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Chapter 9: Cellular Respiration: Harvesting Chemical Energy
Living is work
-Cell organizes small organic molecules into polymers
-proteins
-DNA
-Pumps substances across membranes
-Grows, reproduces
-Maintains complex structures
-Work to maintain complex structure, order in intrinsically unstable
-Energy enters ecosystems in the form of sunlight
-Plants and other photosynthetic organisms
-Animals obtain fuel by eating plants or other organisms that eat plants
-How do cells harvest energy in organic molecules?
-How do they use it to regenerate ATP
-ATP is the molecule that drives cellular work
I. The Principles of Energy Harvest
-Metabolic pathway with many steps
A. Cellular Respiration and fermentation are catabolic, energy-yielding pathways
1.With the help of enzymes cell degrades complex molecules
a. Rich in PE
b. Waste products have less energy
c. Some energy used to do work
d. Some dissipates as heat
-Catabolic pathways: Metabolic pathways that release stored energy by breaking
down complex molecules
2. Fermentation: Partial degradation of sugars that occurs without the help of
oxygen
3. Cellular respiration: Catabolic pathway in which oxygen is consumed as a
reactant along organic fuel
a. Mitochondria houses the equipment
b. Compared to combustion of gasoline in car
a. Food is the fuel
b. CO2 is the exhaust
c. Organic + Oxygen  Carbon + Water + Energy
compounds
d. Organic compounds: Carbohydrates, fats, proteins
a. Traditionally: Glucose
-Exergonic: Δ G = -686 kcal/mol
(products store less energy than
reactants
-Main thought: How cells use energy stored in food molecules to make
ATP
-ATP is then used to move flagella, pump solutes across
membranes, polymerize monomers, etc.
B. Cells recycle ATP
1.ATP has high chemical energy
2.Chemical spring relaxes by losing a phosphate
3.Phosphorylation: Enzymes transfer phosphate groups to other compounds
a. Primes molecule to undergo change that performs work
b. Loses phosphate at the end of process
4.ADP (inorganic phosphate) has less stored energy than ATP
a. Muscle cell recycles 10 million molecules/s
C. Redox reactions release energy when electrons move closer to electronegative atoms
1.The relocation of electrons releases energy stored in for molecules
a. Used to synthesize ATP
2. Redox reactions (oxidation-reduction reactions): Transfer of electrons from
one reactant to another
a. Oxidation: The loss of electrons from one substance
b. Reduction: The addition of electrons to another substance
c. Reducing agent: The electron donor
d. Oxidizing agent: The electron acceptor
3. Fig. 9.3
a. The more electronegative the atom, the more energy required to pull
electrons away from it
b. Electrons lose PE as they shift from a less electronegative atom to a
more electronegative atom
b. A reaction that shifts electrons closer to O2 releases energy that can be
put to work
D. Electrons “fall” from organic molecules to oxygen
1.Cellular respiration is a redox process
a. Glucose is oxidized
b. Oxygen is reduced
1.Electrons lose potential energy
2.Organic molecules that have an abundance of H
a. Bonds are source of electrons that “fall” closer to O2
b. H is transferred from glucose to oxygen
c. Energy is taken out of storage and made available for
ATP synthesis
d. Main energy foods: Carbohydrates and fats
1.Reservoirs of electrons associated with H
2.Body temp is not high enough to initiate burning
a .Rapid oxidation of fuel , enormous release of heat
b. Swallow honey, molecules reach cells, enzymes lower activation
energy, sugar oxidizes slowly
E. The “fall” of electrons during respiration is stepwise
-The explosion of a gasoline tank cannot drive a car very far
-Cellular respiration does not oxidize glucose in a single explosive step that
transfers all the H from the fuel to the oxygen at one time
-Broken down gradually in a series of steps
-Each one catalyzed by an enzyme
-At key steps H is stripped from glucose, but not transferred directly to O2
F.NAD+ (nicotinamide adenine dinucleotide): Functions as an oxidizing agent during
cellular respiration
a. Dehydrogenases remove a pair of H atoms from the substrate (sugar or fuel)
1.2 e- and 2 protons
2.Delivers 2 e- and 1 proton to NAD+
a. The other proton is released as a H+ into surrounding solution
3.H—C—OH + NAD+ Dehydrogenase> C==O + NADH + H+
a. NADH:
1.Has been reduced
2.Oxidizing agent
3.e- acceptor
4.Most versatile e- acceptor in cellular respiration
a. Functions in many of the redox steps in the break
down of sugar
5.e lose little PE when transferred to NAD+
a. NADH: Stored energy
b. Tapped to make ATP when e- fall to O2
b. How do e- extracted from food and stored by NADH reach O2?
