There is a constant reaction in the human body between atoms and molecules. Cells control these reac-tions to stay alive. In the chemical reaction, new bonds form between atoms or present bonds break down to form a different compound. The term me-tabolism refers to all the chemical reactions that oc-cur in the body. When a chemical reaction occurs, en-ergy may be expended or released.
What is energy? Energy is the capacity to work, and work is movement or a change in the physical struc-ture of matter. Energy can be in two forms—potentialenergy orkinetic energy. For example, imagine anelastic band stretched across two poles. The stretched elastic band has potential energy. If the band comes undone from one pole, it springs back to its original length. This is kinetic energy. Of course, kinetic energy was initially used to stretch the elastic band and tie it to the two poles. Remember that energy cannot be lost, it is only converted from one form to another.
During chemical reactions, much of the energy in the body is converted to heat, which maintains the core body temperature. When the body is cold, me-tabolism (chemical reactions) increases and more heat is produced. That’s why we shiver. The muscles quickly contract and relax (shivering), and the chem-ical reactions that occur during this process generate the needed heat.
The body “captures” energy in the form of high-energy compounds. These compounds require energy to build up; however, when broken down, they release a lot of energy. This high-energy compound is adeno-sine triphosphate (ATP). ATP is formed from thechemicals adenosine monophosphate (AMP) and adenosine diphosphate (ADP) by combining withphosphorus. For example, chemical reactions in the body break glucose down into smaller compounds. The energy that is released is “captured” by combin-ing ADP with organic phosphate to form ATP. When mechanical energy is needed to walk, ATP is broken down to release energy and ADP and P.
Many types of reactions take place in the body. Some reactions occur to break down compounds into smaller bits. This is a decomposition reaction.
AB → A + B
This is what happens when food is broken down and digested. Similarly, when a person loses weight, fat is broken down into smaller fragments. Within the cell, chemical reactions break down substances and the energy released is used to do work. This process is known as catabolism.
Building up, or synthesis, is the opposite of decom-position. In this process, kinetic energy is invariably used to form compounds from fragments. The kinetic energy is converted to potential energy to be used later in a decomposition reaction when work is needed. The process of building up is known as anabolism.
A B → AB
Another reaction that occurs in the body is ex-change. In this process, the fragments get shuffled.
AB+CD → AD+CB
Some reactions can proceed in both ways. The di-rection in which the reaction proceeds is altered by many factors, referred to as reversible reactions.
AB → A+B → AB
Reversible reactions may be represented as:
AB ↔ A+B
The various chemical reactions in the body would proceed too slowly to be of any use if they did not have mechanisms in place to speed up the reaction. The enzyme is one of the mechanisms that help that process. Enzymes are proteins and, although they do not actually participate in the chemical reaction it-self, facilitate the reaction. Enzymes do not get con-sumed or altered in the process. The body has nu-merous enzymes that speed up specific chemical reactions. The importance of specific enzymes is re-alized when one of them is deficient in the body.
Enzyme activity can be modified by various factors, such as temperature, acidity, or alkalinity. For exam-ple, the activity of many enzymes is significantly re-duced when the temperature drops, slowing down chemical reactions. Similarly, an acidic environment is detrimental to enzymes. When muscle activity is in-creased, many chemical reactions are triggered to produce energy for contraction. One of the metabo-lites formed, especially if oxygen supply is inadequate, is lactic acid. If this metabolite is not rapidly removed, the muscle environment becomes acidic and the ac- tivity of various enzymes slows down or stops and muscle fatigue results.