Energy, exergonic reactions, and endergonic reactions [take a quiz on this topic]

Chapter Four: Cell Nutrition and Respiration Endocytosis and exocytosis Energy, exergonic reactions, and endergonic reactions How cells store energy: ATP Glycolysis Anaerobic processes The Krebs cycle and electron transport chain Chemiosmosis Energy yield for aerobic respiration Overview of photosynthesis Noncyclic electron flow and the Calvin cycle

Energy, which is usually discussed in another branch of science called physics, is defined as the "ability to do work." What does that mean, you ask? Well, let's discuss what the word "work" means. If you lift a heavy box, or even a light box, you've just done work. Turning on your computer is also work. Notice that in these examples, when work was done, something in the environment changed. Basically, anything that results in a change of some sort is work. Since energy is the "ability to do work", it is what allows you to cause these changes. Cells needs energy to perform their processes, because all processes (like movement or reproduction) result in a change.

Energy can come in many forms and can easily change from one form to another. For example, electricity is a type of energy. A toaster changes electrical energy to heat energy in order to toast bread. You may have also heard of solar energy: energy that comes from the sun in the form of light and can be converted into electrical energy. Another type of energy is called chemical energy. As discussed in Chapter Two, bonds can form between atoms to form molecules. The formation of bonds requires energy, and when the bonds are broken, that energy is released. When cells break down food molecules, they release energy which can be stored and used later on for all of the cell's processes.

In any chemical reaction, bonds are either formed or broken. Since bonds are a form of energy, all reactions result in either the absorption or release of energy. Exergonic reactions are ones that release energy; these reactions will usually occur spontaneously since they do not require energy to occur. On the other hand, endergonic reactions absorb energy to form bonds, so they do not occur spontaneously. Instead, they occur only if energy is available to be used in the reaction.

Some exergonic reactions do not occur spontaneously. Instead, they require a small input of energy from some outside source to cause the reaction. This outside energy is called the activation energy of the reaction. Endergonic reactions also have an activation energy which is considerably higher than the activation energy for most exergonic reactions. Many reactions in cells are endergonic, so cells require a method of storing energy until it is needed in a chemical reaction. This method of storage is discussed in the next section.