Chapter 7
How Cells Release
Chemical Energy
Albia Dugger • Miami Dade College
7.1 Mighty Mitochondria
• More than forty disorders related to defective mitochondria
are known (such as Friedreich’s ataxia); many of those
afflicted die young
A Mitochondrion
Two Main Metabolic Pathways
• Aerobic metabolic pathways (using oxygen) are used by
most eukaryotic cells
• Anaerobic metabolic pathways (which occur in the absence
of oxygen) are used by prokaryotes and protists in anaerobic
habitats
Aerobic Respiration
• In modern eukaryotic cells, most of the aerobic respiration
pathway takes place inside mitochondria
• Like chloroplasts, mitochondria have an internal folded
membrane system that allows them to make ATP efficiently
• Electron transfer chains in this membrane set up hydrogen
ion gradients that power ATP synthesis
• At the end of these chains, electrons are transferred to
oxygen molecules
INTERACTION: Structure of a
mitochondrion
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7.2 Overview of
Carbohydrate Breakdown Pathways
• Photoautotrophs make ATP during photosynthesis and use it
to synthesize glucose and other carbohydrates
• Most organisms, including photoautotrophs, make ATP by
breaking down glucose and other organic compounds
energy
Photosynthesis
CO2
glucose
H2O
O2
Aerobic Respiration
energy
Figure 7-2 p118
Overview of Aerobic Respiration
• Three stages
• Glycolysis
• Acetyl-CoA formation and Krebs cycle
• Electron transfer phosphorylation (ATP formation)
→
C6H12O6 (glucose) + O2 (oxygen)
CO2 (carbon dioxide) + H2O (water)
• Coenzymes NADH and FADH2 carry electrons and
hydrogen
Aerobic Respiration
glucose
In the Cytoplasm
2 ATP
Glycolysis
4 ATP (2
net)
2 NADH 2 pyruvate
Krebs
Cycle
6 CO2
2 ATP
8 NADH, 2 FADH2
oxygen
Electron Transfer
Phosphorylation
In the Mitochondrion
H2O
32 ATP
Figure 7-3 p119
ANIMATED FIGURE: Overview of aerobic
respiration
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Aerobic Respiration vs.
Anaerobic Fermentation
• Aerobic respiration and fermentation both begin with
glycolysis, which converts one molecule of glucose into two
molecules of pyruvate
• After glycolysis, the two pathways diverge
• Fermentation is completed in the cytoplasm, yielding 2
ATP per glucose molecule
• Aerobic respiration is completed in mitochondria, yielding
36 ATP per glucose molecule
Glycolysis
Carbohydrate
breakdown pathways
start in the cytoplasm,
with glycolysis.
Fermentation
concludes in
cytoplasm.
In eukaryotes,
aerobic respiration
concludes inside
mitochondria.
Figure 7-4 p119
ANIMATED FIGURE: Where pathways start
and finish
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Take-Home Message: How do cells access the
chemical energy in carbohydrates?
• Most cells convert the chemical energy of carbohydrates to
chemical energy of ATP by aerobic respiration or fermentation
• Aerobic respiration and fermentation pathways start in
cytoplasm, with glycolysis
• Fermentation is anaerobic and ends in the cytoplasm
• Aerobic respiration requires oxygen. In eukaryotes, it ends in
mitochondria
3D ANIMATION: Cellular Respiration
7.3 Glycolysis –
Glucose Breakdown Starts
• The reactions of glycolysis convert one molecule of glucose to
two molecules of pyruvate for a net yield of two ATP
• An energy investment of ATP is required to start glycolysis
Glycolysis
• Two ATP are used to split glucose and form 2 PGAL, each
with one phosphate group
• Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH
• Four ATP are formed by substrate-level phosphorylation
(net 2 ATP)
• Glycolysis ends with the formation of two three-carbon
pyruvate molecules
ANIMATED FIGURE: Glycolysis
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ATP-Requiring Steps
1 An enzyme (hexokinase) transfers a phosphate group
from ATP to glucose, forming glucose-6-phosphate.
2 A phosphate group from a second ATP is transferred to
the glucose-6phosphate. The resulting molecule is unstable,
and it splits into two three carbon molecules. The molecules
are interconvertible, so we will call them both PGAL
(phosphoglyceraldehyde). Two ATP have now been invested
in the reactions.
ATP-Generating Steps
3 Enzymes attach a phosphate to the two PGAL, and
transfer two electrons and a hydrogen ion from each PGAL
to NAD+. Two PGA (phosphoglycerate) and two NADH are the
result.
4 Enzymes transfer a phosphate group from each PGA to
ADP. Thus, two ATP have formed by substrate-level
phosphorylation. The original energy investment of two ATP
has now been recovered.
5 Enzymes transfer a phosphate group from each of two
intermediates to ADP. Two more ATP have formed by
substrate-level phosphorylation. Two molecules of pyruvate
form at this last reaction step.
