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Metabolism: Understanding the Interactions and Transformations in Living Cells

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Metabolism: Understanding the Interactions and Transformations in Living Cells METABOLISM: UNDERSTANDING THE INTERACTIONS AND TRANSFORMATIONS IN LIVING CELLS WHAT IS METABOLISM? molecule. The vast majority of chemical Metabolism is all the life-sustaining chemi- reactions that occur in the body are cal reactions that occur in living organisms catalyzed (initiated and accelerated) by and transform one molecule into another enzymes, which are proteins that facilitate one. The chemical reactions may rearrange reactions without themselves being altered atoms within a molecule, add atoms to a so they can perform the same reaction over molecule, or remove atoms from a and over again. UNDERSTANDING METABOLISM KEY IDEAS Metabolism is all the life-sustaining chemical reactions that occur in living organisms that convert one molecule into another molecule. Many metabolic processes fall into one of two broad categories, catabolism and anabolism. Catabolism is the breakdown of large molecules into smaller ones, and is generally accompanied by the release of energy. Anabolism is the synthesis of large molecules from smaller ones, requiring an input of energy that is often obtained from the oxidation of fuels that are provided by catabolic reactions. Since digestion occurs within the lumen of the gastrointestinal tract, which is fundamentally still outside the body, digestion is not considered to be metabolism. How Do Enzymes Work? Starch 1 Starch binds to Enzymes can catalyze the enzyme. chemical reactions only with specific compounds that can bind at their active site. To bind and initiate a reaction, Enzyme active site the shape of this 2 The enzyme breaks the amylase enzyme bond between two Enzyme must exactly glucose molecules. The enzyme can now 4 match that of the act again on another starch molecule. starch molecule. EnzymesE are proteins that facilitate cchemical reactions without being altered themselves. This enzyme 3 Two starch fragments (amylase) breaks down starch, a long are released. chain of sugar (glucose) molecules. ? Why won’t this enzyme break down lipids or proteins? POP_14867_met_MET0-MET14_PP2.indd 2 10/24/18 5:50 PM 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. Coenzyme Substrate (vitamin) binds to enzyme Enzyme is inactive without the required coenzyme Inactive enzyme Active enzyme Active enzyme With the substrates and The reaction occurs coenzyme in place, the and the products reaction can proceed. are released. ? Describe the basic function of a coenzyme. The molecule that an enzyme acts on is ones, and is generally accompanied by the referred to as a substrate, and the modifi ed release of energy. Catabolic processes supply molecule that the reaction yields is called the the fuels that are needed to drive anabolism, product. Many enzymes require small, and they can also provide the substrates organic, nonprotein molecules called needed for a number of anabolic processes. coenzymes to function. Coenzymes bind at a The balance between all anabolic and cata- location on the enzyme known as the active bolic processes over the course of several days site and form an active enzyme, which is only will determine if an individual’s weight will then capable of catalyzing its designated remain stable, or whether he or she will expe- reaction. Vitamins C and K, and all of the B rience a change in body weight. For example, vitamins function as coenzymes. if the total number of catabolic processes Many of the chemical transformations that exceeds that of anabolic processes, an indi- occur within cells require multiple individual vidual’s body weight would decrease, as adi- reactions to be completed in a series. For this pose tissue and muscle mass are lost. reason, cellular metabolism is typically orga- Energy metabolism is the chemical reac- nized into metabolic pathways. Each pathway tions that are involved in storing fuels, or transforms its original substrate into a fi nal breaking them down to provide the energy product or products through a sequence of necessary to drive a variety of chemical reac- linked enzyme-catalyzed reactions. At each step tions and other cellular processes (such as in the pathway the product formed in one reac- active transport and muscle contractions). tion becomes the substrate for the next reaction This energy comes from one of two main fuel in the pathway until the fi nal product is formed. sources: glucose and fatty acids. Both of these Many metabolic processes fall into one of fuels are rich in chemical energy, stored in the two broad categories: anabolism and chemical bonds that hold each molecule catabolism. Anabolism is the synthesis of together. As fuels are slowly metabolized