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UNIT 5 – METABOLISM: ENERGY AND ENZYMES
ENERGY
! Energy – The ability to do work. ! Living organisms need to acquire energy for survival ! Cells require energy to:
! Maintain organization ! Carry out reactions ! Develop, Grow and Reproduce
FORMS OF ENERGY
! Kinetic Energy – energy of motion ! Potential Energy – stored energy ! Source of Potential Energy
! FOOD – called Chemical Energy
! Organisms convert chemical energy into a form of kinetic energy called Mechanical Energy.
ENERGY FLOW
! Is unidirectional in ecosystems (no cycling) ! Law of thermodynamics explains this: ! 1st Law: Energy cannot be created or destroyed,
but can be transformed. ! 2nd Law: Energy transformation results in a loss
of usable energy. ! Loss of usable energy is typically in the form of
heat.
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FLOW OF ENERGY
CELLS AND ENTROPY
! The term entropy is used to indicate the relative state of disorganization.
! Cells need a constant supply of energy to maintain their internal organization.
! Complex molecules tend to break apart into their building blocks. ! Ex: Glucose " Carbon Dioxide + Water ! Greater Organization = less stable
! The result is a loss of potential energy and an increase in entropy.
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CELLS AND ENTROPY
METABOLIC REACTIONS AND ENERGY TRANSFORMATIONS ! Metabolism is the sum of all the chemical
reactions that occur in a cell. ! Reactants are substances that participate in a
reaction; products are substances that form as a result of a reaction.
! A reaction will occur spontaneously if it increases entropy.
! Biologists use the term “free energy” instead of entropy for cells.
! Free energy, ∆G, is the amount of energy to do work after a reaction has occurred.
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! ∆G (change in free energy) is calculated by subtracting the free energy of reactants from that of products.
! A negative ∆G means the products have less free energy than the reactants, and the reaction will occur spontaneously.
! Exergonic reactions have a negative ∆G and energy is released. (EXIT)
! Endergonic reactions have a positive ∆G and occur only if there is an input of energy. (ENTER)
! Energy released from exergonic reactions is used to drive endergonic reactions inside cells.
! ATP is the energy carrier between exergonic and endergonic reactions.
ATP: ENERGY FOR CELLS
! ATP (adenosine triphosphate) is the energy currency of cells.
! ATP is constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell.
! Use of ATP by the cell has advantages: ! It can be used in many types of reactions. ! When ATP --> ADP + P, energy released is
sufficient for cellular needs and little energy is wasted.
! ATP is coupled to endergonic reactions in such a way that it minimizes energy loss.
THE ATP CYCLE
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COUPLED REACTIONS
! In coupled reactions, energy released by an exergonic reaction drives an endergonic reaction.
COUPLED REACTIONS
FUNCTION OF ATP
! Cells make use of ATP for:
1. Chemical work – ATP supplies energy to synthesize macromolecules, and therefore the organism
2. Transport work – ATP supplies energy needed to pump substances across the plasma membrane
3. Mechanical work – ATP supplies energy for cellular movements
METABOLIC PATHWAYS AND ENZYMES
! Cellular reactions are usually part of a metabolic pathway, a series of linked reactions, illustrated as follows:
E1 E2 E3 E4 E5 E6
A " B " C " D " E " F " G
! A-F are reactants or substrates ! B-G are the products in the various reactions ! E1-E6 are enzymes.
! An enzyme is a protein molecule that functions as an organic catalyst to speed a chemical reaction.
! An enzyme brings together specific molecules and causes them to react.
! The reactants in an enzymatic reaction are called the substrates for that enzyme.
ENERGY OF ACTIVATION
! The energy that must be added to cause molecules to react with one another is called the energy of activation (Eact).
! The addition of an enzyme does not change the free energy of the reaction, rather an enzyme lowers the energy of activation.
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ENERGY OF ACTIVATION (EA)
ENZYME-SUBSTRATE COMPLEXES
! Every reaction in a cell requires a specific enzyme.
! Enzymes are named for their substrates:
SUBSTRATE ENZYME
Lipid Lipase
Urea Urease
Maltose Maltase
Ribonucleic acid Ribonuclease
! Only one small part of an enzyme, called the active site, complexes with the substrate(s).
! The active site may undergo a slight change in shape, called induced fit, in order to accommodate the substrate(s).
! The enzyme and substrate form an enzyme-substrate complex during the reaction.
