Bioenergetics for BScAg PDF

Bioenergetics (the study of energy transformations in living systems) Ashrafi Hossain, DOB, SAU, Dhaka 1  Thermodynamics Introduction-Transformation of energy into work and vice-versa Fo...

Bioenergetics (the study of energy transformations in living systems) Ashrafi Hossain, DOB, SAU, Dhaka 1  Thermodynamics Introduction-Transformation of energy into work and vice-versa Formulated by Physicists and engineers to explain the efficiency of steam engines Relevance in Chemistry-everyday chemical transformations, equilibrium, electrochemical cells (biological cells) energy into electrical energy 2  Bioenergetics Bond making and breaking involves changes in energy Body simultaneously emits and absorbs energy, with these respective amounts of energy being equal and opposite In biological system, energy is obtained by chemical linkage (or coupling) to oxidation reactions. The simplest type of coupling may be represented by the equation. A + C→ B + D + Heat 3 Coupling of reactions 4  Basic Concepts of energy Conservation of energy-different forms of energy Energy changes occur during chemical or physical process Exchange of energy with its surroundings in terms of heat or work Keeping track of heat output for physical or chemical process at constant pressure Heat capacities 5 Some of the common examples of transformations of energy in biological systems (a) The running horse represents conversion of chemical energy to mechanical energy. (b) The electric fish (Torpedinidae) converts chemical energy to electrical energy (electric discharge, ranging from 8 to 220 volts). (c) The phosphorescent bacteria convert chemical energy into light energy. (d) Fireflies –Chemical to light energy 6  System, Surroundings & Universe System-part of world in which we have special interest or a system is a matter within a defined region. Surroundings are where we make observations or the matter in the rest of the universe is called the surroundings. Thus, the system plus the surroundings constitute the universe -reaction flask in constant temperature bath 7 8  Systems Open, closed and isolated systems Biological cell ? 9 The First Law : Principle of Conservation of Energy Create energy from nothing so enormous work could be produced endlessly Energy can neither be created nor destroyed -conservation of energy -not its creation nor annihilation Any chemical change must involve only conservation of energy 10  Conservation of energy ΔE = EB – EA = Q – W …………………(1) ΔE = change in internal energy EA = energy of a system at the start of the process EB = energy of a system at the end of the process Q = heat absorbed by the system W = work done by the system Limitation of 1st law: Can’t predict spontaneity of the reaction To determine the spontaneity another function Entropy was considered 11  Conservation of energy Entropy, S Quantitative expression for the randomness or disorder in a system or Entropy is energy in a state of randomness or disorder. It is unavailable, useless energy. In other words, entropy is a measure of the degree of randomness or disorder of a system. The entropy of a system increases (i.e., Δ S is positive) when it becomes more disordered. Entropy becomes maximum in a system as it approaches equilibrium. When equilibrium is attained, no further change can occur spontaneously unless additional energy is supplied from outside the system. 12  The second law of thermodynamics The second law of thermodynamics states that a process can occur spontaneously only if the sum of the entropies of the system and its surroundings increases. Δssystem + ΔSsurroundings> 0 for a spontaneous process …………………(2) Thus, the total entropy of a system must increase if a process is to occur spontaneously. In chemical reaction, S is not readily measurable for both system and surroundings. Difficult, not suitable for biochemical process Problem solved by another function G, Free energy What is G? 13 What is exergonic and endergonic reaction?  Combination of the 1st and 2nd law ΔG = ΔH – TΔS …………………(3) ΔG = the change in free energy of a reacting system ΔH = the change in heat content or enthalpy of this system T = absolute temp. Δ S = the change in entropy of this system Enthalpy? # heat content of the reacting system #reflects the number and kinds of chemical bonds in the reactants and products 14 ΔH = ΔE + PΔV …………………(4) ΔE = the change in the internal energy of a reaction ΔV= the change in volume of this reaction ΔG = ΔE - TΔS …………………(5) (When ΔV is very small, and ΔH = ΔE) 15 Exothermic and endothermic reactions? H (-) value H (+) value 16  If the energy of the products is higher than the energy of the reactants, it is endothermic. When reactants having lower energy change to products having higher energy, the extra energy needed is absorbed by the reactants from the surroundings. So the reaction is endothermic. If the energy of the products is lower than the energy of the reactants, it is exothermic. When reactants having higher energy change to products with lesser energy content, the extra energy that the reactants had is given out into the surroundings. So the reaction is exothermic. 