and large molecules from smaller ones, requiring broken down, energy is released and the prod- an input of energy. Common anabolic pro- uct of each reaction contains less energy than cesses are those that synthesize proteins from the starting substrate. This released energy is amino acids, glycogen from glucose, and tri- not in a form the body cells can use; it must glycerides from sources of excess calories be converted into a molecule called adenosine (such as glucose and amino acids). Cell divi- triphosphate (ATP). sion and growth are also anabolic processes. Commonly referred to as the cell’s energy Catabolism, on the other hand, is the currency, ATP stores chemical energy in the breakdown of large molecules into smaller bonds of its three phosphate groups. When MET-1 POP_14867_met_MET0-MET14_PP2.indd 1 10/24/18 5:50 PM METABOLISM 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. Energy Triphosphate Diphosphate Phosphate Adenosine P P P Adenosine P P + P ATP ADP Energy is released when the bond between the second and third phosphate of ATP is broken. A portion of the released energy can be captured to do work, such as fueling muscle contractions. ADP is then used to reform ATP. our cells need energy, they typically break the coffee with it. ATP, however, is like bills and bond between the last two phosphates, releas- coins—it’s energy your cells can actually ing the stored energy and forming adenosine spend. The primary metabolic pathways diphosphate (ADP). (The “di” in diphosphate involved in producing ATP are glycolysis, the means “two,” as in two phosphates; the “tri” in citric acid cycle, and the electron transport “triphosphate” refers to its three phosphates.) chain (ETC). You can think of the energy in glucose The reactions of energy metabolism pri- and fat as the value of a gold brick: It’s worth marily occur in two cellular compartments, a lot of money, but you can’t buy a cup of the cytosol and the mitochondria. Recall that cells are surrounded by a cell mem- The cytoplasm includes brane. Within the cell membrane is an aque- the cytosol and all the ous fl uid called the cytosol, as well as a organelles except the nucleus. number of cellular organelles and other structures. The membrane-enclosed organ- elles (such as mitochondria, endoplasmic reticulum, and the nucleus) carry out a vari- ety 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. Nucleus All other pathways involved in the produc- Rough tion of ATP occur in mitochondria, which endoplasmic reticulum produce the majority of ATP in most cells. Mitochondria are organelles surrounded by a Mitochondria double-membrane system composed of an Cell membrane inner and outer membrane. The space that is enclosed by the inner membrane is called the matrix. See the Cellular Respiration illustra- tion on page 7. The majority of the functions Cells are the smallest functional unit of living organisms. This is an example of the most common type of cell (an carried out by mitochondria occur in the Enterocyte) found in the lining of the small intestine. A matrix, and in this location the oxygen few of the cellular organelles are labeled. we breathe is used to generate ATP. MET-2 POP_14867_met_MET0-MET14_PP2.indd 2 10/24/18 5:50 PM The breakdown of glucose Oxidation-reduction reactions involve the transfer of electrons between compounds and play a vital role in energy metabolism. Compound OXIDATION Compound A A Memory hint: Oxidation is the loss of electrons and As the addition of e− The loss of an electron reduction is the gain of electrons. Because (or a hydrogen atom) is a negative number an oxidation reaction. reduces the sum, electrons do not fl oat around free in solution the addition of a − e negatively charged they are always transferred from one sub- electron reduces stance to another, so that as one substance Compound REDUCTION Compound the substance. B B loses an electron another substance simulta- The gain of an electron (or a hydrogen atom) is neously gains that electron. The substance a reduction reaction. that loses an electron is oxidized and the sub- stance that gains an electron is reduced. Oxidation-Reduction Reaction OVERVIEW OF ENERGY METABOLISM Compound Compound OXIDIZED As fuels provided by our diet are broken down A A − into smaller and smaller molecules they e undergo a series of oxidation reactions. Energy to produce ATP is extracted from the fuels as e− they are oxidized in what is essentially a con- trolled burn. This gradual breakdown and oxi- Compound Compound REDUCED B B dation of fuels allows us to capture some of the chemical energy in those fuels to do work, Because free electrons are not stable, their removal (oxidation) from one substance such as physical movement, anabolic reac- must be accompanied by their acceptance (reduction) by another substance.
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