How Cells Release
Chemical Energy
Chapter 7
Impacts, Issues
When Mitochondria Spin Their
Wheels



Mitochondria are the organelles responsible for
releasing the energy stored in foods
In Luft’s syndrome, the mitochondria are active
in oxygen consumption, but with little ATP
formation to show for it
In Friedreich’s ataxia, too much iron in the
mitochondria causes an accumulation of free
radicals that attack valuable molecules of life
The Impact
Proper,
or improper,
functioning of mitochondria
is the difference between
health and disease
Section 7.1
Overview of
Energy-Releasing Pathways
Producing the Universal
Currency of Life
All energy-releasing pathways:
 require
characteristic starting materials
 yield
predictable products and byproducts
 produce
ATP
ATP Is Universal
Energy Source
 Photosynthesizers
sun
get energy from the
 Animals
get energy second- or thirdhand from plants or other organisms
 Regardless,
the energy is converted
to the chemical bond energy of
ATP
Making ATP
 Plants
make ATP during
photosynthesis
 Cells
of all organisms make ATP by
breaking down carbohydrates, fats, and
protein
Main Types of
Energy-Releasing Pathways
Anaerobic pathways
Aerobic pathways
Evolved first
 Don’t require
oxygen
 Start with glycolysis
in cytoplasm
 Completed in
cytoplasm


Evolved later
 Require oxygen
 Start with
glycolysis in
cytoplasm
 Completed in
mitochondria
Energy-Releasing Pathways
Overview of Aerobic Respiration
C6H1206 + 6O2
glucose
oxygen
6CO2 + 6H20
carbon dioxide water
Overview of Aerobic Respiration
glucose
cytoplasm
2
ATP
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
mitochondrion
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs
Cycle
2
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
Main Pathways Start
with Glycolysis
 Glycolysis
occurs in cytoplasm
 Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
p.106a
Three Series of Reactions Are
Required for Aerobic Respiration
 Glycolysis
is the breakdown of
glucose to pyruvate
 Small
amounts of ATP are generated
Three Series of Reactions Are
Required for Aerobic Respiration
 The
Krebs cycle degrades pyruvate to
CO2 and water;
and FAD accept H+ ions and
electrons to be carried to the electron
transfer chain
 Small amounts of ATP are generated
 NAD
Three Series of Reactions Are
Required for Aerobic Respiration
 Electron
transfer phosphorylation
processes the H+ ions and electrons to
generate lots of ATP
 Oxygen
is the final electron acceptor
The Role of Coenzymes

NAD+ and FAD accept electrons and hydrogen
from intermediates during the first two stages

When reduced, they are NADH and FADH2

In the third stage, these coenzymes deliver the
electrons and hydrogen to the transfer chain
Overview of Aerobic Respiration
glucose
cytoplasm
2
ATP
ATP
GLYCOLYSIS
energy input to
start reactions
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
mitochondrion
2 NADH
8 NADH
2 FADH2
e-
e- + H+
2 CO2
e- + H+
4 CO2
e- + H+
Krebs
Cycle
2
ELECTRON
TRANSPORT
PHOSPHORYLATION
H+
32
ATP
ATP
water
e- + oxygen
TYPICAL ENERGY YIELD: 36 ATP
Section 7.2
The First Stage: Glycolysis
Glucose
A
simple sugar
(C6H12O6)
 Atoms
held
together by
covalent bonds
Glycolysis Occurs
in Two Stages
 Energy-requiring
 ATP
steps
energy activates glucose and its
six-carbon derivatives
Glycolysis Occurs
in Two Stages
 Energy-releasing
steps
 The
products of the first part are split
into 3-carbon pyruvate molecules
 ATP
and NADH form
Energy-Requiring Steps
ENERGY-REQUIRING STEPS
OF GLYCOLYSIS
glucose
ATP
2 ATP invested
ADP
P
glucose–6–phosphate
P
fructose–6–phosphate
ATP
ADP
P
fructose–1,6–bisphosphate
P
DHAP
Energy-Releasing Steps
ENERGY-RELEASING STEPS
OF GLYCOLYSIS
PGAL
PGAL
NAD+
Pi
NAD+
NADH
Pi
P
P
P
1,3-bisphosphoglycerate
ADP
NADH
ATP
P
1,3-bisphosphoglycerate
ADP
ATP
substrate-level
phosphorylation
2 ATP produced
P
P
3-phosphoglycerate
3-phosphoglycerate
P
P
2-phosphoglycerate
H2O
2-phosphoglycerate
H2O
P
P
PEP
PEP
ADP
ATP
ADP
ATP
substrate-level
phosphorylation
2 ATP produced
pyruvate
pyruvate
to second set of reactions
Glycolysis in a Nutshell
Glucose is first phosphorylated in energyrequiring steps, then split to form two
molecules of PGAL
 Enzymes remove H+ and electrons from
PGAL to change NAD+ to NADH (which
is used later in electron transfer

