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Introduction to Energy Transfer

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Introduction to Energy Transfer 97818_ch05.qxd 8/3/09 5:42 PM Page 118 CHAPTER 5 Introduction to Energy Transfer CHAPTER OBJECTIVES ➤ Describe the first law of thermodynamics related ➤ Differentiate between photosynthesis and respira- to energy balance and work within biologic tion and give the biologic significance of each systems ➤ Identify and give examples of the three forms of ➤ Define potential energy and kinetic energy and biologic work give examples of each ➤ Describe how enzymes and coenzymes affect ➤ Discuss the role of free energy in biologic work energy metabolism ➤ Give examples of exergonic and endergonic ➤ Differentiate between hydrolysis and condensa- chemical reactions within the body and indicate tion and explain their importance in physiologic their importance function ➤ State the second law of thermodynamics and give ➤ Discuss the role of redox chemical reactions in a practical application of this law energy metabolism ➤ Discuss the role of coupled reactions in biologic processes 118 97818_ch05.qxd 8/3/09 5:42 PM Page 119 CHAPTER 5 Introduction to Energy Transfer 119 The capacity to extract energy from the food macronutrients and continually transfer it at a high rate to the contractile elements of skeletal muscle determines one’s capacity for swimming, running, or skiing long distances. Likewise, Potential energy specific energy-transferring capacities that demand all-out, dissipates to kinetic energy as the water “explosive” power output for brief durations determine suc- Potential energy flows down the hill cess in weightlifting, sprinting, jumping, and football line play. Although muscular activity represents the main frame of reference in this text, all forms of biologic work require power generated from the direct transfer of chemical energy. Kinetic energy Work The sections that follow introduce general concepts results from harnessing about bioenergetics that form the basis for understanding en- potential ergy metabolism during physical activity. energy Heat energy ENERGY—THE CAPACITY FOR WORK Unlike the physical properties of matter, one cannot define Lower potential energy in concrete terms of size, shape, or mass. Rather the energy term energy reflects a dynamic state related to change; thus, energy emerges only when change occurs. Within this con- text, energy relates to the performance of work—as work in- creases so also does energy transfer and thus change. From a Newtonian (mechanical) perspective, work is the product of a given force acting through a given distance. In the body, cells more commonly accomplish chemical and electrical work Figure 5.1 • High-grade potential energy capable of than mechanical work. Because it is possible to exchange and performing work degrades to a useless form of kinetic convert energy from one form to another, we commonly ex- energy. In the example of falling water, the waterwheel press biologic work in mechanical units. harnesses potential energy to perform useful work. For the Bioenergetics refers to the flow and exchange of energy falling boulder, all of the potential energy dissipates to within a living system. The first law of thermodynamics kinetic energy (heat) as the boulder crashes to the surface describes a principle related to biologic work. Its basic tenet below. states that energy cannot be created or destroyed but trans- forms from one form to another without being depleted. In essence, this law describes the important conservation of before it flows downstream. In the example of flowing water, energy principle that applies to both living and nonliving the energy change is proportional to the water’s vertical systems. In the body, chemical energy within the bonds of the drop—the greater the vertical drop, the greater the potential macronutrients does not immediately dissipate as heat during energy at the top. The waterwheel harnesses a portion of the energy metabolism; instead, a large portion remains as chem- energy from the falling water to produce useful work. In the ical energy, which the musculoskeletal system then changes case of the falling boulder, all potential energy transforms to into mechanical energy (and ultimately to heat energy). The kinetic energy and dissipates as unusable heat. first law of thermodynamics dictates that the body does not Other examples of potential energy include bound produce, consume, or use up energy; instead it transforms it energy within the internal structure of a battery, a stick of from one state into another as physiologic systems undergo dynamite, or a macronutrient before releasing its stored en- continual change. ergy in metabolism. Releasing potential energy transforms it into kinetic energy of motion. In some cases, bound en- INTEGRATIVE QUESTION ergy in one substance directly transfers to other substances to increase their potential energy. Energy transfers of this Based on the first law of thermodynamics, why is type provide the necessary energy for the body’s chemical it imprecise to refer to energy “production” in the work of biosynthesis. In this process, specific building- body? block atoms of carbon, hydrogen, oxygen, and nitrogen become activated and join other atoms and molecules to synthesize important biologic compounds and tissues. Potential and Kinetic Energy Some newly created compounds provide structure as in bone or the lipid-containing plasma membrane that en- Potential energy and kinetic energy constitute the total en- closes each cell. Other synthesized compounds such as ergy of a system. FIGURE 5.1 shows potential energy as energy adenosine triphosphate (ATP) and phosphocreatine (PCr) of position, similar to a boulder tottering atop a cliff or water serve the cell’s energy requirements. 