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A Brief Idea On Metabolism

Home > Study Notes > A Brief Idea On Metabolism
Published in: Biology
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This is an introductory idea on biochemistry of mammalian metabolism.

Unni K / Delhi

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  1. Lecture 1 Introduction Using chemistry one can understand the chemical composition if a biological molecule. The physicist will most likely be interested in its three dimensional structure. However, it is not sufficient enough to understand how these biological molecules are assembled into organisms or how they function to sustain life. One needs to understand the mechanism of metabolism. Metabolism is the overall process through which any living system acquire and use free energy to sustain life. Metabolism occurs in two forms Catabolism Also called degradation. Nutrients and cell components are broken down into simpler constituents to generate energy. Carries out exergonic oxidation of nutrient molecules to release free energy. Oveview Anabolism Also referred to as biosynthesis. Biomolecules are synthesized from simpler components. The free energy so released are used to drive endergonic processes. A. Nutrition involves food intake and utilization of food. For the nutritional nourishment of an organism, one needs to consider the source of their metabolic energy. For instance, consider two types of prokaryotes — autotrophs and heterotrophs. Autotrophs Self feeders. Synthesizes all their cellular constituents from HO, H2S, C02, NH3. For synthesizing energy from the above inorganic compounds, there are two possible free energy sources available. The two possible free energy sources for autotrophs are, Heterotrophs Non autonomous. Obtain free energy through the oxidation of organic compounds such as carbohydrates, lipids, fats etc. In other words, they rely on autotrophs or/and phtotrophs for those organic substances. a). Chemoslithotrophs: They obtain their energy released from the oxidation of inorganic compounds. 2NH3 + 402 -Y 2HN03 + 2H20. b). Photoautotrophs: They obtain energy through the process of photosynthesis. Light energy is used to transfer electrons from organic donors to C02 to produce (CH20)n charbohydrates.
  2. Classifications based on oxidizing agents Obligate Aerobes Can only survive in the presence of oxygen. Uses oxygen as the oxidizing agent. Obligate Anerobes Can only survive in the absence of oxygen. Uses nitrates and sulfates as oxidizing agents. B. Animals are obligate aerobic heterotrophs whose nutrition solely depends on the balanced intake of macro nutrients such as proteins, carbohydrates and lipids. The digestive system further breaks down the macro nutrients into simpler compounds such as amino acids, glycerol, monosaccharaides, and fatty acids. These micronutrients are then transported to the tissues by the circulatory system for cellular metabolism along with oxygen, water, vitamins and minerals. Vitamins and minerals assist metabolic reaction Vitamins Water soluble B7(Bi0tin), Folic Acid, Ascorbic acid(C) Most water soluble vitamins are converted to coenzymes and growth factors for microorganism and helps cure nutritional deficiency diseases. Fat soluble Vitamin A, D, E, K. They dissolve in fat and can be stored in the liver and fat tissues until needed. Lasts longer in the body as they are not required everyday. They are absorbed through the small intestine. Rate of excretion is very slow and hence might cause toxicity if consumed in excess. Minerals and trace elements are also necessary for metabolism. For example, Mg2+ is involved in almost all reactions that involves ATP including synthesis of DNA, RNA and proteins. C. Metabolic pathways consists of series of enzymatic reactions. Metabolic pathways are a series of steps formed in biochemical reactions that help convert molecules or substrates, such as sugars, into more readily usable materials. Such biochemical reactions occur inside of a cell where enzymes "break up" molecules. Ina metabolic pathway, all the reactants, intermediates and products are referred to as "Metabolites". It is important to note that there are 4000 known metabolic reactions, with each reaction catalyzed by a distinct enzyme.
  3. Catabolism Pathway model (Hypothetical) Enzyme 1 0 Complex metabolites ADP + HP042- NAD + NADH ATP Simple products Metabolic Pathways Enzyme 2 c Anabolism Enzyme 3
  4. Few degradative pathways involved are: i. ii. Cellular respiration: Sugar is taken by the cell, broken down into simpler compounds and releases energy(ATP) allowing us to live. Citric/Krebs cycle: Acetate from macro nutrients undergo oxidation to produce C02. When one substance is oxidized, another must be reduced. Thus the citric acid cycle produces the reduced NADH and FADH2 which "oxidative phosphorylation". How NAD+ is formed? Cells obtain energy during cellular respiration by oxidizing food molecules such as glucose (for instance). The energy released from this oxidation reaction is used to form ATP. Oxidation can be defined as the removal of hydrogen from a molecule. Since a hydrogen consists of proton and an electron, a proton and an electron is removed during oxidation. Oxidation involves removal of hydrogen During a redox reaction in the cell, an enzyme is involved in the transferring of the hydrogen (proton and electron) to a coenzyme called NAD+. The enzyme has active binding sites for both the substrate and the NAD+. Once the substrate and the NAD + are bound with the enzyme, the hydrogen is transferred from the substrate to the NAD + substrate substrate substrate substrate The substrate is oxidized (loses the hydrogen) and the NAD + is reduced (gains the hydrogen) to become NADH (nicotineamide adenine dinucleotide).
  5. Need for enzymes in metabolic pathways It catalyzes the inter-conversion of metabolites in degrative and biosynthetic pathways. In the absence of an enzyme, the reactions would occur far too slowly to support life. Enzymes guarantee the efficiency of metabolic reactions by preventing the formation of useless or toxic by-products. Enzyme provides a mechanism for coupling an endergonic chemical reaction (which does not occur on its own) with an energetically favorable reaction. END

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