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Bioenergetics Energy is the ability to do work Types of Energy: Electrical, Chemical, thermal, etc. Unit of energy (Heat) is cal/kcal OR J/kJ. Calorie (cal) is the amount of energy required to raise the temperature of one gram of water by 1 °C. Joule (J) is the amount of energy needed to apply a 1-newton force over a distance of 1 meter. 1 kJ = 0.239 kcal Energy can be converted from one form to another Energy is transferred through the universe from one system to another Sun CO2 + H2O Energy Photosynthetic Cell Heterotrophic Cell plants animals CHO CO2 + H2O C6H12O6 O2 ATP ADP + Pi Stages of energy extraction Production of ATP Photosynthesis Plants, Some algae, Oxidative Phosphorylation All living organisms Why we need Energy (e.g. ATP) To do work that can be used as: 1. Chemical work; e.g. to build macromolecules 2. To preserve ionic equilibrium across membranes; e.g. active transport. 3. Mechanical work; e.g. for movement. Steady State & Equilibrium • The glucose ingested is circulating in blood and will be taken by cells. Now, less glucose in blood. • A fresh supply of glucose is produced by the liver, so that your blood glucose concentration is more or less constant over the whole day. • The constancy of concentration is the result of a dynamic steady state, which is different from the term equilibrium. Closed and Open system Closed system is a theoretical assumption that we can separate the system completely from its surroundings. Open system is the actual life or the reality of reactions. Definition of Entropy The key descriptors of entropy are randomness and disorder, manifested in different ways. Case 1: The Teakettle and the Randomization of Heat (teakettle (system) + kitchen (surrounding) = universe) We know that steam generated from boiling water can do useful work. Heat passes from the teakettle to the surroundings, raising the temperature of the surroundings (the kitchen) by an infinitesimally small amount until complete equilibrium is attained. At this point all parts of the teakettle and the kitchen are at precisely the same temperature. Moreover, the increase in entropy of the kitchen (the surroundings) is irreversible. G = H - TS = difference Free energy content, G, Enthalpy, H, The absolute temperature, T (in degrees Kelvin) Entropy, S G > 0 endergonic G < 0 exergonic A process tends to occur spontaneously if G is negative. To carry out thermodynamically unfavorable, energy-requiring (endergonic) reactions, cells couple them to other reactions that liberate free energy (exergonic reactions), so that the overall process is exergonic. The sum of the free energy changes is negative. •When a system is at equilibrium, the rate of product formation exactly equals the rate at which product is converted to reactant. •The energy change as the system moves from its initial state to equilibrium, with no changes in temperature or pressure, is given by the free-energy change, G. • The magnitude of G depends on the particular chemical reaction and on how far from equilibrium the system is initially. Each compound involved in a chemical reaction contains a certain amount of potential energy, related to the kind and number of its bonds. Also means that amount of energy needed by the muscle cell to make ATP, ie to reverse the reaction Also means that amount of energy needed by the muscle cell to make ATP, ie to reverse the reaction Because the concentrations of ATP, ADP, and Pi differ from one cell type to another, G for ATP hydrolysis likewise differs among cells. Moreover, in any given cell, G can vary from time to time, depending on the metabolic conditions in the cell and how they influence the concentrations of ATP, ADP, Pi, and H+ (pH). To further complicate the issue, the total concentrations of ATP, ADP, Pi, and H may be substantially higher than the free concentrations, which are the thermodynamically relevant values. The difference is due to tight binding of ATP, ADP, and Pi to cellular