Energy for Metabolic Reactions

Energy for Metabolic Reactions

Energy is the capacity to produce changes in matter or the ability to move something, that is, to do work. Therefore, energy is recognized by what it can do. Common forms of energy include heat, light, sound, electrical energy, mechanical energy, and chemical energy.
 

Although energy cannot be created or destroyed, it can be converted from one form to another. Sunlight is converted to heat when it is absorbed by skin; an ordinary- incandescent light bulb changes electrical energy to heat and light; and an automobile engine converts the chemical energy in gasoline to heat and mechanical energy.

Whenever changes take place, energy is being transferred from one part to another. Thus, all metabolic reactions depend on the presence of energy in one form or another.

Release of Chemical Energy

Most metabolic processes use chemical energy. This form of energy is held in the bonds between the atoms of molecules and is released when these bonds are broken. For example, the chemical energy of many substances can be released by burning.
 

Such a reaction must be started, and this is usually accomplished by applying heat to activate the burning process.
 

As the substance burns, molecular bonds are broken, and energy escapes as heat and light.

Similarly, glucose molecules are "burned" in cells, although the process is more correctly called oxidation. The energy released by the oxidation of glucose is used to promote cellular metabolism. There are, however, some important differences between the oxidation of substances inside cells and the burning of substances outside them.
 

Burning usually requires a relatively large amount of energy to activate the process, and most of the energy released escapes as heat or light. In cells, the oxidation process is initiated by enzymes that reduce the amount of energy needed. Also, by transferring energy- to special energy-earning molecules, cells are able to capture about half of the energy released. The rest escapes as heat, which helps maintain body temperature.

The process by which energy is released from molecules such as glucose and is transferred to other molecules is quite complex The process is called cellular respiration and it involves a number of chemical reactions that must occur in a particular sequence, each one controlled by a separate kind of enzyme. Some of these enzymes are in the cells cytoplasm, while others are in the mitochondria.