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Chapter 8: Cellular Respiration
(Outline)
 NAD+ and FAD
 Phases of Cellular Respiration
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Glycolysis
Preparatory (prep) Reaction
Citric Acid Cycle
Electron Transport System (ETC)
 Fermentation
 Catabolic and Anabolic Reactions
Cellular Respiration
 A cellular process that requires oxygen and
gives off carbon dioxide
 Usually involves breakdown of glucose to
carbon dioxide and water
 Energy extracted from glucose molecule:
 Released slowly step-wise
 Allows ATP to be produced efficiently
 Oxidation-reduction enzymes include NAD+
and FAD as coenzymes
Cellular Respiration
 Glucose is oxidized and thus releases energy, while
oxygen is reduced to form water
 The carbon atoms of the sugar molecule are
released as carbon dioxide (CO2)
 Glucose is high-energy molecule; CO2 and H2O are
low-energy molecules thus energy is released
Cellular Respiration
 Cells carry out cellular respiration in order to
build up ATP molecules
 Energy is released slowly, step-wise, through
many enzymatic reactions in different parts of
the cell, why?
 Breakdown of glucose realizes a maximum
yield of 36 or 38 ATP; this preserves 39% of
energy available in glucose
NAD+ and FAD
 Each metabolic reaction in cellular respiration
is catalyzed by its own enzyme
 NAD+ is a redox coenzyme that can
 Oxidize a metabolite by accepting two
electrons and a hydrogen ion; results in NADH
 Reduce a metabolite by giving up electrons
 FAD is another redox coenzyme
 Sometimes used instead of NAD+
 Accepts two electrons and two hydrogen ions
(H+) to become FADH2
NAD+ and FAD
 Electrons received by
NAD+ and FAD are highenergy electrons and are
usually carried to the
electron transport system
 Only a small amount of
NAD+ is needed in cells,
because each NAD+
molecule is used over &
over again
Phases of Cellular Respiration
 Cellular respiration involves four phases
 Glycolysis:
 Occurs in the cytoplasm
 Does not require oxygen; occurs under
aerobic or anaerobic conditions
 Glucose broken down into two 3-carbon
molecules of pyruvate
 Enough energy is released for immediate
buildup of two ATP molecules
Phases of Cellular Respiration
 Preparatory (Prep) reaction:
 Takes place inside the mitochondrion
 Pyruvate is oxidized to a 2-carbon (C2)
acetyl group
 Electron energy is stored in NADH
 CO2 is released as waste product
 Occurs twice per glucose molecule
Phases of Cellular Respiration
 Citric acid cycle:
 Series of oxidation reactions occurring in the
matrix of mitochondrion (i.e. aerobic)
 Electron energy is stored in NADH and
FADH2
 Produces two immediate ATP molecules per
glucose molecule
 Four carbons are released as CO2
 The cycle turns twice per glucose molecule
Phases of Cellular Respiration
 Electron transport chain:
 A series of carriers on the cristae of the
mitochondria
 Extracts energy from NADH & FADH2
 Electrons pass from higher to lower energy
states, energy is released and stored for
ATP production, how?
 Produces 32 or 34 molecules of ATP per
glucose molecule
Glucose Breakdown:
Overview of the Four Phases
Pyruvate
 Pyruvate is a pivotal metabolite in cellular
respiration
 If O2 is not available to the cell,
fermentation, an aerobic process, occurs
 During fermentation, glucose is
incompletely metabolized to lactate or CO2
and alcohol
 Fermentation results in a net gain of only
two ATP per glucose molecule
Glucose Breakdown:
Glycolysis
 Proceeds in the cytosol
 Occurs universally in organisms (i.e. most
likely evolved before the citric acid cycle and
the electron transport system)
 Energy Investment Steps:
 The addition of two phosphate groups from ATP
to activate glucose in 2 separate reactions
 Glucose splits into two (C3) G3P molecules, each
with a phosphate group
glucose + 2 ATP → 2 G3P + 2 ADP
Glycolysis
 Energy Harvesting Steps:
 In duplicated reactions, NAD+ accepts two
electrons and one H+ ion resulting in two NADH
 Four ATP molecules are formed by substratelevel ATP synthesis
 Net gain of two ATP from glycolysis, why?
