Bioenergetics: The Flow of Energy in the Cell PDF

Bioenergetics: The Flow of Energy in the Cell Benjamin T. Nolasco, M.D. The Importance of Energy All living systems require an ongoing supply of energy, which is often defined as the capacity to do work energy is the capacity to cause specific physical or chemical changes Because...

Bioenergetics: The Flow of Energy in the Cell Benjamin T. Nolasco, M.D. The Importance of Energy All living systems require an ongoing supply of energy, which is often defined as the capacity to do work energy is the capacity to cause specific physical or chemical changes Because life is characterized first and foremost by change, this definition emphasizes the total dependence of all forms of life on the continuous availability of energy Cells Need Energy to Perform Six Different Kinds of Work Six categories of change define six kinds of work that require an input of energy: synthetic work mechanical work concentration work electrical work generation of heat and light Synthetic Work: Changes in Chemical Bonds. results in the formation of new chemical bonds and the synthesis of new molecules This activity is especially obvious in a population of growing cells, where additional molecules must be synthesized for cells to increase in size or number or both synthetic work is required to maintain existing cellular structures Because most structural components of the cell are in a state of constant turnover, the molecules that make up these structures are continuously being degraded and replaced Mechanical Work: Changes in the Location or Orientation of a Cell or a Subcellular Structure a physical change in the position or orientation of a cell or some part of the cell movement of a cell with respect to its environment. This movement often requires one or more appendages such as cilia or flagella - require energy to propel the cell forward Muscle contraction - involving not just a single cell but also many muscle cells working together the streaming of cytoplasm, the movement of organelles and vesicles along microtubules, and the translocation of a ribosome along a strand of messenger RNA Concentration Work: Moving Molecules Across a Membrane Against a Concentration Gradient either to accumulate substances within a cell or organelle or to remove potentially toxic by­products of cellular activity Diffusion of molecules, which always proceeds from areas of high concentration to areas of low concentration, is a spontaneous process and requires no added energy Therefore, the opposite process, concentration, requires an input of energy Electrical Work: Moving Ions Across a Membrane Against an Electrochemical Gradient electrical work is considered a specialized case of concentration work because it also involves movement across membranes ions are transported, and the result is not just a change in concentration. A charge difference, known as an electrical potential or a membrane potential, is also established across the membrane A difference in the concentration of protons on either side of the mitochondrial or chloroplast membrane forms an electrical potential that is essential to produce ATP in both cellular respiration and photosynthesis Electrical work is also important in the transmission of impulses in nerve cells Heat: An Increase in Temperature That Is Useful to Homeothermic Animals Living organisms do not use heat as a form of energy in the same way that a steam engine does Heat production is a major use of energy in homeotherms (animals that regulate and maintain their body temperature independent of the environment) Heat is released as a by­product of many chemical reactions Homeotherms take advantage of this by­product every day Bioluminescence and Fluorescence: The Production of Light Bioluminescence, the production of light using ATP or chemical oxidation as an energy source Fluorescence, the production of light following absorption of light of a shorter wavelength Organisms Obtain Energy Either from Sunlight or from the Oxidation of Chemical Compounds Not all organisms can obtain energy from sunlight directly organisms (and cells) can be classified as either phototrophs (literally, “light feeders”) or chemotrophs (“chemical feeders”) can also be classified as autotrophs (“self-feeders”) or heterotrophs (“other feeders”), depending on whether their source of carbon is CO2 or organic molecules Most organisms are either photoautotrophs (plants, algae, some bacteria) or chemoheterotrophs (all animals, fungi, protozoa, and most bacteria) Phototrophs Capture light energy from the sun using light­absorbing pigments and then transform this light energy into chemical energy, storing the energy in the form of ATP Use solar energy to produce all their necessary carbon compounds from CO2 during photosynthesis Include plants, algae, cyanobacteria, and photosynthetic bacteria Photoheterotrophs (some bacteria) harvest solar energy to power cellular activities, but they must rely on the intake of organic molecules for their carbon needs Chemotrophs get energy by oxidizing chemical bonds in organic or inorganic molecules Chemoautotrophs (a few bacteria) oxidize inorganic compounds such as HS2 , H2 gas, or inorganic ions for energy and synthesize all their organic compounds from CO2 Chemoheterotrophs ingest and use chemical compounds such as carbohydrates, fats, and proteins to provide both energy and carbon for cellular needs All animals, protozoa, fungi, and many bacteria are chemoheterotrophs Energy Flows Through the Biosphere Continuously Oxidation is the removal of electrons from a substance usually involves the removal of hydrogen atoms (a hydrogen ion plus an electron) and the addition of oxygen atoms release energy Energy Flows Through the Biosphere Continuously Reduction is he addition of electrons to a substance— and it usually involves the addition of hydrogen atoms (and a loss of oxygen atoms) Reduction reactions require an input of energy Bioenergetics The