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Chap 8 - Metabolism.docx

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University of Lethbridge
BIOL 1010
Brent Sellinger

Reece et al. 9 Edition Chapter 8 Page1 An Introduction to Metabolism I. Metabolism, Energy and Life Metabolism • Metabolism is all of an organism'schemical reactions (an emergent property that arises from interactions of molecules in the orderly environment of the cell) • Metabolism is very important for the management of cellular material and energy resources Metabolic reactions 1* Metabolic reactions are organized into pathways of enzyme controlled chemical reactions. Enzymes are required to make the reactions occur at a rate fast enough to support life. 2* Cells need a supply of molecules (food) and energy 3* Cells need to get rid of waste products 4* Metabolic pathways begin with a specific molecule(s) and end with a product 1. Catabolic pathways: 5* Break down complex molecules into simple molecules 6* Energy stored in complex molecules is made available to do work or transformed into readily usable chemical forms (i.e.,ATP) 7* small molecules resulting from the catabolism of complex energy rich molecules may be used by the cell to build new molecules Energy stored in compounds (potential energy) can be used to do WORK Types of cellular work i. mechanical ii. transport iii. chemical - [i.e., endergonic reactions] 2. Anabolic pathways 8* Anabolic pathways use energy to synthesize complex molecules from simple molecules. 9* free energy (G) - a measure of available energy under the conditions of a biochemical reaction - energy that can perform work Energy extracted from the environment is used to create order and carry out life processes (Bioenergetics – the study of energy flows through living organisms) How is energy extracted? Recall the following types of metabolism • Photoautotrophy • Chemoautotropy • Photoheterotrophy 2 • Chemoheterotrophy II. Forms of Energy Energy = capacity to cause change • Energy exists in various forms, some of which can perform work • Kinetic energy is energy associated with motion • Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules • Potential energy is energy that matter possesses because of its location or structure • Chemical energy is potential energy available for release in a chemical reaction • Energy can be converted from one form to another III. Laws of Energy Transformations or Thermodynamics Thermodynamics = the study of energy transformations that occur in a collection of matter. • An isolated system, such as that approximated by liquid in a thermos, is isolated from its surroundings • In an open system, energy and matter can be transferred between the system and its surroundings • Organisms are open systems 1st Law - Energy can be transferred and transformed but cannot be created or destroyed. 2nd Law - Every energy transfer or transformation increases the entropy (i.e., randomness) of the universe. • Note: The laws of thermodynamics identify what will and will not happen under given conditions but indicate nothing about the rate of reactions IV. Free Energy and Metabolism • Biologists want to know which reactions occur spontaneously and which require input of energy • To do so, they need to determine energy changes that occur in chemical reactions • Free energy (G) is the portion of a system’s energy that can perform work when temperature and pressure are uniform throughout the system.A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell Change in free energy (▯G) ▯G = G final statinitial state • The free energy change of a reaction indicates whether or not the reaction occurs spontaneously or requires input of energy from the outside. Reece et al. 9 Edition Chapter 8 Page3 • Only processes with a negative ▯G are spontaneous – spontaneous processes can be harnessed to do work Chemical reactions can be classified as exergonic or endergonic based on their free energy changes. (Recall: a chemical reaction is the breaking and reforming of bonds resulting in the change in composition of matter) Exergonic reaction = proceeds with a net release of free energy (▯G < 0). These reactions are energetically favourable and will occur spontaneously Endergonic reaction = a reaction that absorbs free energy from its surroundings. This type of reaction stores free energy in molecules. These reactions will not occur spontaneously and have a positive change in free energy (▯G > 0) Where does a cell get the energy needed for anabolic processes? Why are cells open systems? Reactions in a closed system eventually reach equilibrium and can then do no more work. If a cell is in a metabolic equilibrium then it is dead! V. ATP and Energy Coupling How is the energy stored in energy rich molecules used to drive cellular work? Energy coupling - energy released by catabolic processes is used to drive anabolic processes Adenosine triphosphate (ATP - Figure 8.8) 10* ATP powers cellular work by coupling exergonic (i.e., energy releasing) reactions to endergonic (i.e., energy storing) reactions 11*ATP mediates most but not all energy coupling in cells 12* ATP stores energy in unstable “high energy” bonds that can be hydrolyzed to release energy ATP + H O2▯ ADP + Pi (where Pi or inorganic phosphate is HOPO ) 32- ▯G= -7.3 kcal/mole in test tube 13* energy released through the hydrolysis ofATP is coupled through production of phosphorylated intermediates. 14* Aphosphorylated intermediate is more reactive than the original unphosphorylated molecule ATP is regenerated by energy coupling (ATP cycle - Figure 8.11) ADP + Pi ▯ ATP + H O 2 ▯G= 7.3 kcal/mole in test tube 4 VI. The Rate of Chemical Reactions • While the laws of thermodynamics enable us to identify what will or will not happen under a given set of conditions, these same laws provide no information about the rate of a reaction. • Many chemical reactions will occur spontaneously (▯G < 0) but the rates of these reactions may not be rapid enough to sustain life. Why is this the case
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