VI. Reactions for every occasion - Journal of ... - ACS Publications

Dec 1, 1978 - Reactions for every occasion. Henry A. Bent. J. Chem. Educ. , 1978, 55 (12), p 796. DOI: 10.1021/ed055p796. Publication Date: December ...
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VI. Reactions for Every Occasion Henry A. Bent North Carolina State University Raleigh. 27650

Muscle Metabolism The chemistry of a person's muscle is, in some respects, strikingly like the chemistry of a car's engine. Both devices require fuel. Both need cooling. Both use oxygen in extended operation, yet both can function for hrief periods of time without it: a car can run off its battery (which, however, will eventually have t o be recharged, with consumption of fuel and oxygen); and a muscle can acttwitch-with the most rigorous exclusion of oxygen, even with the entire prohibition of oxidation by cyanide poisoning. On the other hand, whereas a car's engine can he run at full throttle for long periods of time, a muscle stimulated to maximum output is soon fatigued (Part I, Box 2), and its owner left breathless. Once turned off, however, car engines, however hard-run, stop breathing instantly. Fatigue and hreathlessness are chemically interrelated phenomena. Oxidation of fuel during the rapid breathing of breathlessness charges up phosphate reserves discharged during fatiguing exercise. During the first momenta of strenuous physical activity (power output perhaps 6 hp or more, chiefly from fast-twitch, white muscle fibers), motive power is provided by the splitting of reserve ATP, to ADP and Pi (Box 16). Immediately on near exhaustion of ATP, within a second or less, mechanical output stovs. or d r o ~ sMuscular . movement devends on the snlittinr of l'o&r m;tpul from mus&s ¬ long run ahead of the rate at which spiir A'rP is resynthesi~nlfrom ADI'and

ATP.

-F'.

.-During the time a trained sprinter requires to run 200 m,

rapid resynthesis of ATP can be driven by the splitting of small stores of a second high energy phosphate ester, creatine phosphate, CP. During continued maximum-level activity, however, C P reserves are soon exhausted. Exhaustion of muscle cells' high energy phosphate reserves by strenuous exercise, like exhaustion of a planet's oil reserves by fast-living,requires (after 20 sec or so for the human body) a further decrease in the system's power output. At the diminished hut still fast pace of 3 hp, phosphate reserves are resynthesized by "glycolic," anaerobic breakdown of glycogen to lactic acid. Rapid but relatively inefficient glycolysis of glycogen-like soot-producing combustion-so pollutes the cells' environment with pH-lowering lactic acid that within a minute or less physical activity must again come to a standstill in skeletal n~&cies(though not in heart muscles, whirh mn mctahdize Inrtic wid), unless, again, power uutput is funher diminishd so as to match the rate at which ATP can be resvnthesized via a still slower hut thermodynamically more effkent and environmentally less polluting process: oxidative breakdown of glycogen. At a work output of 1.5 hp, oxidation of glycogen in athletes trained for endurance--and, increasingly important with time, oxidation of fat-resynthesizes ATP as fast as it is split. In that steady-state, no-growth-of-pollution regime chief chemical by-products are two environmentally benign, easily recycled compounds: carbon dioxide and water. Biochemical Life-styles of Olympic Medalists Sports' fans may have noticed that each major source of impelling power-splitting of creatine phosphate (CP), gly796 1 Journal of Chemical Education

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Box 16. Swrce of Motive Power versus Length of Exercise

.

