Akki Thakur product Energetics The production of energy, its storage, and its use are as central to the economy of the cell as they are to the management of the world’s resources. Cells require energy to do all their work, including the synthesis of sugars from carbon dioxide and water in photosynthesis, the contraction of muscles, and the replication ofDNA. Energy may be defined as the ability to do work, a concept that is easy to grasp when it is applied to automobile engines and electric power plants. When we consider the energy associated with chemical bonds and chemical reactions within cells, however, the concept of work becomes less intuitive. Living Systems Use Various Forms of Energy, Which Are Interconvertible There are two principal forms of energy: kinetic and potential. Kinetic energy-y is the energy of movement—the motion of molecules potential energy- or stored energy, is more important in the study of biological or chemical systems Kinetic Energy Heat, or thermal energy, is a form of kinetic energy—the energy of the motion of molecules. For heat to do work, it must flow from a region of higher temperature—where the average speed of molecular motion is greater—to one of lower temperature. Differences in temperature often exist between the internal and external environments of cells; however, cells generally cannot harness these heat differentials to do work. Even in warm-blooded animals that have evolved a mechanism for thermoregulation, the kinetic energy of molecules is used chiefly to maintain constant organismic temperatures. Radiant energy is the kinetic energy of photons, or waves of light, and is critical to biology. Radiant energy can be converted to thermal energy, for instance when light is absorbed by molecules and the energy is converted to molecular motion. In the process of photosynthesis, light energy is absorbed by chlorophyll and is ultimately converted into other types of energy, such as that stored in covalent chemical bonds. One of the major forms of electric energy is also kinetic—the energy of moving electrons or other charged particles. Potential Energy Several forms of potential energy are biologically significant. Central to biology is the potential energy stored in the bonds connecting atoms in molecules. Indeed, most of the biochemical reactions described in this book involve the making or breaking of at least one covalent chemical bond. We recognize this energy when chemicals undergo energy-releasing reactions. The sugar glucose, for example, is high in potential energy. Cells degrade glucose continuously, and the energy released when glucose is metabolized is harnessed to do many kinds of work. A second biologically important form of potential energy, to which we shall refer often, is the energy in aconcentration gradient. When the concentration of a substance on one side of a permeable barrier, such as amembrane, is different from that on the other side, the result is a concentration gradient. All cells form concentration gradients between their interior and the external fluids by selectively exchanging nutrients, waste products, and ions with their surroundings. Also, compartments within cells frequently contain different concentrations of ions and other molecules; the concentration of protons within a lysosome, as we saw in the last section, is about 500 times that of the cytosol. A third form of potential energy in cells is an electric potential—the energy of charge separation. For instance, there is a gradient of electric charge of ≈200,000 volts per cm across the outer, or “plasma,” membrane of virtually all cells.
Posted on: Thu, 08 Aug 2013 13:08:30 +0000