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Cellular Respiration: How
Cells Release Chemical
Energy
sunlight energy
photosynthesis
organic
compounds,
oxygen
carbon
dioxide,
water
aerobic
respiration
p.92
Aerobic Respiration
• Uses products of photosynthesis
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
ATP:
Universal Energy Source
• Photosynthesizers get light energy from
the sun, store it as chemical energy,
and produce ATP
• Animals eat plants or other animals and
transform chemical energy to ATP
Making ATP
• Plants make ATP during photosynthesis
• Prokaryotes make ATP by fermentation
• Cells of most organisms make ATP by
aerobic respiration of carbohydrates,
fats, and protein
Aerobic Respiration
•
Occurs in three steps:
1. Glycolysis; partial breakdown of glucose
Occurs in the cytoplasm
Produces 2 ATP
2. Krebs cycle (citric acid cycle)
Break down of glycolysis byproducts to CO2
produces NADH and FADH
3. Electron Transport Chain
uses NADH and FADH from Krebs cycle
to produce ATP
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+
Electron
Transfer
Phosphorylation
H+
32
2
ATP
ATP
water
e- + oxygen
Typical Energy Yield: 36 ATP
Summary Equation for Aerobic
Respiration
C6H12O6 +
GLUCOSE
6O2
OXYGEN
6CO2 +
CARBON
DIOXIDE
6H2O +
WATER
ATP
The Role of Coenzymes
• NAD+ and FAD accept electrons and
hydrogen
• Become NADH and FADH2
• Deliver electrons and hydrogen to the
electron transfer chains
Glycolysis Occurs
in Two Stages
• Energy-requiring steps
– ATP energy activates glucose and its 6-carbon
derivatives
• Energy-releasing steps
– The products of the first part are split into 3-carbon
pyruvate molecules
– ATP and NADH form
glucose
GLYCOLYSIS
pyruvate
to second stage of aerobic
respiration or to a different
energy-releasing pathway
Fig. 6-2, p.84
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
Energy-Requiring
Steps
Energy-Requiring Steps of Glycolysis
2 ATP invested
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
P
fructose1,6-bisphosphate
P
PGAL
P
PGAL
Energy Releasing Steps of Glycolysis
P
NAD+
Pi
P
PGAL
NADH
NAD+
Pi
PGAL
NADH
P
P
Energy1,3-bisphosphoglycerate
ADP
Releasing
ATP
P
Steps
3-phosphoglycerate
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
Glycolysis: Net Energy Yield
Energy requiring steps:
2 ATP used
Energy releasing steps:
2 NADH formed
4 ATP formed
Net yield: 2 ATP + 2 NADH
Mitochondria
• Organelles where the next two
phases of aerobic respiration
proceed (Krebs cycle and
electron transport chain)
• Produces 34 more energy
molecules ATP
Mitochondria
Second Stage Reactions
• Preparatory reactions
– Pyruvate is oxidized into 2-carbon acetylCoA + CO2
– NAD+ is reduced
• Krebs cycle
– Acetyl-CoA is oxidized to two CO2
– NAD+ and FAD are reduced
Second Stage
Reactions
glucose
GLYCOLYSIS
pyruvate
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
CYTOPLASM
2
glucose
Second Stage
Reactions
Glycolysis
4
ATP
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 +
4 CO2
e- + H +
Electron
Transfer
Phosphorylation
H+
32
2
ATP
ATP
water
e- + oxygen
ELECTRON TRANSFER
Typical Energy Yield: 36 ATP
PHOSPHORYLATION
Preparatory Reactions
pyruvate
NAD+
coenzyme A (CoA)
NADH
O
CoA
acetyl-CoA
O carbon dioxide
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
Fig. 6-3, p.86
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 CO2
• Coenzymes are reduced (they pick up
electrons and hydrogen)
• One molecule of ATP is formed
• 4-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
glucose
Third Stage
GLYCOLYSIS
pyruvate
KREBS
CYCLE
ELECTRON TRANSFER
PHOSPHORYLATION
Electron Transfer
Phosphorylation
• Occurs in mitochondria
• Coenzymes deliver electrons to electron
transfer systems
• 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
ATP
INNER
COMPARTMENT
ADP
+
Pi
Importance of Oxygen
• Electron transport phosphorylation
requires oxygen
• Oxygen withdraws spent electrons from
the electron transport system, then
combines with H+ to form water
glucose
2
ATP
2 PGAL
4 ATP
2 NAD+
2 NADH
2 pyruvate
Glycolysis
2 FADH2
e–
2 CO2
2 acetyl2 NADH
CoA
2 ATP
6 NADH
Krebs
Cycle
2 FADH2
4CO2
ATP
ATP
32 ATP
Electron Transfer
phosphorylation
oxygen
accepts
“spent”
electrons
H+
H+
H+
H+
H+
H+
ADP
+ Pi
H+
H+
H+
Fig. 6-5 p.87
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
Anaerobic Pathways
• Alcoholic Fermentation
Fermentation Pathways
• Begin with glycolysis
• Are anaerobic: don’t require oxygen
• Yield only 2 ATP from glycolysis
• Steps after glycolysis only regenerate NAD+
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
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
Alternative Energy Sources
• Carbohydrates, fats, and proteins
– are digested and enter aerobic respiration
Alternative Pathways
FOOD
fats
fatty
acids
glycerol
glycogen
complex
carbohydrates
proteins
simple sugars
amino acids
glucose-6-phosphate
NH3
GLYCOLYSIS
PGAL
pyruvate
acetyl-CoA
KREBS
CYCLE
urea
carbon
backbones
Evolution of Metabolic Pathways
• Earliest organisms used anaerobic
pathways
• Later, noncyclic pathway of
photosynthesis increased atmospheric
oxygen
• Aerobic respiration evolved due to
selective pressure by oxygen
Anaerobic Archaeans
• Use hydrogen sulfide as energy source