97818_ch05.qxd 8/3/09 5:42 PM Page 120 120 Section 2 Energy for Physical Activity Energy-Releasing and Energy- form water releases 68 kCal per mole (molecular weight of a Conserving Processes substance in grams) of free energy in the following reaction: ϩ : Ϫ⌬ и Ϫ1 The term exergonic describes any physical or chemical H2 O H2O G 68 kCal mole process that releases (frees) energy to its surroundings. Such In the reverse endergonic reaction, ⌬G remains positive reactions represent “downhill” processes because of a decline because the product contains more free energy than the reac- in free energy—“useful” energy for biologic work that en- tants. The infusion of 68 kCal of energy per mole of water compasses all of the cell’s energy-requiring, life-sustaining causes the chemical bonds of the water molecule to split processes. Within a cell, where pressure and volume remain apart, freeing the original hydrogen and oxygen atoms. This relatively stable, free energy (denoted by the symbol G to “uphill” process of energy transfer provides the hydrogen and honor Willard Gibbs [1839–1903] whose research provided oxygen atoms with their original energy content to satisfy the the foundation of biochemical thermodynamics) equals the principle of the first law of thermodynamics—the conserva- potential energy within a molecule’s chemical bonds (called tion of energy. enthalpy, or H) minus the energy unavailable because of ran- H + O ; H O ϩ⌬G 68 kCal и moleϪ1 domness (S) times the absolute temperature (°C ϩ 273). The 2 2 equation G ϭ H Ϫ TS describes free energy quantitatively. Energy transfer in cells follows the same principles as Chemical reactions that store or absorb energy are ender- those in the waterfall–waterwheel example. Carbohydrate, gonic; these reactions represent “uphill” processes and proceed lipid, and protein macronutrients possess considerable potential with an increase in free energy for biologic work. Exergonic energy within their chemical bonds. The formation of product processes sometimes link or couple with endergonic reactions substances progressively reduces the nutrient molecule’s origi- to transfer some energy to the endergonic process. In the body, nal potential energy with a corresponding increase in kinetic coupled reactions conserve in usable form a large portion of the energy. Enzyme-regulated transfer systems harness or conserve chemical energy stored within the macronutrients. a portion of this chemical energy in new compounds for use in FIGURE 5.2 illustrates the flow of energy in exergonic and biologic work. In essence, living cells serve as transducers with endergonic chemical reactions. Changes in free energy occur the capacity to extract and use chemical energy stored within a when the bonds in the reactant molecules form new product compound’s atomic structure. Conversely, and equally impor- molecules with different bonding. The equation that expresses tant, cells also bond atoms and molecules together to raise them these changes, under conditions of constant temperature, to a higher level of potential energy. pressure, and volume, takes the following form: The transfer of potential energy in any spontaneous process always proceeds in a direction that decreases the ca- ⌬G ϭ⌬H Ϫ T⌬S pacity to perform work. The tendency of potential energy to The symbol ⌬ designates change. The change in free en- degrade to kinetic energy of motion with a lower capacity for ergy represents a keystone of chemical reactions. In exergonic work (i.e., increased entropy) reflects the second law of reactions, ⌬G is negative; the products contain less free thermodynamics. A flashlight battery provides a good illus- energy than the reactants, with the energy differential released tration. The electrochemical energy stored within its cells as heat. For example, the union of hydrogen and oxygen to slowly dissipates, even if the battery remains unused. The Endergonic Exergonic Product Reactant Energy supplied Energy Energy Reactant Energy Product released A Reaction Progress B Reaction Progress Figure 5.2 • Energy flow in chemical reactions. A. Energy supply prepares an endergonic reaction to proceed because the reaction’s product contains more energy than the reactant. B. Exergonic reaction releases energy, resulting in less energy in the product than in the reactant. 97818_ch05.qxd 8/3/09 5:42 PM Page 121 CHAPTER 5 Introduction to Energy Transfer 121 energy from sunlight also continually degrades to heat energy the potential energy of another source.
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