Metabolism, Energy and Life

Transcription

1 BSC Exam I Lectures and Text ages I. Intro to Biology (2-29) II. Chemistry of Life Chemistry review (30-46) Water (47-57) Carbon (58-67) Macromolecules (68-91) III. Cells and Membranes Cell structure (92-123) Membranes ( ) IV. Introductory Biochemistry Energy and Metabolism ( ) Cellular Respiration ( ) hotosynthesis ( ) Energy & Metabolism Metabolism, Energy and Life Metabolism = all of an organism s chemical rxns and energy conversions Metabolism transforms the energy and material resources of a cell Cells use energy to perform various types of work Metabolic reactions occur in specific pathways, catalyzed by specific enzymes (proteins) The Energy of Life The living cell Is a miniature factory where thousands of reactions occur Converts energy in many ways and uses energy to perform work, such as active transport. Some organisms convert energy to light, as in bioluminescence Figure 8.1 1

2 Organization of the Chemistry of Life into Metabolic athways A metabolic pathway has many steps That begin with a specific molecule and end with a product That are each catalyzed by a specific enzyme Enzyme 1 Enzyme 2 Enzyme 3 A B C D Reaction 1 Reaction 2 Reaction 3 Starting roduct molecule Catabolic athways Break down larger molecules into smaller ones Release energy that can be captured in the bonds of AT Example: Cellular Respiration (glucose CO 2 + H 2 O + AT) Anabolic athways Synthesize complicated molecules from simpler ones Consume energy (use AT or another E source) are biosynthetic pathways (use energy) Example: hotosynthesis (CO 2 + H 2 O + sunlight energy O 2 + glucose) 2

3 Energy Coupling Anabolism is fueled by the energy released from catabolism Anabolic pathways use the energy produced by catabolic pathways. The transfer of energy is accomplished by AT. Forms of Energy Energy Is the capacity to cause change, to do work, to move or rearrange matter Exists in various forms, of which some can perform work Kinetic Energy Is the energy associated with motion/work Ex = leg muscles turning a bike wheel Heat = thermal energy = kinetic energy assoc. w/ random movement of molecules Moving matter does work by transferring its motion to other matter. 3

4 otential Energy Is energy stored in the location or structure of matter Includes chemical energy potential energy available from a reaction that is stored in the arrangement of atoms in molecules Energy Can Be Converted From one form to another lants convert light energy (kinetic) into chemical energy (potential) in sugars. On the platform, a diver has more potential energy. Diving converts potential energy to kinetic energy. Figure 8.2 Climbing up converts kinetic energy of muscle movement to potential energy. In the water, a diver has less potential energy. Metabolism is Subject to the Laws of Thermodynamics An organism s metabolism transforms matter and energy into various forms, but always subject to the laws of thermodynamics 4

5 The Laws of Energy Transformation Thermodynamics Is the study of energy transformations A System = matter under study a. Closed system is isolated from surroundings b. Open system = energy can be transferred between the system & surroundings Ex = organisms The First Law of Thermodynamics According to the first law of thermodynamics energy cannot be created or destroyed The energy of the universe is constant. This is the principle of the conservation of energy So energy can be transferred & transformed but NOT created or destroyed Figure 8.3 Chemical energy (a) First law of thermodynamics: Energy can be transferred or transformed but Neither created nor destroyed. For example, the chemical (potential) energy in food will be converted to the kinetic energy of the cheetah s movement in (b). The Second Law of Thermodynamics According to the second law of thermodynamics, spontaneous changes that do not require outside energy increase the entropy, or disorder, of the universe Heat co 2 + H 2O Figure 8.3 (b) Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, disorder is added to the cheetah s surroundings in the form of heat and the small molecules that are the by-products of metabolism. 5

6 The Second Law of Thermodynamics Every energy transformation/transfer increases the entropy (disorder) of the universe. No energy transfer is 100% efficient, some is always converted to heat. Heat (kinetic energy) has a high entropy value. It is the least ordered form of energy. Order can increase locally (energy input), but is constantly decreasing globally Organisms are low-entropy islands in an increasingly random universe. They live at the expense of free energy. Organisms are open systems and can become more ordered, but only with the input of energy, and that is at the expense of the surroundings. Organisms take in food (a highly ordered form of energy) and put out heat, water, and carbon dioxide. Biological Order and Disorder Living systems Increase the entropy of the universe Use energy to maintain order 50µm Figure 8.4 Buttercup root cross-section Organisms Live at the Expense of FREE ENERGY The reactions in our bodies use up energy. Reactions will only run without the input of additional outside energy IF they are spontaneous reactions. Non-spontaneous reactions require the input of outside energy. a. Reactions that happen on their own (w/out energy input) are spontaneous. Spontaneous processes always increase entropy. b. How do we know if a rxn is spontaneous? It occurs without the input of external energy and it will only do so if it decreases the free energy of the system. 6

7 Organisms Live at the Expense of FREE ENERGY Gibbs free energy (G) is the portion of a system s energy that is available to do work when temperature and pressure are held constant. Reactions with a - G are spontaneous. Those reactions decrease the total free energy and/or increase the disorder (entropy [S]) and thereby increase the stability of the system. Change in Free Energy The change in free energy, G during a biological process Is related directly to the enthalpy change ( H) and the change in entropy G = H T S H = enthalpy (total energy in biological systems) S = entropy (disorder, energy not available for work) T = temperature (K = C + 273) An unstable system is rich in free energy and has a tendency to change to a more stable state and potentially perform work in the process. Free Energy and Equilibrium or Why Care About Spontaneity? In terms of rxn equilibriums (see Ch 2): a. G = G final -G initial (smaller value is more stable) b. Equilibrium = state of maximum stability c. When equilibrium reached G is lowest in that system d. In chemical reactions, equilibrium is when the forward and backward reactions proceed at the same rate and G = 0, so no net free energy change. e. A cell that has reached equilibrium is DEAD ( G is lowest & no work can be done) Lack of equilibrium (life) maintained by making products of one rxn the reactants of another (with a steady supply of glucose + O 2 ) Organisms are open systems. Life is constantly supplied with free E from the sun. That energy must be added to the system to move it away from equilibrium (death). 7

