Carbohydrate Metabolism

Glycolysis

Glycolysis runs in the cytoplasm of every cell, needs no oxygen, and splits glucose into two pyruvate: $\text{Glucose} + 2\,\text{ADP} + 2\,P_i + 2\,\text{NAD}^+ \to 2\,\text{Pyruvate} + 2\,\text{ATP} + 2\,\text{NADH}$ The three irreversible, regulated steps are hexokinase (inhibited by glucose-6-phosphate), phosphofructokinase-1 — the committed, rate-limiting step, activated by AMP and fructose-2,6-bisphosphate, inhibited by ATP and citrate — and pyruvate kinase.

Fates of Pyruvate

With oxygen, pyruvate dehydrogenase converts pyruvate to acetyl-CoA for the TCA cycle. Without oxygen, lactate dehydrogenase reduces pyruvate to lactate, regenerating the NAD$^+$ that keeps glycolysis running (red blood cells and sprinting muscle rely on this).

Gluconeogenesis and Glycogen

Gluconeogenesis (liver, fasting) rebuilds glucose from lactate, alanine, and glycerol, bypassing the three irreversible steps with pyruvate carboxylase, PEP carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase. Glycolysis and gluconeogenesis are reciprocally regulated so both never run at once. Glycogen stores glucose in liver (for blood sugar) and muscle (for its own use); the pentose phosphate pathway supplies NADPH and ribose-5-phosphate.

Key Takeaways

• PFK-1 is the committed step and master control point of glycolysis.
• Anaerobic lactate production exists to regenerate NAD$^+$.
• Gluconeogenesis bypasses, never reverses, the irreversible glycolytic steps.
• Liver glycogen serves the blood; muscle glycogen serves the muscle.