Download Bio150 Chapter 7

Respiration
• Organisms can be classified based on how
they obtain energy:
• Autotrophs
– Able to produce their own organic molecules
through photosynthesis
• Heterotrophs
– Live on organic compounds produced by other
organisms
• All organisms use cellular respiration to
extract energy from organic molecules
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Cellular respiration
• Cellular respiration is a series of reactions
• Aerobic respiration (in presence of
oxygen)
– Final electron receptor is oxygen (O2)
• Anaerobic respiration (without oxygen)
– Final electron acceptor is an inorganic
molecule (not O2)
• Fermentation (without oxygen – non
humans)
– Final electron acceptor is an organic molecule
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• Aerobic respiration (in presence of
oxygen)
– Final electron receptor is oxygen (O2)
• Anaerobic respiration (without oxygen)
– Final electron acceptor is an inorganic
molecule (not O2)
• Fermentation (without oxygen – non
humans)
– Final electron acceptor is an organic molecule
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Cellular Energy
•In order to sustain life (steady state), cells constantly
expend energy in the form of ATP hydrolysis
–the hydrolysis of ATP yields a molecule of ADP
(adenosine diphosphate) and a Phosphate group
•ATP → ADP + P + E
•Following ATP hydrolysis, the cell synthesizes new
molecules of ATP using ADP and P
–ADP + P + E → ATP
–“recycling energy”
–cells use the energy stored in the covalent bonds of
monosaccharides, fatty acids and amino acids as
an energy source (fuel) to resynthesize ATP
Cellular Respiration
-Most cells of the body use glucose in the presence of
O2 that we inhale, as fuel source to synthesize
molecules of ATP in a process called cellular
respiration
•The products include molecules of carbon dioxide that
we exhale, and water
-C6H12O6 + 6O2 + 38 ADP +38 P  6H2O + 6CO2 + 38 ATP
–note that the number of atoms in the reactants equal
the number of atoms in the products
–That is the reaction is balanced
Cellular Respiration
The process of cellular respiration is divided into 3
sequential phases
1. Glycolysis
occurs in the cytoplasm
2. Krebs cycle
occurs in the mitochondrial matrix
3. The electron transport chain and oxidative
phosphorylation
occurs across the inner mitochondrial
membrane - between the matrix and the
intermembrane space
•During the first 2 phases of glycolysis and the Krebs
cycle, a molecule of glucose is gradually broken apart in
20 sequential biochemical reactions which transforms
the glucose into different carbohydrate intermediates as
bonds are broken or rearranged
-4 molecules of ATP are synthesized as a result of
energy released from the breaking of bonds of the
carbohydrate intermediates
–the 12 hydrogen atoms of the original glucose
molecule are removed and used to create a large
H+ gradient across the inner mitochondrial
membrane
-the H+ gradient is used as an energy source by
the electron transport chain and oxidative
phosphorylation to synthesize the remaining 34
molecules of ATP
Summary of ATP Production
Glycolysis
-Glycolysis = sugar breaking
•The first 7 biochemical reactions of cellular respiration
are catalyzed by enzymes in the cytoplasm
•Begins with 1 molecule of glucose (6 carbons) and
ends with 2 molecules of pyruvic acid (3 carbons x 2 = 6
carbons)
•During glycolysis:
–2 molecules of ATP are hydrolyzed
–one 6 carbon carbohydrate intermediate is split into
two 3 carbon carbohydrate intermediates (PGAL)
–in one reaction, each of the 3 carbon carbohydrate
intermediates molecules lose a H (oxidized) which is
transferred to a molecule of NAD+ (reduced to NADH)
–4 molecules of ATP are synthesized
Glycolysis
Glycolysis
-The products of glycolysis are:
–2 molecules of ATP
•4 are synthesized, but 2 are hydrolyzed at the
beginning of glycolysis
-2 molecules of pyruvic acid (3 carbon molecule)
–2 molecules of NADH
•move from the cytosol to the mitochondrial
matrix
•Only 2 of 38 molecules of ATP are synthesized during