1.H2 and O2 to form water
a. Provide spark for activation energy
b. Gases combine explosively
c. Release of energy as e-s of H fall closer to O
2.Cellular respiration brings H and O together to form water
a. H derived from organic molecules, not H2
b. Electron transport chain: Sequence of e- carrier molecules
(membrane proteins) that shuttle e-s during the redox reactions that
release energy used to make ATP
1.Breaks the fall of electrons to oxygen into several energy
releasing steps
a. Instead of 1 explosive reaction
2.Consists mostly of proteins built into the inner
mitochondrion membrane
a. e- removed from food are shuttled by NADH to
top of chain
c. Bottom: O2 captures e-s along with H+ forming water
3.E- transfer from NADH to Oxygen: Exergonic ΔG = -222 kJ/mol
a. Single explosive step: Wasteful
b. Cascade down chain from one carrier molecule to the next
1.Losing a small amount of energy with each step
2.Oxygen: Final e- acceptor
3.Each carrier is more electronegative than the one uphill
4.Oxygen: Most electronegative
5.Oxygen pulls e-s down the chain in an energy-yielding
tumble
6.Food  NADH  electron transport chain  Oxygen
II. The process of Cellular Respiration
A. 3 Metabolic stages
1.Glycolysis: Breaks glucose into 2 molecules of pyruvate (cytosol)
2.Krebs cycle: Decomposes a derivative of pyruvate to CO2 (mitochondrial matrix)
a. Catabolic pathways that decompose glucose and other organic fuels
3. Electron transport chain:
a. Oxidative phosphorylation: The production of ATP using energy derived
from the redox reactions of an e- transport chain.
1.Inner membrane of mitochondrion
2.Accounts for almost 90% of ATP generated by cellular respiration
b. Substrate-level phosphorylation: The formation of ATP by directly
transferring a phosphate group to ADP from an intermediate substrate in
catabolism.
1.Substrate: Organic molecule generated during the catabolism of glucose
2.Fore each molecule of glucose degraded to CO2 and H2O
a. Cell makes up to 38 molecules of ATP
B. Glycolysis (splitting of sugar)
1.Glucose (6-C sugar) split into 2 3-C sugars
a. Oxidized, remaining atoms rearranged to form 2 pyruvates
b. Ionized form of 3-C acid (pyruvic acid)
2.Divided into 2 phases
a.1st 5 steps: Energy investment phase
1.Spends ATP to phosphorylate fuel molecules
b.2nd 5 steps: Energy payoff phase
1.ATP produced
2.NAD+ is reduced to NADH
3.Yield: 1 glucose: 2 ATP + 2 NADH
c. All C accounted for; no CO2 released
1.Can occur whether or not O2 is present
2.If present:
a. Energy stored in NADH is converted to ATP by etransport chain and oxidative phosphorylation
