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Metabolism and Energy Mrs. Stahl AP Biology The Energy of Life • The living cell is a miniature chemical factory where thousands of reactions occur • The cell extracts energy stored in sugars and other fuels and applies energy to perform work • Some organisms even convert energy to light, as in bioluminescence Figure 8.1 Forms of Energy • Energy is the 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 A diver has more potential energy on the platform than in the water. Figure 8.2 Climbing up converts the kinetic energy of muscle movement to potential energy. Diving converts potential energy to kinetic energy. A diver has less potential energy in the water than on the platform. The Laws of Energy Transformation • Thermodynamics is the study of energy transformations • An isolated system, such as that approximated by liquid in a thermos, is unable to exchange energy or matter with its surroundings • In an open system, energy and matter can be transferred between the system and its surroundings • Organisms are open systems The First Law of Thermodynamics • According to the first law of thermodynamics, the energy of the universe is constant – Energy can be transferred and transformed, but it cannot be created or destroyed • The first law is also called the principle of conservation of energy • Plants do not produce energy, they transform light energy to chemical energy. During every transfer, some energy is converted to heat -> a system can use heat to do work only when there is a difference that results in heat flowing from warmer locations to cooler ones. If heat is uniform as in a living cell, heat can only be used to warm the organism. Figure 8.3 Heat H2O Chemical energy (a) First law of thermodynamics CO2 (b) Second law of thermodynamics The Second Law of Thermodynamics • During every energy transfer or transformation, some energy is unusable, and is often lost as heat • According to the second law of thermodynamics – Every energy transfer or transformation increases the entropy (disorder) of the universe – Entropy- measure of disorder or randomness – Increase entropy = increase heat • Living cells unavoidably convert organized forms of energy to heat • Spontaneous processes occur without energy input; they can happen quickly or slowly. – Living systems create ordered structures from less ordered starting materials. – Ex- amino acids are ordered into polypeptide chains – Ex- structure of a multicellular body is organized and complex. • For a process to occur without energy input, it must increase the entropy of the universe Highly Ordered Living Organisms Do Not Violate the Second Law of Thermodynamics • Organisms also take in organized forms of matter and energy from its surrounding and replaces them with less ordered forms. – Ex- animal consumes organic molecules as food and catabolizes (breaks down -> metabolism) them to low energy molecules such as carbon dioxide and water. • The evolution of more complex organisms does not violate the second law of thermodynamics because Earth and organisms are open systems. We get our energy from the sun and we can evolve and create order by increasing the disorder of the universe. • Entropy (disorder) may decrease in an organism, but the universe’s total entropy increases The free-energy change of a reaction tells us whether or not the reaction occurs spontaneously • 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 Change, G • A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell • The change in free energy (∆G) during a process is related to the change in enthalpy, or change in total energy (∆H), change in entropy (∆S), and temperature in Kelvin units (T) ∆G = ∆H - T∆S • ∆H= enthalpy, in a spontaneous reaction it gets smaller or decreases because energy is released. • ∆S= entropy, measure of the disorder . In a spontaneous reaction entropy increases. • T= temperature, when temperature increases the spontaneous reaction is more likely to happen. • What makes ∆G decrease? – A decrease in ∆H – An increase in ∆S – An increase in T • Only processes with a negative ∆G are spontaneous also known as exergonic ( releases free energy as heat) • ∆G > 0 = endergonic reaction, energy must be added • ∆G = 0 = equilibrium • Increases in temperature amplify the entropy. Think about a cherry bomb-> add heat and it blows up. • Not all the energy in a system is available for work because the entropy component must be subtracted from the enthalpy component. What remains is the free energy, referred to as the stability of the system. Figure 8.5b (a) Gravitational motion (b) Diffusion (c) Chemical reaction Exergonic and Endergonic Reactions in Metabolism • Exergonic Reaction: – ∆G is negative, Spontaneous – The products of the reaction contain less free energy than reactants. Bond is lower or disorder is higher or both. – Energy is released. Ex- Cellular Respiration • Endergonic reaction: – absorbs free energy from its surroundings and is non-spontaneous – Energy must be supplied, ex- photosynthesis (a) Exergonic reaction: energy released, spontaneous Free energy Reactants Amount of energy released (∆G < 0) Energy Products Progress of the reaction (b) Endergonic reaction: energy required, nonspontaneous Free energy Products Reactants Energy Progress of the reaction Amount of energy required (∆G > 0) If all chemical reactions that release free energy tend to occur spontaneously, why haven’t all such reactions already occurred? • Most reactions require an input of energy