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Biology
Concepts and Applications | 9e
Starr | Evers | Starr
Chapter 7
How Cells Release
Chemical Energy
©
Learning2015
2015
© Cengage
Cengage Learning
Cells break organic molecules apart in
small steps (Review)
• The bonds of organic molecules hold a lot
of energy
• Removed electrons are carried by electron
carrier molecules.
• Redox: one molecule accepts electrons
(it’s reduced) from another molecule (it’s
oxidized)
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Electron Transfers (Review)
• Electron transfer chain (ETC): array of
molecules that accept and give up
electrons
• The energy of the electrons is released
with each step of the sequence by moving
(H+) across membrane
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7.1 How Do Cells Access the Chemical
Energy in Sugars?
• Cells use the energy stored in molecules,
• There are two mechanisms by which
organisms break down sugars to make
ATP
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7.3 What Is Glycolysis?
• Glycolysis found in ALL cells
• Net yield of 2 ATP per glucose
• Electrons from glucose are transferred to 2
electron carrier NADH
• Produces 2 three-carbon pyruvate
molecules
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What Is Glycolysis? (cont’d.)
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Aerobic Respiration and Fermentation
Compared (cont’d.)
• Aerobic respiration follows glycolysis when
oxygen is present
• Produces more ATP
• Main energy-releasing pathway in nearly
all eukaryotes and some bacteria
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7.4 What Happens During the Second
Stage of Aerobic Respiration?
• Aerobic part occurs inside mitochondria
• Breaks down the pyruvate produced
during glycolysis
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What Happens During the Second Stage
of Aerobic Respiration? (cont’d.)
mitochondrion
cytoplasm
2 pyruvate
outer membrane
inner membrane
2 acetyl–CoA
matrix
6 CO2
2
8
2
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The breakdown of 2 pyruvate
(from glycolysis) to 6 CO2
yields 2 ATP and 10 reduced
coenzymes (8 NADH and 2
FADH2).
Electrons carried by the
coenzymes will power ATP
formation in the third stage of
aerobic respiration.
Acetyl–CoA Formation
• Each pyruvate is split into CO2 and a twocarbon acetyl group (Acetyl CoA)
• Electrons are removed combine with
NADH
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The Krebs Cycle (a.k.a. Citric acid cycle)
• Each acetyl groups are transferred forming
citric acid
• ATP formed
• Three NADH form and 1 FADH2
• Two CO2 released
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The Krebs Cycle (cont’d.)
2nd stage of
aerobic respiration
pyruvate (2)
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carbon dioxide (6)
7.5 What Happens During the Third Stage
of Aerobic Respiration?
• Last stage occurs on the inner
mitochondrial membrane
• NADH and FADH2 deliver electrons and H+
to electron transfer chains
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What Happens During the Third Stage of
Aerobic Respiration? (cont’d.)
• E.T.C. move H+ actively across the inner
membrane
• Ion gradient causes the ions to flow
through the ATP synthase
• Oxygen accepts electrons at the end of
mitochondrial electron transfer chains
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Summary of aerobic respiration:
• For each glucose molecule, 4 ATP form in
the first- and second-stage reactions
• The twelve electron carriers produce 32
additional ATP during the third stage
• 36 net ATP are produced in total
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What Happens During the Third Stage of Aerobic Respiration? (cont’d.)
glucose
Stage 1
Glycolysis in cytoplasm splits a glucose
molecule into 2 pyruvate; 2 NADH and 4
ATP also form. An investment of 2 ATP
began the reactions, so the net yield is 2
ATP.
2
Stage 2
Acetyl–CoA formation and the Krebs cycle
in the mitochondrial matrix break down
the pyruvate to CO2, which leaves the cell.
Ten additional coenzymes are reduced. Two
ATP form.
Stage 3
In electron transfer phosphorylation, the reduced
coenzymes give up electrons and hydrogen ions
to electron transfer chains in the inner mitochondrial membrane. Energy lost by the electrons as
they move through the chains is used to move H+
across the membrane. The resulting gradient
causes H+ to flow through ATP synthases,
which drives ATP synthesis.
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4
2 NADH
2 pyruvate
2 NADH
2 NADH
2 acetyl–CoA
2 CO2
4 CO2
6 NADH
2 FADH2
2
32
oxygen H2O
2 (net)
7.6 What Is Fermentation?
• Pyruvate is not fully broken down to CO2
• No additional ATP forms
• The net yield is two ATP
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What Is Fermentation? (cont’d.)
• Alcoholic fermentation: pathway that
produces ATP, CO2, and ethanol
• Lactate fermentation: pathway that
produces ATP and lactate
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Alcoholic Fermentation (cont’d.)
NADH
NAD+
+
pyruvate
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carbon
dioxide
acetaldehyde
ethanol
Lactate Fermentation (cont’d.)
NADH
pyruvate
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NAD+
lactate
Lactate Fermentation (cont’d.)
B
C
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7.7 Can the Body Use Any Organic
Molecule for Energy?
• 36 ATP by fully oxidizing glucose
• Other carbohydrates, fats, and proteins
can be converted to molecules that enter
glycolysis or the Krebs cycle
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Food
7.7 Can the Body Use Any Organic
Molecule for Energy? (cont’d.)
a triglyceride (fat)
glycerol
head
Fats
fatty acids
2
acetyl–CoA
glycerol
3
Complex Carbohydrates
Proteins
glucose, other simple sugars
amino acids
1
4
acetyl–CoA
PGAL
fatty acid
tails
NADH pyruvate
intermediate
of Krebs cycle
NADH, FADH2
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7.8 Application:
Mitochondrial Malfunction
• Sometimes when oxygen enters an
electron transfer chain, it escapes as a free
radical
– Free radicals cause damage by oxidizing
biological molecules and breaking carbon
backbones
• Antioxidants in the cytoplasm detoxify free
radicals
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Application: Mitochondrial
• A genetic disorder or encounter with a toxin
Malfunction (cont’d.)
can result in a missing antioxidant or
defective electron transfer chain
– Free radicals accumulate and destroy first the
function of mitochondria, then the cell
• This tissue damage is called oxidative stress
– Hundreds of incurable disorders are
associated with such defects
• Cancer, hypertension, Alzheimer’s, and Parkinson’s
diseases
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Application: Mitochondrial Malfunction (cont’d.)
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