Download 9.1 Catabolic Pathways yield energy by oxidizing organic fuels

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Chapter 9 Cellular Respiration & Fermentation
The energy stored in the organic molecules of food ultimately comes from the sun.
Energy flows into an ecosystem as sunlight and exits as heat
The chemical elements esse ntial to life are recycled
Photosynthesis generates oxygen and organic molecules used by the mitochondria of
eukaryotes as fuel for cellular respiration. Respiration breaks this fuel down, generating
ATP.
The waste products of this type of respiration, carbon dioxide and water, are the raw materials for
photosynthesis
cells harvest the chemical energy stored in organic molecules and use it to generate ATP
9.1 Catabolic Pathways yield energy by oxidizing organic fuels
 catabolic processes, ones that release stored energy by breaking down complex molecules, are
central to cellular respiration. Electron transfer plays a major role in these pathways.
Catabolic Pathways and the Production of ATP
 organic compounds possess potential energy as a result of the arrangement of electrons in the
bonds between their atoms. Compounds that can participate in exergonic reactions act as fuels.
 With the help of enzymes, a cell can break down complex organic molecules that are rich in PE
to simpler waste products that have less PE. Some of the energy is converted to do work, while
the rest is dissipated as heat
 One catabolic process, fermentation (anaerobic respiration), is a partial degradation of sugars
or other organic fuel that occurs without the use of oxygen
 Aerobic respiration is the most prevalent and efficient catabolic pathway in which oxygen is
consumed as a reactant along with the organic fuel.
 Cellular respiration includes both aerobic & anaerobic processes
Organic Compounds + Oxygen  Carbon Dioxide + Water + Energy
 The breakdown of organic fuel is exergonic (-G means that the products store less energy than
the reactants & the reaction can occur spontaneously, without input of energy)
 Catabolism is linked to work by ATP. To keep working, cells must regenerate its supply of ATP
from ADP & P1.
Redox Reactions: Oxidation and Reduction
 Catabolic pathways that decompose organic fuel yields energy due to the transfer of electrons during the
chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this
energy is used to synthesize ATP
The Principle of Redox (oxidation-reduction reactions)
 In a redox reaction, the loss of electrons from one substance is oxidation, and the addition of electrons to
another substance is reduction.
 The electron donor is the reducing agent. It loses an electron, becoming more positive, so it is oxidized.
 The electron acceptor is the oxidizing agent. It oxidizes the reducing agent by taking its electron,
becoming more negative, so it is reduced
 Not all redox reactions involve the complete transfer of electrons from one substance to another. Some
modify the degree of electron sharing in covalent bonds
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The atoms in methane are about equally electronegative.
When CH4 reacts with O2, forming CO2, electrons end up
shared less equally between CO2 & its O2 covalent
partners, which are very electronegative. So the C atom
has partially lost its shared eWhen O2 reacts with the H in CH4, forming H2O, the eof the covalent bonds spend more time near the O2, so O2
has partially gained e-.
O2 is one of the most powerful oxidizing agents since it
is so electronegative
E must be added to pull an e- away from an atom, so the more electronegative an atom, the more
energy is required to take an e- away from it. An e- loses PE when it shifts from a less
electronegative atom towards a more electronegative one.
A redox reaction that moves e- closer to oxygen releases chemical energy that can be put to work
Oxidation of Organic Fuel Molecules During Cellular Respiration
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The organic fuel is oxidized and the O2 is reduced. The e- lose PE along the way, and E is
released
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Organic molecules that have a lot of H are great fuels because their bonds are a source of
“hilltop” e-, whose E may be released as these e- “fall” down an E gradient when they’re
transferred to O2.
H is transferred from the organic fuel to O2. The E state of the e- changes as H (along with its e-)
is transferred to O2.
Oxidation of organic fuel transfers e- to a lower E state, freeing E that becomes available for ATP
synthesis
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The main energy-yielding foods, carbs and fats, are reservoirs of e- associated with H.
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Once organic fuel is taken in, enzymes in cells will lower the barrier of activation energy that
holds back the flood of e- to a lower E state, allowing the fuel to be oxidized in a series of steps
Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
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Cell respiration doesn’t oxidize glucose in a single explosive step, because it can’t be harnessed
efficiently for constructive work.
