Q.33 In humans, the largest energy reserve is
Options:
(A) liver glycogen
(B) muscle glycogen
(C) blood glucose
(D) adipose tissue triacylglycerol
Human energy homeostasis maintains reserves for varying durations: minutes (ATP), hours (glycogen), and months (fat). Adipose tissue triacylglycerols provide ~150,000 kcal in lean adults vs. mere ~2,000 kcal total from carbohydrates, making fat the overwhelming energy storehouse.
The correct answer is (D) adipose tissue triacylglycerol, storing 85-90% of total body energy as 9 kcal/g vs. 4 kcal/g carbohydrates.
Why (D) Adipose Triacylglycerol Dominates Energy Storage
Average 70kg male (~15% body fat = 10.5kg fat) holds ~94,500 kcal. Even obese individuals (30% fat = 21kg) carry ~189,000 kcal. Glycogen storage limited by osmotic constraints (~500g total), while fat expands/contract virtually unlimited. Triacylglycerols’ hydrophobic nature enables massive caloric density.
Quantitative Energy Reserve Comparison
| Option | Reserve | Typical Amount | kcal Stored | Duration @ 2500 kcal/day |
|---|---|---|---|---|
| A | Liver glycogen | ~100g | ~400 kcal | ~3 hours |
| B | Muscle glycogen | ~400g | ~1600 kcal | ~15 hours |
| C | Blood glucose | ~5g | ~20 kcal | ~30 minutes |
| D | Adipose TAG | 10-30kg | 90,000-270,000 kcal | 1-4 months |
Explanation of All Options
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(A) Liver glycogen (~100g, 400 kcal): Maintains blood glucose homeostasis (6-8mM). Rapid mobilization via glycogenolysis, but tiny reserve.
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(B) Muscle glycogen (~400g, 1600 kcal): Fuel for anaerobic glycolysis during exercise. Muscle-specific, cannot directly contribute to blood glucose.
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(C) Blood glucose (~5g, 20 kcal): Immediate fuel, tightly regulated. Hypoglycemia (<4mM) triggers counter-regulatory hormones.
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(D) Adipose triacylglycerol (90,000+ kcal): Primary reserve. Hormone-sensitive lipase mobilizes fatty acids via β-oxidation yielding 106 ATP/fatty acid vs. 30 ATP/glucose.
Biotech relevance: Adipose tissue engineering targets obesity therapeutics. Lipolysis assays measure hormone signaling (insulin vs. glucagon). Memory trick: “Fat = Phat energy stacks; Glycogen = quick snacks.” Evolution favors fat storage for famine survival.
