ap bio unit 3 2

AP BioAP BioUnit 3:Unit 3:Cellular EnergeticsCellular Energetics

Big Ideas: ENE, SYIScience Practices: 1, 3, 4, 6

EnzymesEnzymesBiological catalysts◦ Speed up a chemical reaction without the catalyst

being altered◦ The catalyst does not cause the reaction to occur, it

only speeds it upMost enzymes are proteins, but a few are RNA molecules called ribozymesThe enzyme binds the reactants in a chemical reaction, but this participation does not permanently change the enzyme◦ At the end of the reaction, the enzyme is unchanged

and available to catalyze other reactions

Properties of EnzymesProperties of EnzymesFolded chains of amino acids with a specific shapeThis shape is determined by the sequence of amino acids held together by Hydrogen bondsEnzymes are specific - they have a specific shape ◦ only a certain substrate will fit its active site

the part of the enzyme where the substrate attaches

Properties of EnzymesProperties of Enzymes

Properties of EnzymesProperties of EnzymesTheories of enzyme action◦ Lock and Key

The lock and key theory states that only a certain substrate will fit a certain active site, just like a key fits a lock

◦ Induced Fit Induced fit, states that enzymes wrap around substrates, attracted to each other by opposite charges, forming an enzyme substrate complex.

Properties of EnzymesProperties of Enzymes

CatalysisCatalysis

Enzymes neither initiate the reaction nor affect the equilibrium ratio of reactants and productsRather, enzymes accelerate the rate of reaction 108 to 1012 times in both directions to attain the equilibrium position◦ Rate of forward and reverse reactions are

equal

CatalysisCatalysis

The kinetic or collision theory states that for molecules to react they must collide and must possess sufficient energy to overcome the energy barrier for reactionThe minimum amount of free energy required to overcome the energy barrier, so that substrates transform into the transitional state, is called activation energy

CatalysisCatalysis

Exergonic reaction – releases energy◦ Free energy (G) – the energy that is available

to do work

Endergonic reaction – absorbs energy◦ May be driven by the free energy released by

an exergonic reaction◦ Activation energy (Ea) – the energy needed to

start a reactionReduced by enzymes

CatalysisCatalysisActivation energy increases kinetic energy of substrates and brings about the forceful collisions between Enzyme (E) and substrates (S)The difference in energy level between the substrate and product is called the change in Gibbs free energy (G)◦ Negative ∆G – exergonic

Releases energy◦ Positive ∆G – endergonic is positive

requires input of energy

CatalysisCatalysis

Factors that Affect Enzyme FunctionFactors that Affect Enzyme Function

Temperature◦ Usually the activity of the enzymes is optimum at

normal body temperature ◦ At very low temperature (0°C) the activity of the

enzymes is minimum◦ An increase in temperature up to a certain limit

increases the enzyme activity, maximum being at about 45°C after which the enzyme activity is retarded

Beyond 60°-70°C usually their activity is permanently stopped due to the denaturation (change in the sequence of amino acids, which changes the shape) of enzymes.


pH ◦ Enzymes are active only over a limited range of

pHSome enzymes are active in high pH Some enzymes are active in low pH

Water◦ In absence of water the enzyme activity is

suppressed so much so the enzymes are almost inactive◦ Proper hydration of the cells is necessary for

enzyme activity because water provides medium for enzyme reaction to take placein many cases it is one of the reactants.


Substrate Concentration◦ Increase in the concentration of the substrate brings

about an increase in the activity of the enzyme till all the active sites of the enzyme molecules are saturated with substrate

After this the rate of enzyme reaction becomes steady and addition of the substrate will not have positive effect

Enzyme Concentration◦ Usually a very small amount of the enzyme can

consume large amount of the substrate◦ An increase in the concentration of the enzyme will

increase the rate of reaction catalyzed by it provided there is enough concentration of substrate

Factors that Affect Enzyme FunctionFactors that Affect Enzyme FunctionInhibitors◦ Presence of inhibitors in the reaction mixture

inhibits (slows down) the activity of the enzymes partially or completely depending upon the nature of the inhibitors◦ Inhibitors are less effective when the

concentration of the enzyme and substrate is higher◦ Types

Competitive InhibitorsSuch inhibitors have structural similarity with the substrate both of which compete for the same active site of enzyme


If a competitive inhibitor pre-occupies the active site, the substrate molecule will be unable to combine with the enzyme and hence, the enzyme activity will be inhibited

removal of the competitive inhibitor restores the activity of the enzyme.

