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How Cells Release
Chemical Energy
Chapter 7
Producing the Universal
Currency of Life
All energy-releasing pathways
– require characteristic starting materials
– yield predictable products and by-products
– produce ATP
ATP Is Universal
Energy Source
• Photosynthesizers get energy from the
sun
• 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
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
Main Pathways Start
with Glycolysis
• Glycolysis occurs in cytoplasm
• Reactions are catalyzed by enzymes
Glucose
(six carbons)
2 Pyruvate
(three carbons)
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
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
Glycolysis Occurs
in Two Stages
• Energy-requiring steps
– ATP energy activates glucose and its six-
carbon derivatives
• Energy-releasing steps
– The products of the first part are split into
three-carbon pyruvate molecules
– ATP and NADH form
Energy-Requiring Steps
ATP
ENERGY-REQUIRING STEPS
OF GLYCOLYSIS
glucose
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 invested
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 invested
pyruvate
pyruvate
to second set of reactions
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
Second-Stage Reactions
• Occur in the
mitochondria
• Pyruvate is broken
down to carbon
dioxide
• More ATP is formed
• More coenzymes
are reduced
inner
mitochondrial
membrane
outer
mitochondrial
membrane
inner
outer
compartment compartment
Two Parts of Second Stage
• Preparatory reactions
– Pyruvate is oxidized into two-carbon acetyl
units and carbon dioxide
– NAD+ is reduced
• Krebs cycle
– The 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
Coenzyme A
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
Coenzyme Reductions during
First Two Stages
• Glycolysis
• Preparatory
reactions
• Krebs cycle
2 NADH
2 FADH2 + 6 NADH
• Total
2 FADH2 + 10 NADH
2 NADH
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
Second Stage of
Aerobic Respiration
Acetyl-CoA
Formation
pyruvate
coenzyme A
(CO2)
NAD+
NADH
CoA
acetyl-CoA
Krebs Cycle
CoA
oxaloacetate
citrate
NAD+
NADH
NADH
NAD+
FADH2
NAD+
FAD
NADH
ATP
ADP +
phosphate
group
Electron Transfer
Phosphorylation
glucose
GLYCOLYSIS
pyruvate
• Electron transfer chains
are embedded in inner
mitochondrial
compartment
• NADH and FADH2 give up electrons that they picked up
in earlier stages to electron transfer chain
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
• 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 CO2
2 FADH2
e–
2 acetyl-CoA
2 NADH
H+
H+
2
ATP
6 NADH
Krebs
Cycle
KREBS
CYCLE
ATP
2 FADH2
4 CO2
H+
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
– 2 ATP formed by substrate-level
phosphorylation
• Krebs cycle and preparatory reactions
– 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
Efficiency of
Aerobic Respiration
• 686 kcal of energy are released
• 7.5 kcal are conserved in each ATP
• When 36 ATP form, 270 kcal (36 X 7.5) are
captured in ATP
• Efficiency is 270 / 686 X 100 = 39 percent
• Most energy is lost as heat
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 ellipsoideus
– Used to make beer and wine
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
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
Carbohydrate Breakdown
and Storage
• Glucose is absorbed into blood
• Pancreas releases insulin
• Insulin stimulates glucose uptake by cells
• Cells convert glucose to glucose-6-phosphate
• This traps glucose in cytoplasm where it can
be used for glycolysis
Making Glycogen
• If glucose intake is high, ATP-making
machinery goes into high gear
• When ATP levels rise high enough, glucose6-phosphate is diverted into glycogen
synthesis (mainly in liver and muscle)
• Glycogen is the main storage polysaccharide
in animals
Using Glycogen
• When blood levels of glucose decline,
pancreas releases glucagon
• Glucagon stimulates liver cells to convert
glycogen back to glucose and to release it to
the blood
• (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
• 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
acetyl-CoA, which enters Krebs cycle
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
Evolution of Metabolic
Pathways
• When life originated, atmosphere had little
oxygen
• Earliest organisms used anaerobic pathways
• Later, noncyclic pathway of photosynthesis
increased atmospheric oxygen
• Cells arose that used oxygen as final
acceptor in electron transfer
Processes
Are Linked
Aerobic Respiration
Photosynthesis
• Reactants
• 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