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Instead, the organic fuels are broken down in a series of steps, each one catalyzed by an enzyme
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At key steps, e- are stripped from the glucose. Each e- travels with a H+ (proton)
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The H aren’t transferred directly to O2, but instead are passed to an electron carrier, a coenzyme
called NAD+, which can cycle easily between oxidized (NAD+) and reduced (NADH) states. It is
an oxidizing agent during respiration. NAD+ is the most versatile e- acceptor in cell respiration
and functions in several redox steps during the breakdown of glucose
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NAD+ traps electrons from organic molecules. Enzymes called dehydrogenases remove a pair of
H atoms (2 e- & 2 p+) from the substrate, thereby oxidizing it. The enzyme delivers the 2 e- & 1
p+ to its coenzyme, NAD+. The other p+ is released as an H+ into the surrounding solution
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By receiving 2 e- but only 1+, NAD+ has its charge neutralized when it’s reduced to NADH
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e- lose very little of their PE when they’re transferred from glucose to NAD+. Each NADH
molecule formed during respiration represents stored E that can be tapped to make ATP when the
e- complete their “fall” down an E gradient from NADH to O2.
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The e- that are extracted from glucose & stored as PE in NADH reaches O2
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Cell respiration brings H & O2 together to form water, but the H that reacts with O2 is derived
from organic molecules. Instead of occurring in one explosive reaction, respiration uses an
electron transport chain to break the fall of e- to O2 into several energy-releasing steps.
An electron transport chain consists of a number of molecules, mostly proteins, built into the
inner membrane of the mitochondria of eukaryotic cells. e- removed from glucose are shuttled by
NADH to the “top,” higher-E end of the chain. At the “bottom,” lower-E end, O2 captures these
e- along with H+, forming H2O
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e- transfer from NADH to O2 is an exergonic
reaction with DG = -53 kcal/mol. Instead of this
E being released & wasted in a single explosive
step, e- cascade down the chain from one
carrier molecule to the next in a series of redox
reactions, losing a small amount of energy with
each step until they finally reach O2, the
terminal e- acceptor.
Each “downhill” carrier is more electronegative
(more capable of oxidizing) than its “uphill”
neighbor. The e- removed from glucose by
NAD+ fall down an E gradient in the electron
transport chain to a far more stable location in
the electronegative O2.
Most e- travel down this downhill route: glucose
O2 pulls e- down the chain in an E-yielding
→ NADH →electron transport chain→O2
tumble
The Stages of Cellular Respiration: A Preview
 The harvesting of energy from glucose by cell respiration is a cumulative function of 3 metabolic
stages
1. Glycolysis- begins degradation process in cytosol by breaking glucose into two
molecules of pyruvate, which enters the mitochondria.
 Note: cell respiration includes only steps 2 &3. Glycolysis is not a part of cell
respiration, but respiring cells derive energy from glucose using glycolysis to
produce the starting material for the citric acid cycle
2. a. Pyruvate oxidation- pyruvate enters the mitochondria & is oxidized into
acetacetyl CoA, which enters the citric acid cycle
b. Citric acid cycle- the breakdown of glucose to CO2 is completed.
 Substrate-level phosphorylation- mode of ATP synthesis that occurs when an
enzyme transfers a phosphate group from a substrate molecule to ADP, rather than
adding an inorganic P1 to ADP as in oxidative phosphorylation; ATP is generates as
an intermediate during the catabolism of glucose; produces some ATP
3. Oxidative phosphorylation- mode of ATP synthesis powered by the redox reactions of
the e- transport chain
a. e- transport- the e- transport chain accepts the e- from the breakdown products
of the first 2 stages (NADH) and passes these e- from one molecule to another.
At the end of the chain, the e- are combined with O2 & H+, forming H2O.
b. chemiosmosis- 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
 In eukaryotic cells, the inner membrane of the mitochondria is the site of etransport & chemiosmosis. In prokaryotes, these happen in the plasma
membrane.
 This stage amounts for 90% of the ATP generated by respiration.
 For each molecule of glucose degraded into CO2 & H2O by respiration, the cell makes up to 32
molecules of ATP, each with 7.3 kcal/mol of G.