principles governing energy flow are the subject of an area of science known as thermodynamics Considers other forms of energy and processes that converted energy from one form to another Describes the energy transactions that accompany most physical processes and all chemical reactions Bioenergetics, in turn, can be thought of as applied thermodynamics—the application of thermodynamic principles to reactions and processes in the biological world Systems, Heat, and Work Energy is distributed throughout the universe The restricted portion of the universe that is being considered at any given moment is called the system All the rest of the universe is referred to as the surroundings Systems can be either open or closed, A closed system is sealed from its environment and can neither take in nor release energy in any form. An open system, on the other hand, can have energy added to it or removed from it. State A system is said to be in a specific state if each of its variable properties (such as temperature, pressure, and volume) is held constant at a specified value. In such a situation, the total energy content of the system, while not directly measurable, has some unique value. If such a system then changes from one state to another as a result of some interaction between the system and its surroundings, the change in its total energy is determined uniquely by the initial and final states of the system. Heat and Work The exchange of energy between a system and its surroundings occurs either as heat or as work. Heat is energy transfer from one place to another as a result of a temperature difference and occurs spontaneously from the hotter place to the colder place generally, not a useful source of energy for cells—although it can be used for such purposes as maintaining body temperature Work is the use of energy to drive any process other than heat flow work is performed when the muscles in your arm expend chemical energy to lift this book, Heat and Work Energy changes are usually expressed in terms of calorie which is defined as the amount of energy required to warm 1 gram of water 1 degree centigrade at a pressure of 1 atmosphere. One kilocalorie (kcal) equals 1000 calories. Joule is preferred by physicists, 1 cal = 4.184 J, 1 J = 0.239 cal. First Law of Thermodynamics: Law of Conservation States that in every physical or chemical change, the total amount of energy in the universe remains constant, although the form of the energy may change. Or, in other words, energy can be converted from one form to another but can never be created or destroyed. the total amount of energy that leaves the system must be exactly equal to the energy that enters the system minus any energy that remains behind and is therefore stored within the system First Law of Thermodynamics: Law of Conservation The total energy stored within a system is called the internal energy of the system, represented by the symbol E. Change in internal Energy, ΔE, that occurs during a given process. is the difference in internal energy of the system before the process and after the process First Law of Thermodynamics: Law of Conservation For Biological processes, enthalpy is used or heat content. Enthalpy is represented by the symbol H (for heat) and is related to the internal energy E by a term that combines both pressure (P) and volume (V): Unlike many chemical reactions, biological reactions generally proceed with little or no change in either pressure or volume. So, for biological reactions, both and are usually zero (or at least negligible) First Law of Thermodynamics: Law of Conservation The enthalpy change that accompanies a specific reaction is simply the difference in the heat content between the reactants and the products of the reaction: The value for a specific reaction or process will be either negative or positive. If the heat content of the products is less than that of the reactants, heat is released, ΔH will be negative, and the reaction is said to be exothermic. For example, the burning (oxidation) of gasoline in your car is exothermic because the heat content of the products ( and ) is less than the heat content of the reactants (gasoline and ). First Law of Thermodynamics: Law of Conservation If the heat content of the products is greater than that of the reactants, ΔH will be positive, and the reaction is endothermic. Heat energy is absorbed, as in the melting of an ice cube—the heat content of the resulting liquid water is greater than the heat content of the ice before melting. Second law of Thermodynamics: Law of thermodynamic spontaneity Thermodynamic spontaneity is a measure of whether a reaction or process can go, but it says nothing about whether it will go. In every physical or chemical change, the universe always tends toward greater disorder or randomness (entropy). The second law is useful for our purposes because it allows us to predict in what direction a reaction will proceed under specified conditions, how much energy the reaction will release as it proceeds, and how the energetics of the reaction will be affected by specific changes in the conditions. An important point to note is that no process or reaction disobeys the second law of thermodynamics. Entropy and Free Energy Thermodynamic spontaneity—whether a reaction can go— can be measured by changes in either of two parameters: entropy or free energy Entropy Entropy is represented by the symbol S. For any system, the change in entropy, ΔS , represents a change in the degree of randomness or disorder of the components of the system. There is an important link between spontaneous events and entropy changes because all processes or reactions that occur spontaneously result in an increase in the total entropy of the universe. Entropy and Free Energy Free energy one of the most useful thermodynamic concepts in biology For biological reactions at constant pressure, volume, and temperature, the free energy change, ΔG , is dependent on the