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Inquiry into LifeEleventh Edition

Sylvia S. MaderChapter 6

Lecture Outline

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Cells and the flow of energy• Forms of energy

– Kinetic energy- energy of movement– Potential energy-stored energy

• Laws of thermodynamics– First law of thermodynamics

• Energy cannot be created or destroyed, but it can be changed from one form to another

– Second law of thermodynamics• Energy cannot be changed from one

form to another without a loss of usable energy

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Cells and the flow of energy cont’d.• Cells and entropy

Cell metabolism involves energy transformations• There is a loss of usable energy at each

step– All living organisms require an outside

source of energy that ultimately comes from the sun

– Entropy- relative amount of disorder or disorganization. Organization requires energy.

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• The term entropy is used to indicate the relative state of disorganization.

• Cells need a constant supply of acquired energy to maintain their internal organization, and to carry out reactions that allow cells to develop, grow, and reproduce.

• Complex molecules like glucose tend to break apart into their building blocks, in this case carbon dioxide and water.

• This is because glucose is more organized, and thus less stable, than its breakdown products.

• The result is a loss of potential energy and an increase in entropy.

Cells and Entropy

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Cells and entropy

• Fig 6.2

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6.2 Metabolic reactions and energy transformations

• Metabolism - All of the chemical reactions that occur in cells– Reactants are the substances that

participate in chemical reactions– Products- substances that are formed in a

chemical reaction

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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.•ΔG (change in free energy) is calculated by subtracting the free energy of reactants from that of products.

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• A negative ΔG means the products have less free energy than the reactants, and the reaction will occur spontaneously and release energy. These are called Exergonic reactions.

• A positive ΔG means the products have more free energy than the reactants, and the reaction needs energy added to proceed. These are called Endergonic reactions.

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Metabolic reactions and energy transformations cont’d.

• In Coupled Reactions energy released from exergonic reactions is used to drive endergonic reactions inside cells.

• ATP is the energy carrier between exergonic and endergonic reactions.

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The ATP cycle

• Fig 6.3

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Metabolic reactions and energy transformations cont’d.

• ATP- energy currency for cells– Adenosine triphosphate– Generated from ADP + P

• Avantages of ATP as energy carrier:– Provides a common energy currency for many

reactions– Breakdown of ATP to ADP+P releases sufficient

energy for biological processes- little wasted– ATP breakdown is coupled to endergonic reactions in

such a way that it minimizes energy loss

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Metabolic reactions and energy transformations cont’d.

• ATP structure– Modified nucleotide– Adenine, ribose, and 3 phosphates– “high energy” compound because the terminal

phosphate group is easily removed– Releases 7.3 kcal per mole

• Coupled reactions– Energy from exergonic breakdown of ATP is used

to drive endergonic reactions

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Coupled reactions

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Metabolic reactions and energy transformations cont’d.

• Function of ATP– Transport-supplies energy for active transport

mechanisms– Mechanical-supplies energy for muscle

contraction, cilia movement, chromosome migration, etc.

– Chemical-supplies energy for synthesis of macromolecules

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6.3 Metabolic pathways and enzymes

• Metabolic pathways– Series of linked reactions– Begin with a specific reactant and produce an end

product– More efficient means of capturing metabolic energy

than releasing it all in one step– Each step is catalyzed by a specific enzyme

• Enzymes– Protein molecule which functions as a catalyst to

speed up rate of chemical reaction– Reactants in an enzymatic reaction are called

substrates

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• 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• Here, the letters A-F are reactants or

substrates, B-G are the products in the various reactions, and E1-E6 are enzymes.

Metabolic reactions and energy transformations cont’d.

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Metabolic reactions and energy transformations cont’d.

• Energy of activation

–Energy which must be added to make molecules react

–Enzymes lower the energy of activation of specific reactions but do not change the Free Energy of the reaction.

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• Every reaction in a cell requires a specific enzyme.

• Enzymes are named for their substrates:Substrate EnzymeLipid LipaseUrea UreaseMaltose MaltaseRibonucleic acid Ribonuclease

Enzyme-Substrate Complexes

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Energy of activation

• Fig 6.5

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Metabolic reactions and energy transformations cont’d.

• How enzymes function– Enzyme binds with substrate to form an enzyme-substrate complex– E + S ES E + P– Binding occurs only at the active site which is part of

the tertiery structure of the enzyme– The enzyme is not changed by the reaction, and it is

free to act again.

• Induced fit model– Substrate and active site shapes don’t match exactly– Active site is induced to undergo a slight change in

shape to accommodate substrate binding

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Induced fit model

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Enzymatic action

• Fig 6.6

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Enzymatic reaction

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Enzymatic reaction

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Metabolic reactions and energy transformations cont’d.

Factors affecting speed of enzymatic reactions

• Enzyme concentration – more enzyme can combine with more available substrate – Limited by quantity of substrate available Cell regulates metabolism by regulating which enzymes are active

• Genes producing enzymes can be turned on or off to regulate enzyme concentration

• In some cases a signaling molecule is used to activate an enzyme

– Substrate concentration– Temperature– PH

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• 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.

• Maximum rate is achieved when all active sites of an enzyme are filled continuously with substrate

• Rate becomes limited by how quickly active sites become available

Factors Affecting Enzymatic Speed

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Metabolic reactions and energy transformations cont’d.

• Factors affecting enzymatic speed cont’d.

–Temperature• Activity increases as temperature increases which leads to increased enzyme-substrate collisions.

• High heat will denature an enzyme by altering it’s shape.

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Metabolic reactions and energy transformations cont’d.

• Factors affecting enzymatic rate cont’d.

–pH

• Each enzyme has an optimal pH• Enzyme structure is pH dependent• Extremes of pH can denature an exzyme by

altering its structure

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Effect of Temperature and pH on rate of reaction

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Metabolic reactions and energy transformations cont’d.

• Factors affecting enzymatic rate cont’d.Enzyme inhibition Occurs when enzyme cannot bind its substrate

• Activity of cell enzymes is regulated by feedback inhibition• Ex: when product is abundant it binds to the enzyme’s active site

and blocks further production• When product is used up, it is removed from the active site• In a more complex type of inhibition, product binds to a site other

than the active site, which changes the shape of the active site• Poisons are often enzyme inhibitors

– Enzyme cofactors-molecule which helps enzyme function. Copper and zinc are examples of inorganic cofactors

• Organic nonprotein cofactors are called coenzymes

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6.4 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.

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Oxidation-reduction and the flow of energy cont’d.

• Photosynthesis energy + 6CO2+6H2O C6H12O6 + 6O2

– Hydrogen atoms are transferred from water to carbon dioxide and glucose is formed

– Energy is required and this comes in the form of light energy from the sun

– Chloroplasts convert solar energy to ATP which is then used along with hydrogen to reduce carbon dioxide to glucose

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Oxidation-reduction and the flow of energy cont’d.

• Cell respiration

C6H12O6 + 6O2 6CO2 + 6H2O + energy

– Glucose is oxidized (lost hydrogen atoms)– Oxygen is reduced to form water– Complete oxidation of a mole of glucose

produces 686 kcal of energy– This energy is used to form ATP– The oxidation of glucose to form ATP is done in a

series of small steps to increase efficiency

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Oxidation-reduction and the flow of energy cont’d.

• Organelles and the flow of energy– Cycling of molecules between chloroplasts and

mitochondria allows energy to flow from sun to all living things

– Chloroplasts use light energy from the sun to make carbohydrates

– Mitochondria break down carbohydrates to form ATP

– Cellular respiration produces carbon dioxide and water which are used in photosynthesis