Amino acids can be used as building blocks of new proteins or broken down further for the production of ATP. Proteins, which are polymers, can be broken down into their monomers, individual amino acids.

About one-half of excess fat is stored in adipocytes that accumulate in the subcutaneous tissue under the skin, whereas the rest is stored in adipocytes in other tissues and organs. Excess glucose is either stored as an energy reserve in the liver and skeletal muscles as the complex polymer glycogen, or it is converted into fat (triglyceride) in adipose cells (adipocytes).Īmong the lipids (fats), triglycerides are most often used for energy via a metabolic process called β-oxidation. Among the monosaccharides, glucose is the most common fuel for ATP production in cells, and as such, there are a number of endocrine control mechanisms to regulate glucose concentration in the bloodstream. Sugar catabolism breaks polysaccharides down into their individual monosaccharides. They take the form of either complex carbohydrates, polysaccharides like starch and glycogen, or simple sugars (monosaccharides) like glucose and fructose. Of the four major macromolecular groups (carbohydrates, lipids, proteins, and nucleic acids) that are processed by digestion, carbohydrates are considered the most common source of energy to fuel the body. These building blocks are then used for the synthesis of molecules in anabolic reactions. ATP, ADP, and P i are constantly being cycled through reactions that build ATP and store energy, and reactions that break down ATP and release energy.įigure 24.3 Sources of ATP During catabolic reactions, proteins are broken down into amino acids, lipids are broken down into fatty acids, and polysaccharides are broken down into monosaccharides. The products of this reaction are a molecule of adenosine diphosphate (ADP) and a lone phosphate group (P i). It is the first bond that catabolic enzymes break when cells require energy to do work. The chemical bond between the second and third phosphate groups, termed a high-energy bond, represents the greatest source of energy in a cell. Structurally, ATP molecules consist of an adenine, a ribose, and three phosphate groups ( Figure 24.2). The remaining 60 percent of the energy released from catabolic reactions is given off as heat, which tissues and body fluids absorb. ATP can also be stored to fulfill future energy demands. This includes building new tissue and repairing damaged tissue. ATP, the energy currency of cells, can be used immediately to power molecular machines that support cell, tissue, and organ function. Approximately 40 percent of energy yielded from catabolic reactions is directly transferred to the high-energy molecule adenosine triphosphate (ATP). The amount of energy released is less than the total amount contained in the molecule. These energy releases (conversions) are not 100 percent efficient. Catabolic ReactionsĬatabolic reactions break down large organic molecules into smaller molecules, releasing the energy contained in the chemical bonds. On the other hand, if the net energy change is negative (catabolic reactions release less energy than anabolic reactions use), the body uses stored energy to compensate for the deficiency of energy released by catabolism. If the net energy change is positive (catabolic reactions release more energy than the anabolic reactions use), then the body stores the excess energy by building fat molecules for long-term storage. Both sets of reactions are critical to maintaining life.īecause catabolic reactions produce energy and anabolic reactions use energy, ideally, energy usage would balance the energy produced. Conversely, anabolic reactions use the energy produced by catabolic reactions to synthesize larger molecules from smaller ones, such as when the body forms proteins by stringing together amino acids. The reactions governing the breakdown of food to obtain energy are called catabolic reactions. Metabolism is the sum of all of the chemical reactions that are involved in catabolism and anabolism. Metabolic processes are constantly taking place in the body. Describe the hormones that regulate anabolic and catabolic reactions.Describe the process by which monomers are combined into polymers.Describe the process by which polymers are broken down into monomers.By the end of this section, you will be able to:

Catabolic processes