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10/25/2010
Life is Work
CHAPTER 9
CELLULAR
RESPIRATION
y Living cells require transfusions of energy from outside
sources to perform their many tasks:
y Chemical work
y Transport work
y Mechanical work
y Energy stored in the organic molecules of food
ultimately comes from the sun
Energy flows into an ecosystem as
sunlight and leaves as heat
y In contrast, chemical elements essential to life are recycled
y Photosynthesis generates oxygen and organic molecules
y Respiration breaks this fuel down, generating ATP
y In this chapter, we will focus on the key pathways of aerobic
respiration:
y Glycolysis
y Citric acid cycle
y Oxidative phosphorylation
Catabolic pathways = oxidizing fuels
y Catabolic pathways = metabolic pathways that release stored
energy by breaking down complex molecules
Types of cellular respiration
y Aerobic Respiration (‘cellular respiration’):
y Oxygen is consumed as a reactant along with the organic
fuel
y Compounds that can participate in exergonic reactions can act as
fuels
y With the help of enzymes, a cell can systematically degrade
macromolecules that are rich in potential energy
y Energy taken out of chemical storage can be used to do work
y Cells of most eukaryotes and prokaryotes
y Anaerobic Respiration:
y Example: fermentation
y Degradation of organic fuel without the use of oxygen
y Cells of some prokaryotes and some eukaryotes
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The Stages of Cellular Respiration
Overall process:
Organic
compound
Oxygen
Carbon
dioxide
y Glycolysis:
Water
Energy
y
y
y
y
Technically not part of cellular respiration
Catabolic pathway that occurs in cytosol
Breaks down glucose to two smaller molecules
Dehydrogenases transfer e- from NAD+ to NADH
y We will learn the steps of cellular respiration by tracking the degradation
of the sugar glucose:
C6H12O6 + 6 O2
y Citric Acid Cycle
y Takes p
place in the mitochondria ((matrix)) eukaryots
y
y Takes place in cytosol in prokaryots
y Dehydrogenases transfer e- from NAD+ to NADH
6 CO2 + 6 H2O + Energy (ATP, heat)
y Oxidative phosphorylation: electron transport and chemiosmosis
y Glucose is the fuel that cells most often use
y Accepts e- from NADH (H+ + e-)
y Pass through many molecules and the end it is combined with oxygen to
y Exergonic process where ∆G = -686 Kcal/mol
y The energy released makes ATP
form water
REDOX REACTIONS
y Loss of electrons = oxidation
y Gain of electrons = reduction (reduce + charge)
y Reducing agent = electron donor
y Oxidizing agent = electron acceptor
NAD+ and NADH
y Dehydrogenase removes a pair of hydrogen atoms (2eand 2p+) from the substrate and oxidizes it
y NAD+ is an electron carrier (a coenzyme)
y It acts as an oxidizing agent during respiration
y The enzyme delivers 2 e- and 1p+ to the coenzyme
y Oxidation = loss of electrons
(NAD+)
y Oxidizing agent = takes away electrons
y The other p+ (H+) is released to the surrounding
H+
2H
solution
H
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GLYCOLYSIS
y Oxidizes glucose to pyruvate
y Glycolysis Overall:
y Glucose (6 C sugar) is split to two 3 C sugars
y Pyruvate
y Two phases:
y Energy investment
y Energy payoff
y Oxygen is not
necessary
y CO2 is not released
y NET ATP YIELD = 2 ATP + 2 NADH
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Citric Acid Cycle
y Glycolysis releases less than a 1/4 of the chemical
energy stored in glucose
y Most
M t off the
th energy remains
i in
i the
th two
t
molecules
l
l off
pyruvate
y If oxygen is available, pyruvate enters a mitochondrion
(eukaryotes)
y Active transport
y Acetyl CoA is very unstable (potential energy? Ender/exergonic?)
