What is Glycogen? — Biology

Definition 7.8 High School Biology · Chapter 7 — Exercise and the Body’s Energy Needs

Glycogen is the animal form of stored carbohydrate: a branched chain of glucose units, held as granules in the muscle cells (about $400\,\mathrm{g}$ in an adult) and in the liver (about $100\,\mathrm{g}$ ). Muscle glycogen fuels the muscle that stores it; liver glycogen is broken down into glucose released into the blood, keeping the blood glucose near $1\,\mathrm{g}/\mathrm{L}$ for the brain and the other tissues. Fat, stored in adipose cells, is the far larger reserve (Chapter 1), but it can be burned only slowly and only with oxygen.

The energy chain of sustained exercise: fuels from food and body stores, oxygen from breathing, ATP made in the mitochondria and spent on contraction — three quarters of it leaving as heat.

Examples

Example 7.9 (The marathon wall)

Running consumes about $4\,\mathrm{kJ}$ per kilogram of body mass per kilometre; a $70\,\mathrm{kg}$ runner spends $280\,\mathrm{kJ}/\mathrm{km}$ , and a marathon costs $11\,800\,\mathrm{kJ}$ . The glycogen stores hold about $8500\,\mathrm{kJ}$ . If the runner burns glycogen too fast, the stores run out around the thirtieth kilometre; the muscles must then rely on fat, which supplies energy at barely half the rate, and the pace collapses — "hitting the wall". Trained runners burn a larger share of fat from the start, and eat carbohydrate during the race.

Example 7.11 (An hour of cycling)

One hour at $150\,\mathrm{W}$ on the pedals: metabolic power about $600\,\mathrm{W}$ , energy $600 \times 3600 = 2.16 \times 10^{6}\,\mathrm{J} \approx 2200\,\mathrm{kJ}$ ; oxygen $2200/20 = 110\,\mathrm{L}$ , i.e. $1.8\,\mathrm{L}/\mathrm{min}$ — consistent with the curve. If half comes from carbohydrate: $1100/17 \approx 65\,\mathrm{g}$ of glucose or glycogen, and $1100/38 \approx 29\,\mathrm{g}$ of fat.

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Examples

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