Academic journal article The Science Teacher

Ask the Experts

Academic journal article The Science Teacher

Ask the Experts

Article excerpt

Q When you lose 10 pounds, where does it go?

Marian H. Grogan

USP Technical Support Project


Center for Science Education

Newton, MA

A First, let's distinguish between weight and mass. Mass is a measure of the relative amount of matter; weight is the gravitational force of attraction upon mass. Presumably we are talking about losing 4.5 kg of body mass, and not the weight loss that might accompany childbirth, haircuts, or leaving the Earth's surface. (Even at sea level, weight varies slightly, being somewhat less at the oblate spheroid's equator.)

The idea of "weight loss" is itself a little misleading, since mass is actually conserved in all chemical reactions. The matter one sheds during "weight loss" in its conventional sense is lost by metabolism of fat and other molecules into waste products such as carbon dioxide (C[O.sub.2]) and water [H.sub.2]O. Eating less and exercising more requires the body to rely on energy stored in molecules such as glycogen and fat, which are converted into glucose. Glucose, in turn, is broken down by cellular respiration to C[O.sub.2] and [H.sub.2]O. Although the purpose of respiration is to provide energy, the glucose's mass is not, strictly speaking, converted into energy but rather into carbon dioxide and water, a process that also releases energy as chemical bonds are broken and reformed. …

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