Fermentation: lactic acid and alcoholic template

Lactic acid fermentation (pyruvate → L-lactate, lactate dehydrogenase) and alcoholic fermentation (pyruvate → acetaldehyde + CO₂, pyruvate decarboxylase with TPP; acetaldehyde → ethanol, alcohol dehydrogenase), with the NAD⁺/NADH loop and the 2 ATP per glucose yield.

What the template shows

• Without O₂, cells reduce pyruvate to regenerate the NAD⁺ that glycolysis needs at the GAPDH step
• Lactic acid fermentation (muscle, red blood cells, lactic acid bacteria)
• Alcoholic fermentation (yeast)
• Two enzymes: the decarboxylation needs TPP and Mg²⁺; humans lack pyruvate decarboxylase
• Why ferment? NAD⁺ is recycled
• glucose → 2 pyruvate · GAPDH uses 2 NAD⁺
• LDH (muscle, red cells) or ADH (yeast)
• No O₂ needed: NAD⁺ returns to GAPDH, so glycolysis keeps making 2 ATP per glucose
• Lactate: glucose + 2 ADP + 2 Pᵢ → 2 L-lactate + 2 ATP + 2 H₂O (H⁺ left out). No net oxidation: the 2 NADH made by GAPDH are spent by LDH.
• Ethanol: glucose + 2 ADP + 2 Pᵢ → 2 ethanol + 2 CO₂ + 2 ATP + 2 H₂O; the CO₂ raises bread and carbonates beer.
• Lactate from muscle goes to the liver and is made back into glucose (Cori cycle).
• Pasteur showed that yeast ferments sugar without air; acids are drawn neutral (lactic acid, pyruvic acid).

Drawn with 24 atoms, 3 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Nelson & Cox, Lehninger Principles of Biochemistry, 8th ed., 2021, ch. 14 Textbook
• Berg, Tymoczko, Gatto & Stryer, Biochemistry, 9th ed., 2019 Textbook
• Voet & Voet, Biochemistry, 4th ed., Wiley 2011 Textbook