Making and storing glucose

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Predict whether carbon is being stored, released, or rebuilt into glucose by following the substrate, tissue, and regulatory state.

About 18 minutes, including the liver-versus-muscle application and checks.

Start with the direction of carbon flow

Glycolysis breaks glucose down. Glycogenesis stores glucose equivalents in glycogen. Glycogenolysis mobilizes glycogen. Gluconeogenesis builds glucose from noncarbohydrate precursors such as lactate, glycerol, and glucogenic amino-acid carbon skeletons. The useful MCAT move is to identify which pool supplies carbon and which product the tissue needs.

These pathways are regulated rather than allowed to run maximally in opposite directions at the same time. A low-energy state inside a cell generally favors fuel use, whereas hormonal signals can coordinate whole-body fuel distribution. The liver is especially important because it can produce and release glucose to help maintain blood glucose; skeletal muscle primarily uses its glycogen for its own energy needs.

Concept: gluconeogenesis bypasses irreversible glycolytic steps

Gluconeogenesis is not simply glycolysis played backward. The strongly favorable glycolytic steps require bypass reactions. In a standard accounting from two pyruvate to one glucose, gluconeogenesis consumes ATP and GTP as well as reducing power. That energetic cost helps make glucose synthesis directional rather than a free reversal of glucose breakdown.²

Lactate can be oxidized to pyruvate and enter the gluconeogenic route. Glycerol can enter at the triose-phosphate level after conversion to an appropriate intermediate. Glucogenic amino-acid carbon skeletons can enter as pyruvate or citric-acid-cycle intermediates. The exact entry point matters because not every carbon source can produce net glucose.

Pattern: glycogen has different jobs in liver and muscle

Glycogen is a branched glucose polymer that can be synthesized without a nucleic-acid template. Both liver and skeletal muscle can store glycogen, but the resulting glucose units serve different physiological roles. Liver contains glucose-6-phosphatase and can convert glucose-6-phosphate to free glucose for export. Skeletal muscle lacks significant glucose-6-phosphatase activity and instead retains glucose-6-phosphate for local metabolism.

That difference means “glycogen breakdown increased” does not imply the same downstream outcome in every tissue. In liver, glycogenolysis can support blood-glucose release. In contracting muscle, glycogen-derived glucose-6-phosphate can enter glycolysis without first being released into the circulation.

Worked application: one signal, two tissues

An original experiment exposes isolated liver and skeletal-muscle preparations to epinephrine. Both tissues contain glycogen and respond by increasing glycogen phosphorylase activity. The experiment also verifies that glucose-6-phosphatase activity is high in liver and below detection in muscle.

Both tissues mobilize glycogen, so the glycogen decrease is not enough to identify the tissue. The discriminating evidence is downstream. Liver can dephosphorylate glucose-6-phosphate and release free glucose. Muscle cannot do so efficiently and instead sends the phosphorylated carbon toward glycolysis; under the supplied contracting condition, increased lactate is consistent with greater glycolytic flux.

If a problem adds a liver-specific inhibitor of glucose-6-phosphatase, glycogen can still be broken down to glucose-6-phosphate. What changes is the ability to convert that intermediate into exportable free glucose. This is a localization question: the block lies downstream of glycogen mobilization.

Rule: acetyl-CoA does not give net glucose from even-chain fatty acids

In humans, acetyl-CoA produced from even-chain fatty acids does not provide net carbon for glucose synthesis. Pyruvate dehydrogenase is effectively irreversible in the direction from pyruvate to acetyl-CoA, and the two carbons entering the citric acid cycle as acetyl-CoA are balanced by carbon loss from the cycle rather than creating a net pool of oxaloacetate for glucose production.

This does not mean fat contributes nothing to fasting glucose metabolism. Glycerol released from triglycerides can support gluconeogenesis, and fatty-acid oxidation supplies energy that helps drive gluconeogenic metabolism. Odd-chain fatty acids can also yield propionyl-CoA, which can contribute carbon to pathways feeding the citric acid cycle. The narrow rule is specifically about net glucose from acetyl-CoA derived from even-chain fatty-acid carbon.

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Continue learning

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Sources

1. AAMC. Content Category 1D: Principles of bioenergetics and fuel molecule metabolism.
2. NCBI Bookshelf. Physiology, Gluconeogenesis.