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• ENERGY FLOW WITHIN THE CELL (2) LEARNING OBJECTIVES Justify the role of mitochondria in generation of energy (structure, enzymes in membranes and matrix, own DNA). • List the enzymes of electron transport chain in the increasing redox potential order. • Justify that electron transport chain is also termed as respiratory chain. • Explain that oxidation is linked with phosphorylation of ATP. • • • • • • • • Citric acid cycle. CITRIC ACID CYCLE:- Also known as TCA cycle or tricarboxylic acid cycle or Krebs cycle. It is a cyclic process. The cycle involves a sequence of compounds interrelated by oxidation – reduction and other reactions which finally produce CO2 and H2O. It is a final common pathway of breakdown or catabolism of carbohydrate, fats and proteins. Acetyl-CoA derived mainly from oxidation of either glucose, or from fatty acids or partly from certain amino acids combine with oxaloacetate to form citric acid or citrate (first reaction of TCA cycle). In this reaction the acetyl-CoA transfer its acetyl group (2 C) to oxaloacetate. By step wise dehydrogenation and loss of two molecule of CO2, accompanied by series of reaction the citric acid is reconverted to oxaloacetate, which again starts the cycle by taking up another acetyl group from acetyl-CoA. Enzymes are located in mitochondrial matrix, either free or attached to the inner surface of the inner mitochondrial membrane, which facilitates the transfer of reducing equivalents to the adjacent enzymes of the respiratory chain. The whole process is aerobic, requiring O2 as a final oxidant of the reducing equivalents. • • • • • • • ELECTRON TRANSPORT CHAIN The living Cell and organism must perform work to stay alive, to grow and reproduce. The ability to take energy and channel it to biological work is a fundamental property of all living organism. They convert one form of energy to another, they use chemical energy in fuel to bring about the synthesis of complex highly ordered macro molecules from simple pre-cursors. Oxidation of a molecule involve the loss of electron. The reduction of a molecule involve gain of electron. BIOLOGICAL OXIDATION DEFINITION :- this is the final common pathway in aerobic cells in which electron derived from various substances are transferred to oxygen. Electron transport chain (ETC) is a series of highly organized oxidation- reduction enzymes. Location :- the ETC is localized in mitochondria. MITOCHONDRIA Mitochondria is a power house of cell, since it is within this organelles that most of the captured energy derives from respiratory oxidation. The system that couples respiration to the generation of high energy intermediate ATP in mitochondria is called oxidative phosphorylation. • • • • • • • The main part of the electron transport chain consist of four large protein complexes embedded in the inner mitochondrial membrane called NADH dehydrogenase , the cytochrome bc1 complex and cytochrome oxidase. Electron flows from NADH to oxygen through these complexes. Each complex contain several electron carriers, two small electron carriers are also needed to link these large complexes, ubiquinone which is also called co enzyme Q and cytochrome c. Complex I NADH- COQ Reductase Complex II or succinate dehydrogenase. Complex III or COQ cytochrome C reductase complex. Cytochrome bc1 complex to cytochrome C to cytochrome oxidase. Complex IV cytochrome oxidase to oxygen. Complex I NAD Dehydrogenase • Removes two electrons from NADH • Transfers them to ubiquinone (Q). • The reduced product is called ubiquinol (QH2) and can freely move about the membrane. • Moves four protons from the mitochondrial matrix to the inter-membrane space, beginning the production of a proton gradient. Complex II Succinate Dehydrogenase • Removes electrons from succinate and transfers them to ubiquinone • Via FAD. • Does not contribute to the proton gradient. Complex III Cytochrome bc1 Complex • Removes two electrons form QH2 • Transfers them to two molecules of the electron carrier, cytochrome c. • Moves four protons across the inner mitochondrial membrane, further contributing to the proton gradient. Complex IV Cytochrome Oxidase • Removes two electrons from the two molecules of cytochrome c and transfers them to molecular oxygen (O2), producing water. • Moves two electrons across the inner mitochondrial membrane, adding to the proton gradient. OXIDATIVE PHOSPHORYLATION • • • Equilibrium and Metabolism Reactions in a closed system eventually reach equilibrium and then do no work Cells are not in equilibrium; they are open systems experiencing a constant flow of materials A catabolic pathway in a cell releases free energy in a series of reactions An Analogy For Cellular Respiration – Glucose Catabolism • • • • • • • • Energy Coupling Living organisms have the ability to couple exergonic and endergonic reactions: Energy released by exergonic reactions is captured and used to make ATP from ADP and Pi ATP can be broken back down to ADP and Pi, releasing energy to power the cell’s endergonic reactions. The Structure and Hydrolysis of ATP ATP (adenosine triphosphate) – Is the cell’s energy shuttle – Provides energy for cellular functions Energy is released from ATP when the terminal phosphate bond is broken Energy Coupling - ATP / ADP Cycle Releasing the third phosphate from ATP to make ADP generates energy (exergonic): Linking the phosphates together requires energy - so making ATP from ADP and a third phosphate requires energy (endergonic), Catabolic pathways drive the regeneration of ATP from ADP and phosphate • • • • • • How ATP Performs Work ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant The recipient molecule is now phosphorylated The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP ATP drives endergonic reactions by phosphorylation, transferring a phosphate to other molecules - hydrolysis of ATP Activation Energy All reactions, both endergonic and exergonic, require an input of energy to get started. This energy is called activation energy The activation energy, EA – Is the initial amount of energy needed to start a chemical reaction – Activation energy is needed to bring the reactants close together and weaken existing bonds to initiate a chemical reaction. – Is often supplied in the form of heat from the surroundings in a system. *************************************************************************************