 PGAL

is converted eventually to pyruvate
By substrate-level phosphorylation, four
ATP are produced
Substrate-level What?
 Substrate-level
phosphorylation means
that there is a direct transfer of a
phosphate group from the substrate of a
reaction to some other molecule – in
this case, ADP
 Substrate is a reactant in a reaction –
the substance being acted upon, for
example, by an enzyme
Net Energy Yield
from Glycolysis
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed
Net yield is 2 ATP and 2 NADH
Section 7.3
Second Stage of Aerobic
Respiration
Second-Stage Reactions
Occur in the
mitochondria
 Pyruvate is
broken down to

inner
mitochondrial
membrane
outer
mitochondrial
membrane
carbon dioxide
More ATP is
formed
 More coenzymes
are reduced

inner
outer
compartment compartment
Fig. 7-5b, p.112
Second
Stage of
Aerobic
Respiration
Two Parts of Second Stage
 Preparatory
reactions
 Pyruvate
is oxidized into two-carbon
acetyl units and carbon dioxide
 NAD+ is reduced
 Krebs
 The
cycle
acetyl units are oxidized to carbon
dioxide
 NAD+ and FAD are reduced
Preparatory Reactions
pyruvate + coenzyme A + NAD+
acetyl-CoA + NADH + CO2

One of the carbons from pyruvate is released in
CO2

Two carbons are attached to coenzyme A and
continue on to the Krebs cycle
What Is Acetyl-CoA?
A
two-carbon acetyl group linked to
coenzyme A
CH3
Acetyl group
C=O
S
Coenzyme A
Second
Stage of
Aerobic
Respiration
=CoA
acetyl-CoA
KREBS CYCLE
CoA
oxaloacetate
citrate
NADH
H2O
NAD+
H2O
malate
NAD+
H2O
FADH2
isocitrate
NADH
fumarate
O
O
a-ketoglutarate
FAD
NAD+
succinate
NADH
CoA
O
O
succinyl-CoA
ATP
ADP +
phosphate group
Stepped Art
Fig. 7-7a, p.113
The Krebs Cycle
Overall Reactants
Overall Products
Acetyl-CoA
 3 NAD+
 FAD
 ADP and Pi


Coenzyme A
 2 CO2
 3 NADH
 FADH2
 ATP
Results of the Second Stage
 All
of the carbon molecules in
pyruvate end up in carbon dioxide
 Coenzymes are reduced (they pick up
electrons and hydrogen)
 One molecule of ATP is formed
 Four-carbon oxaloacetate is
regenerated
Two pyruvates cross the inner
mitochondrial membrane.
Krebs
Cycle
2
NADH
6
NADH
2
FADH2
2
6 CO2
ATP
outer mitochondrial
compartment
inner mitochondrial
compartment
Eight NADH, two FADH 2,
and two ATP are the payoff
from the complete breakdown of two pyruvates in the
second-stage reactions.
The six carbon atoms from two pyruvates diffuse out
of the mitochondrion, then out of the cell, in six CO
Fig. 7-6, p.112
Coenzyme Reductions during
First Two Stages
Glycolysis
 Preparatory
reactions
 Krebs cycle