 Both G3Ps are oxidized to pyruvates
 Pyruvate enters mitochondria if oxygen is
available and aerobic respiration follows
 If oxygen is not available, glycolysis becomes a
part of fermentation and pyruvate is reduced
Inputs and Outputs of Glycolysis
Glycolysis:
Substrate-level ATP synthesis
 A phosphate group is
transferred to ADP
giving one ATP molecule
 During glycolysis, the C3
substrate BPG gives up
a phosphate group to
ADP
 Occurs twice per glucose
molecule
Glycolysis
Inside the Mitochondria
 Aerobic Respiration – involves the preparatory
reaction, the citric acid cycle and the electron
transport system
 Mitochondrion Structure and Function
 Double membrane with an intermembrane space
between the outer and inner membrane
 Cristae - inner folds of membrane
 Matrix - the innermost compartment filled with a
gel-like fluid
 Produces most of the ATP from cellular respiration
(i.e. powerhouse of the cell)
Mitochondrion:
Structure and Function
Glucose Breakdown:
The Preparatory (Prep) Reaction
 Preparatory reaction connects glycolysis to the
citric acid cycle
 End product of glycolysis (i.e. pyruvate) enters
the mitochondrial matrix
 Pyruvate is converted to a C2 acetyl group
 Attached to CoA to form acetyl CoA
 Electrons picked up by 2 NAD+ to give 2 NADH
 CO2 is released and transported out of
mitochondria into the cytoplasm
 Reaction occurs twice per glucose molecule
Preparatory Reaction
Glucose Breakdown:
Citric Acid Cycle
 Also known as the Krebs cycle; a cyclic pathway
occurring in the matrix of mitochondria
 Both (C2) acetyl-CoA groups from the prep
reaction:
 Joins with a C4 molecule to give citrate (C6)
 Each acetyl group is oxidized to two CO2 molecules
 Electrons are accepted by NAD+ in three instances
(forming 3 NADH) and by FAD in one instance
(forming FADH2)
 ATP is formed (per acetyl group) by substrate-level
ATP synthesis
Citric Acid Cycle
Inputs and Outputs of The Citric
Acid Cycle

Krebs cycle turns twice per glucose molecule
Electron Transport Chain (ETC)
 Location:
 Eukaryotes – cristae of the mitochondria
 Aerobic Prokaryotes – plasma membrane
 Series of carrier molecules:
 Pass energy rich electrons along
 Protein carriers such as cytochrome molecules
 Cytochromes - proteins with a central iron (heme)
group; the group is the one being oxidized &
reduced
 Receives electrons from NADH and FADH2
ETC (cont.)
 Oxygen is the final electron acceptor in the ETC
 Lack of oxygen blocks the entire ETC – no
additional ATP is produced leading to death
 Some poisons also inhibit normal activity of
cytochromes
 Example: Cyanide binds to iron in cytochrome,
blocking ATP production
Cycling of Carries
 The fate of the hydrogens:
 H+ from NADH deliver enough energy to make 3
ATPs
 Those from FADH2 have only enough for 2 ATPs
 Recycling of coenzymes increases efficiency
 Once NADH delivers H+, it returns (as NAD+) to
pick up more H+
 However, hydrogen atoms must be combined
with oxygen to make water
 If O2 is not present, NADH cannot release H+
 No longer recycled back to NAD+
ETC
Carriers on Cristae of a
Mitochondrion
 The ETC consists of 3 protein complexes and 2
carriers
 The 3 protein complexes include:
 NADH-Q reductase complex
 Cytochrome reductase complex
 Cytochrome oxidase complex
 The other 2 carriers are coenzyme Q and
cytochrome c
 H+ carried by NADH and FADH2 are pumped by
the protein complexes into the intermembrane
space; thus creating H+ gradient
Organization and Function of
Cristae
ATP Production
 ATP synthase complex – channel protein (in
cristae) that serves as an enzyme for ATP
synthesis
 Protons diffuse from the intermembrane space
(high conc.) to the matrix (low conc.) through
the enzyme complex ATP synthase
 This catalyzes the phosphorylation of ADP to
form ATP – Chemiosmosis
 ATP molecules them move out of the
mitochondria to perform cellular work
Mitochondria in Active Tissue
 ATP production must be constant in order to
sustain life
 Active tissues (e.g. muscles) require greater
amounts of ATP and contain more mitochondria
than less active cells
 Example – Dark meat of chickens contains more
mitochondria than the white meat of the breast
Energy Yield from Glucose
Metabolism
 Complete breakdown of glucose to CO2 and H2O
yields 36 to 38 ATPs
 Substrate-level ATP synthesis
 2 ATP from glycolysis
 2 AP from the citric acid cycle
 Total of four ATP are formed outside of the
electron transport system
 32 to 34 ATP from the electron transport chain
and chemiosmosis
Energy Yield from Glucose
Metabolism (cont.)
