Respiration Calculators

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Respiration is the metabolic process by which cells break down organic molecules (primarily glucose) to release energy in a form usable by the cell (ATP). Aerobic respiration uses oxygen and yields approximately 30–32 ATP per glucose through three stages: glycolysis (cytoplasm), the tricarboxylic acid cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane). Anaerobic respiration (fermentation) occurs without oxygen, yielding only 2 ATP per glucose but allowing continued NAD⁺ regeneration so glycolysis can continue. Cellular respiration is fundamental to all eukaryotic life.

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Aerobic Respiration Stages

Overall equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30-32 ATP

  1. Glycolysis (cytoplasm): Glucose (6C) → 2 pyruvate (3C); produces 2 net ATP + 2 NADH; does not require oxygen
  2. Pyruvate decarboxylation: 2 pyruvate → 2 acetyl-CoA + 2 CO₂; produces 2 NADH
  3. TCA cycle (matrix): 2 acetyl-CoA → 4 CO₂ + 6 NADH + 2 FADH₂ + 2 GTP (per glucose)
  4. Oxidative phosphorylation (inner membrane): NADH and FADH₂ feed electrons to ETC → proton gradient → ATP synthase → ~28 ATP

ATP Yield Summary

  • Glycolysis: 2 net ATP + 2 NADH (→ ~5 ATP in mitochondria)
  • Pyruvate decarboxylation: 2 NADH (→ ~5 ATP)
  • TCA cycle: 2 GTP + 6 NADH + 2 FADH₂ (→ 2 + 15 + 3 = 20 ATP)
  • Total: ~30–32 ATP per glucose

Anaerobic Respiration (Fermentation)

Lactic acid fermentation (muscles, bacteria): pyruvate + NADH → lactate + NAD⁺. Alcoholic fermentation (yeast): pyruvate → ethanol + CO₂ + NAD⁺. Both regenerate NAD⁺ to allow glycolysis to continue → only 2 ATP net.

Respiratory Quotient (RQ)

RQ = VCO₂/VO₂: glucose oxidation RQ = 1.0; fat oxidation RQ = 0.71; protein RQ = 0.82.

Glossary

Aerobic Respiration
Complete oxidation of glucose to CO₂ + H₂O using oxygen; three stages: glycolysis, TCA cycle, oxidative phosphorylation; yields ~30–32 ATP per glucose.
Glycolysis
The first stage of cellular respiration; occurs in the cytoplasm; glucose → 2 pyruvate; yields 2 net ATP + 2 NADH; does not require oxygen; shared by aerobic and anaerobic pathways.
Fermentation
Anaerobic regeneration of NAD⁺ from pyruvate; lactic acid fermentation (pyruvate → lactate) or alcoholic fermentation (pyruvate → ethanol + CO₂); yields 0 extra ATP beyond glycolysis.

Frequently Asked Questions

Aerobic cellular respiration has three main stages: (1) Glycolysis (cytoplasm): one glucose (6C) split into two pyruvate (3C); requires 2 ATP, produces 4 ATP (net: +2 ATP) + 2 NADH. Does not require oxygen. (2) TCA cycle / Krebs cycle (mitochondrial matrix): 2 acetyl-CoA (each 2C) enter; per turn: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂. Two turns per glucose = 6 NADH + 2 FADH₂ + 2 GTP. (3) Oxidative phosphorylation (inner mitochondrial membrane): electrons from NADH and FADH₂ flow through ETC; proton gradient drives ATP synthase; NADH → ~2.5 ATP each; FADH₂ → ~1.5 ATP each. Total ≈ 30–32 ATP per glucose.

Without oxygen, the electron transport chain cannot accept electrons (O₂ is the final electron acceptor). NADH produced in glycolysis cannot be reoxidized → NAD⁺ pool depleted → glycolysis halts (requires NAD⁺ as cofactor). Solution: fermentation regenerates NAD⁺ without oxygen: Lactic acid fermentation: pyruvate + NADH → lactate + NAD⁺ (in muscle, RBCs, lactic acid bacteria). Alcoholic fermentation: pyruvate → acetaldehyde + CO₂ → ethanol + NAD⁺ (in yeast). Both only yield 2 ATP per glucose (from glycolysis alone) — much less efficient than aerobic respiration (~30–32 ATP). However, fermentation allows rapid, short-term energy production during intense exercise (anaerobic threshold).

The TCA (tricarboxylic acid) cycle, also called the Krebs cycle, occurs in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) → citrate (6C) → various intermediates → oxaloacetate regenerated. Per turn: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂. Two turns per glucose → 6 NADH + 2 FADH₂ + 2 GTP + 4 CO₂. The TCA cycle does not directly produce much ATP — its main role is generating NADH and FADH₂ that carry electrons to the ETC. Key regulated enzymes: isocitrate dehydrogenase and α-ketoglutarate dehydrogenase (inhibited by NADH, activated by ADP — regulate the cycle in response to energy status).

Glucose (C₆H₁₂O₆): ~30–32 ATP per molecule. Fat (e.g., palmitate, C₁₆H₃₂O₂): ~129 ATP per molecule. Per unit mass: fat yields ~9 kcal/g vs. carbohydrate ~4 kcal/g — more than twice the energy density. This is why fats are the primary long-term energy storage molecule. The higher ATP yield from fat comes from: more reduced carbon (more C-H bonds per carbon); beta-oxidation produces more NADH and FADH₂ per carbon atom than glucose catabolism. Trade-off: fat oxidation requires more O₂ per ATP produced (lower RQ = 0.71 vs. 1.0 for glucose); fat cannot be used anaerobically (beta-oxidation requires mitochondria and O₂); fat mobilization is slower, so glucose is used preferentially during high-intensity exercise.