6 Summing up, glycolysis yields two NADH, two ATP (net),
Stepped Art
and two pyruvate for each glucose molecule. Depending on
the type of cell and environmental conditions, the pyruvate
may enter the second stage of aerobic respiration or it may
be used in other ways, such as in fermentation.
Figure 7-5 p121
Take-Home Message:
What is glycolysis?
• Glycolysis is the first stage of carbohydrate breakdown in both
aerobic respiration and fermentation
• The reactions of glycolysis occur in the cytoplasm
• Glycolysis converts one molecule of glucose to two molecules
of pyruvate, with a net energy yield of two ATP; two NADH
also form
3D ANIMATION: Cellular Respiration
ANIMATION: Energy inputs and release in
glycolosis
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7.4 Second Stage of Aerobic Respiration
• The second stage of aerobic respiration completes the
breakdown of glucose that began in glycolysis
• Occurs in mitochondria
• Includes two sets of reactions: acetyl CoA formation and the
Krebs cycle (each occurs twice in the breakdown of one
glucose molecule)
Acetyl CoA Formation
• In the inner compartment of the mitochondrion, enzymes split
pyruvate, forming acetyl CoA and CO2 (which diffuses out of
the cell)
• NADH is formed
The Krebs Cycle
• Krebs cycle
• A sequence of enzyme-mediated reactions that break
down 1 acetyl CoA into 2 CO2
• Oxaloacetate is used and regenerated
• 3 NADH and 1 FADH2 are formed
• 1 ATP is formed
Second Stage of Aerobic Respiration
cytoplasm
outer
membrane
inner
membrane
matrix
The breakdown of 2
pyruvate to 6 CO2 yields 2
ATP and 10 reduced
coenzymes (8 NADH, 2
FADH2). The coenzymes will
carry their cargo of
electrons and hydrogen
ions to the third stage of
aerobic respiration.
ANIMATED FIGURE: The Krebs Cycle details
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Acetyl–CoA Formation and the Krebs Cycle
1 An enzyme splits a pyruvate
coenzyme A NAD+ molecule into a
two-carbon acetyl group and CO2.
Coenzyme A binds the acetyl group
(forming acetyl–CoA). NAD+
combines with released hydrogen
ions and electrons,
forming NADH.
2 The Krebs cycle starts as one
carbon atom is transferred from
acetyl–CoA tooxaloacetate.
Citrate forms, and coenzyme
A is regenerated.
3 A carbon atom is
removed from an
intermediate and leaves the
cell as CO2. NAD+ combines
with released hydrogen ions
and electrons, forming
NADH.
4 A carbon atom is
removed from another
intermediate and leaves the
cell as CO2, and another
NADH forms.
Pyruvate’s three carbon
atoms have now exited the
cell, in CO2.
8 The final steps
of the Krebs cycle
regenerate
oxaloacetate.
Krebs
Cycle
7 NAD+ combines
with hydrogen ions
and electrons,
forming NADH.
6 The coenzyme
FAD combines with
hydrogen
ions and electrons,
forming FADH2.
5 One ATP forms
by substrate-level
phosphorylation.
Stepped Art
Figure 7-7 p123
Take-Home Message: What happens during the
second stage of aerobic respiration?
• The second stage of aerobic respiration, acetyl–CoA
formation and the Krebs cycle, occurs in the inner
compartment (matrix) of mitochondria
• The pyruvate that formed in glycolysis is converted to acetyl–
CoA and CO2; the acetyl–CoA enters the Krebs cycle, which
breaks it down to CO2
• For two pyruvate molecules broken down in the second-stage
reactions, two ATP form, and ten coenzymes (eight NAD+;
two FAD) are reduced
7.5 Aerobic Respiration’s Big Energy Payoff
• Many ATP are formed during the third and final stage of
aerobic respiration
• Electron transfer phosphorylation
• Occurs in mitochondria
• Results in attachment of phosphate to ADP to form ATP
Electron Transfer Phosphorylation
• Coenzymes NADH and FADH2 donate electrons and H+ to
electron transfer chains
• Active transport forms a H+ concentration gradient in the outer
mitochondrial compartment
• H+ follows its gradient through ATP synthase, which attaches
a phosphate to ADP
• Finally, oxygen accepts electrons and combines with H+,
forming water
Electron Transfer Phosphorylation
Summary: The Energy Harvest
• Typically, the breakdown of one glucose molecule yields 36
ATP
• Glycolysis: 2 ATP
• Acetyl CoA formation and Krebs cycle: 2 ATP
• Electron transfer phosphorylation: 32 ATP
Figure 7-9 p125
ANIMATED FIGURE: Third-stage reactions
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Take-Home Message: What happens during
the third stage of aerobic respiration?
• In electron transfer phosphorylation, energy released by
electrons flowing through electron transfer chains is captured
in the attachment of phosphate to ADP; a typical net yield of
aerobic respiration is thirty-six ATP per glucose
• The reactions begin when coenzymes that were reduced in
the first and second stages of reactions deliver electrons and
hydrogen ions to electron transfer chains in the inner
mitochondrial membrane
Take-Home Message: (cont.)