y Citric Acid Cycle =
Krebs Cycle
(1930s)
y Oxidizes organic
fuel derived from
pyruvate
y The cycle
generates:
y 1 ATP
y 3 NADH
y 1 FADH2
y 1 NADH (junction)
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Oxidative phosphorylation and
chemiosmosis
y So far, glycolysis and the citric acid cycle have
produced only 4 ATP molecules per glucose molecule
(substrate-level phosphorylation)
y 2 ATP from glycolysis + 2 NADH
y 2 ATP from citric acid cycle + 4 NADH + FADH2
y NADH and FADH2 molecules account for most of the
energy extracted from the glucose molecule
Electron Transport
y The electron transport chain is a collection of
molecules embedded in the inner membrane of the
mitochondrion (eukaryotic cells)
y Most components are proteins, which exist in
complexes (I – IV)
y Other non protein components are essential for the
functioning of enzymes, they are called prosthetic
groups
y Electrons from NADH are
transferred to first molecule:
y flavoprotein (flavin
mononucleotide)
y Iron-sulfer protein
y Coenzyme
y
Q
y Cytochromes (heme
prosthetic groups)
y Oxygen (very
electronegative)
Why not release all the
energy in just one step?
+ energy
2 H2
O2
2 H2 O
Chemiosmosis: energy-coupling
mechanism
y Many copies of ATP synthase populate the inner
membrane of the mitochondrion
y It uses energy of an existing ion gradient to power ATP
y
synthesis
y In the mitochondrial inner membrane there is a pump
that creates a difference in H+ concentration between
the matrix and the intermembrane space
y Also considered a difference in pH
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Chemiosmosis
y Process in which energy stored in the form of a
hydrogen ion gradient across a membrane is used to
drive cellular work, such as the synthesis of ATP
y ATP synthase is a
multisubunit complex with
four main parts:
y Stator
y Rotor
y Internal Rod
y Catalytic knob
y To read: Fig. 9.15 Inquiry
How do we maintain the H+ gradient?
y That is the function of the electron transport chain
y The exergonic flow of electrons from NADH (or FADH2)
down to oxygen is used to pump H+ across the
membrane
y ATP synthase is the only route through the membrane
(for H+)
y Certain members of the electron transport chain accept
and release protons (H+)
y The H+ gradient that results is referred to as protonmotive force
Peter Mitchell
y Chemiosmosis is an energy-coupling mechanism that
uses energy stored in the form of an H+ gradient
across a membrane to drive cellular work
y Examples:
was awarded the Nobel Prize in 1978
for originally proposing the
chemiosmotic model
y Chloroplasts: use it to generate ATP during
phothosynthesis
y Prokaryotes: use it to make ATP and also to rotate their
flagella and pump nutrients and waste across the
membrane
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… ATP production?
y During respiration, most energy flows:
y Glucose > NADH > ET Chain > Proton MF > ATP
Electron shuttles
span membrane
CYTOSOL
or
2 FADH2
2 NADH
Glycolysis
y 1 NADH = 1 pair electrons = pumps 10 H+
MITOCHONDRION
2 NADH
Glucose
6 NADH
2 NADH
2
Pyruvate
2 FADH2
Citric
acid
cycle
2
Acetyl
CoA
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
y 3 – 4 H+ = 1 ATP
y 1 NADH = 2.5 – 3.3 ATP = ~ 3
y 1 FADH2 = 1.5 – 2 ATP = ~ 2
+ 2 ATP
+ 2 ATP
Maximum per glucose:
+ about 32 or 34 ATP
About
36 or 38 ATP
y READ: p. 176-177 Reasons that affect ATP yield
Fermentation and Anaerobic
Respiration
Anaerobic Respiration
y The estimated ATP yield from aerobic respiration is
y Takes place in certain prokaryotic organisms
contingent on an adequate supply of oxygen to the cell
(environments without O2)
y These organisms have electron transport chains, but
y However,
However cells can oxidize organic fuel and generate
ATP without the use of oxygen:
y Anaerobic respiration
y Fermentation
Fermentation
y It harvests chemical energy without using oxygen or
any electron transport chain
y Glycolysis oxidizes glucose to two molecules of
the end product is a different molecule
y Ex: SO42y H2S is a by-product (instead of water)
Fermentation
y There must be a sufficient supply of NAD+
y Electrons can be transferred to pyruvate instead
pyruvate (oxidizing agent NAD+)
y Glycolysis is exergonic and makes ATP by substratelevel phosphorylation
y Fermentation expands glycolysis so that it continuously
y Types of fermentation (end product):
y Alcohol Fermentation
y Lactic Acid Fermentation
generates ATP
y How??
