Download C9 Cellular Respiration (Video)

C9 Cellular Respiration
C9 Cellular Respiration (Video)
Cellular respiration (CR) – most common and efficient catabolic
pathway, in which organic compounds and oxygen yield energy,
water, and CO2. Occurs in mitochondria. Reactions are exergonic
with G = -686 kcal/mol of glucose.
C6H12O6 + 6O2  6CO2 + 6H2O + ATP + heat
Fermentation – anaerobic catabolic process that produces limited
ATP from glucose, common in fungi and bacteria.
 Alcoholic fermentation – pyruvate converted to ethanol and
CO2.
 Lactic acid fermentation – pyruvate reduced to make lactate
as waste product. No CO2 produced. In human muscles, lactic
acid carried to liver by blood where it is converted back to
pyruvate.
Stages of CR (Net total ATP = 38):
1. Glycolysis – in the cytosol; aerobic or anaerobic; net 2 ATP
2. Krebs cycle (citric acid cycle) – in mitochondrion; aerobic; 2
ATP
3. ETC – in mitochondria; aerobic; 34 ATP
Oxidative phosphorylation – producing ATP with energy from redox
reactions of an ETC; accounts for almost 90% of ATP via respiration.
Substrate-level phosphorylation – producing ATP by directly
transferring a P group to ADP from an intermediate substrate in
catabolism.
Glycolysis – splitting sugar; 6-C glucose splits into 2 3-C sugars.
The 3-C sugars are oxidized and rearranged to form 2 pyruvate
molecules. (10 steps). Glycolysis is source of 2 net ATP and 2
NADH molecules/glucose molecule and organic molecules for further
oxidation in Krebs cycle. NO CO2 released from glycolysis.
If oxygen present, pyruvate enters mitochondrion. Pyruvate
converted to acetyl CoA in 3 steps prior to actual Krebs cycle:
1. Carboxyl group removed and given off as CO2.
2. 2-C fragment oxidized to form acetate. An enzyme transfers
the electrons to NAD+, forming NADH.
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C9 Cellular Respiration
3. Coenzyme A is attached to the acetate by unstable bond that
makes the acetyl group very reactive. (Now acetyl CoA).
Krebs cycle – Hans Krebs, 1930’s. 8 steps, each catalyzed by a
specific enzyme in the mitochondrial matrix. CO2 given off in steps 3
and 4. Most energy conserved as NADH. For each acetate that
enters, 3 NAD+ molecules are reduced to NADH. Step 6, electrons
are transferred to FAD. FADH2 also donates its electrons to the ETC,
but at a lower level. Step 5, ATP formed by substrate-level
phosphorylation. Nets 2 ATP/glucose molecule.
Oxidation-reduction reactions (redox reactions) – transfer of 1or >
electrons from one reactant to another.
o Oxidation – loss of electrons
o Reduction – addition of electrons
Ex. Na + Cl  Na+ + ClNa was oxidized; Cl was reduced.
Na is reducing agent; Cl is oxidizing agent.
Because oxygen is so electronegative, it is one of the most potent
oxidizing agents. Electrons “fall” from organic molecules to oxygen
during cellular respiration.
NAD+ - nicotinamide adenine dinucleotide, coenzyme that functions
as oxidizing agent during respiration. NAD+ picks up 2 electrons and
1 proton to become NADH. NADH represents stored/carried energy
to make ATP.
Electron transport chain (ETC) – made of several proteins (except
ubiquinone), built in inner membrane of mitochondrion.
Cytochromes (cyt) are iron containing proteins. ETC breaks the fall
of electrons to oxygen into several energy-releasing steps instead of
1 explosive reaction. NADH drops off electrons at top; oxygen
captures these electrons with H to form H2O. Electron transfer of
NADH to oxygen is exergonic with G = -53 kcal/mol.
Most electrons travel this downhill route:
Food  NADH  ETC  oxygen
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C9 Cellular Respiration
ATP synthase – enzyme that makes ATP. Power source is
concentration gradient of H+ across the inner mitochondrial
membrane. (pH difference). ETC uses exergonic flow of electrons to
pump H+ across the membrane, from matrix into intermembrane
space. Only the ATP synthases are permeable to the H+ which flows
back through their channels. The H+ gradient couples the redox
reactions of the ETC to ATP synthesis.
Chemiosmosis – energy coupling mechanism that uses energy
stored in the form a H+ gradient across a membrane to drive cellular
work, such as ATP synthesis.
Proton-motive force – potential energy stored in the form of an
electrochemical gradient, generated by the pumping of H+’s across
biological membranes during chemiosmosis.
Most respiration energy flow:
Glucose  NADH  ETC  proton-motive force  ATP
1 NADH yields ~2-3 ATP
1 FADH2 yields ~2 ATP, efficiency of respiration ~40% (best
automobiles ~25%)
Evolutionary significance of glycolysis: ancient prokaryotes
probably used glycolysis before O2 present in Earth’s atmosphere
(~2.5 billion years ago). Glycolysis is most widespread metabolic
pathway.
Catabolism controlled by speeding up respiration if more ATP
needed. Phosphofructokinase, enzyme that catalyzes step 3 of
glycolysis, acts as a switch because it is the earliest step that
commits substrate irreversibly to the glycolytic pathway.
Phosophofructokinase is pacemaker of respiration. It is an allosteric
enzyme inhibited by ATP and stimulated by AMP. It is also sensitive
to citrate (first product of Krebs cycle). If citrate accumulates in
mitochondria, some of it passes into the cytosol and inhibits
phosphofructokinase. This mechanism helps synchronize rates of
glycolysis and Krebs cycle.
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