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METABOLISM: UNDERSTANDING THE INTERACTIONS AND TRANSFORMATIONS IN LIVING CELLS

UNDERSTANDING   METABOLISM

WHAT IS METABOLISM?

Metabolism is all the life-sustaining chemical reactions that occur in living organisms and transform one molecule into another one. The chemical reactions may rearrange atoms within a molecule, add atoms to a molecule, or remove atoms from a molecule. The vast majority of chemical reactions that occur in the body are catalyzed (initiated and accelerated) by enzymes, which are proteins that facilitate reactions without themselves being altered so they can perform the same reaction over and over again.

The molecule that an enzyme acts on is referred to as a substrate, and the modified molecule that the reaction yields is called the product. Many enzymes require small, organic, nonprotein molecules called coenzymes to function. Coenzymes bind at a location on the enzyme known as the active site and form an active enzyme, which is only then capable of catalyzing its designated reaction. Vitamins C and K, and all of the B vitamins function as coenzymes.

How Do Enzymes Work?

How Do Coenzymes Work? Coenzymes associate with enzymes to form an active complex that is capable of catalyzing a chemical reaction. Some vitamins function as coenzymes.

Many of the chemical transformations that occur within cells require multiple individual reactions to be completed in a series. For this reason, cellular metabolism is typically organized into metabolic pathways. Each pathway transforms its original substrate into a final product or products through a sequence of linked enzyme-catalyzed reactions. At each step in the pathway the product formed in one reaction becomes the substrate for the next reaction in the pathway until the final product is formed.

Many metabolic processes fall into one of two broad categories: anabolism and catabolism. Anabolism is the synthesis of large molecules from smaller ones, requiring an input of energy. Common anabolic processes are those that synthesize proteins from amino acids, glycogen from glucose, and triglycerides from sources of excess calories (such as glucose and amino acids). Cell division and growth are also anabolic processes.

Catabolism, on the other hand, is the breakdown of large molecules into smaller ones, and is generally accompanied by the release of energy. Catabolic processes supply the fuels that are needed to drive anabolism, and they can also provide the substrates needed for a number of anabolic processes. The balance between all anabolic and catabolic processes over the course of several days will determine if an individual’s weight will remain stable, or whether he or she will experience a change in body weight. For example, if the total number of catabolic processes exceeds that of anabolic processes, an individual’s body weight would decrease, as adipose tissue and muscle mass are lost.

Energy metabolism is the chemical reactions that are involved in storing fuels, or breaking them down to provide the energy necessary to drive a variety of chemical reactions and other cellular processes (such as active transport and muscle contractions). This energy comes from one of two main fuel sources: glucose and fatty acids. Both of these fuels are rich in chemical energy, stored in the chemical bonds that hold each molecule together. As fuels are slowly metabolized and broken down, energy is released and the product of each reaction contains less energy than the starting substrate. This released energy is not in a form the body cells can use; it must be converted into a molecule called adenosine triphosphate (ATP).

Commonly referred to as the cell’s energy currency, ATP stores chemical energy in the bonds of its three phosphate groups. When our cells need energy, they typically break the bond between the last two phosphates, releasing the stored energy and forming adenosine diphosphate (ADP). (The “di” in diphosphate means “two,” as in two phosphates; the “tri” in “triphosphate” refers to its three phosphates.)

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Adenosine Triphosphate (ATP) is produced during energy metabolism. ATP has a high energy content and is often referred to as the energy currency of cells.

You can think of the energy in glucose and fat as the value of a gold brick: It’s worth a lot of money, but you can’t buy a cup of coffee with it. ATP, however, is like bills and coins—it’s energy your cells can actually spend. The primary metabolic pathways involved in producing ATP are glycolysis, the citric acid cycle, and the electron transport chain (ETC).

The reactions of energy metabolism primarily occur in two cellular compartments, the cytosol and the mitochondria. Recall that cells are surrounded by a cell membrane. Within the cell membrane is an aqueous fluid called the cytosol, as well as a number of cellular organelles and other structures. The membrane-enclosed organelles (such as mitochondria, endoplasmic reticulum, and the nucleus) carry out a variety of specialized functions. The cytoplasm includes the cytosol and all the organelles except the nucleus. Glucose oxidation (glycolysis) is the only ATP-producing pathway that occurs in the cytosol.

All other pathways involved in the production of ATP occur in mitochondria, which produce the majority of ATP in most cells. Mitochondria are organelles surrounded by a double-membrane system composed of an inner and outer membrane. The space that is enclosed by the inner membrane is called the matrix. See the Cellular Respiration illustration. The majority of the functions carried out by mitochondria occur in the matrix, and in this location the oxygen we breathe is used to generate ATP.

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Oxidation-reduction reactions involve the transfer of electrons between compounds and play a vital role in energy metabolism. Oxidation is the loss of electrons and reduction is the gain of electrons. Because electrons do not float around free in solution they are always transferred from one substance to another, so that as one substance loses an electron another substance simultaneously gains that electron. The substance that loses an electron is oxidized and the substance that gains an electron is reduced.