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Biology, Section 3
Notes
Date Submitted: 1/6/12
Chapter 7 Section 2 Notes
Cellular Respiration: Aerobic Respiration
Aerobic Respiration
A. 2 major stages: Krebs cycle & Electron Transport Chain (associated w/ chemiosmosis)
B. Krebs cycle (matrix)
a. Glucose oxidizes  NAD+ reduced to NADH
C. Electron Transport Chain (inner membrane)
a. NADH makes ATP
D. Krebs cycle also produces small amt. of ATP; most come from ETC
and Chemiosmosis
E. In prokaryotes, reactions of Krebs cycle & ETC take place in cytosol
F. In eukaryotes, reactions take place inside mitochondria
G. Pyruvic acid from glycolysis diffuse across double membrane of
mitochondrion  mitochondrial matrix (space inside inner
membrane) – contains enzymes to catalyze Krebs cycle
H. When pyruvic acid enters matrix, it reacts w/ molecule called
coenzyme A to form acetyl coenzyme A (acetyl CoA)
a. Acetyl part of acetyl CoA contains 2C atoms
b. Pyruvic acid is 3C compound
c. C atoms lost in conversion is released in 1 CO2; reaction
reduces 1 NAD+ to 1NADH
Krebs cycle
A. Krebs cycle: biochem. Pathway that breaks down acetyl CoA  produces CO2, H atoms, ATP
B. By Hans Krebs (1900-1981), German biochemist
C. 5 main steps; in eukaryotic cells - all steps occur in mitochondrial matrix
1) 2C molecule of acetyl CoA + 4C compound oxaloacetic acid  6C citric acid
a. Regenerates coenzyme A
2) Citric acid releases 1CO2 + H atom  5C compound
a. By losing H atoms w/ its electron, citric acid is oxidized
b. e- in H atom is transferred to NAD+  reduced to NADH
3) 5C compound releases 1CO2 + H atoms  4C compound
a. NAD+ reduced to NADH
b. ATP is synthesized from ADP
4) 4C compound releases H atom  another 4C compound
a. H atom is used to reduce FAD to FADH2
b. FAD (flavin adenine dinucleotide) is molecule ~ to NAD+ = accepts e- during redox
reactions
5) 4C compound releases H atom to regenerate oxaloacetic acid
a. keeps Krebs operating
b. e- in H atom reduces NAD+ to NADH
D. In glycolysis, 1 glucose produces 2 PA, which can form 2 acetyl CoA
E. 1 glucose can be broken down into 2 turns of Krebs cycle  produce 4 CO2, 2 ATP, and H atoms to
make 6 NADH, 2 FADH2
a. CO2 = waste product, diffused out of cell
b. 1 glucose yields only 2 ATP (same as glycolysis)
F. Bulk of energy released by oxidation of glucose still hasn’t been transferred to ATP
G. Glycolysis of 1 glucose produces 2 NADH, 2PA  acetyl CoA  +2 NADH + 6NADH from
Krebs = 10 NADH molecules for every glucose that is oxidized + 2 FADH2 from Krebs drives next
stage of aerobic resp. (most energy conversion occurs)
ETC & Chemiosmosis
A. ETC linked w/ chemiosmosis
B. ETC = series of molecules in membrane that transfer electrons
a. In eukaryotic cells, ETC and enzyme ATP synthase are embed in inner membrane of
mitochondria called cristae
b. In prokaryotes, ETC is in cell membrane
c. ATP is produced by ETC when NADH and FADH2 release H atoms, regenerating NAD+ and
FAD
1) Electrons in H atoms from NADH + FADH2 are given up to ETC
a. NADH donates electrons @ beginning
b. FADH2 donates later
c. Also give up protons (H+)
2) Electrons are passed down chain, losing energy
3) Energy lost from electrons is used to pump H+ from matrix  build high [ ] of H+ in
intermembrane space  [ ] gradient & electrical gradient as protons carry + charge
4) [ ] and electrical gradients of protons driven ATP synthesis by chemiosmosis (process that
generates ATP in photosynthesis)
a. ATP synthase molecules embedded in inner membrane near ETC make ATP from ADP
and phosphate as protons move thru ATP synthase down [ ] and electrical gradients
5) O2 is final electron acceptor & accepts protons from H atoms from NADH and FADH2
a. Protons + electrons + oxygen  water
C. Importance of O2
a. ATP can be made by chemiosmosis ONLY if electrons continue to move in ETC
b. Last molecule must pass electrons on to final electron acceptor
c. Otherwise, ETC halts
d. Ex: cars kept entering dead-end, one-way street  no more cars
e. If last molecule couldn’t unload electrons it accepts, no more electrons could enter ETC &
ATP synthesis would stop
Efficiency of CR
A. Glycolysis, Krebs  2ATP each directly for every glucose oxidized
a. Each NADH that supplies ETC can generate 3ATP
b. Each FADH2 from glycolysis can generate 2ATP
B. 10 NADH + 2 FADH2 from glycolysis, conversion of pyruvic acid  acetyl CoA, Krebs cycle can
produce up to 34 ATP by ETC and chemiosmosis
C. Adding 4 ATP from glycolysis and Krebs gives max 38 ATP per glucose
D. Actual number of ATP thru CR varies from cell to cell
a. In most eukaryotic cells, NADH made in cytosol during glycolysis can’t diffuse thru inner
membrane of mitochondrion
b. must be actively transported into matrix
c. active transport of NADH consumes 2ATP
d. total for eukaryotic cells = 36 ATP
E. Efficiency of CR can vary depending on conditions
a. Efficiency when 38 ATP generated:
F. CR is nearly 20x more efficient than glycolysis alone
G. Automobile engine is ~25% efficient in extracting energy from gasoline to move car; most of
remaining energy released from gasoline is lost as heat
Summary of CR
A. Glycolysis – Glucose  PA; produces: small amount of ATP and NADH
B. Aerobic respiration – PA  CO2 and H2O, produces: large amount of ATP
C. Complete oxidation of glucose in CR:
D. In addition to glucose, many other compounds can be used as fuel in CR
E. Molecules derived from breakdown of fats, proteins, carbohydrates can enter glycolysis or Krebs
cycle @ diff. points  more energy
F. CR is NOT reverse of PS; 2 processes involve different biochem pathways, occur @ different sites
G. Cells also need specific organic compounds to build macromolecules; can’t be found in food
a. Molecules formed @ diff. steps in glycolysis and Krebs = often used by cells to make
compounds missing in food
b. = can be diverted into other biochem pathways for cell to make required molecules
i. Ex: approx. 10 of a.a. needed by our body can be made w/ compounds diverted from
Krebs cycle