Total
2 NADH
2 NADH
2 FADH2 + 6 NADH
2 FADH2 + 10 NADH
Section 7.4
Third Stage of Aerobic
Respiration – The Big
Energy Payoff
Electron Transfer
Phosphorylation
 Occurs
in the mitochondria
 Coenzymes deliver electrons to
electron transfer chains
 Electron transfer sets up H+ ion
gradients
 Flow
of H+ down gradients powers
ATP formation
Electron Transfer
Phosphorylation

Electron transfer chains
are embedded in inner
mitochondrial
compartment
glucose
GLYCOLYSIS
pyruvate
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
• NADH and FADH2 give up electrons that they picked up
in earlier stages to electron transfer chain
• Electrons are transferred through the chain
• The final electron acceptor is oxygen
Creating an
+
H
Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
ATP Formation
ATP
INNER
COMPARTMENT
ADP
+
Pi
Summary of Transfers
glucose
ATP
2 PGAL
ATP
2 NADH
2 pyruvate
glycolysis
2 CO 2
2 FADH 2
e–
2 acetyl-CoA
2 NADH
H+
H+
2
ATP
6 NADH
Krebs
Cycle
H+
ATP
2 FADH 2
4 CO 2
H+
ATP
36 ATP
electron
transfer
phosphorylation
H+
H+
ADP
+ Pi
H+
H+
H+
Importance of Oxygen
 Electron
transfer phosphorylation
requires the presence of oxygen
 Oxygen
withdraws spent electrons
from the electron transfer chain, then
combines with H+ to form water
Summary of Energy Harvest
(per molecule of glucose)

Glycolysis


Krebs cycle and preparatory reactions


2 ATP formed by substrate-level phosphorylation
2 ATP formed by substrate-level phosphorylation
Electron transfer phosphorylation

32 ATP formed
Energy Harvest from
Coenzyme Reductions

What are the sources of electrons used
to generate the 32 ATP in the final
stage?
4
ATP - generated using electrons released
during glycolysis and carried by NADH
 28 ATP - generated using electrons formed
during second-stage reactions and carried
by NADH and FADH2
Energy Harvest Varies
NADH formed in cytoplasm cannot enter
mitochondrion
 It delivers electrons to mitochondrial
membrane
 Membrane proteins shuttle electrons to
NAD+ or FAD inside mitochondrion
 Electrons given to FAD yield less ATP
than those given to NAD+

Energy Harvest Varies
Liver, kidney, heart cells

Electrons from first-stage reactions are delivered to
NAD+ in mitochondria

Total energy harvest is 38 ATP
Skeletal muscle and brain cells

Electrons from first-stage reactions are delivered to FAD
in mitochondria

Total energy harvest is 36 ATP
Section 7.5
Fermentation Pathways
Anaerobic Pathways

Do not use oxygen

Produce less ATP than aerobic pathways

Two types of fermentation pathways

Alcoholic fermentation

Lactate fermentation
Fermentation Pathways

Begin with glycolysis

Do not break glucose down completely to
carbon dioxide and water

Yield only the 2 ATP from glycolysis

Steps that follow glycolysis serve only to
regenerate NAD+
Alcoholic Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 ADP
2 NAD+
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
ethanol
formation
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Yeasts
 Single-celled
fungi
 Carry out alcoholic fermentation
 Saccharomyces cerevisiae
 Baker’s
yeast
 Carbon dioxide makes bread dough rise
 Saccharomyces
 Used
ellipsoideus
to make beer and wine
Lactate Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
lactate
formation
electrons, hydrogen
from NADH
2 lactate
Lactate Fermentation

Carried out by certain bacteria

Electron transfer chain is in bacterial plasma
membrane

Final electron acceptor is compound from
environment (such as nitrate), not oxygen

ATP yield is low
Lactate Fermentation

Lactobacillus and some other bacteria
produce lactate
 This
produces cheeses, yogurt, buttermilk and
other dairy products

Fermenters also are used to cure meats and
in pickling
 Sauerkraut
is an example
 Sour taste due to lactic acid (form of lactate)
Slow-twitch v. Fast-twitch muscles