glycolysis
Pyruvic Acid
-The presence or absence of O2 in the cell determines
how the cell will use the 2 molecules of pyruvic acid at
the end of glycolysis
–If O2 is present pyruvic acid will be used in the
process of aerobic respiration
-36 more molecules of ATP are synthesized as
cellular respiration continues through the Krebs cycle
and the electron transport chain and oxidative
phosphorylation
•If O2 is absent the pyruvic acid will be used in the
process of anaerobic respiration (fermentation)
•produces no more ATP
Anaerobic Fermentation (Anaerobic Respiration)
•In the absence of O2 (oxygen debt), the 2 molecules of
pyruvic acid from the end of glycolysis are converted into
2 molecules of lactic acid
•Anaerobic fermentation most commonly occurs in active
skeletal muscle cells when they use O2 faster than it can
be delivered by the respiratory and circulatory systems
•The accumulation of lactic acid causes:
–muscle proteins to partially denature
•leading to muscle weakening (fatigue)
–muscle soreness
Aerobic Respiration
-In the presence of O2, the 2 molecules of pyruvic
acid from the end of glycolysis are transported into
the mitochondrial matrix
•Each molecule of pyruvic acid is converted into a
molecule of Acetyl Coenzyme A (Acetyl CoA)
-required step between glycolysis and the Krebs
cycle
2 Pyruvic Acid → 2 Acetyl CoA
During the conversion of each of the 2 molecules of
pyruvic acid to Acetyl CoA the following occurs:
•Decarboxylation (CO2 is removed) of pyruvic acid
making acetic acid (2 carbons)
-Acetic acid is oxidized while NAD is reduced to NADH
-Coenzyme A in the mitochondrial matrix is added to the
acetic acid to make acetyl CoA
During this step the products are:
–2 molecules of CO2
–2 molecules of NADH
–2 molecules of Acetyl CoA
•used in the Krebs Cycle
Pyruvic Acid → Acetyl CoA
Krebs Cycle (Citric Acid Cycle)
•A series of 10 biochemical reactions which begins and
ends (cyclic) with a molecule of citric acid
-Each acetic acid (2 carbons) is combined with a
molecule of oxaloacetic acid (4 carbons) to make citric
acid (6 carbons)
•Each citric acid is decarboxylated and oxidized as it is
converted into carbohydrate intermediates
–NAD+ and FAD are reduced to NADH and FADH2
–energy released from the hydrolysis of bonds is
used to synthesize ATP
•These reactions produce:
–3 molecules of NADH
–1 molecule of FADH2
–2 molecules of CO2
–1 molecule of ATP
–1 molecule of oxaloacetic acid
Krebs Cycle
•For each molecule of glucose entering glycolysis, two
molecules of acetyl CoA enter the Krebs cycle yielding :
Products of Krebs Cycle
–6 NADH
–2 FADH2
–4 CO2
–2 ATP
–2 oxaloacetic acid
•reenter the Krebs cycle of reactions after
combined with another acetic acid
Products of Glycolysis → Krebs Cycle
•4 ATP
–2 from glycolysis
–2 from Krebs
•10 NADH (10 H from original glucose molecule)
–2 from glycolysis
–2 from (2) Pyruvic Acid → (2)Acetyl CoA
–6 from Krebs
•2 FADH2 (2 H from original glucose molecule)
–2 from Krebs
•6 CO2 (6 C from original glucose molecule)
–2 from (2) Pyruvic Acid → (2)Acetyl CoA
–4 from Krebs
Electron Transport Chain
A group of 5 integral and peripheral membrane
enzymes associated with the inner mitochondrial
membrane called the electron transport chain (ETC)
creates a H gradient between the intermembrane space
and the matrix of the mitochondria using the H of
molecules of NADH and FADH2 that have accumulates
in the mitochondrial matrix as products of glycolysis
and the Krebs cycle
•3 Enzyme complexes of the ETC oxidizes the NADH
and FADH2 (remove the H) in the matrix and split it into
an electron (e-) and a proton (H+)
•The e- is passed from one enzyme of the ETC to the
next until it reaches the last enzyme of the ETC
•NADH is oxidized by Enzyme complex 1 of the ETC
•FADH2 is oxidized by Enzyme complex 2 of the ETC
-The electron that is shuttled from enzyme to enzyme in
the ETC initially has a high amount of energy
-the electron energizes the 1st, 2nd and 3rd Enzyme
complexes which provides enough energy to each
complex to pump a free proton from the mitochondrial
