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.