediated reactions that break
down 1 acetyl CoA (transition stage) into 2 CO2
• Oxaloacetate (last intermediate) is used and
regenerated
• 3 NADH and 1 FADH2 are formed, 1 ATP is formed
• Substrate level phosphorylation occurs
• Electrons and hydrogens are transferred to
coenzymes in both glycolysis and the Kreb cycle
• Energy, CO2, and H+ are released
• Cycle turns 2x to break down glucose
A An enzyme splits a
pyruvate molecule into a
two-carbon acetyl group
and CO2. Coenzyme A
binds the acetyl group
(forming acetyl–CoA). NAD+
combines with released
hydrogen ions and
electrons, forming NADH.
Acetyl–CoA
Formation
pyruvate
transition from
glyco
NAD+
coenzyme A
NADH
CO2
B The Krebs cycle starts as
one carbon atom is
transferred from acetyl–
CoA to oxaloacetate. Citrate
forms, and coenzyme A is
regenerated.
acetyl–CoA
coenzyme A
H The final steps of the
Krebs cycle regenerate
oxaloacetate.
citrate
C A carbon atom is removed
from an intermediate and
leaves the cell as CO2. NAD+
combines with released
hydrogen ions and
electrons, forming NADH.
D A carbon atom is
removed from another
intermediate and leaves the
cell as CO2, and another
NADH forms.
CO2
oxaloacetate
Krebs
Cycle
NAD+
G NAD+ combines with
hydrogen ions and
electrons, forming NADH.
NADH
NADH
NAD+
CO2
NAD+
FADH2
FAD
NADH
Pyruvate’s three carbon atoms have
now exited the cell, in CO2.
ADP + Pi
ATP
F The coenzyme FAD combines with hydrogen ions
and electrons, forming
FADH2.
E One ATP forms by
substrate-level
phosphorylation.
Stepped Art
Fig. 8-6, p. 129
Animation: The Krebs Cycle - details
8.4 Aerobic Respiration’s
Big Energy Payoff
 Many ATP’s are formed during the third and final
stage of aerobic respiration (Chemiosmotic
Theory = production of ATP’s)
 Electron transfer phosphorylation
• Occurs in mitochondria (inner membrane)
• Results in attachment of phosphate to ADP to
form ATP
• Generates a hydrogen concentration (build up
of hydrogen ions between 2 membranes)
gradient
Electron Transfer Phosphorylation
Summary: The Energy Harvest
 Typically, the breakdown of one glucose
molecule yields 36 ATP
• Glycolysis: 2 ATP
• Acetyl CoA formation and Krebs cycle: 2 ATP
• Electron transfer phosphorylation: 32 ATP
Animation: Third-stage reactions
8.5 Anaerobic
Energy-Releasing Pathways
 Fermentation pathways break down
carbohydrates without using oxygen (Ex.
Bacteria that causes botulism)
 The final steps in these pathways regenerate
NAD+
Fermentation Pathways
 Glycolysis is the first stage of fermentation
• Forms 2 pyruvate, 2 NADH, and 2 ATP
 Pyruvate turns to lactic acid using e- from NADH
(Lactate/Lactose Fermentation)
 Pyruvate is converted to ethanol producing
acetaldehyde and CO2 (Alcoholic Fermentation)
Two Pathways of Fermentation
 Alcoholic fermentation
• Pyruvate produce acetaldehyde and CO2 when
converted to ethanol
• Acetaldehyde receives electrons and hydrogen
from NADH, forming NAD+ and ethanol
 Lactate fermentation
• Pyruvate receives electrons and hydrogen from
NADH, forming NAD+ and lactate (muscle cells,
sour cream, etc)
Glycolysis
2
ATP
glucose
2 NAD+
2 NADH
Glycolysis
2
ATP
2 NADH
4 ATP
4 ATP
pyruvate
pyruvate
Alcoholic
Fermentation
glucose
2 NAD+
2 CO2
acetaldehyde
Lactate
Fermentation
2 NADH
2 NAD+
2 NADH
2 NAD+
lactate
ethanol
Stepped Art
Fig. 8-9, p. 132
8.6 The Twitchers
 Slow-twitch muscle fibers (“red” muscles) make
ATP by aerobic respiration
• Have many mitochondria
• Dominate in prolonged activity
 Fast-twitch muscle fibers (“white” muscles) make
ATP by lactate fermentation
• Have few mitochondria and no myoglobin
• Sustain short bursts of activity
• Human muscles have a mixture of both fibers
8.7 Alternative
Energy Sources in the Body
 In humans and other mammals, the entrance of
glucose and other organic compounds into an
energy-releasing pathway depends on the kinds
and proportions of carbohydrates (our main
source of energy), fats and proteins in the diet
The Fate of Glucose at Mealtime
and Between Meals
 When blood glucose concentration rises, the
pancreas increases insulin (hormone) secretion
• Cells take up glucose faster, more ATP is formed
 When blood glucose concentration falls, the
pancreas increases glucagon (hormone)
secretion (between meals)
• Stored glycogen is converted to glucose, the
brain continues to receive glucose
• Triglycerides are tapped as an energy alternative.
Energy From Fats
 About 78% of an adult’s energy reserves is
stored in fat (mostly triglycerides)
 Excess glucose(carbs) in the diet = FAT; acetyl
Co A exits the Kreb Cycle and enters a
pathway that makes fatty acids
 Enzymes cleave fats into glycerol and fatty acids
• Glycerol products enter glycolysis
• Fatty acid products enter the Krebs cycle
Energy from Proteins
 Enzymes split dietary proteins into amino acid
subunits, which enter the bloodstream
• Used to build proteins or other molecules
 Excess amino acids are broken down into
ammonia (NH3) and various products that can
enter the Krebs cycle
FOOD
FATS
fatty acids
glycerol
acetyl–CoA
PGAL
COMPLEX CARBOHYDRATES
PROTEINS
glucose, other simple sugars
amino acids
acetyl–CoA
Glycolysis
NADH
pyruvate
Krebs
Cycle
oxaloacetate or
another intermediate
of the Krebs
NADH, FADH2
Electron Transfer
Phosphorylation
Fig. 8-12, p. 135