Lecture 23 - Aerobic Cellular Respiration, Krebs, ETC

MB 251, Exam 2 | Lee, Spring 2025

Krebs cycle

series of biochemical rxns in which a large amt of potential chem energy stored in acetyl-CoA is released step-by-step

Krebs cycle

pyruvate produced in glycolysis is oxidized to CO₂

Krebs cycle

elucidated by Sir Hans Krebs

Krebs cycle

2nd stage of glucose metabolism - can occur w/o O₂

pre-Krebs molecules

have specific pathways that leads to Krebs

pre-Krebs molecules

molecules are converted into products that can enter Krebs

pre-Krebs molecules

e.g. carbs → acetyl-CoA via glycolysis

e.g. fatty acids → acetyl-CoA via beta oxidation pathway

bridge/transition step

pyruvic acid decarboxylated to form 2-C acetyl-CoA

energy in the bridge/transition step

some transferred w/ e⁻ to NADH

some used to energize acetyl by adding CoA

Krebs step 1

formation of citric acid

Krebs step 2

citric acid isomerized to isocitric acid

Krebs step 3

isocitric acid decarboxylated to alpha ketoglutaric acid and is simultaneously oxidized

Krebs step 4

alpha ketoglutaric acid oxidized and decarboxylated to succinyl CoA

Krebs step 5

ATP made by SLP using the energy released when the high energy bond is broken

CoA of succinic-CoA removed, leaves succinic acid

Krebs step 6

oxidation of succinic acid forms fumaric acid

electrons dumped to FAD+ to make FADH₂

Krebs step 7

rearrangement of fumaric acid to make malic acid

Krebs step 8

final oxidation step converts malic acid to oxaloacetic acid → ready to enter another round of Krebs

products of Krebs (after 2 cycles)

6 CO₂ (one from bridge step)

products of Krebs (after 2 cycles)

8 NADH (3 in cycle, one in bridge step)

products of Krebs (after 2 cycles)

2 ATP (from SLP step 5)

products of Krebs (after 2 cycles)

3 FADH₂ (step 6)

1 NADH yields

3 ATP

1 FADH₂ yields

2 ATP

electron transport chain (ETC)

releases energy as e⁻ transferred from higher-energy compounds to lower-energy compounds

electron transport chain (ETC)

stepwise release of energy as e⁻ move through the ETC

electron transport chain (ETC)

consists of a series of membrane-bound carrier molecules passing e⁻ from one to another

eukaryotic e⁻ carriers

contained in inner membrane of mitochondria

prokaryotic e⁻ carriers

found in plasma membrane

flavoproteins

e⁻ carrier class that have flavin - coenzyme made in riboflavin

cytochromes

e⁻ carrier class of proteins w/ an iron-containing group

cytochromes

e⁻ carrier class capable of alternating as the reduced Fe₂⁺ or the oxidized Fe₃⁻

ubiquinones (coenzyme Q) (Q)

small nonprotein carriers

chemiosmosis

general term for the use of potential energy in ion gradients to phosphorylate ADP into ATP

e⁻ carriers arranged into 3 complexes for H⁺

H⁺ to be pumped across the membrane in 3 points

e⁻ carriers arranged into 1 of 3 complexes

NADH dehydrogenase complex

e⁻ carriers arranged into 1 of 3 complexes

Cytochrome b-c₁ complex

e⁻ carriers arranged into 1of 3 complexes

Cytochrome oxidase complex

H⁺ pumping in prokaryotes

pumped across plasma membrane from cytoplasmic side

H⁺ pumping in eukaryotes

pumped from matrix side of mitochondrial membrane to opposite side

ETC step 1

1) NADH enters ETC at NADH dehydrogenase complex and is oxidized into NAD+

2) e⁻ passed to first carrier molecule flavin mononucleotide (FMN) which is reduced when it receives the e⁻

ETC step 2

1) one directional proton pumping establishes a proton gradient (and electrical charge gradient)

2) excess H⁺ on one side makes that side positively charged

3) resulting potential energy creates a proton motive force (PMF)

ETC step 3

1) e⁻ passed from FMN to next e-carrier protein Q that is reduced

2) simultaneously pumping out protons

3) e⁻ continue down the chain to reduce cytochrome b-c₁

ETC step 4

1) continues until e⁻ reaches cytochrome oxidase complex

2) O₂ is final e⁻ acceptor during aerobic cellular respiration

ETC step 5

buildup of H⁺ that wants to get back into cytoplasmic space can diffuse across the membrane through special protein channels w/ ATP synthase enzyme

ETC step 5 (OXPHOS)

energy is released and used by the enzyme to make ATP from ADP and inorganic phosphate

ATP synthase (ATPase)

huge molecular complex embedded in cytoplasmic membrane of prokaryotes and in inner membrane of eukaryotes

ATP synthase (ATPase)

convert energy of H⁺ moving down their conc. gradient into the synthesis of ATP

ATP synthase (ATPase)

can also reverse itself and hydrolyze ATP to pump ions against an electrochemical gradient

membrane-spanning portion of ATPase (F₀)

ion channel

membrane-spanning portion of ATPase (F₀)

uses transmembrane electrochemical gradient in forward to generate a rotary torque to drive ATP synthesis in F₁

membrane-spanning portion of ATPase (F₀)

uses transmembrane electrochemical gradient in reverse to use rotary torque from F₁ to pump ions against their gradient

soluble portion of ATPase (F₁)

are catalytic sites

soluble portion of ATPase (F₁)

in forward: generates rotary torque by hydrolyzing ATP at its 3 catalytic sites

soluble portion of ATPase (F₁)

in reverse: use rotary torque from F₀ to synthesize ATP

aerobic respiration

final e⁻ acceptor in ETC is O₂

anaerobic respiration

final e⁻ acceptor in ETC is not O₂

anaerobic respiration

yields less energy than aerobic respiration