Slow-twitch muscles make ATP only by
aerobic respiration (no fermentation)
 Slow-twitch
muscles are for light, steady,
prolonged activity
 Slow-twitch muscles are red because they
have lots of myoglobin, a pigment used to
store oxygen
 They also have many mitochondria
Fast-twitch Muscles





These pale (lighter colored) muscles have few
mitochondria and no myoglobin
Fast-twitch muscles, which are used for
immediate and intense energy demands, use
lactate fermentation to produce ATP
It works quickly, but not for long
Chickens have fast-twitch breast muscles used
for quick flights (white meat)
Ducks fly long distances – what color is their
breast meat?
Alcoholic Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 ADP
2 NAD+
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
ethanol
formation
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Lactate Fermentation
glycolysis
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
lactate
formation
electrons, hydrogen
from NADH
2 lactate
Section 7.6
Alternative Energy Sources
in the Body
The Fate of Glucose
After eating, glucose is absorbed into the
blood
 Insulin levels rise, causing greater uptake
of glucose by cells

 Glycolysis

will follow
Excess glucose is converted into glycogen
 Glycogen
is known as “animal starch,” and is
the main storage polysaccharide in animals
 Stored in the muscles and the liver
Between Meals

When blood levels of glucose decline, pancreas releases
glucagon, a hormone

Glucagon stimulates liver cells to convert glycogen back
to glucose and to release it to the blood

Glycogen levels are adequate, but can be depleted in 12
hours

(Muscle cells do not release their stored glycogen)
Energy Reserves

Glycogen makes up only about 1 percent of the
body’s energy reserves

Proteins make up 21 percent of energy reserves

Fat makes up the bulk of reserves (78 percent)
Energy from Fats

Most stored fats are triglycerides in adipose tissue


Triglycerides are “three-tailed” fats
Triglycerides are broken down to glycerol and fatty
acids

Glycerol is converted to PGAL, an intermediate of
glycolysis

Fatty acids are broken down and converted to acetylCoA, which enters Krebs cycle
glucose
ATP
2 PGAL
ATP
2 NADH
2 pyruvate
glycolysis
2 CO 2
2 FADH 2
e–
2 acetyl-CoA
2 NADH
H+
H+
2
ATP
6 NADH
Krebs
Cycle
H+
ATP
2 FADH 2
4 CO 2
H+
ATP
36 ATP
electron
transfer
phosphorylation
H+
H+
ADP
+ Pi
H+
H+
H+
Energy from Proteins

Proteins are broken down to amino acids

Amino acids are broken apart

Amino group is removed, ammonia forms,
is converted to urea and excreted

Carbon backbones can enter the Krebs
cycle or its preparatory reactions
Reaction Sites
FOOD
fats
fatty
acids
glycogen
glycerol
complex
carbohydrates
proteins
simple sugars
(e.g., glucose)
amino acids
NH3
glucose-6phosphate
urea
carbon
backbones
PGAL
2
glycolysis
ATP
4 ATP
(2 ATP net)
NADH
pyruvate
Acetyl-CoA
NADH
NADH,
FADH2
CO2
Krebs
Cycle
2 ATP
CO2
e–
ATP
ATP
ATP
H+
e– + oxygen
many ATP
fats
Section 7.7
Perspective on Life
Evolution of Metabolic Pathways

When life originated, atmosphere had little oxygen

Earliest organisms used anaerobic pathways

Later, cyclic pathway (simple form) of photosynthesis
increased atmospheric oxygen

Much more efficient cells arose that used oxygen as
final acceptor in electron transfer
Processes
Are Linked
Aerobic Respiration


Reactants
Photosynthesis

Reactants

Sugar

Carbon dioxide

Oxygen

Water
Products

Products

Carbon dioxide

Sugar

Water

Oxygen
Life Is System
of Prolonging Order

Powered by energy inputs from sun, life continues
onward through reproduction

Following instructions in DNA, energy and materials
can be organized, generation after generation

With death, molecules are released and may be cycled as
raw material for next generation