Cells and Energy
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Britannica LaunchPacks | Cells and Energy Cells and Energy For Grades 9-12 This Pack contains: 8 ARTICLES 2 IMAGES 3 VIDEOS © 2020 Encyclopædia Britannica, Inc. 1 of 41 Britannica LaunchPacks | Cells and Energy adenosine triphosphate (ATP) energy-carrying molecule found in the cells of all living things. ATP captures chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes. Adenosine triphosphate, or ATP, is the primary carrier of energy in cells. The water-mediated… Encyclopædia Britannica, Inc. Cells require chemical energy for three general types of tasks: to drive metabolic reactions that would not occur automatically; to transport needed substances across membranes; and to do mechanical work, such as moving muscles. ATP is not a storage molecule for chemical energy; that is the job of carbohydrates, such as glycogen, and fats. When energy is needed by the cell, it is converted from storage molecules into ATP. ATP then serves as a shuttle, delivering energy to places within the cell where energy-consuming activities are taking place. Examples of members of the four families of small organic molecules: sugars (e.g., glucose), amino… Encyclopædia Britannica, Inc. ATP is a nucleotide that consists of three main structures: the nitrogenous base, adenine; the sugar, ribose; and a chain of three phosphate groups bound to ribose. The phosphate tail of ATP is the actual power source which © 2020 Encyclopædia Britannica, Inc. 2 of 41 Britannica LaunchPacks | Cells and Energy the cell taps. Available energy is contained in the bonds between the phosphates and is released when they are broken, which occurs through the addition of a water molecule (a process called hydrolysis). Usually only the outer phosphate is removed from ATP to yield energy; when this occurs ATP is converted to adenosine diphosphate (ADP), the form of the nucleotide having only two phosphates. ATP is able to power cellular processes by transferring a phosphate group to another molecule (a process called phosphorylation). This transfer is carried out by special enzymes that couple the release of energy from ATP to cellular activities that require energy. The three processes of ATP production include glycolysis, the tricarboxylic acid cycle, and… Encyclopædia Britannica, Inc. Although cells continuously break down ATP to obtain energy, ATP also is constantly being synthesized from ADP and phosphate through the processes of cellular respiration. Most of the ATP in cells is produced by the enzyme ATP synthase, which converts ADP and phosphate to ATP. ATP synthase is located in the membrane of cellular structures called mitochondria; in plant cells, the enzyme also is found in chloroplasts. The central role of ATP in energy metabolism was discovered by Fritz Albert Lipmann and Herman Kalckar in 1941. EB Editors Citation (MLA style): "Adenosine triphosphate." Britannica LaunchPacks: Cells and Energy, Encyclopædia Britannica, 12 Mar. 2020. packs-preview.eb.com. Accessed 10 Aug. 2021. While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. cellular respiration the process by which organisms combine oxygen with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water. Organisms that do not depend on oxygen degrade foodstuffs in a process called fermentation. (For longer treatments of various aspects of cellular respiration, seetricarboxylic acid cycle and metabolism.) © 2020 Encyclopædia Britannica, Inc. 3 of 41 Britannica LaunchPacks | Cells and Energy Cellular respiration releases stored energy in glucose molecules and converts it into a form of… Encyclopædia Britannica, Inc. During the process of glycolysis in cellular respiration, glucose is oxidized to carbon dioxide and… Encyclopædia Britannica, Inc. Role of mitochondria The three processes of ATP production include glycolysis, the tricarboxylic acid cycle, and… Encyclopædia Britannica, Inc. One objective of the degradation of foodstuffs is to convert the energy contained in chemical bonds into the energy-rich compound adenosine triphosphate (ATP), which captures the chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes. In eukaryotic cells (that is, any cells or organisms that possess a clearly defined nucleus and membrane-bound organelles) the enzymes that catalyze the individual steps involved in respiration and energy conservation are located in highly organized rod- shaped compartments called mitochondria. In microorganisms the enzymes occur