Chapter 8 Overview - Loudoun County Public Schools



Chapter 8 OverviewEnergy is a requirement of life. Living is hard work; energy must be expended to perform all the processes that occur in organisms. Chemical reactions, cellular functioning, running, growing, reproducing-all require energy. Without a steady source of usable energy, organisms would die. As we will see in the next two chapters, photosynthetic organisms use the sun as their energy source and convert solar energy to chemical energy. That chemical energy is then used by the photosynthesizers themselves and by essentially all other organisms. In this chapter we learn the fundamentals of energy and metabolism: what energy is, how it can be transformed and transferred, how chemical energy and reactions operate in living organisms, and how such systems are controlled. Chapter 8 Section 1 and Section 28.1, 8.2 Key Termsp. 144: bioenergetics, metabolism, anabolism, catabolism, energy, kinetic energy, potential energy, thermodynamics, kilocalorie (kcal), oxidationp. 145: reduction, oxidation-reduction (redox) reactions, cellular respiration, First Law of Thermodynamics p. 146: heat, Second Law of Thermodynamics, entropy (S), p. 147: free energy (G), enthalpy (H), change in free energy (_G), exergonic, endergonicp. 148: activation energy, catalysis, catalysts p. 149: enzymes, substrates p. 150: carbonic anhydrase, ribozymes, active sites p. 151: induced fit p. 152: temperature optimum, pH optimum, inhibitor, competitive inhibitors, noncompetitive inhibitors, allosteric site p. 152: allosteric inhibitor, activatorsp. 153: cofactors, coenzyme, nicotinamide adenine dinucleotide (NAD+)5th edition8.0 IntroductionLife Viewed as Constant Flow of Energy Required for Each of the Significant Properties of Life fig 8.1 Bioenergetics: How Energy Behaves in Living Systems 8.1 The laws of thermodynamics describe how energy changes The Flow of Energy in Living Things Energy Is the Ability to Do Work Exists in two states fig 8.2Kinetic energy: Energy of motion Potential energy: Stored energy that has the capacity of moving Living organisms transform potential energy into kinetic energy Thermodynamics Is the Study of Energy Energy is readily measured by its conversion into heat Unit of heat: 1000 calories = 1 kilocalorie (kcal) Oxidation-Reduction Life exists on earth because it is able to capture energy from the sun Energy from the sun transformed into chemical energy Process called photosynthesis Done by plants, algae and certain bacteria Combine water and carbon dioxide to make sugars Energy stored in covalent bonds between sugar atoms Certain reactions pass electrons from one molecule to another Oxidation: Atom or molecule loses an electron, becomes oxidized fig 8.3Oxygen strongly attracts electrons Oxygen is most common electron acceptor in biological systems Reduction: Atom or molecule gains an electron and is reduced Redox reactions occur together, electron transfers from one atom to other fig 8.4 Reactions play key role in flow of energy through biological systems Light adds energy and boosts electron to higher energy level The Laws of Thermodynamics First Law of Thermodynamics Energy can be transformed but not created or destroyed Total amount of energy in the universe remains constant Animals transfer food potential energy into their own chemical bonds Energy is not lost but may be changed into other forms Converted to kinetic energy, light, electricity Also dissipated as heat Heat harnessed to do work only via heat gradient Temperature difference between two areas Cells too small to maintain substantial internal heat differences Second Law of Thermodynamics All objects tend to become less ordered, disorder is increasing Spontaneous conversion from order/low stability to disorder/stability fig 8.3 Entropy Measure of disorder of a system = S Universe has progressively become disordered since beginning, increasing entropy Free Energy Bonds Between Atoms Hold Molecules Together Free energy: Energy available to break and form chemical bonds = G Enthalpy: Energy within a cell that is available to do work = H Temperature = T Free Energy = Ordering Influences - Disordering Influences G = H – TS Change in free energy: fig 8.6Positive Endergonic reactions Products contain more free energy than the reactants Reactions do not occur spontaneously, requires input of energy Negative Exergonic reactions Products contain less free energy or more disorder than reactants Reactions occur spontaneously, release excess usable free energy Activation Energy Reactions Require an Input of Energy to Get Started Must break chemical bonds before new bonds can be created Activation energy: Required to destabilize existing chemical bonds fig 8.7aRate of exergonic reaction depends on activation energy needed to start reaction Large activation energy, reaction proceeds slowly Catalysis Stressing chemical bonds makes them easier to break fig 8.7bCatalyst: Substance that carries out catalysis Cannot violate basic laws of thermodynamics Accelerates reaction in both forward and reverse directions Direction of reaction dependent on free energy Analogy of bowling ball rolling down hill 8.2 Enzymes are biological catalysts Enzymes Enzymes Carry Out Catalysis in Living Organisms Are generally proteins (or RNA) with specialized shapes Permit temporary associations with the molecules that are reacting Lower activation energy required for new bonds to form Bring two substrates together in the correct orientation Stress particular bonds of a substrate Example: Formation of carbonic acid from carbon dioxide and water Reaction proceeds in either direction Reaction is slow because of a great activation energy Carbonic anhydrase: Enzyme that speeds the reaction Enzymes given the name of their substrate with the ending -ase Thousands of Different Enzymes Exist Each enzyme catalyzes a different reaction Different cells contain different complements of enzymes How Enzymes Work