 Respiration cashes in the large quantity of E stored in a single molecule of glucose for the small
change of many molecules of ATP
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9.2 Glycolysis Harvests Chemical Energy by Oxidizing
Glucose to Pyruvate
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Glycolysis is sugar splitting. Glucose, a 6-C sugar, is split
into 2 3-C sugars, which are then oxidized and their
remaining atoms rearrange to form 2 molecules of
pyruvate
 Glycolysis is divided into two phases:
1. Energy investment phase- cell spends ATP.
 Phosphofructokinase, the pacemaker of glycolysis,
transfers a phosphate group from ATP to the opposite
end of the sugar. This is a key step for regulation of
glucose. Kinases cause phospholrylation.
 ATP allosterically inhibits phosphofructokinase
 2 ATP are invested, 2 ADP are produced
All the C originally present in
 an intermediate is split apart, and it generates 2 3-C
glucose is accounted for in the 2
molecules, which are isomers of each other.
molecules of pyruvate; no C is
2. Energy payoff phase- ATP is produced by substrate-level
released as CO2 during
phosphorylation & NAD+ is reduced to NADH by eglycolysis. Occurs regardless of
released from the oxidation of glucose. This yields 2 ATP whether O2 is present or not
+ 2 NADH
 4 ATP and 2 NADH are produced for each pyruvate
9.3 After Pyruvate is Oxidized, the Citric Acid Cycle Completes the E-Yielding Oxidation of
Organic Molecules
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If O2 is present, the pyruvate enters a mitochondria (in eukaryotic cells and cytosol of
prokaryotic) where the oxidation of glucose is completed
Oxidation of Pyruvate to Acetyl CoA (link reaction)
 Upon entering the mitochondria via active transport, pyruvate is first converted to acetyl CoA.
This step links glycolysis & the citric acid cycle, and it is carried out by a multi-enzyme
complex that catalyzes 3 reactions:
1. Pyruvate’s carboxyl group, which is already fully oxidized & thus has little chemical
E, is removed & given off as a molecule of CO2.
2. The remaining 20C fragment is oxidized, forming acetate. The extracted e- are
transferred to NAD+, storing E in the form of NADH
(Hint: anytime CO2 leaves, the molecule has to rearrange, so NAD+ needs to come in
and reduce to NADH)
3. Coenzyme A (CoA), a sulfur-containing compound from a B-vitamin, is attached via
its sulfur atom to the acetate, forming acetyl CoA, which has a high PE. The reaction
of acetyl CoA to yield lower-E products is very exergonic.
Quick review of structure of mitochondria:
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 The folds, cristae, help increase surface area, thus
increasing respiration.
 The inner-membrane space is between the matrix
and the outer membrane
The Citric Acid Cycle
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Oxidizes organic fuel derived from pyruvate
A transport protein actively transports pyruvate into the mitochondrial matrix, where the Kreb’s
cycle occurs
end products of the cycle are the starting products
Acetyl CoA (from the oxidation of pyruvate) adds its 2-C acetyl group to oxaloacetate (4-C),
producing a 6-C citrate. The CoA leaves
In order to get to a 4-C molecule end product, 2 CO2s are lost, and 2 NAD+ are reduced to NADH.
CoA is transferred by a phosphate group, which is added to GDP to form GTP, which can generate
ATP.
2 H are transferred from an intermediate to FAD, forming FADH2
The addition of H2O rearranges the bonds in the substrate
The substrate is oxidized, reducing NAD+ to NADH & regenerating oxaloacetate
The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical E
is transferred to NAD+ & a related e- carrier, coenzyme FAD during redox reactions.
The reduced coenzymes, NADH & FADH2, shuttle their cargo of high-E e- into the e- transport
chain
9.4 During Oxidative Phosphorylation, Chemiosmosis Couples Electron Transport to ATP Synthesis
The Pathway of Electron Transport
 The e- transport chain is a collection of molecules embedded in the inner membrane
 the folding of the inner membrane to form cristae increases its surface area, providing space for
more copies of the chain in each mitochondria.
 Most components of the chain are protein complexes number I – IV. Tightly bound to these
proteins are prosthetic groups, nonprotein compounds essential for the catalytic function of
certain enzymes
 During e- transport along the chain, e- carriers alternate between reduced & oxidized states as they
accept & donate e-.
 Each component of the chain becomes reduced when it accepts e- from its uphill neighbor, which is
less electronegative
 It then returns to its oxidized form as it passes e- to its downhill, more electronegative neighbor
 e- are carried by NADH and are transferred to flavoprotein, the first molecule in the e- transport
chain.
 the e- continue along the chain, passing through a Fe-S protein then through a small hydrophobic ecarrier, ubiquinone,
 Most of the remaining e- carriers between Q & O2 are proteins called cytochromes, which have
prosthetic groups (heme group) that accept and donate e-.