• Energy released by electrons as they pass through electron
transfer chains is used to pump H+ from the mitochondrial
matrix to the intermembrane space
• The H+ gradient that forms across the inner mitochondrial
membrane drives the flow of hydrogen ions through ATP
synthases, which results in ATP formation
ANIMATION: Mitochondrial chemiosmosis
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7.6 Fermentation
• Fermentation pathways break down carbohydrates without
using oxygen
• The final steps in these pathways regenerate NAD+ but do not
produce ATP
Fermentation
• Glycolysis is the first stage of fermentation
• Forms 2 pyruvate, 2 NADH, and 2 ATP
• Pyruvate is converted to other molecules, but is not fully
broken down to CO2 and water
• Regenerates NAD+ but doesn’t produce ATP
• Provides enough energy for some single-celled anaerobic
species
Two Fermentation Pathways
• Alcoholic fermentation
• Pyruvate is split into acetaldehyde and CO2
• Acetaldehyde receives electrons and hydrogen from
NADH, forming NAD+ and ethanol
• Lactate fermentation
• Pyruvate receives electrons and hydrogen from NADH,
forming NAD+ and lactate
Glycolysis
glucose
2 NAD+
2
2
4
pyruvate
Alcoholic
Fermentation
2 CO2
acetaldehyde
2
2 NAD+
ethanol
Figure 7-10a p127
Figure 7-10b p127
Glycolysis
glucose
2 NAD+
2
2
4
pyruvate
Lactate
Fermentation
2 CO2
2
2 NAD+
lactate
Figure 7-11a p127
ANIMATED FIGURE: Fermentation
pathways
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Red and White Muscle Fibers
• Red muscle fibers make ATP by aerobic respiration
• Have many mitochondria
• Myoglobin stores oxygen
• Sustain prolonged activity
• White muscle fibers make ATP by lactate fermentation
• Have few mitochondria and no myoglobin
• Sustain short bursts of activity
Figure 7-11b p127
Figure 7-11c p127
Take-Home Message:
What is fermentation?
• ATP can form by carbohydrate breakdown in fermentation
pathways, which are anaerobic
• The end product of lactate fermentation is lactate. The end
product of alcoholic fermentation is ethanol
• Both pathways have a net yield of two ATP per glucose
molecule; the ATP forms during glycolysis
• Fermentation reactions regenerate the coenzyme NAD+,
without which glycolysis (and ATP production) would stop
7.7 Alternative Energy Sources in Food
• Aerobic respiration can produce ATP from the breakdown of
complex carbohydrates, fats, and proteins
• As in glucose metabolism, many coenzymes are reduced,
and the energy of the electrons they carry ultimately drives
the synthesis of ATP in electron transfer phosphorylation
Energy From Complex Carbohydrates
• Enzymes break starch and other complex carbohydrates
down to monosaccharide subunits
• Monosaccharides are taken up by cells and converted to
glucose-6-phosphate, which continues in glycolysis
• A high concentration of ATP causes glucose-6-phosphate to
be diverted away from glycolysis and into a pathway that
forms glycogen
Energy From Fats
• Enzymes cleave fats into glycerol and fatty acids
• Glycerol products enter glycolysis
• Fatty acids are converted to acetyl Co-A and enter the
Krebs cycle
• Compared to carbohydrates, fatty acid breakdown yields
more ATP per carbon atom
• When blood glucose level is high, acetyl CoA is diverted from
the Krebs cycle and into a pathway that makes fatty acids
Energy from Proteins
• Enzymes split dietary proteins into amino acid subunits, which
are used to build proteins or other molecules
• The amino group is removed and converted into ammonia
(NH3), a waste product eliminated in urine
• Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle
forms, depending on the amino acid
glucose
starch (a complex carbohydrate)
A Complex carbohydrates are broken down to their
monosaccharide subunits, which can enter glycolysis.
1
Figure 7-12a p128
a triglyceride (fat)
glycerol head
fatty acid tails
Figure 7-12b p128
Food
Fats
fatty acids
2
acetyl–CoA
glycerol
3
Complex Carbohydrates
Proteins
glucose, other simple
sugars
1
amino acids
4
acetyl–CoA
PGAL
Glycolysis
NADH pyruvate
intermediate of
Krebs cycle
Krebs
Cycle
NADH, FADH2
Electron Transfer
Phosphorylation
Figure 7-12b p128
alanine (an amino acid)
pyruvate
Figure 7-12c p128
ANIMATED FIGURE: Alternative energy
sources
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Take-Home Message: Can organic molecules
other than glucose be used for energy?
• Complex carbohydrates, fats, and proteins can be oxidized in
aerobic respiration to yield ATP
• First the digestive system and then individual cells convert
molecules in food into intermediates of glycolysis or the Krebs
cycle