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Alcohol fermentation
y Piruvate is converted to
ethanol in two steps:
y Release of CO2 =
acetaldehyde
y Acetaldehyde is
reduced
d
db
by NADH =
ethanol
y Ex: yeast: brewing,
winemaking and baking
Lactic Acid Fermentation
y Pyruvate is reduced
directly by NADH to
form lactate
y No release of CO2
y Used in the dairy
insudstry to make
yogurt and cheese
y Human muscles (when
O2 is scarce)
Fermentation and Aerobic
Respiration Compared
Obligate / Facultative
y Obligate Anaerobes:
y Some organisms carry out only fermentation or anaerobic
y Both processes use glycolysis to oxidize glucose and other
respiration
y They cannot survive in the presence of oxygen
y The processes have different final electron acceptors: an
y Facultative Anaerobes:
y Yeasts and many bacteria can make enough ATP to
survive using either fermentation or respiration
organic fuels to pyruvate
organic molecule (such as pyruvate or acetaldehyde) in
fermentation and O2 in cellular respiration
y Cellular respiration produces 38 ATP per glucose molecule;
fermentation produces 2 ATP per glucose molecule
y Our muscle cells behave as facultative anaerobes
Fig. 9-19
Glucose
y Obligate anaerobes carry out fermentation or anaerobic
CYTOSOL
respiration and cannot survive in the presence of O2
y Yeast and many bacteria are facultative anaerobes,
Glycolysis
Pyruvate
No O2 present:
Fermentation
meaning that they can survive using either fermentation or
cellular respiration
O2 present:
Aerobic cellular
respiration
y In a facultative anaerobe, pyruvate is a fork in the
metabolic road that leads to two alternative catabolic
routes
MITOCHONDRION
Ethanol
or
lactate
Acetyl CoA
Citric
acid
cycle
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The Evolutionary Significance of
Glycolysis
y Glycolysis occurs in nearly all organisms
y Oldest fossil bacteria date to ~3.5 bya
y But atmospheric O2 only started to accumulate ~ 2.7
2 7 bya
y Glycolysis probably evolved in ancient prokaryotes before
there was oxygen in the atmosphere
Other Catabolic pathways
y Free glucose molecules are not common in the diets of
humans and other animals
y Most our calories com from fats, proteins, sucrose and
starch
y All these molecules can be used by cellular respiration
to make ATP
y Carbohydrates:
y Starch can be hydrolyzed
y Glycogen can also be hydrolyzed
y Proteins:
y First they are digested (aa)
y Aa can be used to build proteins or can be converted to
intermediates of glycolysis and the Krebs cycle
(d
(deamination
i ti + enzymes))
y Fats:
y Digested to glycerol and fatty acids
y Glycerol = glyceraldehyde-3-phosphate (glycolysis)
y Fatty acids via Beta oxidation = 2 C fragments = enter
Krebs cycle as Acetyl CoA (NADH and FADH2 also
generated)
Anabolic Pathways
y Cells need substance as well as energy
y Food must also provide the atoms necessary to make
new molecules
Example
y Dihydroxyacetone phosphate (intermediate compound
generated during glycolysis) can be converted to one of
the precursors of fat
y Intermediates of glycolysis and the citric acid cycle can
be diverted into anabolic pathways as precursors for
other molecules
y So, when we eat more food than we need, we store fat
even if our diet is fat-free.
y Humans can make about half of the 20 essential aa,
the rest come from diet
y Metabolism is remarkably versatile and adaptable
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Feedback Mechanisms
y Feedback inhibition is the most common control
mechanism for anabolic pathways
Phosphofructokinase
y Important switch in metabolic control
y The cell can speed up or slow down the entire catabolic
process by controlling the first step of the glycolytic
pathway
y An end product of the anabolic pathway inhibits the
enzyme that catalyzes an early step in the pathway
y Catabolism is also regulated through positive and
negative feedback
y Phosphofructokinase is an allosteric enzyme with
receptor sites for specific inhibitors and activators:
y Inhibited by ATP
y Stimulated by AMP
y Also sensitive to citrate (1st product Citric Acid Cycle)
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