 The last cytochrome of the chain, cyt a3, passes its e- to O2, which is the most electronegative
 FADH2 is another source of e- for the transport chain. It adds e- to complex II, at a lower E level
than NADH does.
 Although NADH & FADH2 each donate 2 electrons for O2 reduction, the e- transport chain
provides less energy for ATP synthesis when the e- donor is FADH2.
 The e- transport chain eases the fall of e- from food to O2, breaking a large DG into a series of
smaller steps that release E in manageable amounts.
Chemiosmosis: the Energy-Coupling Mechanism
 ATP synthase is an enzyme that makes ATP from ADP & inorganic phosphate; many copies
populate the inner membrane of the mitochondria
 ATP synthase works like an ion pump running in reverse. Ion pumps use ATP as an energy source
to transport ions against their gradients.
 ATP synthase uses the E of an existing ion gradient to power ATP synthesis. The power source for
the ATP synthase is a difference in the concentration of H+ on opposite sides of the inner
mitochondrial membrane.
 chemiosmosis- E stored in the form of a H+ gradient across a membrane that’s used to drive
cellular work (such as the synthesis of ATP)
 ATP synthase is a multisubunit complex with 4 main parts. Protons move one by one into binding
sites on one of the parts, causing it to spin so that it catalyzes ATP production from ADP &
inorganic phosphate.
 The inner mitochondrial membrane generates & maintains the H+ gradient that drives ATP
synthesis
 A main function of the e- transport chain is establishing the H+ gradient. The chain is an E
converter that uses the exergonic flow of e- from NADH & FADH2 to pump H+ across the
membrane, from the matrix to the intermembrane space. The H+ has a tendency to move back
across the membrane, diffusing with its gradient. And the ATP sythases are the only sites that
provide a route through the membrane for H+. The E stored in a H+ gradient across a membrane
couples the redox reactions of the e- transport chain to ATP synthesis.
 E- transport chain pumps H+ ions because certain members of the e- transport chain accept &
release H+ along with e-.
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At certain steps along the chain, e- transfers cause H+ to be taken up & released into the
surrounding solution. H+ is accepted from the matrix & deposited into the intermembrane space.
The H+ gradient that results is the proton motive force. This force drives H+ back across the
membrane through the H+ channels provided by ATP synthases
Chemiosmosis is an E-coupling mechanism that uses E stored in the form of an H+ gradient across
a membrane to drive cell work
An Accounting of ATP Production by Cellular Respiration
 the overall function of cell respiration is harvesting the E of glucose for ATP synthesis
 During respiration, the E flows in this sequence: glucose  NADH  e- transport chain 
proton-motive force  ATP
 4 ATP are produced directly by substrate-level phosphorylation during glycolysis & the Kreb’s
cycle
 Each NADH that transfers a pair of e- from glucose to the e- transport chain contributes enough to
the proton-motive force to generate a max of 3 ATP
 Each FADH2 can be used to generate a max of 2 ATP
 There are three reasons that we cannot state an exact number of ATP molecules generated by one
molecule of glucose.
1. Phosphorylation and the redox reactions are not directly coupled to each other, so the ratio
of number of NADH to number of ATP is not a whole number.
2. The ATP yield varies slightly depending on the type of shuttle used to transport electrons
from the cytosol into the mitochondrion
• The mitochondrial inner membrane is impermeable to NADH, so the two electrons
of the NADH produced in glycolysis must be conveyed into the mitochondrion by
one of several electron shuttle systems
3. The proton-motive force generated by the redox reactions of respiration may drive other
kinds of work, such as mitochondrial uptake of pyruvate from the cytosol
 Complete oxidation of glucose releases 686 kcal/mol.
 About 34% of the potential chemical energy in glucose has been transferred to ATP
 Cell respiration is very efficient at its energy conversion
 the rest of the E stored in glucose is lost as heat
9.5 Fermentation & Anaerobic Respiration Enable Cells to Produce ATP without the Use of Oxygen
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without the electronegative O2 to pull e- down the transport chain, oxidative phosphorylation
ceases.
in anaerobic respiration, which takes place in prokaryotes that live without oxygen, the organisms
have an e- transport chain but don’t use O2 as the final e- acceptor. Less electronegative substances
can also serve as final e- acceptors.