as components of the cellmembrane. A liver cell has about 1,000 mitochondria; large egg cells of some vertebrates have up to 200,000. Main metabolic processes © 2020 Encyclopædia Britannica, Inc. 4 of 41 Britannica LaunchPacks | Cells and Energy Main metabolic processes Biologists differ somewhat with respect to the names, descriptions, and the number of stages of cellular respiration. The overall process, however, can be distilled into three main metabolic stages or steps: glycolysis, the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation (respiratory-chain phosphorylation). Glycolysis The generation of pyruvate through the process of glycolysis is the first step in fermentation. Encyclopædia Britannica, Inc. Glycolysis (which is also known as the glycolytic pathway or the Embden-Meyerhof-Parnas pathway) is a sequence of 10 chemical reactions taking place in most cells that breaks down a glucose molecule into two pyruvate (pyruvic acid) molecules. Energy released during the breakdown of glucose and other organic fuel molecules from carbohydrates, fats, and proteins during glycolysis is captured and stored in ATP. In addition, the compound nicotinamide adenine dinucleotide (NAD+) is converted to NADH during this step (see below). Pyruvate molecules produced during glycolysis then enter the mitochondria, where they are each converted into a compound known as acetyl coenzyme A, which then enters the TCA cycle. (Some sources consider the conversion of pyruvate into acetyl coenzyme A as a distinct step, called pyruvate oxidation or the transition reaction, in the process of cellular respiration.) © 2020 Encyclopædia Britannica, Inc. 5 of 41 Britannica LaunchPacks | Cells and Energy Tricarboxylic acid cycle The eight-step tricarboxylic acid cycle. Encyclopædia Britannica, Inc. The TCA cycle (which is also known as the Krebs, or citric acid, cycle) plays a central role in the breakdown, or catabolism, of organic fuel molecules. The cycle is made up of eight steps catalyzed by eight different enzymes that produce energy at several different stages. Most of the energy obtained from the TCA cycle, however, is captured by the compounds NAD+ and flavin adenine dinucleotide (FAD) and converted later to ATP. The products of a single turn of the TCA cycle consist of three NAD+ molecules, which are reduced (through the process of adding hydrogen, H+) to the same number of NADH molecules, and one FAD molecule, which is similarly reduced to a single FADH molecule. These molecules go on to fuel the third stage of cellular 2 respiration, whereas carbon dioxide, which is also produced by the TCA cycle, is released as a waste product. Oxidative phosphorylation In the oxidative phosphorylation stage, each pair of hydrogen atoms removed from NADH and FADH provides a 2 pair of electrons that—through the action of a series of iron-containing hemoproteins, the cytochromes— eventually reduces one atom of oxygen to form water. In 1951 it was discovered that the transfer of one pair of electrons to oxygen results in the formation of three molecules of ATP. The series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of… Encyclopædia Britannica, Inc./Catherine Bixler © 2020 Encyclopædia Britannica, Inc. 6 of 41 Britannica LaunchPacks | Cells and Energy Oxidative phosphorylation is the major mechanism by which the large amounts of energy in foodstuffs are conserved and made available to the cell. The series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain. Some descriptions of cellular respiration that focus on the importance of the electron transport chain have changed the name of the oxidative phosphorylation stage to the electron transport chain. EB Editors Citation (MLA style): "Cellular respiration." Britannica LaunchPacks: Cells and Energy, Encyclopædia Britannica, 13 Mar. 2020. packs- preview.eb.com. Accessed 10 Aug. 2021. While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions. gluconeogenesis also called Glucogenesis, formation in living cells of glucose and other carbohydrates from other classes of compounds. These compounds include lactate and pyruvate; the compounds of the tricarboxylic acid cycle, the terminal stage in the oxidation of foodstuffs; and several amino acids. Gluconeogenesis occurs principally in the liver and kidneys; e.g., the synthesis of blood glucose from lactate in the liver is a particularly active process during recovery from intense muscular exertion. Although several of the reactions in the gluconeogenetic