 ETC and Chemiosmosis
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Per glucose molecule, 10 NADH and two FADH2
provide electrons and H+ ions to electron transport
system
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For each NADH formed within the mitochondrion,
3 ATP are produced
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For each FADH2 formed by Krebs cycle, 2 ATP
result since FADH2 delivers electrons after NADH
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For each NADH formed in the cytoplasm, 2 ATP
are formed as electrons are “shuttled” across the
mitochondrial membrane and delivered to FAD
Energy Yield from Glucose
Breakdown
 Total NADH = 8 x 3 ATP = 24 ATP
 Total FADH2 = 2 x 2 ATP = 4 ATP
 ETC yields 28 ATP
 28 (ETC) + 4 (substrate level) = 32 ATP
 Other 4 ATP comes from NADH produced by
glycolysis which transport electrons via shuttle
molecule to 2 FADH2, thus 2 x 2ATP = 4 ATP
 Now we have 32 + 4 = 36 ATP molecules
Efficiency of Cellular Respiration
Glucose  O 2 
 CO 2  H 2 O
Reactants
Products
 Energy difference (∆G) = 686 kcal
 One ATP phosphate bond has an energy
content of 7.3 kcal
 36 ATP produced during glucose breakdown
(36 x 7.3) = 263 kcal
 Efficiency is 263/686 x 100 = 39% of available
energy from glucose
 The rest of the energy is lost as heat
Overall Energy Yielded per Glucose
Molecule
Fermentation
 When oxygen is limited:
 Spent hydrogens have no final acceptor
 NADH can’t recycle back to NAD+
 Glycolysis stops because NAD+ is required
 Fermentation:
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“Anaerobic” pathway
Can provide rapid burst of ATP in the absence of O2
Provides NAD+ for glycolysis
NADH combines with pyruvate to yield NAD+
NAD+ is then free to return and pick up more e- during
earlier reactions of glycolysis
Fermentation
Fermentation (cont.)
 Pyruvate is reduced by NADH to:
 Lactate (Animals & anaerobic bacteria)
 In muscles, pyruvate is reduced to lactate when
it is produced faster than it can be oxidized by
Krebs cycle
 Cheese & yogurt
 Industrial chemicals (i.e. isoprpanol, butyric
acid, propionic acid & acetic acid)
 Ethanol & carbon dioxide (Yeasts)
 Bread & alcoholic beverages
Advantages and Disadvantages of
Fermentation
 Despite the low yield of 2 ATP, it provides a
quick burst of energy for muscular activity
 Allows glycolysis to proceed faster than O2 can
be obtained
 Anaerobic exercise
 Lactic acid accumulates (toxic to cells)
 Causes cramping and oxygen debt
 When O2 restored, lactate is broken down to
pyruvate; then respired or converted into
glucose
Efficiency of Fermentation
 Two ATP produced per glucose molecule during
fermentation gives (2 x 7.3)= 14.6 kcal
 Complete glucose breakdown to CO2 and H2O
during cellular respiration = 686 kcal of energy
 Efficiency is 14.6/686 x 100 = 2.1%
 Much less efficient than complete breakdown of
glucose
Catabolic and Anabolic Reactions
 Metabolism – sum of all chemical reactions
within a living organism
 Catabolism – chemical reactions that result in
the breakdown of complex organic compounds
into simpler substances; thus releases energy
 Example: Cellular Respiration
 Anabolism – chemical reactions that build new
molecules from simpler substances; thus
requires energy
 Example: Photosynthesis