! The enzyme is not changed by the reaction, and it is free to act again.
ENZYMATIC REACTION
INDUCED FIT MODEL
FACTORS AFFECTING ENZYMATIC SPEED ! Enzymatic reactions proceed with great speed
provided there is enough substrate to fill active sites most of the time.
! Enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme.
TEMPERATURE AND PH
! As the temperature rises, enzyme activity increases because more collisions occur between enzyme and substrate.
! If the temperature is too high, enzyme activity levels out and then declines rapidly because the enzyme is denatured.
! Each enzyme has an optimal pH at which the rate of reaction is highest.
RATE OF AN ENZYMATIC REACTION AS A FUNCTION OF TEMPERATURE AND PH
! A cell regulates which enzymes are present or active at any one time.
! Genes must be turned on or off to regulate the quantity of enzyme present.
! Another way to control enzyme activity is to activate or deactivate the enzyme.
! Phosphorylation is one way to activate an enzyme.
ENZYME INHIBITION
! Enzyme inhibition occurs when an active enzyme is prevented from combining with its substrate.
! When the product of a metabolic pathway is in abundance, it binds competitively with the enzyme’s active site, a simple form of feedback inhibition.
! Other metabolic pathways are regulated by the end product binding to an allosteric site on the enzyme.
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FEEDBACK INHIBITION
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ENZYME COFACTORS
! Presence of enzyme cofactors may be necessary for some enzymes to carry out their functions.
! Inorganic metal ions, such as copper, zinc, or iron function as cofactors for certain enzymes.
! Organic molecules, termed coenzymes, must be present for other enzymes to function.
! Some coenzymes are vitamins.
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OXIDATION-REDUCTION AND THE FLOW OF ENERGY
! Oxidation is the loss of electrons and reduction is the gain of electrons.
! Because oxidation and reduction occur simultaneously in a reaction, such a reaction is called a redox reaction.
! Oxidation also refers to the loss of hydrogen atoms, and reduction refers to the gain of hydrogen atoms in covalent reactions in cells.
! These types of oxidation-reduction, or redox, reactions are exemplified by the overall reactions of photosynthesis and cellular respiration.
! The two pathways of photosynthesis and cellular respiration permit the flow of energy from the sun though all living things.
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PHOTOSYNTHESIS
! The overall reaction for photosynthesis can be written:
6CO2 + 6H2O + energy ---> C6H12O6 + 6O2 ! During photosynthesis, hydrogen atoms are
transferred from water to carbon dioxide, and glucose is formed.
! Water has been oxidized; carbon dioxide has been reduced.
! Energy to form glucose comes from the sun.
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CELLULAR RESPIRATION
! The overall equation for cellular respiration is opposite that of photosynthesis:
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy ! In this reaction, glucose is oxidized and oxygen
is reduced to become water. ! The complete oxidation of a mol of glucose
releases 686 kcal of energy that is used to synthesize ATP.
GLYCOLYSIS
Citric Acid Cycle
ELECTRON TRANSPORT CHAIN
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ORGANELLES AND THE FLOW OF ENERGY
! During photosynthesis, chloroplasts capture solar energy and use it to convert water and carbon dioxide into carbohydrates that provide food for other living things.
! Cellular respiration, the breakdown of glucose into carbon dioxide and water, occurs in mitochondria.
! It is the cycling of molecules between chloroplasts and mitochondria that allows a flow of energy from the sun through all living things.
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RELATIONSHIP OF CHLOROPLASTS TO MITOCHONDRIA
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CHAPTER SUMMARY
! Two laws of thermodynamics state that energy cannot be created or destroyed, and energy transformations result in a loss of energy, usually as heat.
! As a result of these laws, we know the entropy of the universe is ever increasing, and that it takes energy to maintain the organization of living things.
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! Metabolism refers to all the chemical reactions in the cell.
! Only reactions with a negative free energy occur spontaneously.
! Endergonic reactions are thus coupled with exergonic reactions.
! Energy is stored in cells in ATP molecules. ! Metabolic pathways are a series of enzyme-catalyzed
reactions.
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! Each reaction requires a specific enzyme. ! Substrate concentration, temperature, pH, and
enzyme concentration affect the rates of reactions. ! Most metabolic pathways are regulated by feedback
inhibition. ! Photosynthesis and cellular respiration involve
oxidation-reduction reactions and account for the flow of energy through all living things.