b. Chemical energy left in pyruvate can be extracted by
Krebs cycle
3.Fig. 9.9
C. The Krebs cycle
-Glycolysis releases less than a quarter of the chemical energy stored in glucose
-Most energy remains in pyruvate
-If oxygen is present: Pyruvate enters mitochondrion
1.Pyruvate is converted to acetyl CoA: acetyl coenzyme A
a. Junction between glycolysis and Krebs cycle
b. Multienzyme complex catalyzes 3 reactions
1.Pyruvates’s carboxyl group is given off as 1 CO2
2.Remaining 2-C fragment is oxidized
a. Acetate (ionized form of acidic acid)
b. Enzyme transfers e-s to NAD+ forming NADH
3.Coenzyme A (a sulfur containing compound derived from B
vitamin) is attached to the acetate
a. Unstable bond
b. Acetyl group: Very reactive
c. Acetyl CoA is now ready to feed its acetate into Krebs
cycle
2.Hans Krebs (German-British scientist, 1930s)
a.8 steps, each catabolized by a specific enzyme
b. Fig 9.11
1.For each turn, 2 C enter in the relatively reduced form of acetate
2. 2 different C leave in the oxidized form of CO2
3.Acetate joins by enzymatic addition to oxaloacetate forming
citrate
4. Subsequent steps decompose citrate back to oxaloacetate
a.CO2 for exhaust
5.Krebs cycle enzymes are located in mitochondrial matrix
a. Except for enzyme that catalyzes step 6
1.Inner mitochondrial membrane
6. For each acetate that enters cycle: 2 NAD+ are reduced to
NADH
a.(3,4,8)
7.Step 6: e- are transferred to a different e- acceptor
a. FAD (flavin adenine dinucleotide)
1.Derived from riboflavin (B vitamin)
2.FADH2 donates e- to e- transport chain
8. Step in step 5: Forms ATP by substrate-level phosphorylation
a. Similar to glycolysis
D. The inner mitochondrial membrane
-Glycolysis produces 2 ATP
-Krebs cycle produces 2 ATPs
-Output: 3 molecules of CO2 (Including 1 released by pre-Krebs)
-1 ATP/turn (substrate phosphorylation)
-Most chemical energy transferred to NAD+ and FAD
-The reduced coenzymes NADH and FADH2 shuttle their high energy e-s
to the e- transport chain
-Per glucose, multiply by 2 (glucose is split)
-Folding of inner membrane to form cristae increases surface area
1. Thousands of copies of e-transpsort chain in each mitochondria
a. Most components: Proteins
1. Prosthetic groups: Nonprotein components essential for catalytic
functions of certain enzymes
2. Alternate between reduced and oxidized states
a. Accept and donate e-s
b. e removed from food during glycolysis and Krebs cycle
1. Transferred by NADH to first molecule of e- transport chain
a. Flavoprotein:
1.Prosthetic group: Flavin mononucleotide (FMN)
