HOW CELLS RELEASE STORED ENERGY

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HOW CELLS RELEASE STORED ENERGY
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I. Introduction to Cellular Respiration
A. Breathing and cellular
respiration

1. Breathing - the process by which
the body exchanges gases with the
environment (external respiration)




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2. Cellular respiration - the chemical
breakdown of organic compounds to
release energy for cellular work (internal
respiration)
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B. Two main types of energy releasing pathways
1. Aerobic respiration - occurring in the
presence of of free oxygen
a. About 50 efficient b. Yields about
36ATPs
2. Anaerobic respiration - occurring without
free oxygen
a. About 2 efficient b. Yields 2
ATP
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C. All energy-releasing pathways start with
glycolysis
1. Occurs in the cytoplasm without the use of
oxygen
2. Glucose is split into two pyruvate
molecules
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II. Molecular Overview of Aerobic Respiration
A. Summary equation
B. Three stages of aerobic cellular
respiration (Figure 8.3)
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1. Glycolysis - breakdown of glucose (6C) into
two molecules of pyruvate (3C)

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2. Krebs cycle - degrades pyruvate to carbon
dioxide, hydrogen ions (H), and electrons
(e-)
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3. Electron transfer phosphorylation - processes
H and e- to generate high yields of ATP
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C. Cellular respiration is an oxidation-reduction
process
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1. glucose carbon dioxide
Oxidation - glucose loses electrons and
hydrogens (H and e-)

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2. oxygen water
Reduction - oxygen gains electrons and
hydrogens (H and e-)
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D. Coenzymes serve as intermediates in
transferring electrons
1. NAD accepts electrons to become reduced
to NADH
2. FAD accepts electrons to become reduced to
FADH2
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III. Glycolysis (in cytoplasm) -- (Figure 8.4)
A. Net result Glucose (6C) is
broken into two molecules of
pyruvate (3C)

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B. Energy-requiring steps (Figure 8.4 a-d)
1. Requires input of 2 ATP

2. Glucose (6C) is transformed into
two molecules of PGAL (3 C)

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C. Energy-releasing steps (Figure 8.4 e-k)
1. H and e- are removed from PGAL
and transferred to NAD,
reducing it to NADH





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2. Four ATPs are formed by substrate level
phosphorylation





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3. Two molecules of pyruvate are
formed

D. End products 2 pyruvate 2 NADH 2
ATP




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IV. Krebs Cycle
A. Mitochondrion - site of Krebs cycle and
electron transport (Figure 8.5)
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1. Enzymes regulating the Krebs cycle are in the
inner compartment
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2. Enzymes regulating electron transport
phosphorylation are in the inner mitochondrial
membrane
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(Figure 8.5c - Functional zones in a
mitochondrion)
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pyruvate
B. Preparatory steps (Figure 8.6 a)
coenzyme A (CoA)
NAD
(CO2)
NADH
CoA
AcetylCoA
1. Pyruvate (3C) is stripped of carbon,
producing acetate (2C) and releasing
CO2
CoA
oxaloacetate
citrate
H2O
NADH
H2O
NAD
2. NAD is reduced to NADH
isocitrate
malate
NAD
H2O
NADH
fumarate
a-ketogluterate
3. Acetate is coupled with coenzyme A
to form Acetyl-CoA
FADH2
CoA
NAD
FAD
NADH
succinate
CoA
succinylCoA
ADP phosphate group (from GTP)
ATP
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pyruvate
C. Krebs cycle (Figures 8.6 b-g)
coenzyme A (CoA)
NAD
(CO2)
NADH
1.Acetyl-CoA (2C) combines with
oxaloacetate (4C) to form citrate (citric
acid) (6C)
CoA
AcetylCoA

CoA
oxaloacetate
citrate
2. Citrate is cycled back to
oxaloacetate
H2O
NADH
H2O
NAD

isocitrate
malate
NAD
3. CO2 is released
H2O
NADH
4. Hydrogen (H and e-) is released to
carriers NAD and FAD to form NADH and
FADH2
fumarate
a-ketogluterate

FADH2
CoA
NAD
FAD
NADH
succinate
CoA
5. 2 ATPs are formed
succinylCoA
ADP phosphate group (from GTP)
ATP
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V. Electron Transfer Phosphorylation (Figure 8.7)
A. Consists of a series of protein complexes
and and electron carriers (enzymes and
cytochromes) embedded in the inner
mitochondrial membrane
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B. Electrons from NADH and FADH2 are transported
down an electron transfer chain
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C. ATP is generated by chemiosmosis
D. Electrons and hydrogen ions are finally
accepted by oxygen and released as water

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E. Net gain of 32 ATPs
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VI. ATP Synthesis in Aerobic Respiration
Glycolysis 2 ATP
Krebs Cycle 2 ATP
Electron Transport 32 ATP
TOTAL 36 ATP

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VII. Anaerobic Respiration - oxygen is not the
final electron acceptor
A. Fermentation - the breakdown if glucose
in which organic substances are the
final electron acceptor
1. Glycolysis is the initial stage
a. Yields 2 ATPs b. NADH is oxidized
back to NAD
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2. Alcoholic fermentation - forms carbon dioxide
and ethanol (ethyl alcohol)
a. Used in baking and to produce alcoholic
beverages
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3. Lactate fermentation - pyruvate is converted
to lactate (lactic acid)
a. Used to produce food products, such as
cheese, yogurt, sauerkraut
b. Can occur in human muscle cells
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B. Anaerobic electron transfer - the final
electron acceptor is an inorganic substance
other than oxygen, such as sulfates (SO4)
or nitrates (NO3)
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X. Alternative Energy Sources
A. Krebs cycle intersects other
biochemical pathways (Figure 8.12)
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B. Energy form glycogen
1. Excess glucose is stored as glycogen in
liver and muscles
2. Glycogen reserves are tapped when free
glucose supply is low
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glycogen --gt glucose --gt glycolysis
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C. Energy from lipids
1. Fats are stored as triglycerides in
adipose tissue
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2. Glycerol can enter glycolysis
3. Fatty acids can enter the Krebs cycle
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D. Energy from proteins

1. Amino acids can be fed into the Krebs
cycle
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pyruvate
amino acids acetyl Co-A
Krebs cycle