to get started such as endergonic reactions (photosynthesis). Activation Energy • Before any bonds can form, they have to be broken by energy input. • Defined as the extra energy needed to destabilize existing chemical bonds and initiate / start a chemical reaction. • Exergonic rate depends on the activation energy required for the reaction to begin. • Rates of reactions are increased by: – Increasing the energy of reacting molecules – Lowering activation energy Catalysis / Catalysts • A substance that lowers the activation energy – Ex- enzymes – They cannot make an endergonic reaction proceed spontaneously. Adenosine Triphosphate- ATP • Main energy currency in all living cells • 1. Makes sugars • 2. Supplying activation energy for chemical reactions • 3. Actively transporting substances across membranes • 4. Moving through the environment and growing The Structure and Hydrolysis of ATP • ATP (adenosine triphosphate) is the cell’s energy shuttle • ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups How does ATP store energy? • The energy is stored in the bonds between the triphosphates. These groups repel each other due to their negative charges, and the covalent bonds joining the phosphates are unstable and can break. • They are easily broken by hydrolysis and when they break they release and transfer a large amount of energy which can be used. Figure 8.9a Adenine Triphosphate group (3 phosphate groups) (a) The structure of ATP Ribose Figure 8.9b Adenosine triphosphate (ATP) H 2O Energy Inorganic phosphate Adenosine diphosphate (ADP) (b) The hydrolysis of ATP How does ATP become ADP? • The bonds are broken on the third phosphate, releasing energy. • ATP -> ADP +Pi • Energy = 7.3kcal / mol • This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves Why does the hydrolysis of ATP yield so much energy? • The release of energy initially comes from the chemical change to a state of lower free energy. • Each phosphate group has a negative charge. Three like charges are crowded together and their mutual repulsion contributes to the instability of the molecule. How the Hydrolysis of ATP Performs Work • The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP • In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction • Overall, the coupled reactions are exergonic • ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant • The recipient molecule is now called a phosphorylated intermediate • Transport and mechanical work in the cell are also powered by ATP hydrolysis • ATP hydrolysis leads to a change in protein shape and binding ability Transport protein Solute Figure 8.11 ATP ADP P Pi Pi Solute transported (a) Transport work: ATP phosphorylates transport proteins. Vesicle ATP Cytoskeletal track ATP Motor protein ADP Protein and vesicle moved (b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed. Pi What reactions does ATP Drive? • Endergonic Reactions Figure 8.12 ATP Energy from catabolism (exergonic, energyreleasing processes) ADP H2O Pi Energy for cellular work (endergonic energy-consuming processes) The Regeneration of ATP • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) • The energy to phosphorylate ADP comes from catabolic reactions in the cell • The ATP cycle is a revolving door through which energy passes during its transfer from catabolic to anabolic pathways Enzymes speed up metabolic reactions by lowering energy barriers • A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction • Substrate- The molecules that will undergo the reaction • An enzyme is a catalytic protein • Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction Figure 8.UN02 Sucrase Sucrose (C12H22O11) Glucose (C6H12O6) Fructose (C6H12O6) Functions of Enzymes • 1. Lowers the activation energy required for new bonds to form • 2. Speeds up the rate of reaction • 3. Regulates metabolic pathways What is the importance of carbonic acid in vertebrate red blood cells? • Vertebrate RBC’s have an enzyme called carbonic anhydrase, which aids in breaking down carbon dioxide in our blood. 600,000 molecules of carbonic acid form every second. This enzyme increases the rate of reaction one million times. The Active Site • They are pockets on the enzyme where the substrates fit perfectly. Catalysis in the Enzyme’s Active Site • In an enzymatic reaction, the substrate binds to the active site of the enzyme • The active site can lower an EA barrier by – Orienting substrates correctly – Straining substrate bonds – Providing a favorable microenvironment – Covalently bonding to the substrate Enzyme Substrate Complex • A substrate molecule binds with an enzyme at its active site. Chemical reactions occur and bonds are either broken or new ones are formed. The substrates have been changed into products. The products leave the enzyme and the process starts all over again. Induced Fit • When the active site changes its shape slightly so that it can bind onto the substrate more tightly. Figure 8.15 Substrate Active site Enzyme Enzyme-substrate complex Where are most enzymes found? • Cytoplasm • Cell membranes • Organelles Multienzyme Complexes and Advantages • They allow a plethora of chemical reactions to occur-> molecular machine • Advantages: – 1. All reactions can be controlled as a unit – 2. No unwanted side reactions – 3. Rate of the enzyme is limited The Activation Energy Barrier • Every chemical reaction between molecules involves bond breaking and bond forming • The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) • Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings A B Figure 8.13 C D Free energy Transition state A B C D EA Reactants A B ∆G < O C D Products Progress of the reaction Animation: How Enzymes Work 1 Substrates enter Figure 8.16-1 2 Substrates are active site. Substrates Enzyme-substrate complex held in active site by weak interactions. 