39. Catabolism, respiration and fermentation

Question 1

what are the phases of aerobic respiration?

Answer 1

• glycolysis
  – oxidative decarboxylation of pyruvate
• krebs cycle
• electron transport chain

Question 2

what does aerobic glycolysis do?

Answer 2

• converts glucose to pyruvate
  – generating 2 ATP by substrate level phosphorylation
• pyruvate decarboxylated to acetyl-coenzyme A (acetyl-CoA) and CO2

Question 3

what happens in the krebs cycle?

Answer 3

• acetyl CoA enters krebs cycle
  – 2 CO2, 3 NADH, 1 FADH, 1 ATP

Question 4

what happens in the electron transport chain?

Answer 4

• generates ATP
• regenerates NAD+
  – oxygen is terminal electron acceptor

Question 5

what is oxidative decarboxylation of pyruvate?

Answer 5

• not part of krebs cycle
  – but main source of acetyl-CoA, also from catabolism of many lipids, carbohydrates and amino acids
• pyruvate converted to acetyl-CoA and CO2
  – enzyme containing coenzyme A removes CO2
  – coenzyme A binds to C2 intermediate forming acetyl-CoA
  – NAD+ reduced to NADH

Question 6

how is pyruvate oxidation regulated?

Answer 6

• irreversible conversion step
  – controls amount of acetyl-CoA enters krebs
• catalysed by enzyme pyruvate dehydrogenase
  – inhibited by acetyl-CoA
  – activated by pyruvate
  – ensures acetyl-CoA only made when needed and plenty of pyruvate available
• ATP and NADH inhibit enzyme, ADP activates
  – acetyl-CoA formed when energy stores low

Question 7

what is the krebs cycle?

Answer 7

• tricarboxylic acid (TCA) cycle
  – or citric acid cycle
• mitochondrial matrix (eukaryotes)
• cytoplasmic matrix (prokaryotes)
• complete oxidation of glucose
• generates carbon skeletons for biosynthesis
  – precursor metabolites

Question 8

what is the process of the krebs cycle?

Answer 8

• first reaction
  – condensation of acetyl-CoA, C4 intermediate, oxaloacetate
  – forms citrate (C6)
• citrate oxidised and decarboxylated twice producing:
  – 2 molecules CO2 (waste)
  – 2 NADH
• citrate converted back to oxaloacetate
  – series of enzymatically catalysed steps
  – produces 1 ATP, 1 FADH, 1 additonal NADH

Question 9

how is the krebs cycle regulates?

Answer 9

• pyruvate dehydrogenase
  – enzyme that produces acetyl-CoA
• two additonal steps
  – CO2 molecules released
  – NADH produces
• isocitrate dehydrogenase turns C6 to C5
  – inhibited by ATP and NADH
  – activated by ADP
• alpha-ketoglutarate dehydrogenase turns C5 to C4 bound by coenzyme (A) (succinyl-CoA)
  – inhibited by ATP, NADH, and other molecules including succinyl-CoA

Question 10

what is produced by the krebs cycle?

Answer 10

• 2 molecules CO2
• three NAD+ reduced to NADH
• one FAD reduced to FADH2
• one ATP generated
• since two molecules pyruvate produced by one glucose
  – krebs cycle occurs twice

Question 11

what happens post glycolysis, decarboxylation, krebs cycle?

Answer 11

• all carbon and oxygen from glucose released as:
  – CO2
  – H+ atoms and electrons reduce NAD+ and FAD and delivered to electron transport chain

Question 12

how are electrons transported and oxidative phosphorylation occur?

Answer 12

• glycolysis and krebs generate 4 ATP
  – by substrate-level phosphorylation
  – also generate 10 NADH, 2 FADH2
• most ATP generated during aerobic respiration from oxidation
  – electron carriers in ETC
• ETC consistes of series of tranmembrane proteins
  – prokaryotes: cell membrane
  – eukaryotes: mitochondrial membrane
  – transfer of e- from donors to acceptors
• H+ separated into protons and e-
  – e- transported through chain, through series of donor/acceptor molecules (falling to successively lower energy states)
  – released energy pump protons across membrane (Eukaryotes=intermembrane space of mitochondria; prokaryotes=outside cell)
• forms proton gradient (pH) and charge gradient
  – chemical/electrical potential differences (PMF=proton motive force)
  – used to perform work (ATP generation) when protons flow back across membrane, down gradients
• oxygen is terminal electron acceptor
  – forms H2O with H+

Question 13

how is ATP generated?

Answer 13

• flow of protons down gradients released energy
  – used to phosphorylate ADP to ATP
  – oxidative phosphorylation
• use PMF to generate ATP, catalysed by ATP synthase (ATPase)
  – transmembrane protein
  – smallest biological motor
  – protons pumped back across membrane through ATPase down gradients causing physical rotation in socket
• drives coupling of ADP with Pi to form ATP
  – NADH - 3 ATP
  – FADH2 - 2 ATP

Question 14

what does aerobic respiration result in?

Answer 14

• complete oxidation of glucose
  – to CO2 in presence of oxygen
• 38 ATP generated
  – 4 ATP = substrate-level phosphorylation
  – 34 ATP = oxidative phosphorylation

Question 15

what is anaerobic metabolism?

Answer 15

• metabolism of glucose without oxygen
  – anaerobic respiration
  – fermentation
• for aerobic organisms
  – temporary, facultative reactions to ‘weather storm’ whilst oxygen concentrations low
  – if O2 remains low, organisms not able to survive

Question 16

what does anaerobic respiration involve?

Answer 16

• terminal electron acceptor
  – nitrate, nitrite and sulphate
• produces less ATP than aerobic
  – slower growth

Question 17

what is fermentation?

Answer 17

• doesn’t require O2
• releases energy from oxidation of organic molecules
  – sugars, organic acids, amino acids, purines, pyramidines
• glycolysis common pathway
  – no krebs cycle or ETC
  – pyruvate used as terminal electron acceptor from NADH (regenerating NAD+ for glycolysis)

Question 18

what are the types of fermentation?

Answer 18

• lactic acid fermentation
  – bacteria, protists, animal skeletal muscle
• alcohol fermentation
  – fungi, protists, bacteria
  – saccharomyces cerevisiae (yeast)

Question 19

what is lactic acid fermentation?

Answer 19

• NADH reduces pyruvate
  – generating lactate
• NAD+ cycle back to glycolysis
• net yield of 2 ATP per molecule glucose

Question 20

what is alcohol fermentation?

Answer 20

• pyruvate broken down into C2 and CO2
• NADH reduces C2
  – generates ethanol
• NAD+ cycled back to glycolysis
• net yield of 2 ATP per molecule glucose

Question 21

what are the differneces between respiration and fermentation?

Answer 21

• oxidation
  – complete
  – partial
• electrons
  – from NADH go to O2 / other terminal e- acceptors
  – NAD+ regenerated by pyruvate reduction
• ATP
  – large amount generated through ETC and oxidative phosphorylation
  – few ATP generated