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Cell Respiration Introduction This guide is an introduction to the processes underlying cellular metabolism and respiration. Metabolism is the process by which living organisms acquire energy from external sources and utilize it internally in order to carry out necessary cellular activities. Respiration is a means of acquiring energy through biol ogical degradation of food molecules. In reviewing these processes, we will learn how eukaryotic animals convert ingested food into energy they can use to function. We will also learn about the essential role of molecular oxygen in the respiratory and metabolic processes. These concepts provide an understanding of how and why food and oxygen are such critical resources to most living organisms. In this introduction to the topic, we will introduce some of the key concepts in the study of cellular metabolism and respiration. Before we jump into our discussions of metabolism and respiration, we will first explore the ultimate goals of metabolism: energy. We will discuss the main energy source of humans, adenosine triphosphate (ATP), as well as other essential biomolecules. We will next discuss some of the broad concepts behind metabolism and respiration. In Cell Metabolism and Respiration following this introduction, we will make a more detailed examination of the various reactions that occur the delivery of a food source to a cell through its conversion into an energy source: glycol ysis, the metabolism of glucose; the citric acid cycle; and oxidative phosphorylation and electron transport. Terms Acetyl Coenzyme A - A small molecule that carries acetyl functional groups in cells. Composed of an acetyl group attached to a coenzyme A molecule. The starting product of the citric acid cycle. Adenosine Triphosphate (ATP) - The molecule from which cells derive energy. Comprised of an adenosine molecule bonded to three phosphates, each phosphate bond contains energy, especially the third bond. By breaking that one bond and reducing ATP to adenosine diphosphate (ADP), the cell can get the energy to carry out its various processes. Aerobic respiration - A metabolic process involving oxygen in the breakdown of glucose. Anabolic - Term describing enzyme-catalyzed reactions in a cell that involves the synthesis of complex molecules out of simpler subunits and which uses energy. Anaerobic respiration - A metabolic process that does not involve oxygen in the breakdown of glucose. Carbohydrate - A molecular compound containing carbon, hydrogen, and oxygen. Subunits are sugars. Catabolic - Term describing enzyme-catalyzed reactions in a cell that involve the degradation of molecules into more simple subunits with the release of energy. Chemotroph - An organism that derives its energy from the ingestion of food molecules. Citric acid cycle - Also known as the Krebs Cycle; a metabolic pathway found in aerobic organisms that oxidizes acetyl coA groups to carbon dioxide and water. Coenzyme - A molecule that participates in an enzyme-catalyzed reaction and functions to transfer atoms or electrons between itself and various molecules. Elimination reaction - A reaction that involves the ejection of a specific group from a molecule, often resulting in the formation of a carbon-carbon double bond. Glycolysis - A metabolic pathway occurring in the cell *cytosol that during a series of reactions converts glucose to pyruvate and synthesizes ATP**. Isomerization - A reaction that does not change the atomic make-up of a molecule, but rather changes its geometric conformation, yielding a slightly different molecule. Lipid - An organic molecule that is insoluble in water. A main component of cell membranes. Metabolism - All the reactions occurring in an organism that participate in the acquisition or conversion of energy for use in the organism. www.forum.ysapak.com www.forum.ysapak.com Nicotinamide adenine dinucleotide - A coenzyme that participates in oxidation and reduction reactions. An important electron carrier in oxidative phosphorylation. Oxidation - A reaction that involves the overall loss of electrons from a specific m olecule or atom. Can occur with the addition of an oxygen or by the removal of a hydrogen. Oxidative phosphorylation - A process occurring in the mitochondria that results in the formation of ATP from the flow of electrons to oxygen. Photosynthesis - A process in which plants convert sunlight into energy sources that can be used inside the cell to sustain life. Phototroph - Organisms that obtain energy from sun light through photosynthesis. Protein - An essential molecule found in all cells. Composed of amino acid subunits. Reduction - A reaction that results in the overall gain of electrons to a specific molecule or atom. Can occur with the addition of a hydrogen atom or by the removal of an oxygen atom. Respiration - A process that occurs in cells in which cells breakdown food molecules to yield ATP. Can be either aerobic or anaerobic. Cellular Energy Sources The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities. Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. As we have mentioned, food is the external energy source for humans. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources. The distinct means of metabolizing specific subunits all have the same goal, the production of the primary cellular energy source: adenosine triphosphate. Figure %: Chemical structure of ATP www.forum.ysapak.com www.forum.ysapak.com As you can see in the figure above, ATP contains three phosphate groups. These groups are primarily responsible for ATP's role as an energy source. During metabolic reactions, these phosphate groups can be transferred from ATP to yield either adenosine diphosphate (ADP) or adenosine monophosphate (AMP). ATP -> ADP + P + energy, or ATP -> AMP + 2P + energy The release of one or more phosphate groups is energetically favorable: the reaction produces energy. ATP can also undergo a reaction with water to yield ADP or AMP to release energy. The cell can use the energy produced from the breakdown of ATP for whatever purpose is necessary. Often, the energetically favorable breakdown of ATP is often coupled with another, energetically unfavorable reaction that is designed to drive the first reaction forward through the synthesis of additional ATP. ATP synthesis is almost exactly opposite to the process by which ATP is broken down to produce energy: phosphate groups are brought in contact with either ADP or AMP. While this process is not as favorable, it is able to occur with the energy derived from metabolizing foods. In addition to ATP, there are a number of other reactive molecules that are involved in the production of cellular energy. These are called coenzymes and their role is to help transfer other chemical groups like hydrogens. Coenzymes work in conjunction with metabolic enzymes to drive metabolic reactions. Among these are nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A. We will discuss the specific roles of both these molecules more in following sections. Metabolism Basics of Metabolism Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes--they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. Highly ordered and organized conditions are not energetically favorable and require energy to occur. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions. In order to begin any of these processes, cells need an external energy source. The breakdown of the external source can provide the energy that can couple to drive other reactions. Cells acquire this external energy in one of two ways. Phototrophs get their energy from the sun through photosynthesis. Plants are phototrophs. Plants use light energy to convert carbon dioxide and water into carbohydrates and oxygen. Chemotrophs, such as humans, derive energy from the breakdown of organic compounds such as carbohydrates, lipids, and proteins. Our focus in discussing cell respiration and metabolism will be on this second, chemical type of energy acquisition. The relationship between phototrophs and chemotrophs is complimetary: chemotrophs require oxygen and expire carbon dioxide while phototrophs require carbon dioxide and expire oxygen. Additionally, many of the carbohydrates ingested by chemotrophs derive from the metabolic carbohydrate products of phototrophs. Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves deg