matrix into the intermembrane space
–the energy in the electron decreases as it is passed
along the ETC, as each Enzyme complex takes
some of the energy away from the electron using it
to pump protons
–the 3rd Enzyme complex transfers the electron to an
atom of oxygen in the matrix
ETC and Oxidative Phosphorylation
Free Energy of Electrons in the ETC
Proton (H+) gradient
•The movement of protons from the matrix into the
intermembrane space creates a high H+ (pH = 7)
concentration in the intermembrane space and a low H+
(pH = 8) concentration in the matrix (Chemiosmosis)
–this proton gradient becomes the source of energy
used by the mitochondria to synthesize ATP, which
is released as H+ diffuse from the intermembrane
space back into the matrix
•The oxidation of NADH energizes all 3 Enzymes
complexes (1st, 2nd and 3rd) moving 3 H+ across the
inner membrane
-The oxidation of FADH2 energizes only 2 enzymes
(2nd and 3rd) moving 2 H+ across the inner membrane
•The oxidation of NADH contributes more to the proton
gradient than FADH2
ATP Synthase
•ATP synthase is an integral membrane protein in the
inner mitochondrial membrane that has 2 functions:
–it acts as a H+ channel
•allows H+ to diffuse down its gradient into the
matrix
–it acts as an enzyme
•uses the energy that is released by the diffusion
of H+ to synthesize ATP from ADP and P
–this type of ATP synthesis is called oxidative
phosphorylation because, this process requires
the oxidation of NADH and FADH2
- and attaching Phosphate to ADP to make ATP
ATP Synthesis During Oxidative Phosphorylation
-Because the oxidation of NADH contributes more to
the proton gradient than the oxidation of FADH2, more
ATP is synthesized from NADH than FADH2
•On average, for each NADH that is oxidized 3
molecules of ATP are synthesized
–10 NADH x 3 ATP = 30 ATP
•On average, for each FADH2 that is oxidized 1.5
molecules of ATP are synthesized
–2 FADH2 x 2 ATP = 4 ATP
-34 of the 38 molecules of ATP are synthesized by
oxidative phosphorylation
Formation of Water
-After the electrons arrive to the last enzyme of the ETC
they are transferred to atoms of oxygen in the
mitochondrial matrix
-this makes oxygen especially negative
-In the matrix, these oxygen atom are combined with the
H+ that have diffused back into the matrix from the
intermembrane space to form water (H2O)
–the protons become “reunited” with their electron
Oxidation Without O2
1. Anaerobic respiration
– Use of inorganic molecules (other than O2) as
final electron acceptor
– Many prokaryotes use sulfur, nitrate, carbon
dioxide or even inorganic metals
2. Fermentation
– Use of organic molecules as final electron
acceptor (lactic acid or ethyl alcohol)
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Anaerobic respiration
• Methanogens
– CO2 is reduced to CH4 (methane)
– Found in diverse organisms including cows
• Sulfur bacteria
– Inorganic sulphate (SO4) is reduced to
hydrogen sulfide (H2S)
– Early sulfate reducers set the stage for
evolution of photosynthesis
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Fermentation
• Reduces organic molecules in order to
regenerate NAD+
1.Ethanol fermentation occurs in yeast
– CO2, ethanol, and NAD+ are produced
2.Lactic acid fermentation
– Occurs in animal cells (especially muscles)
– Electrons are transferred from NADH to
pyruvate to produce lactic acid
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Catabolism of Fat
• Fats are broken down to fatty acids and
glycerol
– Fatty acids are converted to acetyl groups by
b-oxidation
– Oxygen-dependent process
• The respiration of a 6-carbon fatty acid
yields 20% more energy than 6-carbon
glucose.
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Catabolism of Protein
• Amino acids undergo deamination to
remove the amino group
• Remainder of the amino acid is converted
to a molecule that enters glycolysis or the
Krebs cycle
– Alanine is converted to pyruvate
– Aspartate is converted to oxaloacetate
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