Carbohydrate Metabolism

Core Concepts

Glucose entry and the committed step

Glucose enters cells through glucose transporters (GLUTs); insulin stimulates muscle and adipose tissue to insert GLUT4 into their membranes, which is why those tissues need insulin to take up glucose efficiently. Inside the cell, the first step is phosphorylation: hexokinase (or glucokinase in the liver) converts glucose to glucose-6-phosphate. This is the committed step — the phosphorylated sugar cannot leave the cell, and the cell must now do something with it. There are four fates: (1) energy — glycolysis → pyruvate → Krebs cycle/ETC; (2) storage — glycogenesis to glycogen; (3) NADPH and ribose — the pentose phosphate pathway; (4) excess — conversion to fat (lipogenesis, Topic 3).

Glycolysis and its two endings

Glycolysis (Topic 1) converts glucose to two pyruvate with a commonly taught net yield of 2 ATP and 2 NADH, in the cytosol, without oxygen. The fate of pyruvate depends on oxygen: with oxygen, it becomes acetyl-CoA and is oxidized fully in the Krebs cycle and electron transport chain; without enough oxygen (intense exercise), pyruvate is reduced to lactate, regenerating the NAD+ that glycolysis needs. The liver then recycles lactate back to glucose through the Cori cycle — an elegant loop: muscle does anaerobic work, ships lactate to the liver, and the liver returns glucose.

Glycogenesis and glycogenolysis: the glucose bank

Glycogen is a branched polymer of glucose, the body's short-term storage form, held mainly in the liver and skeletal muscle. Glycogenesis (building glycogen) is promoted by insulin in the fed state. Glycogenolysis (breaking it down) is promoted by glucagon (liver) and epinephrine (liver and muscle) during fasting, stress, and exercise. The crucial difference: the liver can release free glucose into the blood because it has the enzyme glucose-6-phosphatase, which removes the phosphate group; muscle lacks this enzyme, so muscle glycogen is a private fuel supply for the muscle itself and cannot raise blood glucose. Commonly taught ballpark stores: the liver holds roughly 100 g of glycogen and skeletal muscle roughly 400 g, though values vary with body size and training.

Gluconeogenesis: cooking glucose from scratch

When glycogen runs low — overnight fasting, prolonged exercise, low-carbohydrate diets — the liver (and to a lesser extent the kidneys) builds new glucose from non-carbohydrate precursors: lactate (from anaerobic muscle), glycerol (from fat breakdown), and glucogenic amino acids (from protein breakdown). Gluconeogenesis is not simply glycolysis run in reverse: it bypasses the three irreversible steps of glycolysis with different enzymes, so the cell can turn the pathway on and off independently. It is stimulated by glucagon and cortisol and inhibited by insulin. Because fatty acids cannot be converted to glucose in humans (their breakdown product, acetyl-CoA, cannot feed gluconeogenesis net), the body relies on glycerol and amino acids as the carbon sources — a fact with big consequences in fasting (Topic 5).

The pentose phosphate pathway: NADPH and ribose

A parallel cytosolic route, also called the hexose monophosphate shunt, branches glucose-6-phosphate toward two products: NADPH (used for fatty acid synthesis, steroid synthesis, and antioxidant defense — for example, keeping glutathione reduced in red blood cells) and ribose-5-phosphate (the sugar backbone of nucleotides and nucleic acids). It is most active in the liver, adipose tissue, and red blood cells. A classic teaching example is G6PD deficiency, in which red blood cells cannot make enough NADPH and are vulnerable to oxidative damage under certain stresses — commonly taught in physiology courses (educational mention only; diagnosis and management are clinical matters).

Hormonal regulation and blood glucose homeostasis

Blood glucose is kept in a commonly taught reference range of roughly 70–100 mg/dL (about 3.9–5.6 mmol/L) after fasting; verify current standards, as reference ranges vary by source and institution. The two opposing hormones are the core of the system:

• Insulin (pancreatic beta cells; high after meals): promotes glucose uptake (GLUT4), glycolysis, glycogenesis, lipogenesis, and protein synthesis — overall, storage and use.
• Glucagon (pancreatic alpha cells; high during fasting): promotes glycogenolysis and gluconeogenesis in the liver — overall, release of glucose into the blood.

Epinephrine adds a stress response (glycogenolysis, lipolysis), and cortisol supports gluconeogenesis during prolonged stress. When blood glucose falls below the commonly taught threshold of about 70 mg/dL, a person may feel shaky, sweaty, confused, or hungry — the classic hypoglycemia picture; when it stays high, as in diabetes, glucose spills into the urine and the body shifts to fat-based fuels (educational concepts; thresholds and management vary with current guidelines and must be verified). Diabetes is a disorder of this balance: type 1 involves insulin deficiency, and type 2 involves insulin resistance — every symptom traces back to glucose handling.

Common Confusions