How Cells Release
Stored Energy
Chapter 8
8.1 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
Summary Equation for Aerobic
Respiration
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
CYTOPLASM
2
glucose
ATP
4
Glycolysis
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
e- + H +
2 CO2
Overview of Aerobic
Krebs
Respiration
Cycle
2 NADH
8 NADH
2 FADH2
e-
ATP
e - + H+
e-
+
4 CO2
H+
2
Electron
Transfer
Phosphorylation
H+
32
ATP
ATP
water
e- + oxygen
Typical Energy Yield: 36 ATP
Figure 8.3
Page 135
The Role of Coenzymes
• NAD+ and FAD accept electrons and
hydrogen
• Become NADH and FADH2
• Deliver electrons and hydrogen to the
electron transfer chain
8.2 GLYCOLYSIS
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
In-text figure
Page 136
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 of Glycolysis
2 ATP invested
Energy-Requiring
Steps
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
P
fructose1,6-bisphosphate
P
PGAL
P
PGAL
Figure 8.4(2)
Page 137
P
NAD+
Pi
P
PGAL
NADH
NAD+
Pi
PGAL
NADH
P
P
Energy1,3-bisphosphoglycerate
ADP
Releasing
ATP
P
3-phosphoglycerate
Steps
P
P
1,3-bisphosphoglycerate
ADP
ATP
P
3-phosphoglycerate
P
P
2-phosphoglycerate
H2
O
P
2-phosphoglycerate
PEP
PEP
P
ADP
ADP
ATP
ATP
pyruvate
H2
O
pyruvate
Figure 8.4
Page 137
Glycolysis: Net Energy Yield
Energy requiring steps:
2 ATP invested
Energy releasing steps:
2 NADH formed
4 ATP formed
Net yield is 2 ATP and 2 NADH
8.3 Second Stage Reactions
• 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 (CoA)
NAD+
NADH
O
CoA
acetyl-CoA
O carbon dioxide
Krebs
Cycle
=CoA
acetyl-CoA
CoA
oxaloacetate
citrate
NADH
H2O
NAD+
H2O
malate
NAD+
H2O
FADH2
isocitrate
NADH
fumarate
O
a-ketoglutarate
FAD
NAD+
NADH
CoA
O
succinate
succinyl-CoA
Figure 8.6
Page 139
ATP
O
ADP +
phosphate group
O
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 forms
• Four-carbon oxaloacetate regenerates
Coenzyme Reductions during
First Two Stages
• Glycolysis
• Preparatory
reactions
• Krebs cycle
2 NADH
2 FADH2 + 6 NADH
• Total
2 FADH2 + 10 NADH
2 NADH
8.4 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
Creating an H+ Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
Making ATP:
Chemiosmotic Model
ATP
INNER
COMPARTMENT
ADP
+
Pi
Importance of Oxygen
• Electron transport 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 transport phosphorylation
– 32 ATP formed
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+
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
8.5 Anaerobic Pathways
• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types
– Fermentation pathways
– Anaerobic electron transport
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+
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
GLYCOLYSIS
Alcoholic
Fermentation
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
2 pyruvate
energy output
2 ATP net
ETHANOL
FORMATION
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
Anaerobic Electron Transport
• 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
FOOD
fats
fatty
acids
glycogen
glycerol
complex
carbohydrates
proteins
simple sugars
amino acids
glucose-6-phosphate
NH3
GLYCOLYSIS
PGAL
pyruvate
acetyl-CoA
8.6 ALTERNATIVE
ENERGY SOURCES
Figure 8.11
Page 145
KREBS
CYCLE
urea
carbon
backbones
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 transport
8.7 Processes Are Linked
sunlight energy
PHOTOSYNTHESIS
water
+
carbon
dioxide
sugar
molecules
oxygen
AEROBIC
RESPIRATION
In-text figure
Page 146