2. Passes e s to Fe.S (an iron-sulfur protein
3. Passes e- to ubiquinone (Q) lipid (only member that is not a protein)
4. Cytochromes: Proteins containing a prosthetic heme group; 4 organic
rings surrounding a single iron atom
a. Similar to hemoglobin
b. Fe of cytochoromes transfers e-s not O2
c. Several types
1. Last cytochrome (cyt a3) passes its e-s to O2
2. O2 picks up a pair of H+ to form water
3. For every 2 NADH molecules one O2 is reduced to 2
molecules of water
2.FADH2 is another source of ea. Adds e-s at a lower energy level than NADH
1.1/3 less energy for ATP synthesis
3.Chemiosmosis: Energy coupling mechanism that uses energy stored in H+
gradient to drive cellular work, such as the synthesis of ATP
-Osmosis: Flow of H+ across membrane
a. ATP synthase: Protein complex that makes ATP from ADP and
inorganic phosphate
1.Found in inner membrane
2. Uses energy of an existing ion gradient to power ATP synthesis
3. Difference in concentration of H+ on opposite sides of inner
mitochondrial membrane
a. Difference in pH
b. e- transport chain pumps H+ across membrane from
matrix to intermembrane space
c. H+ leaks back across, but only through ATP synthases
1.Pass through channel
2.Use exergonic flow to drive phosphorylation of
ADP
3. How?
4. e transport chain accepts H+ from matrix and deposits it in
space
a. Proton-motive force: H+ gradient that can perform work
5. ATP synthase: Multi-subunit complex (4 parts)
a. Rotor (membrane)
b. Knob (Protrudes into matrix)
c. Internal rod (extends from rotor to knob)
d. Stator (anchored next to knob, holds knob stationary)
6. H+ flows between stator and rotor causing rotor and rod to rotate
a. Causes conformational changes in knob
b. Activates catalytic sites where ADP and inorganic
phosphate combine to make ATP
7. Chemiosmosis in other places
a. Chloroplasts:
-
1. Light rather than chemical energy drives
electron flow down e- transport chain
2. Light drives H+ gradient formation
b. Prokaryotes
1. Generate H+ gradients across membranes
2. Tap proton-motive force to make ATP, pump
nutrients, waste, rotate flagella
8. Peter Mitchell (1961): 2 decades later Nobel Prize
E. Cellular respiration generates many ATP for each sugar molecule
1. Glucose NADH  e- transport chain  proton-motive force ATP
2. 1Glucose  6 CO2
3. Tally: Few from glycolysis and Krebs, many from oxidative phosphorylation of
e- transport chain
a. Each NADH that transfers a pair of e- from food to transport chain
1.Enough proton-motive force to generate about 3 ATPs
a. Average: Between 2 and 3
b. Krebs cycle transfers FADH2: 2 molecules of ATP
4.Some eukaryotic cells
a. NADH lower yield (2)
1. Mitochondrial inner membrane is impermeable to NADH
2. NADH in cytosol is segregated from machinery of oxidative
phosphorylation
a. Must be shuttled to mitochondrion
1.NAD+ (3 ATPs) or
2.FAD (2 ATPs)
5. If NAD+ is the shuttle: Max ATPs (38)
a. Very efficient: 40%
1.Phosphorylation of ADP to form ATP stores 7.3 kcal/mol
2.7.3x 38/686
3.The rest is lost as heat
a. Body temp
b. Dissipate the rest
c. Automobiles convert 25% energy stored in gas to move
car
III. Related Metabolic Processes
-Must have O2 for electron transport to work
A. Fermentation: ATP with no O2 (anaerobic process)
1.Extension of glycolysis: Generates ATP by substrate-level phosphorylation
a. NAD+ is the e- acceptor
1.Recycled from NADH by transfer of e- to transport chain
2.Anerobic: Transfer e-s to pyruvate
2. Alcohol Fermentation: Pyruvate is converted to ethanol in 2 steps
a. CO2 is released from pyruvate yielding a 2 C acetaldehyde
b. Acetaldehyde is reduced by NADH to ethanol
1.Regenerates supply of NAD+
2.Used by yeast in brewing and winemaking
3.Many bacteria, under anaerobic conditions
3.Lactic acid fermentation: Pyruvate is reduced by NADH to form lactate
a. Ionized form of lactic acid
b. No release of CO2
c. Used for making cheese and yogurt
d. Human muscle cells make ATP when O2 is scarce
1.When sugar catabolism for ATP production outpaces muscle’s
supply of O2 from the blood
2. Cells switch from aerobic respiration to lactic acid fermentation
a. Can accumulate as a waste product
1.Fatigue and pain
2.Carried away by blood to liver
a. Converted back to pyruvate
B. Fermentation and respiration compared
Similarities
Differences
