Cellular Respiration Pathways Summary

Glycolysis: Preparatory Phase (Cytosol)

Glycolysis involves the breakdown of glucose in the cytosol.

1. Glucose + ATP → ADP + Glucose 6-phosphate (G6P) + Pi (Enzyme: Hexokinase) (Irreversible)
2. Glucose 6-phosphate ↔ Fructose 6-phosphate (F6P) (Enzyme: Phosphoglucose Isomerase) (Reversible)
3. Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate (F1,6BP) + ADP + Pi (Enzyme: Phosphofructokinase) (Irreversible)
4. Fructose 1,6-bisphosphate ↔ Glyceraldehyde 3-phosphate (G3P) + Dihydroxyacetone Phosphate (DHAP) (Enzyme: Aldolase) (Reversible)
5. Dihydroxyacetone Phosphate ↔ Glyceraldehyde 3-phosphate (G3P) (Enzyme: Triose Phosphate Isomerase) (Reversible)

Glycolysis: Payoff Phase

Oxidation of Glyceraldehyde 3-phosphate and subsequent steps leading to pyruvate, coupled with NADH and ATP production.

6. Glyceraldehyde 3-phosphate + NAD⁺ + Pi → 1,3-Bisphosphoglycerate + NADH + H⁺ (Enzyme: Glyceraldehyde 3-Phosphate Dehydrogenase)
7. 1,3-Bisphosphoglycerate + ADP → ATP + 3-Phosphoglycerate (Enzyme: Phosphoglycerate Kinase)
8. 3-Phosphoglycerate ↔ 2-Phosphoglycerate (Enzyme: Phosphoglycerate Mutase)
9. 2-Phosphoglycerate → H₂O + Phosphoenolpyruvate (PEP) (Enzyme: Enolase)
10. Phosphoenolpyruvate + ADP → Pyruvate + ATP (Enzyme: Pyruvate Kinase)

Glycolysis Net Yield

Net gain per glucose molecule: 2 ATP + 2 NADH. (Assuming 1 NADH = 3 ATP in oxidative phosphorylation, this phase contributes ~8 ATP total to cellular respiration).

Electron Transport Chain (ETC)

A series of protein complexes in the inner mitochondrial membrane that transfer electrons to generate an electrochemical gradient.

Complex I & II

Catalyze electron transfer from different electron donors: NADH (Complex I) and Succinate (Complex II) to Ubiquinone (Q).

Complex III

Transfers electrons from reduced Ubiquinone (QH₂) to Cytochrome c.

Complex IV

Completes the sequence by transferring electrons from Cytochrome c to molecular Oxygen (O₂), forming water.

1/2 O₂ + 2H⁺ → H₂O

Krebs Cycle (Citric Acid Cycle)

A central metabolic pathway occurring in the mitochondrial matrix, oxidizing Acetyl-CoA.

1. Acetyl-CoA (2C) + Oxaloacetate (4C) + H₂O → Citrate (6C) + CoA-SH (Enzyme: Citrate Synthase) (Irreversible)
2. a) Citrate ↔ cis-Aconitate + H₂O (Enzyme: Aconitase) (Dehydration)
   b) cis-Aconitate + H₂O ↔ Isocitrate (Enzyme: Aconitase) (Hydration) (Reversible)
3. Isocitrate + NAD⁺ → α-Ketoglutarate + CO₂ + NADH + H⁺ (Enzyme: Isocitrate Dehydrogenase) (Oxidative Decarboxylation) (Irreversible)
4. α-Ketoglutarate + NAD⁺ + CoA-SH → Succinyl-CoA + NADH + CO₂ (Enzyme: α-Ketoglutarate Dehydrogenase Complex) (Oxidative Decarboxylation) (Irreversible)
5. Succinyl-CoA + GDP + Pi → GTP + Succinate + CoA-SH (Enzyme: Succinyl-CoA Synthetase) (Substrate-Level Phosphorylation) (Reversible)
6. Succinate + FAD → Fumarate + FADH₂ (Enzyme: Succinate Dehydrogenase) (Dehydrogenation) (Reversible)
7. Fumarate + H₂O ↔ Malate (Enzyme: Fumarase) (Hydration) (Reversible)
8. Malate + NAD⁺ → Oxaloacetate + NADH + H⁺ (Enzyme: Malate Dehydrogenase) (Dehydrogenation)

Pyruvate to Acetyl-CoA Conversion

For each glucose (2 pyruvate): 2 NADH produced. (Assuming 1 NADH = 3 ATP, this contributes ~6 ATP).

Krebs Cycle Net Yield (Per Glucose - 2 Turns)

For each glucose (2 turns): 2 ATP (or GTP) + 6 NADH + 2 FADH₂ produced. (Assuming 1 NADH = 3 ATP and 1 FADH₂ = 2 ATP, this contributes ~24 ATP).