Metabolism Calculators

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Metabolism encompasses all biochemical reactions that occur within a living organism to maintain life — obtaining energy from nutrients, building structural molecules, and regulating cellular processes. It is divided into catabolism (breaking down complex molecules to release energy — producing ATP from glucose, fatty acids, and amino acids) and anabolism (building complex molecules using energy — synthesizing proteins, nucleic acids, lipids, and polysaccharides). The major metabolic pathways for energy generation are glycolysis (cytoplasm, anaerobic), the Krebs (citric acid) cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane, producing ~25–30 ATP per glucose aerobically).

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Catabolic Pathways

Glucose oxidation: Glycolysis: glucose → 2 pyruvate; net 2 ATP + 2 NADH (cytoplasm). Pyruvate decarboxylation: 2 acetyl-CoA; 2 NADH. Krebs cycle (×2): 6 NADH + 2 FADH₂ + 2 GTP. Oxidative phosphorylation: NADH → ~2.5 ATP; FADH₂ → ~1.5 ATP. Total: ~30–32 ATP per glucose. Fat oxidation (β-oxidation): highly efficient; palmitate → ~106 ATP. Protein: amino acid catabolism → enters at pyruvate, acetyl-CoA, or Krebs intermediates.

Anabolic Pathways

  • Gluconeogenesis: non-carbohydrate precursors → glucose; occurs in liver/kidney
  • Fatty acid synthesis: acetyl-CoA → fatty acids; uses NADPH; in cytoplasm
  • Protein synthesis: translation (ribosome); uses ATP and GTP

Metabolic Regulation

Allosteric regulation: AMP, ADP → activate AMPK → stimulates catabolism; ATP → inhibits. Hormonal: insulin → anabolism; glucagon → catabolism; cortisol → catabolism. Substrate availability: glucose, fatty acid concentrations drive pathway selection.

Glossary

Metabolism
All biochemical reactions in a cell; catabolism (breakdown → ATP) and anabolism (synthesis ← ATP/NADPH); regulated by allosteric enzymes, AMPK, and hormones (insulin, glucagon).
Catabolism
Degradative metabolism breaking complex molecules to simpler ones; releases energy as ATP, NADH, FADH₂; includes glycolysis, Krebs cycle, β-oxidation; coupled to anabolism.
Oxidative Phosphorylation
ATP synthesis driven by proton flow through ATP synthase; powered by electron transport chain; yields ~2.5 ATP per NADH; requires O₂ as final electron acceptor; ~25–28 ATP per glucose.

Frequently Asked Questions

Metabolism is the total of all biochemical reactions in a living cell or organism. Catabolism: reactions that break down complex molecules into simpler ones; releases energy; produces ATP, NADH, FADH₂. Examples: glycolysis, Krebs cycle, β-oxidation of fatty acids, amino acid catabolism. Anabolism: reactions that build complex molecules from simpler precursors; requires energy (ATP, NADPH, reducing equivalents). Examples: protein synthesis, DNA replication, fatty acid synthesis, gluconeogenesis, glycogen synthesis. The two divisions are coupled: catabolism provides the ATP and reducing power (NADPH) needed for anabolism.

Glycolysis (Embden-Meyerhof-Parnas pathway): occurs in the cytoplasm; anaerobic (no O₂ required). Steps: 10 enzyme-catalyzed reactions convert 1 glucose (6C) to 2 pyruvate (3C). Net yield: 2 ATP (consumed 2, produced 4 = net +2); 2 NADH; 2 pyruvate. Under aerobic conditions: pyruvate → acetyl-CoA → Krebs cycle → oxidative phosphorylation → full ATP yield ~30. Under anaerobic conditions: pyruvate → lactate (in animals/yeast → ethanol); regenerates NAD⁺ to sustain glycolysis; only 2 ATP from glucose. Rate-limiting enzyme: phosphofructokinase-1 (PFK-1); allosterically activated by AMP, ADP, fructose-2,6-bisphosphate; inhibited by ATP and citrate.

Oxidative phosphorylation (OXPHOS): occurs on the inner mitochondrial membrane; the main ATP-producing pathway. Steps: NADH and FADH₂ from glycolysis, PDC, and Krebs donate electrons to Complex I/II of the electron transport chain (ETC). Electrons flow through Complexes I→III→IV; final acceptor is O₂ → H₂O. Complexes I, III, IV pump H⁺ across the inner membrane → creates proton motive force (PMF) = ΔΨ + ΔpH. H⁺ flows back through ATP synthase (Complex V) → drives ATP synthesis from ADP + Pᵢ. Yield: each NADH → ~2.5 ATP (P/O ratio); each FADH₂ → ~1.5 ATP. Total from glucose oxidation: ~30–32 ATP. Uncoupling: protons bypass ATP synthase (via UCP1 in brown adipose tissue) → dissipated as heat.

Metabolic regulation ensures ATP production matches demand and prevents futile cycling: Allosteric regulation: immediate, reversible regulation of enzyme activity by metabolites. Phosphofructokinase-1 (glycolysis): inhibited by high ATP and citrate; activated by AMP and ADP. Pyruvate kinase: activated by fructose-1,6-bisphosphate; inhibited by ATP and alanine. AMPK (AMP-activated protein kinase): activated by high AMP/ATP ratio (energy shortage) → stimulates catabolism; inhibits anabolism. Hormonal regulation: Insulin: released after carbohydrate intake; activates glucose uptake (GLUT4), glycolysis, glycogen synthesis, fatty acid synthesis. Glucagon: released during fasting; activates gluconeogenesis, glycogenolysis, fatty acid oxidation. Cortisol: stress hormone; promotes protein catabolism and gluconeogenesis.