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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 7
How Cells Release
Chemical Energy
(Sections 7.5 - 7.7)
Albia Dugger • Miami Dade College
7.5 Aerobic Respiration’s
Big Energy Payoff
• The third stage of aerobic respiration, electron transfer
phosphorylation, occurs at the inner mitochondrial membrane
• Many ATP are formed in electron transfer phosphorylation
Energy Transfer
• The first two stages of cell respiration (glycolysis and Krebs
cycle) have delivered electrons and hydrogen ions to electron
transfer chains in the inner mitochondrial membrane
• Electrons moving through the chains release energy which is
used to move H+ from the matrix to the intermembrane space
Hydrogen Ion Concentration
• Hydrogen ions that accumulate in the intermembrane space
form a gradient across the inner membrane
• H+ ions follow the gradient back to the matrix through
transport proteins (ATP synthases) that drive ATP synthesis
• Oxygen combines with electrons and H+ at the end of the
transfer chains, forming water
5 Steps in
Electron Transfer Phosphorylation
1. NADH and FADH2 (high-energy molecules created by earlier
steps) deliver electrons to electron transfer chains in the inner
mitochondrial membrane
2. Electron flow through the chains causes hydrogen ions (H+) to
be pumped from the matrix to the intermembrane space
3. The electron transfer chains cause an H+ gradient to form
across the inner mitochondrial membrane
5 Steps in
Electron Transfer Phosphorylation
4. Hydrogen ions flow back to the matrix through ATP
synthases, driving the formation of ATP from ADP and
phosphate (Pi)
5. Oxygen (O2) accepts electrons and hydrogen ions at the end
of mitochondrial electron transfer chains, forming water
Electron Transfer Phosphorylation
1 NADH and FADH2 deliver
electrons to electron transfer
chains in the inner mitochondrial
membrane.
Electron Transfer Phosphorylation
2 Electron flow through the
Oxygen (O2)
cytoplasm
outer membrane
intermembrane space
3 The activity of the electron
3
transfer chains causes a hydrogen
ion gradient to form across the inner
mitochondrial membrane.
inner membrane
2
matrix
1
chains causes hydrogen ions (H+)
to be pumped from the matrix to
the intermembrane space.
5
4
4 Hydrogen ion flow back to the
matrix through ATP synthases
drives the formation of ATP from
ADP and phosphate (Pi).
5 Oxygen (O2) accepts
electrons and hydrogen ions at
the end of mitochondrial
electron transfer chains, so
water forms.
Stepped Art
Fig, 7.8a, p. 114
Summing Up: The Energy Harvest
• Overall, aerobic respiration typically yields 36 ATP for each
glucose molecule
• 32 ATP typically form in the third stage, but yield varies
• Example: Typical yield of aerobic respiration in brain and
skeletal muscle cells is 38 ATP, not 36
Summary:
Aerobic
Respiration
Glycolysis
A First stage:
Glucose is converted
to 2 pyruvate; 2 NADH
and 4 ATP form.
cytoplasm
outer membrane
B Second stage:
10 more
coenzymes accept
electrons and
hydrogen ions
during the
second-stage
reactions.
C Third stage:
Coenzymes that
were reduced in
the first two
stages give up
electrons and
hydrogen ions to
electron transfer
chains. Energy
lost by the
electrons as they
flow through the
chains is used to
move H+ across
the membrane.
intermembrane space
inner membrane
matrix
Krebs
Cycle
oxygen
Fig, 7.9, p. 115
Key Concepts
• Aerobic Respiration
• The final stages of aerobic respiration break down
pyruvate to CO2
• High-energy molecules formed in the first two stages of
cellular respiration deliver electrons and hydrogen ions to
electron transfer chains, where ATP forms by electron
transfer phosphorylation
7.6 Fermentation Pathways
• Anaerobic fermentation pathways begin with glycolysis and
finish in the cytoplasm
• alcoholic fermentation
• lactate fermentation
• A molecule other than oxygen accepts electrons at the end of
these reactions
Key Terms
• alcoholic fermentation
• Anaerobic carbohydrate breakdown pathway that
produces ATP and ethyl alcohol (ethanol)
• Used in wine and breads
• lactate fermentation
• Anaerobic carbohydrate breakdown pathway that
produces ATP and lactate
• Used in cheeses and pickles
Energy of Fermentation
• Breakdown of one glucose molecule in either alcoholic or
lactate fermentation yields only the 2 ATP that form in
glycolysis reactions – enough energy to sustain single-celled
anaerobic species
Alcoholic Fermentation
Products of Alcoholic Fermentation
Lactate Fermentation
Lactate Fermentation in Muscles
• Skeletal muscle has two types of fibers: red and white
• ATP is produced primarily by aerobic respiration in red
muscle fibers, which sustain activities that require endurance
• Lactate fermentation in white fibers supports activities that
occur in short, intense bursts
Lactate in Muscles
Key Concepts
• Fermentation
• Fermentation pathways start with glycolysis
• Substances other than oxygen accept electrons at the end
of the pathways
• Compared with aerobic respiration, the net yield of ATP
from fermentation is small
7.7 Alternative Energy
Sources in the Body
• Simple sugars from carbohydrate breakdown, glycerol and
fatty acids from fat breakdown, and carbon backbones of
amino acids from protein breakdown may enter aerobic
respiration at various reaction steps
Energy From Fats
• When blood glucose gets too high, excess dietary
carbohydrates are converted to fatty acids
• The body stores most fats as triglycerides, which have three
fatty acid tails
• When blood glucose falls, triglycerides provide energy
Energy From Proteins
• When you eat more protein than your body needs, the amino
acids are broken down
Food
Fats
2 fatty acids 1 glycerol
Complex Carbohydrates
Proteins
glucose, other simple sugars3 amino acids
acetyl–CoA intermediate
of glycolysis
acetyl–CoA
Glycolysis
NADH pyruvate
Krebs
cycle
Alternative
Pathways
of Aerobic
Respiration
intermediate
of Krebs
cycle
NADH, FADH2
Electron Transfer
Phosphorylation
Fig, 7.12, p. 118
Key Concepts
• Other Metabolic Pathways
• Molecules other than carbohydrates are common energy
sources in the animal body
• Many different pathways can convert dietary lipids and
proteins to molecules that may enter glycolysis or the
Krebs cycle
When Mitochondria
Spin Their Wheels (revisited)
• At least 83 proteins are directly involved in electron transfer
phosphorylation in mitochondria
• Defects in any of the thousands of other proteins used by
mitochondria (such as frataxin) may be involved in many
illnesses such as diabetes, hypertension, Alzheimer’s,
Parkinson’s disease, and even aging