free energies of the products and the reactants: This free energy change is related to the changes in enthalpy and entropy by the formula where ΔG is the change in free energy, ΔH is the change in enthalpy, T is the temperature of the system in degrees Kelvin (K C 273, and ΔS is the change in entropy. Entropy and Free Energy change in free energy of a reaction, ΔG, will increase when the change in heat content, ΔH, increases or when the change in entropy (randomness), Δ S, decreases. Free Energy Change as a Measure of Thermodynamic Spontaneity. Every spontaneous reaction is characterized by a decrease in the free energy of the system (ΔGsystem < 0) an increase in the entropy of the universe (ΔSuniverse > 0). Changes in Free Energy for the Oxidation and Synthesis of Glucose The exergonic oxidation of glucose shown in (a) has a large negative ΔG that is exactly equal in magnitude but opposite in sign to the large positive ΔG for the endergonic synthesis of glucose shown in (b) Understanding ΔG The Equilibrium Constant Is a Measure of Directionality the ratio of product concentrations to reactant concentrations at equilibrium For the general reaction in which A is converted reversibly into B, the equilibrium constant is simply the ratio of the equilibrium concentrations of A and B:  For example, the equilibrium constant for Reaction 5-9 at 25°C is known to be 0.5. This means that, at equilibrium, there will be one-half as much fructose-6- phosphate as glucose-6-phosphate, regardless of the actual magnitudes of the concentrations:  If the two compounds are present in any other concentration ratio, the reaction will not be at equilibrium and will move toward equilibrium. Thus, a concentration ratio less than Keq means that there is too little fructose6-phosphate present, and the reaction will tend to proceed to the right to generate more fructose-6-phosphate at the expense of glucose-6- phosphate. Conversely, a concentration ratio greater than Keq indicates that the relative concentration of fructose-6- phosphate is too high, and the reaction will tend to proceed to the left Free Energy and Chemical Equilibrium The free energy is lowest at equilibrium and increases as the system is displaced from equilibrium in either direction. The tendency toward equilibrium provides the driving force for every chemical reaction ΔG is the free energy change, in cal/mol, under the specified conditions; R is the gas constant (1.987 cal/mol-K); T is the temperature in kelvins (use 25 298 K unless otherwise specified); [A]pr and [B]pr are the prevailing concentrations of A and B in moles per liter; [A]eq and [B]eq are the equilibrium concentrations of A and B in moles per liter; Keq is the equilibrium constant at the standard temperature of 298 K (25 ); and ln stands for the natural logarithm of (i.e., the logarithm of a quantity to the base of the natural logarithm system, e, which equals approximately 2.718). The Equilibrium Constant Enthalpy Change ( ∆H) The relative heights of the two chambers can be thought of as measures of the enthalpy or heat content (H) Chamber 1 has a higher H value than chamber 2 “Downhill” jump from chamber 1 to chamber 2, it makes sense that ∆H has a negative value for the jumping reaction from chamber 1 to chamber 2 It seems reasonable that ∆H for the reverse reaction should have a positive value because that jump is “uphill.” Entropy Change ( ∆S) The floor area of the chambers can be thought of as a measure of the entropy, or randomness, of the system, S, and the difference between the two chambers can be represented by ∆S Because chamber 2 has a greater floor area than chamber 1, the entropy change is positive for the jumping reaction as it proceeds from left to right under these conditions  Note that for ∆H , negative values are associated with favorable reactions, whereas for ∆S , favorable reactions are indicated by positive values. Summary The Importance of Energy The complexity of cells is possible only due to the availability of energy from the environment. All cells require energy Phototrophs obtain energy directly from the sun and use it to reduce low-energy inorganic molecules such as carbon dioxide and nitrate to high-energy molecules such as carbohydrates, proteins, and lipids Chemotrophs cannot harvest solar energy directly but must obtain their energy by oxidizing the high-energy molecules synthesized by phototrophs There is a unidirectional flow of energy in the biosphere as energy moves from the sun to phototrophs to chemotrophs and is ultimately released into the environment as heat Summary Bioenergetics All living cells and organisms are open systems that exchange energy with their environment. The flow of energy through these living systems is governed by the laws of thermodynamics. The first law specifies that energy can change form but must always be conserved. The second law provides a measure of thermodynamic spontaneity, although this means only that a reaction can occur and says nothing about whether it will actually occur or at what rate. Spontaneous processes are always accompanied by an increase in the entropy of the universe and a decrease in the free energy of the system Summary Understanding ∆G and Keq The equilibrium constant Keq is a measure of the directionality of a particular chemical reaction ∆G′ , which describes the free energy change under specified conditions, is a measure of how far a reaction is from equilibrium. It represents how much energy will be released as the reaction moves toward equilibrium An exergonic reaction has a negative ∆G′ and proceeds spontaneously in the direction written, whereas an endergonic reaction has a positive ∆G′ and requires the input of energy to proceed as written. A negative ∆ ′ G is a necessary prerequisite for a reaction to proceed spontaneously A reaction at equilibrium has a ∆ ′ G = 0, and no useful work can be done by this reaction. Therefore, a cell with all reactions at equilibrium is a dead cell