Metabolic Pathways
Consider the metabolism of sugar. This is a classic example of one of the many cellular processes that use and produce energy. Living things consume sugars as a major energy source, because sugar molecules have a great deal of energy stored within their bonds. For the most part, photosynthesizing organisms like plants produce these sugars. During photosynthesis, plants use energy (originally from sunlight) to convert carbon dioxide gas (CO2) into sugar molecules (like glucose: C6H12O6 ). They consume carbon dioxide and produce oxygen as a waste product. This reaction is summarized as:
Because this process involves synthesizing an energy-storing molecule, it requires energy input to proceed. During the light reactions of photosynthesis, energy is provided by a molecule called adenosine triphosphate (ATP), which is the primary energy currency of all cells. Just as the dollar is used as currency to buy goods, cells use molecules of ATP as energy currency to perform immediate work. In contrast, energy-storage molecules such as glucose are consumed only to be broken down to use their energy. The reaction that harvests the energy of a sugar molecule in cells requiring oxygen to survive can be summarized by the reverse reaction to photosynthesis. In this reaction, oxygen is consumed and carbon dioxide is released as a waste product. The reaction called Cellular Respiration is summarized as:
Both of these reactions involve many steps.
The processes of making and breaking down sugar molecules illustrate two examples of metabolic pathways. A metabolic pathway is a series of chemical reactions that takes a starting molecule and modifies it, step-by-step, through a series of metabolic intermediates, eventually yielding a final product. In the example of sugar metabolism, the first metabolic pathway synthesized sugar from smaller molecules, and the other pathway broke sugar down into smaller molecules. These two opposite processes—the first requiring energy and the second producing energy—are referred to as anabolic pathways (building polymers), and, catabolic pathways (breaking down polymers into their monomers), respectively. Consequently, metabolism is composed of synthesis (anabolism) and degradation (catabolism).
It is important to know that the chemical reactions of metabolic pathways do not take place on their own. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions—those that require energy as well as those that release energy.
Figure 1. Catabolic pathways, shown in the bottom light brown box, are those that generate energy by breaking down larger molecules. To represent this, four green circles together form a large molecule, which is then broken down into four individual molecules and a release of energy. Anabolic pathways, shown in the top blue box, are those that require energy to synthesize larger molecules. To show this, four individual green circles and energy are combined to form the larger molecule. Both types of pathways are required for maintain the cell's energy balance.
CCBY: OpenStax College
Summary
Most processes that occur in the human body are not consciously controlled. They occur continuously to build, maintain, and sustain life. These processes include: organization, in terms of the maintenance of essential body boundaries;metabolism, including energy transfer via anabolic and catabolic reactions; responsiveness; movement; renewal, etc. The cell has to undergo metabolism in order for the tissues, and therefore the organs, to function properly. Energy in its many forms, especially as ATP in our bodies, is necessary to sustain these metabolic processes. Problems with our bodies are actually cellular problems, quite often, so you will need to understand the cell, its needs and its functions, including energy needs and metabolic processes.
Link to Interactive Questions
View this Metabolic Process Location animation (opens in new window) to learn more about metabolic processes. What kind of catabolism occurs in the heart?
