Metabolic Reactions and Energy

Metabolic Reactions and Energy      Metabolic reactions often involve the production or use of energy. Generally, reactions that release energy are catabolic and reactions that require energy are anabolic. Energy Changes in Reactions      Because molecules possess energy and different molecules possess different amounts of energy,  reactions in which molecules change from one kind of molecule into another involve the release or use of energy. Hence, if the reactant molecules possess more energy than the product molecules, energy is released: Reactants ¾® Products + Energy      The energy can take various forms (heat, light, movement) and can be harnessed to perform work.      The energy released in the above reaction can also be expressed as the change of energy between the reactant and product molecules: DE = Eproducts - Ereactants      If the reactant molecules have more energy than the product molecules DE  (delta energy) is negative and energy is released.      If the reaction requires energy: Reactants + Energy ¾® Products DE is positive.      DE is expressed in units of energy, calories (cal) or kilocalories (kcal), or in joules (J) or kilojoules (kJ).      The amount of energy expended or released in a reaction is proportional to the quantity of reactants involved. Hence,  DE is commonly expressed as kcal/mole. Types of Energy Kinetic Energy (Energy of motion)      This energy is due to an objects motion. It is related to an objects mass and how fast it is moving. The kinetic energy associated with the movement of molecules is their thermal energy or heat. Potential Energy      Energy that has the potential to become kinetic energy. Hence, it is stored energy. e.g. Putting tension on the string of a bow creates potential energy that can be released to provide kinetic energy that impels the flight of an arrow.       Energy releasing reactions always go spontaneously in the forward direction. Energy requiring reactions only go forward when energy is put into them. Therefore, the direction of a reaction depends upon DE. Catabolic reactions are energy releasing and should proceed spontaneously. Anabolic reactions are energy requiring and don't proceed without the input of energy.      In cells the energy required for anabolic reactions can be gotten from the energy released by catabolic reactions if the reactions can be coupled.      When the quantity of reactants and products in a reaction do not change the reaction is in equilibrium. In other words, there is no net reaction direction, and DE is zero.