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Chapter 7 CELLS HARVEST CHEMICAL ENERGY ENERGY FOR LIFE Glucose is the most common chemical fuel in cells Cells obtain energy from glucose in a series of metabolic pathways Other fuel molecules are first converted to glucose or other intermediate Principles governing metabolic pathways: complex chemical transformations which occur in a series of separate reactions each reaction is catalyzed by a specific enzyme metabolic pathways are similar in all organisms eukaryote pathways often compartmentalized in organelles each pathway is regulated by key enzymes About half of the energy from glucose is collected in ATP. Breakdown of glucose is highly exergonic , and drives the endergonic formation of ATP Complete breakdown of glucose requires the presence of oxygen gas and involves the four following metabolic pathways: glycolysis , and the three processes of cellular respiration : 1) pyruvate oxidation (transition reaction), 2) Krebs Cycle, 3) electron transport system (respiratory chain) Incomplete breakdown of glucose occurs when oxygen gas is not available and involves the two following metabolic pathways: glycolysis and fermentation (alcoholic or lactate) Glycolysis produces some usable energy + 2 molecules of pyruvate (a 3-carbon sugar) Glycolysis begins glucose metabolism in all cells. Glycolysis does not require O 2 ; it is an anaerobic metabolic process. Cellular respiration uses O 2 and occurs in aerobic (oxygen-containing) environments Pyruvate is converted to CO 2 and H 2 O. Energy stored in covalent bonds of pyruvate used to make ATP molecules Fermentation does not involve O 2 . It is an anaerobic process Pyruvate is converted into lactic acid or ethanol Breakdown of glucose is incomplete Less energy is released than by aerobic cellular respiration Oxidation-reduction reactions ( redox reactions ) - these reactions transfer electron(s) from one reactant to another - this chemical conversion also involves changes in energy * Oxidation = loss of electrons (or Hydrogen atom) from a substance * Reduction = gain of electrons (or Hydrogen atom) by a substance - the electron donor is called the reducing agent and will become oxidized - the electron acceptor is called the oxidizing agent and will become reduced **because oxygen is so electronegative, it is a powerful oxidizing agent , note that as it becomes reduced by the addition of electrons, protons will also flow often resulting in the formation of water.
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** glucose is the reducing agent . Oxidation and reduction always go together (coupled). The coenzyme NAD is an essential electron carrier oxidized form: NAD + reduced form: NADH (+ H + ) the transfer of electrons can provide energy during chemical reactions **There is only a small amount of NAD+ in the cell. Recycling of the reduced coenzyme to the oxidized form is very important in meeting the needs of continued glycolysis.
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