In every human cell, even in the most sedentary individual, thousands of chemical reactions essential to life occur every second. A special type of protein called an enzyme (en'zim) controls the pace of each reaction. The sum total of chemical reactions within the cell constitutes metabolism (me-tab'o-liz-m).
Many metabolic reactions occur one after the other, with the products of one reaction serving as starting materials of another, forming intricate pathways and cycles that may intersect by sharing intermediate compounds. As a result, metabolism in its entirety may seem enormously complex. However, individual pathways of metabolism are fascinating to study because they reveal how cells function—in essence, how chemistry becomes biology. This chapter explores how metabolic pathways supply a cell with energy and how other biochemical processes enable a cell to produce proteins—including the enzymes that make all of metabolism possible.
Metabolic reactions and pathways are of two types. In anabolism (an"ah-bol'lizm), larger molecules are constructed from smaller ones, requiring input of energy. In catabolism (kat"ah-bol-liz-m), larger molecules are broken down into smaller ones, releasing energy.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.