Fermentation harvests chemical E without cell respiration – without the use of O2 or e- transport
chains.
Fermentation is an extension of glycolysis that allows continuous generation of ATP by the
substrate-level phosphorylation of glycolysis. This requires a sufficient supply of NAD+ to accept
e- during the oxidation step of glycolysis. Without being able to recycle NAD+ from NADH,
glycolysis would deplete the cell’s supply of NAD+ by reducing it all to NADH & would shut
itself down for lack of an oxidizing agent.
e- are transferred from NADH to pyruvate.
Types of Fermentation
 Fermentation consists of glycolysis plus reactions
that regenerate NAD+ by transferring e- from NADH
to pyruvate
 The NAD+ can then be reused to oxidize sugar by
glycolysis
 Two common types are alcohol fermentation & lactic
acid fermentation, which differ in the end products
formed from pyruvate
 in alcohol fermentation, pyruvate is converted to
ethanol (ethyl alcohol) in 2 steps.
1. CO2 is released from the pyruvate, which is
converted to the e-C compound acetaldehyde
2. acetaldehyde is reduced by NADH to
ethanol. This regenerates the supply of
NAD+ needed for the continuation of
glycolysis
 many bacteria carry out alcohol fermentation
under anaerobic conditions. Yeast, a fungus,
carries it out. The CO2 bubbles generated by
baker’s yeast allow the bread to rice.
 in lactic acid fermentation, pyruvate is reduced
directly by NADH to form lactate (lactic acid ion) as
an end product, with no release of CO2.
 lactic acid fermentation by certain fungi & bacteria is
used to make cheese & yogurt
 human muscles make ATP by lactic acid
fermentation when O2 is scarce. This occurs during
strenuous exercise, when sugar catabolism for ATP
production outpaces the muscle’s supply of O2 from
the blood. The cells switch to fermentation. The
excess lactate is carried away by the blood to the
liver, where it’s converted back to pyruvate by liver
cells. Because O2 is available, this pyruvate can then
enter the mitochondria in liver cells & complete cell
respiration
Comparing Fermentation with Anaerobic & Aerobic Respiration
Aerobic
the final e- acceptor
is O2
• yields up to 16
times as much ATP
per glucose
molecules than
fermentation does
• up to 32 molecules
of ATP produced
• e- carried by NADH are transferred to an etransport chain, where they move stepwise down a
series of redox reactions to the final e- acceptor
• passage of e- from NADH to the e- transport chain
regenerates the NAD+ required for glycolysis & pays
an ATP bonus when the step-wise e- transport from
this NADH to O2 drives oxidative phosphorylation
• cell pathways for producing ATP by harvesting chemical
energy of food
• use glycolysis to oxidize organic fuels to pyruvate, with
net production of 2 ATP by substrate-level
phosphorylation
•NAD+ is the oxidizing agent that accepts e- from food
during glycolysis
Fermentation
• The final e- acceptor is an organic molecule such
as pyruvate (lactic acid fermentation) or
acetaldehyde (alcohol fermentation)
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Obligate anaerobes carry out only fermentation of
anaerobic respiration. They can’t survive in the
presence of O2, which can be toxic if protective
systems aren’t present in the cell.
Facultative anaerobes can make enough ATP to
survive using either fermentation or respiration.
Pyruvate is a fork in the metabolic road that leads to
two alternative catabolic routes. Under aerobic
conditions, pyruvate can be converted to acetyl CoA,
and oxidation continues in the citric cycle via aerobic
respiration. Under anaerobic conditions, lactic acid
fermentation occurs: pyruvate is diverted from the
citric acid cycle, serving instead as an e- acceptor to
recycle NAD+. To make the same amount of ATP, it
has to consume sugar at a much faster rate when
fermenting than respiring
o ex: yeasts, many bacteria, cells of the
vertebrae brain, muscle cells.
Anaerobic
• the final e- acceptor is
another molecule that
is less electronegative
than O2
The Evolutionary Significance of Glycolysis
 Ancient prokaryotes are thought to have used glycolysis to make ATP long before O2 was present
in Earth’s atmosphere. They generated ATP only from glycolysis.
 The fact that glycolysis is the most widespread metabolic pathway suggests that it evolved very
early in the history of life.