17 How do I know if a reaction graph is endothermic or exothermic? 18 Thus, the change in free energy of a reaction, ΔG depends both on the change in internal energy and on the change in entropy of the system. The ΔG is a valuable criterion in determining whether a reaction can occur spontaneously. Thus, (a)If ΔG is negative in sign, the reaction proceeds spontaneously with loss of free energy, i.e., it is exergonic. If, in addition, ΔG is of great magnitude, the reaction goes virtually to completion and is essentially irreversible. (b) If, however, ΔG is positive, the reaction proceeds only if free energy can be gained, i.e., it is endergonic. If, in addition, ΔG is of high magnitude, the system is stable with little or no tendency for a reaction to occur. (c) If ΔG is zero, the reaction system is at equilibrium and no net change takes place. 19 How do I know if a reaction graph is endergonic or exergonic? 20  Exothermic Exergonic Exothermic reactions are the ones that release heat or These types of reactions occur in favorable conditions energy in one or another form. spontaneously. Although these reactions are similar to exergonic reactions, there is a change in the enthalpy of the There is a decrease in free energy during this reaction, reaction and not in the Gibbs free energy of the leading to a decreased Gibbs free energy, and it is less reaction. The overall change in the enthalpy is negative than zero. for such reactions. The entropy (S) of the systems increases in exothermic The entropy (S) of the system increases during such reactions, which means the randomness increases in reactions. the reaction. The energy released in these reactions is in the form of There is a flow of free energy from the system to its heat or light such as sparks, explosions etc. surroundings. Although there is a release of heat in this reaction, the An exothermic reaction is an exergonic reaction temperature of the surroundings does not increase. If because the change in enthalpy will also lead to the there is an increase in the temperature of the difference in the Gibbs free energy of the system. surroundings, then it becomes an exothermic reaction. Combustion of wood or coal is an apt example of an Breakdowns of sugar is an example of an exergonic exothermic reaction. reaction. Exothermic reactions also increase the temperature of These types of reactions do not need energy; instead, it the surroundings. releases energy in the surroundings. 21 Coupling of exergonic reaction to endergonic reaction Why? to drive otherwise unfavourable reactions. 22  Coupling of exergonic reaction to endergonic reaction Energy coupling in mechanical and chemical processes (a) The downward motion of an object releases potential energy that can do mechanical work. The potential energy made available by spontaneous downward motion, an exergonic process (pink), can be coupled to the endergonic upward movement of another object (blue). (b) In reaction 1, the formation of glucose 6-phosphate from glucose and inorganic phosphate (Pi) yields a product of higher energy than the two reactants. For this endergonic reaction, G1 is positive. In reaction 2, the exergonic breakdown of adenosine triphosphate (ATP) can drive an endergonic reaction when the two reactions are coupled. The exergonic reaction has a large, negative free-energy change (G2), and the endergonic reaction has a smaller, positive free energy change (G1). The third reaction accomplishes the sum of reactions 1 and 2, and the free-energy change, G3, is the arithmetic sum of G1 and G2. Because G23 3 is  Relationship between Keq and ΔG° - (is the Measures of a Reaction’s Tendency to Proceed Spontaneously) In a model reaction, A+B C+D The free energy change, ΔG of this reaction is given by where [A] is the concentration of A, [B] the concentration of B, and so on. ΔG° is the free energy change for this reaction under standard conditions, i.e.,when each of the reactants A, B, C and D is present at a concentration of 1.0 M. Thus, the ΔG of a reaction depends on the nature of the reactants (expressed in ΔG° term) and on their concentrations (expressed in logarithmic terms). At equilibrium, ΔG = 0. 24 The equilibrium constant under standard conditions, K′eq for the reaction A + B  C + D, is given by Now, substituting the value of K’eq 25  Substituting R = 1.98 × 10–3 kcal mol–1 degree–1 and T = 298°K (corresponding to 25°C) gives K′eq = 10–ΔG°/1.36 The units of ΔG° and ΔG are joules per mole (or calories per mole or kcal/mol). A change in equilibrium constant by a factor of 10 results in a change in standard free energy of –1.36 kcal/mol at 25°C. At 37°C, however, the change in standard free energy would be of 1.42 kcal/mol. From this we see that ΔG° is simply a second way (besides K ) of expressing the driving force on a reaction. eq Because K is experimentally measurable, we have a way eq of determining ΔG°, the thermodynamic constant characteristic of each reaction. 26 When K >> 1, ΔG° is large and negative; when K