proteins. For example, the concentration of free ADP in resting muscle has been variously estimated at between 1 and 37 M. Thus, the energy released by ATP hydrolysis is greater than the standard free-energy change, Go`. G = - RT ln Keq A + B (substrates or reactants) Keq C + D (products) High G means that the substrates or reactants are far from equilibrium state. Low G means that the substrates or reactants are near to equilibrium state 0 G means that the substrates or reactants are at equilibrium state The sign of G (+ or -) means exorgenic or endorgenic. Also, means the reaction spontaneous (-) or not (+, i.e., should go in the reverse direction) Go` = - RT ln Keq The standard free-energy change of a chemical reaction is simply an alternative mathematical way of expressing its equilibrium constant. A+BC+D I’m confused G or Go` Go` is the standard G is the actual Reaction is spontaneous (means favor to happen. Also, means it will occur in the forward direction) is due to G or Go`? Due to G. Why? Because G = - RT ln Keq Remember that activation energy is DIFFERENT. ATP hydrolysis gives two benefits: (ATP ADP + Pi + E) 1. Amount of energy to be coupled with another reaction (must be endergonic (G > 0 OR +G) & 2. To release heat into the surrounding so the universe (total) is now increased in entropy (S). So, What is efficiency? Free energy change (G) only tells us the difference and has no connection with the path of the reaction or number of steps involved. S Keq1 G1 Keq3 Keq5 G3 G5 X Keq2 G2 P Y Keq4 G4 Goverall = G1 + G2= G3 + G4 = G5 Keqoverall = Keq1 x Keq2 = Keq3 x Keq4 = Keq5 Example The conversion of glucose to lactic acid has an overall G of -52,000 cal/mol. In an anaerobic cell, this conversion is coupled to the synthesis of 2 moles of ATP per mole of glucose. (a) calculate the G of the overall coupled reaction. (b) calculate the efficiency of energy conservation in the anaerobic cell. (c) At the same efficiency, how many moles of ATP per mole of glucose could be obtained in an aerobic organism in which glucose is completely oxidized to CO2 and H2O (G = -686,000 cal/mol)? (a) Glucose 2 lactic acid G1 = -52,000 cal/mol 2ADP + 2Pi 2ATP G2 = +7,700 cal/mol x 2 = +15,400 cal/mol (Sum) glucose + 2ADP + 2Pi 2 lactic acid + 2ATP G3 = G1 + G2 = (-52,000 cal/mol) + (+15,400 cal/mol) = -36,600 cal/mol energy conserved (b) Efficiency = energy made available Efficiency = 29.6 % x 100% = 15,400 cal/mol x 100% 52,000 cal/mol (c) Glucose + 6O2 6CO2 + 6H2O G4 = -686,000 cal/mol nADP + nPi nATP G1 = n (+7,700 cal/mol) (Sum) glucose + 6O2 + nADP + nPi 6CO2 + 6H2O + nATP At 29.6% efficiency, Total energy that can be conserved is 0.296 X 686,000 cal/mol = 203,000 cal/mol If each mole of ATP requires 7,700 cal for its synthesis, Then: 203,000 cal/mol = 26.4 moles of ATP 7,700 cal/mol = 26 moles of ATP (nearest whole number) Why ATP? ATP = Adenosine Triphosphate Hydrolysis: -30.5 kJ mol-1 OR -7.3 kcal mol-1 ATP ADP + Pi + E 10 6 7 8 5` 4` 3` ATP4- 1 5 2 9 4 1` 2` 3 Mg2+ in the cytosol binds to ATP and ADP, and for most enzymatic reactions that involve ATP as phosphoryl group donor, the true substrate is MgATP2-. ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups The group transferred from ATP is a phosphoryl (PO32-), not a phosphate (OPO32-). ATP Provides Energy by Group Transfers, Not by Simple Hydrolysis. ATP hydrolysis per se usually accomplishes nothing but the liberation of heat, which cannot drive a chemical process in an isothermal system (means that our temp stay at 37oC). ATP conatins 4 negative charge ADP conatins two phosphate groups Go for ATP is -7.3 kcal/mole The terminal PO4- can easily be released because: Electrostatic Repulsion & Resonance Stabilization Advantages of ATP: 1. The released energy (-7.3 kcal/mol) is moderate amongst the biochemical compounds inside the cell. 2. Can transfer energy from compounds with high free energy content to those with less. 3. It acts like a bridge between the chemical reactions e.g. A + ATP x ATP + y + ATP ADP + y~P ATP in aqueous solution is thermodynamically unstable (means easily hydrolysed) and is therefore a good phosphoryl group donor but it is kinetically stable (means its activation energy is high). Because of the huge activation energies (200 to 400 kJ/mol) required for uncatalyzed cleavage of its phosphoanhydride bonds, ATP does not spontaneously donate phosphoryl groups to water or to the hundreds of other potential acceptors in the cell. Only specific enzymes can lower the energy of activation, so the phosphoryl group is now transfered from ATP. The Flow of Electrons Can Do Biological Work 1. Electrons do not move alone. Must be a force to drive them from one position to another. 2. This force is the affinity of each atom to capture the electrons, which is called the electronegativity. 3. The order of electronegativity is H < C < S < N < O. 4. Living cells have a biological “circuit,” with a relatively reduced compound such as glucose as the source of electrons. 5. As glucose is enzymatically oxidized, the released electrons flow spontaneously through a series of electron-carrier intermediates to another chemical species, such as O2. 6. This electron flow is exergonic, because O2 has a higher affinity for electrons than do the electron-carrier intermediates. H+ H+ High-energy electron H+ H+ H+ H+ H+ 3 Pumping out e2 NADH+H+ FADH2 H+ 6 O2 H+ Low-energy electron H+ NADPH+H+ Succinate dehydrogenase 1 Inner mitochondrial membrane (OR Bacterial cell membrane) Electron-carrier 5 4 (A) The electromotive force comes from the movement of the electron from metabolites to O2. It results due to O2 has high affinity to electron than any other compound. This force releases energy that are use to pump out protons. eO2 H+ Energy enough to make ATP ----------------++++++++++++ (B) Generation of the proton-motive force H+ i ii The resulting electromotive force provides energy to a variety of molecular energy transducers (enzymes and other proteins) that do biological work. Examples: 1. In the mitochondrion, membrane-bound enzymes couple electron flow to the production of a trans-membrane pH difference, accomplishing osmotic and electrical work. The proton gradient thus formed has potential energy (the proton-motive force). Another enzyme, ATP synthase in the inner mitochondrial membrane, uses the proton-motive force to do chemical work: synthesis of ATP from ADP and Pi. as protons flow spontaneously across the membrane. 2. Similarly, membrane-localized enzymes in E. coli convert electromotive force to proton-motive force, which is then used to power flagellar motion. more reduced compounds ↑H ↓O Oxidation states of carbon in the biosphere more oxidized compounds Reduction Potentials (E) Measure Affinity for Electrons Eo is + Eo is - e- Reduction Potentials (E) Measure Affinity for Electrons Eo is Eo is + e- Reduction Potentials (E) Measure Affinity for Electrons electrons tend to flow through the external circuit from lower to higher standard reduction potential. Electrons tend to flow to the half-cell with the more positive Eo, and the strength of that tendency is proportional to the difference in reduction potentials, Eo. pH=0 By convention, the half-cell with the stronger tendency to acquire electrons is assigned a positive value of Eo. pH=7 • Nernst equation: F = Farady constant, n = number of electrons • Standard Reduction (Eo) Potentials Can Be Used to Calculate the Free-Energy Change Example Cellular Oxidation of Glucose to Carbon Dioxide Requires Specialized Electron Carriers C6H12O6 + 6O2 6CO2 + 6H2O The complete oxidation of glucose: has a G of 2,840 kJ/mol. This is a much larger release of free energy than is required for ATP synthesis (50 to 60 kJ/mol). Cells convert glucose to CO2 not in a single, high-energy-releasing reaction, but rather in a series of controlled reactions, some of which are oxidations. Electrons removed in these oxidation steps are transferred to coenzymes specialized for carrying electrons, such as NAD and FAD.