 The cytosolic location of glycolysis implies that it doesn’t require any membrane bound organelles
of the eukaryotic cell
9.6 Glycolysis and the Citric Acid Cycle Connect to Many Other Metabolic Pathways
The Versatility of Catabolism
 Free glucose molecules aren’t common in the diets of heterotrophs, who obtain most of their
calories from other organic molecules such as fats, proteins, disaccharides, and starch by cell
respiration to make ATP.
 Glycolysis can accept many carbohydrates.
o Starch (storage polysaccharide for plants) is hydrolyzed to glucose
o Glycogen (storage polysaccharide in animals’ livers) is hydrolyzed to glucose between
meals as fuel for respiration
 Proteins must be digested to their basic amino acids before they can be used for fuel. Many of the
amino acids are used by the organism to build new proteins. Amino acids present in excess are
converted by enzymes to intermediates of glycolysis & the citric acid cycle.
o Before amino acids can feed into glycolysis or the citric acid cycle, their amino groups
must undergo deanimation (remove it). The nitrogenous refuse is excreted from the
animal as waste.
 Catabolism can also harvest E stored in fats from food or storage cells in the body. Fats make great
fuel, mainly due to their chemical structure & high E level of their e-. A gram of fat oxidized by
respiration produces more than twice as much ATP as a gram of carbs. After fats are digested to
glycerol & fatty acids, the glycerol is converted to G3P. Most of the E of a fat is stored in the fatty
acids.
o Beta oxidation is a metabolic sequence that breaks the fatty acids down to 2-C fragments,
which enter the citric acid cycle as acetyl CoA. NADH & FADH2 are also generated; they
can enter the e- transport chain, leading to further ATP production
Biosynthesis (Anabolic Pathways)
 Cells need substance as well as energy. Not all organic molecules of food are destined to be
oxidized as fuel to make ATP.
 Food must provide the C-skeletons that cells require to make their own molecules.
 Some organic monomers from digestion can be used directly, like amino acids from the hydrolysis
of proteins.
 The body needs specific molecules that aren’t present in food. Compounds formed as intermediates
of glycolysis & the citric acid cycle can be diverted into anabolic pathways as precursors from
which the cell can synthesize the molecules it requires.
 Anabolic or biosynthetic pathways consume ATP.
o Ex: humans can make 10 types of amino acids in proteins by modifying compounds tapped
away from the citric acid cycle.
o Ex: Glucose can be made from pyruvate, and fatty acids can be synthesize from acetyl CoA
 Glycolysis & the citric acid cycle function as metabolic interchanges that enable cells to convert
some kinds of molecules to others.
Regulation of Cellular Respiration via Feedback
Mechanisms
 Cells don’t waste energy making excess
substances
o Ex: If there’s an excess of a certain
amino acid, the anabolic pathway that
synthesizes that amino acid from an
intermediate of the citric acid cycle is
switched off
 Feedback inhibition regulates cell
respiration. The end product of the anabolic
pathway inhibits the enzyme that catalyzes an
early step of the pathway. This prevents the
needless diversion of key metabolic
intermediates from uses that are more
important
 Cells control their catabolism. If they’re
working hard & its ATP concentration begins
to drop, respiration speeds up. When there’s
plenty of ATP, respiration slows down,
sparing valuable organic molecules for other
functions.
 Phosphofructokinase is considered as the
pacemaker of respiration. It responds to
inhibitors & activators that help set the pace
of glycolysis & the citric acid cycle. It is an
allosteric enzyme with receptor sites for
specific inhibitors & activators. It is inhibited
by ATP & stimulate by AMP, which comes
from ADP.
o As ATP accumulates, inhibition of the
enzyme slows down glycolysis.
o The enzyme becomes active again as cell
work converts ATP to ADP to AMP
faster than ATP is being regenerated
It’s sensitive to citrate, the 1st product of the citric acid cycle. If citrate accumulates in
mitochondria, some of it passes into the cytosol & inhibits it. This helps coordinate the rates of
glycolysis & the citric acid cycle. As citrate accumulates, glycolysis slows down, & the supply
of acetyl groups to the citric acid cycle decreases. If citrate consumption increases, glycolysis
accelerates & meets the demand.
The E that keeps organisms alive is released, not produced by cell respiration. We are tapping E
that was stored in food by photosynthesis.
o
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