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Metabolic Reactions and Pathways: – Metabolism involves catabolic (breaking down compounds) and anabolic (building up compounds) reactions. – Chemical reactions are organized into metabolic pathways […]

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Metabolic Reactions and Pathways:
– Metabolism involves catabolic (breaking down compounds) and anabolic (building up compounds) reactions.
– Chemical reactions are organized into metabolic pathways facilitated by specific enzymes.
– Metabolic system determines nutritious vs. poisonous substances.
– Basal metabolic rate measures energy consumed by chemical reactions.
– Basic metabolic pathways are similar among different species due to early appearance in evolutionary history.

Key Biochemical Molecules:
– Four basic classes of molecules: amino acids, carbohydrates, nucleic acids, and lipids.
– Proteins, made of amino acids, serve as enzymes or have structural functions.
– Amino acids contribute to cellular energy metabolism.
– Biochemicals can be joined to make essential macromolecules like DNA and proteins.
– Proteins play roles in cell signaling, immune responses, and the cell cycle.

Carbohydrates, Lipids, and Nucleotides:
– Carbohydrates are aldehydes or ketones with hydroxyl groups, existing as chains or rings.
– Lipids are diverse biochemicals used in biological membranes.
– Nucleotides are critical for genetic information storage and interpretation.
– Different types of carbohydrates, lipids, and nucleotides serve various functions in cellular processes.

Coenzymes and Energy Production:
– Metabolism involves chemical reactions transferring functional groups of atoms within molecules.
– Coenzymes facilitate group-transfer reactions in metabolism.
– Adenosine triphosphate (ATP) is a central coenzyme used as energy currency in cells.
– NAD+ and NADH are important coenzymes in metabolism.
– Oxidative phosphorylation and other processes generate energy in the form of ATP.

Energy Sources and Transformation:
– Catabolism breaks down large molecules for energy production.
– Digestion processes break down macromolecules into smaller units for cell metabolism.
– Energy from organic compounds like carbohydrates, fats, and amino acids is utilized for cellular functions.
– Energy transformations from inorganic compounds, light, and carbon fixation are essential for various organisms.
– Anabolism uses energy from catabolism to synthesize complex molecules, building cellular structures.

Metabolism (Wikipedia)

Metabolism (/məˈtæbəlɪzəm/, from Greek: μεταβολή metabolē, "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the conversion of food to building blocks of proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transportation of substances into and between different cells, in which case the above described set of reactions within the cells is called intermediary (or intermediate) metabolism.

Simplified view of the cellular metabolism
Structure of adenosine triphosphate (ATP), a central intermediate in energy metabolism

Metabolic reactions may be categorized as catabolic – the breaking down of compounds (for example, of glucose to pyruvate by cellular respiration); or anabolic – the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

The chemical reactions of metabolism are organized into metabolic pathways, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts – they allow a reaction to proceed more rapidly – and they also allow the regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

The metabolic system of a particular organism determines which substances it will find nutritious and which poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals. The basal metabolic rate of an organism is the measure of the amount of energy consumed by all of these chemical reactions.

A striking feature of metabolism is the similarity of the basic metabolic pathways among vastly different species. For example, the set of carboxylic acids that are best known as the intermediates in the citric acid cycle are present in all known organisms, being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms like elephants. These similarities in metabolic pathways are likely due to their early appearance in evolutionary history, and their retention is likely due to their efficacy. In various diseases, such as type II diabetes, metabolic syndrome, and cancer, normal metabolism is disrupted. The metabolism of cancer cells is also different from the metabolism of normal cells, and these differences can be used to find targets for therapeutic intervention in cancer.

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