Chapter 9: Cellular Respiration and Fermentation
-
See figure 9.2
9.1: Catabolic pathways yield energy by oxidizing organic fuels
Catabolic pathways and production of ATP
- Compounds that can participate in exergonic reactions can act as fuels
- Fermentation – partial degradation of sugars or other organic fuel that
occurs without the use of oxygen
- Aerobic respiration – oxygen is consumed along with organic fuel to produce
carbon dioxide, water and ATP
- Cellular respiration – includes both anaerobic and aerobic processes
- C6H12O6 +O2
CO2 + H2O +Energy
- The
breakdown of glucose is exergonic, has a free
energy change of -686 kcal/mol
- (Recall a negative change in energy indicates the products store less energy
than the reactants and the reaction can occur spontaneously without an
input of energy)
Redox Reactions: Oxidation and Reduction
- The principle of redox
o Transfer of electrons (e-) in a reaction from one reactant to another
o Oxidation – loss of eo Reduction – gain of eo Reducing agent – e- donor
o Oxidizing agent – e- acceptor
o See figure 9.3
- Oxidation of organic fuel molecules during cellular respiration
o In cellular respiration
 Glucose is oxidized
 Oxygen is reduced
o Oxidation of glucose transfers electrons to a lower energy state
- Stepwise energy harvest of NAD+ and the electron transport chain (ETC)
o If energy is released all at once, energy can not be harvested
efficiently for constructive work
o H+ are transferred via electron carriers called NAD+ and FADH
(electrons travel with protons)
o Most electrons flow from glucose – NADH – ETC – oxygen
The stages of cellular respiration: A preview
1. Glycolysis
2. Kreb’s cycle (Citric acid cycle)
3. ETC
- See figure 9.6
- Eukaryotes – step 2 and three occur in mitochondria
- Prokaryotes- step 2 takes place in cytosol, step 3 takes place along the
plasma membrane
-
Oxidative phosphorylation – energy from each step is stored in a form the
mitochondrion can use to make ATP
o Powered by redox reactions of ETC
Substrate-level phosphorylation – enxyme transfers a phosphate group from
a substrate molecule to ADP
The cell makes about 32 molecules of ATP from each molecule of glucose
(each ATP has 7.3 kcal/mol of free energy)
9.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
-
See figure 9.9
Glycolysis – means “sugar splitting”
6-carbon glucose is broken down into 2 3-carbon molecules called pyruvate
To begin glycolysis, 2 molecules of ATP are invested
Net energy yield is 2 ATP (4 are made, but two are spent in the beginning)
2 NADH are formed and transferred to ETC
9.3: After pyruvate is oxidized, the citric acid cycle completes the energyyielding oxidation of organic molecules
- See figure 9.10, 9.11, 9.12
Oxidation of pyruvate to acetyl CoA
- Pyruvate is converted to a compound called acetyl CoA in a transition step
o Releases carbon dioxide (2 total molecules, 1 per pyruvate)
The citric acid cycle
- The cycle generates 1 ATP per turn (2 molecules total) and 3 carbon dioxide
molecules (6 in total, 2 from the conversion of pyruvate to acetyl CoA)
- Most of the chemical energy is transferred to NAD+ and FAD
- 8 NADH and 2 FADH2 are produced from each glucose molecule which is
then transferred to the ETC
9.4: During oxidative phosphorylation, chemiosmosis couples electron
transport to ATP synthesis
The Pathway of Electron Transport
- ETC is a collection of molecules embedded in the inner membrane of the
mitochondria in eukaryotic cells
- Most components are proteins, numbered I through IV
- Cytochromes – electron carriers
- NADH and FADH2 each carry a pair of electrons
o FADH2 provides less energy for ATP synthesis than NADH does
Chemiosmosis: The energy-coupling mechanism
- ATP synthase – enzyme that makes ATP from ADP and inorganic phosphate
o Works like an ion pump running in reverse
o Uses the energy of an existing ion gradient to power ATP synthesis
o Power source is a difference in the [H+]
o See figure 9.14
- Chemiosmosis – energy stored in the form of a hydrogen ion gradient across
a membrane is used to drive cellular work
- Establishing the H+ gradient is a major function of the ETC
- Proton-motive force – H+ gradient that results from H+ being pumped into
intermembrane space
- Chemiosmosis is an energy-coupling mechanism that uses energy stored in
the form of a H+ gradient across a membrane to drive cellular work
o Eukaryotic cells, energy comes from exergonic redox reactions
o ATP synthesis is the work performed
- See figure 9.15
An accounting of ATP production by cellular respiration
- Each NADH molecule generates enough proton-motive force to synthesize
approximately 2.5 ATP molecules
- See figure 9.16
- Each FADH2 generates approximately 1.5 ATP molecules
- Each glucose molecule generates 30-32 molecules of ATP
- Approximately 34% of the energy from glucose is transferred to ATP
production
9.5: Fermentation and anaerobic respiration enable cells to produce ATP
without the use of oxygen
-
Anaerobic respiration and fermentation do not require oxygen to generate
ATP
Types of fermentation
- Glycolysis plus reactions that generate NAD+ by transferring electrons from
NADH to pyruvate
- Alcohol fermentation
o Pyruvate is converted to ethanol in two steps
o See figure 9.17a
o First step releases carbon dioxide from pyruvate
o Second step acetalaldehyde is reduced by NADH to ethanol which
regenerates the supply of NAD+
- Lactic acid fermentation
o Pyruvate is reduced directly by NADH to form lactate with no release
of carbon dioxide
o Humans do this during short bursts of activity when oxygen is scarce
Comparing fermentation with anaerobic and aerobic respiration
- All three pathways use glycolysis to oxidize glucose to pyruvate
- Difference among the three pathways is the mechanisms for oxidizing NADH
back o NAD+
- Obligate anaerobes carry out fermentation or anaerobic respiration only
- Faculative anaerobes make ATP using fermentation or respiration
The evolutionary significance of glycolysis
-
Oldest known fossils of bacteria produced oxygen as a by-product of
photosynthesis
Therefore early prokaryotes may have generated ATP exclusively from
glycolysis
Glycolysis occurred in cytosol which shows that membrane-bound organelles
were not necessary
9.6: Glycolysis and the citric acid cycle connect to many other metabolic
pathways
The versatility of catabolism
- We obtain calories from fat, protein, starch and other sugars
- All of these organic molecules can be used for cellular respiration to make
ATP
- See figure 9.19
- Beta oxidation – breaks fatty acids down to two-carbon fragments
- A gram of fat oxidized by cellular respiration produces more than twice as
much ATP as a gram of carbohydrate
Biosynthesis (Anabolic pathways)
- Some organic monomers are used to provide carbon skeletons that are
required to make other molecules
Regulation of cellular respiration via feedback mechanisms
- See figure 9.20
- Most common mechanism fro control is feedback inhibition – end product of
anabolic pathway inhibits the enzyme that catalyzes an early step of the
pathway
o Prevents needless diversion of metabolic intermediates from uses
that are more urgent
- Catabolic controls are based mainly on regulating the activity of enzymes at
strategic points in the pathway