1 Substrates enter Figure 8.16-2 2 Substrates are active site. Substrates held in active site by weak interactions. Enzyme-substrate complex 3 Substrates are converted to products. 1 Substrates enter Figure 8.16-3 2 Substrates are active site. Substrates held in active site by weak interactions. Enzyme-substrate complex 4 Products are released. 3 Substrates are Products converted to products. 1 Substrates enter Figure 8.16-4 2 Substrates are active site. Substrates held in active site by weak interactions. Enzyme-substrate complex 5 Active site is available for new substrates. Enzyme 4 Products are released. 3 Substrates are Products converted to products. Effects of Local Conditions on Enzyme Activity • An enzyme’s activity can be affected by – General environmental factors, such as temperature and pH – Chemicals that specifically influence the enzyme Effects of Temperature and pH • Each enzyme has an optimal temperature in which it can function • Each enzyme has an optimal pH in which it can function • Optimal conditions favor the most active shape for the enzyme molecule Temperature • Above the optimal temperature= forces are too weak to maintain the enzymes shape -> denatures • Below optimum temperatures= hydrogen bonds and hydrophobic interactions that determine the enzymes shape is not flexible to allow induced fit. • Humans= 35 to 40 Celcius • Prokaryotes= 70 Celcius (hot springs) pH • Interactions are sensitive to the hydrogen ion concentration of the fluid in which the enzyme is dissolved, changing the concentration. Shifts the balance between +/amino acids • Optimum pH= 6 to 8 Optimal temperature for typical human enzyme (37C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77C) Rate of reaction Figure 8.17 0 20 40 60 80 Temperature (C) (a) Optimal temperature for two enzymes Rate of reaction Optimal pH for pepsin (stomach enzyme) 0 1 2 3 4 100 120 Optimal pH for trypsin (intestinal enzyme) 5 6 pH (b) Optimal pH for two enzymes 7 8 9 10 Enzyme Inhibitors- substance that binds to an enzyme and decreases its activity • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective. Binds at the allosteric site. • Examples of inhibitors include toxins, poisons, pesticides, and antibiotics Figure 8.18 (a) Normal binding Substrate Active site (b) Competitive inhibition (c) Noncompetitive inhibition Competitive inhibitor Enzyme Noncompetitive inhibitor • Allosteric enzymes- enzymes that exist as either active or inactive • Allosteric site- on /off chemical switches • Allosteric inhibitors- binds to the allosteric site and reduces enzyme activity • Allosteric Activator- keeps the enzyme active , increases enzyme activity Cofactors • Cofactors are non-protein enzyme helpers that are found around the active site to assist in catalysis. They help weaken bonds and make them easier to break. • Cofactors may be inorganic (such as a metal in ionic form) or organic • An organic cofactor is called a coenzyme • Coenzymes serve as an electron acceptor which then transfers the electrons to a different enzyme, which releases them to the substrates in another reaction (Ex- NADP) The Evolution of Enzymes • Enzymes are proteins encoded by genes • Changes (mutations) in genes lead to changes in amino acid composition of an enzyme • Altered amino acids in enzymes may result in novel enzyme activity or altered substrate specificity • Under new environmental conditions a novel form of an enzyme might be favored – For example, six amino acid changes improved substrate binding and breakdown in E. coli Regulation of enzyme activity helps control metabolism • Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated • A cell does this by switching on or off the genes that encode specific enzymes or by regulating the activity of enzymes Localization of Enzymes Within the Cell • Structures within the cell help bring order to metabolic pathways • Some enzymes act as structural components of membranes • In eukaryotic cells, some enzymes reside in specific organelles; for example, enzymes for cellular respiration are located in mitochondria Metabolism • Totality of an organism’s chemical reaction. Emergent property of life that arises from interactions between molecules within the orderly environment of the cell. • Catabolic pathways release energy by breaking down complex molecules into simpler compounds • Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism • Anabolic pathways consume energy to build complex molecules from simpler ones • The synthesis of protein from amino acids is an example of anabolism • Bioenergetics is the study of how energy flows through living organisms Biochemical Pathways • Organizational units of metabolism. The elements an organism needs / controls to achieve metabolic activity. • They think they came from early oceans, creating an organic soup. Feedback Inhibition • In feedback inhibition, the end product of a metabolic pathway shuts down the pathway • Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is needed Cells! • Cells are not in equilibrium; they are open systems experiencing a constant flow of materials • A defining feature of life is that metabolism is never at equilibrium • A catabolic pathway in a cell releases free energy in a series of reactions