,CP

Splitting

Exercise, Sec

colysis of glycogen, oxidation of glycogen, and oxidation of fats-corresponds to an Olympic trark event, or pair of such events (see beneath the abcissa, Box 16). Splitting of creatine phosphate is the chief source of impelling power in the 100 and 200 m sprints, completed in 10-20 sec. Biochemically, those two short sprints are redundant. Seven times they've been won by the same person, with numerous other duplicate medalists. Glycolysis is the chief source of energy in the 45-sec 400-m sprint. In modern times, no CP-splitting, short-sprint medalist has won a medal in this longer sprint. Both glycolysis and oxidation of glucose are significant sources of energy in the 2-4 min 800- and 1500-m events. Six times those ultralong sprints have been won by the same person. Only twice, however, has an 800 meter medal been won by a 400 meter medalist (and never by 100 and 200 meter medalists). And only once has a 1500-m medal been won by a long distance medalist. Oxidation of glucose is the chief energy source in the 5000and 10,000-m events. Often those two events have the same medalists. In 1972 and again in 1976 Lasse Viren won both events. In the marathon, oxidation of fat is an important source of energy. Only once has a primarily glucose-oxidizing 10,000-m medalist won in the same Olympiad a medal in the marathon. Societies, like athletes, probably cannot pursue simultaneously two widely different life-styles. They may have to decide, Which is our event? Are we on this planet for a short run or a long run? I t is possible to sprint for short periods of time. The US. has shown that, degrading stored fuel and the environment until, perhaps, pollution overwhelms the system. It may he possible to enjoy life as fully by making haste more slowly. Swedes degrade per capita one-half the energy Americans do, the Swiss one-third, New Zealanders one-quarter.

U.S. and World Energy Budgets per year, which correThe U S . expends 1.9 X 10l"cal sponds to 170 kcal per American per minute. 1Yr kcal 1.9 X 1016kcallyr = 1700.21 X 109Am 365 X 24 X 60 min Am yr For comvarison, an average Indian uses enerev.. vear in and year out, at the rate of ah&t 1.7 kcallmin. Energywise, an American is equivalent to about 100 Indians. 1Indian kcal 1American = 179 -X = 100 Indians mln 1.7 kcallmin In total impact on the environment, Americans are (collectively, not individually) equivalent to approximately 100 X 0.21 X 109 = 21 billion Indians-five times the world's present population. "We also-rans must be forgiven," writes Peter Ustinov, English actor, producer, and playwright, "if we sometimes regard America as a luxury that the world can [perhaps] afford, but with increasing difficulty." Over 30% of US. energy usage is for transportation (Box 171, much of it for private trips of less than seven miles, many of which could be, and in other nations would be, taken by bicycle. -?

= $2-kW

kcai

I kW

min Am

14.3 kcallmin

Transportation Industrial Domestic Metabolism Other Total

4 kW (-2 3

W W W W

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Box 17. An American's Energy Expenditures 170 ---X

Box 18. Solar Energy

Am gallday)

2 0.1 3

12

Dri\,ing 10,000 milvs per year is equivalent to a steady energy expenditure o i ~ t 2.:~ kW, d or to thr personal swvices a r ~ m i dthe clock of30 iervmrs each paid one-haliacent per huur, zawline at SO.GO/cal. (The federal tax on casdine in the U.S. is 13 cents per in Europe 70 to 76 cents per gal., in Italy 120 cents per gal.)

H H H H

E E E E

A A A A

T T T T

The 2 kW for domestic purposes, chiefly space-beating, could be greatly diminished by better insulating our homes .and ourselves, by wearing winter-wear indoors, thereby trapping in a natural, benign way the sun's energy (Box 18). he 12 kW per American has made the US., more than any other nation, a fossil fuel junkie. T o help pay for our fix, we ship grain to the USSR who, to help maintain its balance of trade, ships arms to the Arabs, who ship oil to the U S . This new triangular trade keeps many people busy-and everyone on the brink of volitical and ecoloeical disaster. Had man managed to make a mammoth nuclear reactor sited safelv 93 million miles awav: discovered how to store its radiant energy photosynthetic&in a multitude of attractive forms; and learned how to release that stored energy metabolically by one of the most pleasurable of all activities, the whole process proven capable of continuing, cyclically on earth, for millions, perhaps billions of years, he would probably be so pleased with his handiwork he would tend it and promote it ill ever\. pusiil,lc. s n y and nut seek v) anxi,rucly for dangen,us, less attractive ways ut'pro\.iding motive pou,er for human activities. Suggested Readings (1) Margaria, R.."The Sources of Muscular Energy: Sci. A m . 226 131.85 (1972). (2) Keul, J., Doll. E.. and Keppler, D.. "Energy Metabolinm of Human Muscle."University Park Press, Baltimore. 1912. (3) Bent, H. A.,"Hasto Makes Waste-Pollution end Entropy: Chamistry, 44,6 (Octnbe~ 1971,

(4) Rent, H. A.,"Entropy and tho Energy Crisis:

il Sci. Teorh., 44, (51.25 (1977).

Volume 55,Number 12, December 1978 / 797