Glycolysis to oxidize glucose to pyruvate
(F)Final e- acceptor is pyruvate or
Net production of 2 ATP
acetaldehyde
Substrate level phosphorylation
(CR) Final e- acceptor is O2
+
NAD accepts e s from food
1.O2 being the final acceptor
a. Regenerates NAD+ for glycolysis
b. ATP payoff from NADH to O2 and from pyruvate in Krebs cycle
1. Without O2, energy stored in pyruvate is unavailable to
cell
2. Cellular respiration produces 19 x’s more ATP than
fermentation
2. Facultative anaerobes: Yeasts and bacteria that can make enough ATP to
survive using fermentation
a. Pyruvate: Fork in metabolic road (in our bodies)
1. 2 Catabolic routes
a. Aerobic: Converted to acetyl CoA  Krebs cycle
b. Anaerobic: Pyruvate is diverted from Krebs cycle
1. Serves as e- acceptor to recycle NAD+
2. Must consume sugar much faster
C. Evolutionary significance of glycolysis
1. First prokaryotes: May have generated ATP from glycolysis (no O2)
a. Most widespread metabolic pathway
b. Cytosolic location
1.No membrane bound organelles in prokaryotic cells
IV. Glycolysis and Krebs cycle connect to many other metabolic pathways
A. Versatility of catabolism
1.Free glucose molecules are not common in diets of humans and other animals
a. Fats, proteins, sucrose, disaccharides, starch
2.Glycolysis can accept a wide range of carbohydrates for catabolism
a. In digestive tract: Starch is hydrolyzed to glucose
b. Glycogen (stored in liver and muscle) can be hydrolyzed to glucose
between meals
c. Digestion of disaccharides (sucrose) provides glucose and other
monosaccharides
d. Proteins must be digested to amino acids
1. Many used to build new proteins
2. Excess, converted by enzymes to intermediates of glycolysis and
Krebs cycle
a. Before Krebs cycle:
1. Deamination: Amino groups removed from aa
2. Nitrogenous refuse is excreted in the form of
ammonia, urea, or other waste products
e. Fats obtained from food or storage cells (stored as fatty acid)
1. After fats are digested: glycerol is converted to glyceraldehyde
phosphate
a. Intermediate of glycolysis
b. Beta oxidation: Breaks fatty acids down to 2-C
fragments
1.Enter Krebs cycle as acetyl CoA
2.1 g fat produces 2xs as much ATP as 1g
carbohydrate
B. Biosynthesis
1.Cells need substance as well as energy
a. Not all organic molecules of food are destined to be oxidized as fuel to
make ATP
b. C skeletons that cells require to make their own molecules
1.Organic monomers obtained from digestion
a. aa
b. Body needs molecules that are not found in food
1.Compounds formed as intermediates of glycolysis
and Krebs cycle
a. Diverted into anabolic pathways as
precursors
b. Cell can synthesize molecules it requires
c. Humans can make half of 20 aa by
modifying compounds siphoned from Krebs
cycle
2. Glucose can be made from pyruvate
3. Fatty acids can be synthesized from acetyl CoA
4. Consume ATP
2.Glycolysis and Krebs function as metabolic interchanges
a. Enables cells to convert some kinds of molecules to others as we need
them
b. Carbohydrates and proteins can be converted to fats through
intermediates
b. We store fat even if our diet is fat free
V. Feedback mechanisms control cellular respiration
A. Feed back inhibition: The end product of an anabolic pathway inhibits the enzyme that
catalyzes an early step
1.If ATP concentration begins to drop: Respiration speeds up
2.Plenty of ATP to meet demand: Respiration slows down
3.Regulation of activity of enzymes at strategic points
a. Phosphofructokinase: Enzyme that catalyzes step 3 of glycolysis
1.Earliest step that commits substrate irreversibly to glycolytic
pathway
2.By controlling this step: Cell can speed up or slow down entire
process
a. Pacemaker of respiration
3.Allosteric enzyme
a. Receptors for specific inhibitors and activators
b. Inhibited by ATP, stimulated by AMP
1.As ATP accumulates, inhibition of enzyme slows
down glycolysis
c. Becomes active again as cellular work converts ATP to
ADP and AMP faster than ATP is being generated
4.Also sensitive to citrate (first product of Krebs cycle)
a. If accumulates in mitochondria and passes into cytosol
1.Inhibits phosphofructokinase
b. Synchronizes rates of glycolysis and Krebs cycle
1.Citrate accumulates: Glycolysis slows down
a. Supply of acetate to Krebs cycle decreases
2.Citrate consumption increases: Glycolysis
accelerates