Globular Protein Enzymes Possess Surface Clefts Called Active Sites Enzymes are specific in their choice of substrate Form enzyme-substrate complex The substrate must fit precisely into the active site Amino acid side groups of enzyme react with substrate Bond stressed or distorted, activation energy decreased Substrate binding causes enzyme to slightly change shape Induced fit: Binding may induce shape adjustments in the protein Substrate itself may act as activator Enzymes Take Many Forms Multienzyme Complexes Groups of several enzymes that catalyze successive steps of a reaction Assembly is non-covalently bonded Example: Bacterial pyruvate dehydrogenase multienzyme complex fig 8.10Enzymes carry out three sequential reactions in oxidative metabolism Each complex has multiple copies of each enzyme, 60 subunits total Subunits work together Increases catalytic efficiency Product of one reaction delivered to next, if released would diffuse away Eliminates possibility of unwanted side reactions All reactions controlled as one unit Example: Fatty acid synthetase complex Catalyzes synthesis of fatty acids from two-carbon precursors Includes seven enzymes and reaction intermediates Not All Biological Catalysts Are Proteins RNA catalyzes certain reactions involving RNA molecules RNA catalysts called ribozymes Accelerate reactions, show specificity to substrates Two kinds of ribozymes Intramolecular catalysts have folded structures, act upon selves Intermolecular catalysts act on other molecules Catalyzed reactions involve small RNA molecules Chip out unnecessary sections from RNA copies of genes Prepare ribosomes for protein synthesis Facilitate replication of DNA in mitochondria RNA may have evolved before proteins and catalyzed their formationFactors Affecting Enzyme Activity Temperature fig 8.11a Increasing temperature increases random motion and rate of reaction Beyond temperature optimum rate not increased Below optimum Hydrogen bonds and hydrophobic interactions not flexible Does not permit induced fit necessary for catalysis Above optimum Forces too weak to maintain enzyme's shape Enzymes denatures Human enzyme temperature optima range from 35?C to 40?C Hot spring bacteria proteins have more stable enzymes, optima to 70?C pH fig 8.12b Hydrogen ion concentration disrupts bonds between oppositely charged amino acids With more H+ ions fewer negative, more positive charges occur Most enzymes have a pH optimum of 6 to 8 Enzymes that function in acids retain 3-D shape when many H+ present Inhibitors and Activators Activity dependent on presence of specific substances Substances bind to enzyme and change its shape When shape changes activity is altered Inhibitors bind to enzyme and decrease its activity Feedback inhibition: End product inhibits reaction early in pathway Competitive inhibitors bind at same site as substrate Noncompetitive inhibitors bind at different site fig 8.12Allosteric site: Region where non-competitive inhibitor binds Allosteric inhibitor binds to allosteric site to reduce enzyme activity Activators bind to allosteric sites Keep enzymes in active configuration Increase enzyme activity Enzyme Cofactors Cofactors Are Additional Components that Aid Enzyme Action Many metallic trace elements are cofactors Coenzymes are nonprotein organic molecules like vitamins Serve as acceptors for electron pairs in redox reactions, shuttle energy Example: Nicotinamide adenine dinucleotide (NAD+) fig 8.13Structure of NAD+ Composed of nucleotides NMP and AMP AMP acts as core, provides for enzyme shape recognition NMP is active part, contributes site that readily accepts electrons Important biological hydrogen acceptor NAD+ acquires an electron and hydrogen to become reduced NADH NADH carries energy of electron and hydrogen around in cells ReviewNature of chemical bond: Chemical bonds hold atoms together to form molecules. In ionic bonds electrons are transferred between atoms, and the resulting ions are attracted to each other. In covalent bonds pairs of electrons are shared between atoms. Chemical bonds represent stored energy – potential energy. When a chemical bond is broken, energy is released. If the bond being broken is within an organism, the organism may use the released energy to help carry out its life processes. Protein structure: A protein consists of one or more polypeptide chains. Each polypeptide chain consists of a particular sequence of amino acids. This sequence is referred to as the primary structure, and it determines the chemical properties and further structures of the protein. The pleated folding or helical coiling of the chain is called the secondary structure of the protein. The tertiary structure refers to the complex, three-dimensional globular structure assumed by some proteins as their polypeptide chains bend and ball up. Finally, a protein is said to have a quaternary structure if it has more than one polypeptide chain; quaternary structure refers to how these chains are shaped and associated with each other. The structure of a protein is responsible for the way it functions. The enzymes that control the chemistry of living organisms are proteins.Nucleotides: Nucleotides are the building-block units of nucleic acids. Each nucleotide consists of a five-carbon sugar, a phosphate group, and a nitrogenous base. Nucleotides are also components of other biologically important compounds such as ATP and the coenzyme NAD+.Proton pump: The proton pump establishes a proton gradient across a cell or organelle membrane and uses that gradient to produce ATP, the universal energy currency. Protons are actively pumped across the membrane, and their diffusion back through special channels is coupled with the production of ATP. The cell (and organism) can then use the ATP to perform work. The energy needed to pump the protons is supplied by photosynthesis or the breakdown of energy-rich molecules.? ................
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