How Cells Release Stored Energy

1
How Cells Release Stored Energy

• Chapter 8

2
Killer Bees

• Descendents of African honeybees that were
  imported to Brazil in the 1950s
• Africanized bees muscle cells have large
  mitochondria

3
ATP Is Universal Energy Source

• Photosynthesizers get energy from the sun
• Animals get energy second- or third-hand from
  plants or other organisms
• Regardless, the energy is converted to the
  chemical bond energy of ATP

4
Making ATP

• Cells of all organisms make ATP by breaking down
  carbohydrates, fats, and protein

5
Main Types of Energy-Releasing Pathways

• Aerobic pathways
• Evolved later
• Require oxygen
• Start with glycolysis in cytoplasm
• Completed in mitochondria

• Anaerobic pathways
• Evolved first
• Dont require oxygen
• Start with glycolysis in cytoplasm
• Completed in cytoplasm

6
Summary Equation for Aerobic Respiration

• C6H1206 6O2 6CO2 6H20
• glucose oxygen
  carbon water
• dioxide

7
Overview of Aerobic Respiration
CYTOPLASM
glucose
ATP
4
2
ATP
Glycolysis
(2 ATP net)
e- H
2 pyruvate
2 NADH
e- H
2 CO2
2 NADH
e- H
4 CO2
8 NADH
KrebsCycle
e- H
2
ATP
2 FADH2
e-
Electron Transfer Phosphorylation
32
ATP
water
H
e- oxygen
Typical Energy Yield 36 ATP
8
The Role of Coenzymes

• NAD and FAD accept electrons and hydrogen
• Become NADH and FADH2
• Deliver electrons and hydrogen to the electron
  transfer chain

9
Glycolysis Occurs in Two Stages

• Energy-requiring steps
• ATP energy activates glucose
• Energy-releasing steps
• The products of the first part are split into
  three-carbon pyruvate molecules
• ATP and NADH form

10
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

11
Second Stage Reactions

• Preparatory reactions
• Pyruvate forms two-carbon acetyl and carbon
  dioxide
• NAD is reduced to NADH
• Krebs cycle
• The acetyl units form carbon dioxide
• NAD and FAD reduced to NADH and
• FADH2

12
Preparatory Reactions
pyruvate
coenzyme A (CoA)
NAD
carbon dioxide
NADH
CoA
acetyl-CoA
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Krebs Cycle
CoA
acetyl-CoA
CoA
oxaloacetate
citrate
H2O
NADH
NAD
H2O
malate
isocitrate
NAD
H2O
O
O
NADH
fumarate
FADH2
a-ketoglutarate
FAD
NAD
CoA
NADH
succinate
succinyl-CoA
ADP phosphate group
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The Krebs Cycle

• Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP

• Overall Reactants
• Acetyl-CoA
• 3 NAD
• FAD
• ADP and Pi

15
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

16
Coenzyme Reductions during First Two Stages

• Glycolysis 2 NADH
• Preparatory
• reactions 2 NADH
• Krebs cycle 2 FADH2 6 NADH
• Total 2 FADH2 10 NADH

17
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

18
Creating an H Gradient
OUTER COMPARTMENT
NADH
INNER COMPARTMENT
19
Making ATP Chemiosmotic Model
ATP
INNER COMPARTMENT
ADPPi
20
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

21
Summary of Energy Harvest(per molecule of
glucose)

• Glycolysis
• 2 ATP
• Krebs cycle and preparatory reactions
• 2 ATP formed
• Electron transport phosphorylation
• 32 ATP formed

22
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

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Anaerobic Pathways

• Do not use oxygen
• Produce less ATP than aerobic pathways
• Two types Fermentation pathways

24
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

25
Lactate Fermentation
GLYCOLYSIS
C6H12O6
ATP
2
energy input
2 NAD
2 ADP
NADH
2
ATP
4
2 pyruvate
energy output
2 ATP net
LACTATE FORMATION
electrons, hydrogen from NADH
2 lactate
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Alcoholic Fermentation
GLYCOLYSIS
C6H12O6
ATP
2
2 NAD
energy input
2 ADP
NADH
2
ATP
4
2 pyruvate
energy output
2 ATP net
ETHANOL FORMATION
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen from NADH
2 ethanol
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Alternative Energy Sources
FOOD
complex carbohydrates
glycogen
fats
proteins
simple sugars
fatty acids
glycerol
amino acids
glucose-6-phosphate

NH3
carbon backbones
GLYCOLYSIS
urea
PGAL
pyruvate
acetyl-CoA
KREBS CYCLE
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Evolution of Metabolic Pathways

• When life originated, atmosphere had little
  oxygen
• Earliest organisms used anaerobic pathways
• Later, photosynthesis increased atmospheric
  oxygen
• Cells arose that used oxygen as final acceptor in
  electron transport

29
Processes Are Linked
sunlight energy
PHOTOSYNTHESIS
water carbondioxide
sugarmolecules
oxygen
AEROBICRESPIRATION