8 Free Energy, Stability, and Equilibrium Organisms live at the expense of free energy During a spontaneous change Free energy decreases and the stability of a system increases At Maximum Stability The system is at equilibrium More free energy (higher G) Less stable Greater work capacity In a spontaneously change The free energy of the system decreases ( G<0) The system becomes more stable The released free energy can be harnessed to do work. Less free energy (lower G) More stable Less work capacity Figure 8.5 (a) Gravitational motion. Objects (b) Diffusion. Molecules (c) Chemical reaction. In a move spontaneously from a in a drop of dye diffuse cell, a sugar molecule is higher altitude to a lower one. until they are randomly broken down into simpler dispersed. molecules. Free Energy and Metabolism Free E and living systems (metabolism): a. Exergonic rxn (fig 8.6a) = E releasing (negative G) spontaneous The greater the decrease in G greater amt of work can be done Ex: Respiration G for C6H12O6 + O2 6CO2 + 6H2O = -686 kcal/mol b. Endergonic rxn (fig 8.6b) = E absorbing (positive G) NOT spontaneous Sunlight (E) drives photosynthesis (reverse of respiration) G for 6CO2 + 6H2O C6H12O6 + O2 = +686 kcal/mol c. The energy released by an exergonic reaction (- G) is equal to the energy required by the reverse endergonic reaction (+ G) d. Metabolic disequilibrium is essential to life. Respiration and other cell reactions are reversible and could reach equilibrium if the cell did not maintain a supply of reactants and use up or dispose of products. e. The cell must couple the energy of exergonic processes to power endergonic processes. 8

9 Exergonic and Endergonic Reactions in Metabolism An exergonic reaction roceeds with a net release of free energy and is spontaneous Reactants Free energy Energy roducts Amount of energy released ( G <0) rogress of the reaction Figure 8.6 (a) Exergonic reaction: energy released Exergonic and Endergonic Reactions in Metabolism An endergonic reaction Is one that absorbs free energy from its surroundings and is nonspontaneous roducts Free energy Reactants Energy Amount of energy required ( G>0) rogress of the reaction Figure 8.6 (b) Endergonic reaction: energy required Equilibrium and Metabolism Reactions in a closed system Eventually reach equilibrium G < 0 G = 0 Figure 8.7 A (a) A closed hydroelectric system. Water flowing downhill turns a turbine that drives a generator providing electricity to a light bulb, but only until the system reaches equilibrium. 9

10 Maintaining Disequilibrium in Living Systems Cells in our body Experience a constant flow of materials in and out, preventing metabolic pathways from reaching equilibrium (b) An open hydroelectric system. Flowing water keeps driving the generator because intake and outflow of water keep the system from reaching equlibrium. G < 0 Figure 8.7 An analogy for cellular respiration G < 0 G < 0 G < 0 Figure 8.7 (c) A multistep open hydroelectric system. Cellular respiration is analogous to this system: Glucoce is broken down in a series of exergonic reactions that power the work of the cell. The product of each reaction becomes the reactant for the next, so no reaction reaches equilibrium. AT Energy Currency of the Cell AT powers cellular work by coupling exergonic reactions to endergonic reactions Energy coupling Is a key feature in the way cells manage their energy resources to do work A cell does three main kinds of work Mechanical Transport Chemical 10

11 The Structure and Hydrolysis of AT AT (adenosine triphosphate) Is the cell s energy shuttle Adenine NH 2 - N O O O HC O O O O N CH 2 O O- O- O- H H hosphate groups H H Figure 8.8 OH OH C C C N Ribose N CH Energy is released from AT When the terminal phosphate bond is broken Adenosine triphosphate (AT) H 2 O i + Energy Figure 8.9 Inorganic phosphate Adenosine diphosphate (AD) AT hydrolysis (and the energy released) Can be coupled to other reactions Endergonic reaction: G is positive, reaction is not spontaneous NH 2 Glu Glutamic acid + NH 3 Ammonia Glu Glutamine G = +3.4 kcal/mol Exergonic reaction: G is negative, reaction is spontaneous AT + H 2 O AD + G = kcal/mol Figure 8.10 Coupled reactions: Overall G is negative; together, reactions are spontaneous G = 3.9 kcal/mol 11

12 How AT erforms Work AT drives endergonic reactions By phosphorylation, AT transfers a highenergy phosphate to other molecules Energy to perform work becomes available when the phosphate is released from its substrate. The three types of cellular work Are powered by the hydrolysis of AT i Motor protein rotein moved (a) Mechanical work: AT phosphorylates motor proteins AT Membrane protein i Solute Solute transported (b) Transport work: AT phosphorylates transport proteins AD + i Figure 8.11 NH 2 Glu + NH 3 + i Glu Reactants: Glutamic acid roduct (glutamine) and ammonia made (c) Chemical work: AT phosphorylates key reactants The Regeneration of AT Catabolic pathways provide energy to Drive the regeneration of AT from AD and phosphate AT synthesis from AD + i requires energy AT hydrolysis to AD + i yields energy AT Energy from catabolism (exergonic, energy yielding processes) Energy for cellular work (endergonic, energyconsuming processes) Figure 8.12 AD + i 12