Non-Competitive InhibitorsThese are usually poisons which do not compete for the active sites, but destroy the structure of the enzyme and cause permanent or irreversible inhibition of the activity of the enzyme


Accumulation of End Products◦ Accumulation of the end-products retards the

enzymic activity mainly because the active sites of the enzymes are crowded by them and substrate molecules will have less of a chance of combining with the active sites

DenaturationDenaturationChange in the shape of an enzyme due to a change in the sequence of amino acids◦ They are no longer active and cannot functionDue to◦ Extreme temperature◦ Incorrect pH

Usually a very acidic pH

Denaturation changes the shape, therefore changing the active site◦ The substrate will not be able to attach to the

active site

DenaturationDenaturation

Cellular EnergyCellular Energy

Cells can obtain energy in different ways◦ Plants – sunlight◦ Animals – food that we eat

These processes provide the cell with glucose, which is one of the raw materials for cellular respiration◦ Makes ATP (adenosine triphosphate) –

molecule used to store energy

Cellular EnergyCellular Energy

The energy made is used for:◦All metabolic activities

MovementCell divisionReproductionProtein synthesisPhotosynthesis

PhotosynthesisPhotosynthesisAutotrophs ◦ Organisms that are able to harvest the energy of

sunlight along with carbon dioxide (CO2) and water (H2O) to produce glucose (C6H12O6) and oxygen gas (O2)

6CO2 + 6H2O → C6H12O6 + 6O2Endergonic reactionChloroplasts2 pathways◦ Light reactions◦ Carbon-fixation reactions

Light ReactionsLight ReactionsConvert light energy into chemical energy in the form of ATP and the electron carrier NADPHLight is absorbed by the pigment chlorophyll that is contained in the thylakoid membranes (diagram unit 2)◦ When light is absorbed by chlorophyll, the

chlorophyll enters an excited stateElectrons jump up to a higher energy level

◦ Chlorophyll quickly returns to its ground state, releasing most of the absorbed energy

Happens in picoseconds (trillionths of a second)◦ The released energy is passed along chlorophyll

molecules until it reaches the final chlorophyll molecule (Chl)

Light ReactionsLight ReactionsThis time, when the chlorophyll becomes excited, it does not release the absorbed energy

This is where the absorbed light energy is changed into chemical energy

The final chlorophyll gives up its excited electron to an electron acceptor

The chlorophyll is oxidized (loses an electron) and the receptor molecule is reduced (gains an electron)

Chl+

The electrons are then passed along a series of acceptors until it gets to the final acceptor, NADH+, which gets reduced

NADH+ + H+ +2e- → NADPHATP is also produced during the process of electron transport

Light ReactionsLight Reactions

◦ The Chl+ is a strong oxidizing agentIt wants to gain the electron that it lostThe electron comes from splitting the bonds of waterSplits H2O into hydrogen and oxygenThe oxygen is released as O2

Light ReactionsLight Reactions

Dark ReactionsDark Reactions

NADPH and ATP is used to change CO2into carbohydratesOccurs in the stroma of the chloroplastCalled the Calvin CycleThree processes:◦ Fixation ◦ Reduction◦ Regeneration


Fixation◦ CO2 is added to an acceptor molecule,

ribulose 1,5-biphosphate (RuBP)◦ Forms a six-carbon molecule which quickly

breaks down into 2 3-carbon molecules , 3-phosphoglycerate (3PG)

Catalyzed by the enzyme rubiscoWorks very slowly, so a very large quantity is needed for enough photosynthesis to occur

Makes up about half of all the proteins in a leaf

Dark ReactionsDark ReactionsThe extra G3P can be used in 2 ways◦ Leaves the chloroplast and is changed to

glucoseStarting material for cellular respiration to produce energyUsed to make amino acids and other moleculesConverted to sucrose for use in other parts of the plant

◦ Forms starch for use by the plant when it is dark

Provides a supply of glucose to fuel cellular activities

Cellular RespirationCellular RespirationHarvesting the energy stored in the bonds of carbohydrates obtained from foodC6H12O6 + 6O2 → + 6CO2 + 6 H2O + 36 ATP◦ 686 kcal/mol of glucose

234 kcal/mol stored as ATP and the rest is lost as heat

Three pathways◦ Glycolysis◦ Pyruvate oxidation◦ Citric acid cycle

GlycolysisGlycolysisOccurs in the cytosol (aqueous portion of the cytoplasm)Requires 2 ATP to begin10 enzyme-catalyzed reactions◦ Some C-H bonds of glucose are broken releasing

some stored energyFinal products:◦ 2 molecules of pyruvate◦ 2 molecules of ATP◦ 2 molecules of NADHIn the presence of O2, further oxidation can occur◦ Eukaryotes – takes place in the mitochondrial matrix

(diagram Unit 2)

GlycolysisGlycolysis

PyruvatePyruvate OxidationOxidation

Pyruvate is oxidized to a 2-carbon acetate molecule and CO2

The acetate is then bound to coenzyme A (CoA) to form Acetyl CoAExergonic - releases NADHAcetyl CoA starts the citric acid cycle

PyruvatePyruvate OxidationOxidation

Citric Acid CycleCitric Acid CycleAlso known as the Kreb’s CyclePathway of 8 reactions that completely oxidizes the acetyl group to 2 molecules of CO2◦ The CoA is released once the acetal group

enters the cycle2 turns of the cycle are completed for each glucose (1 for each pyruvate)At the end of the cycle, oxaloacetate is regenerated and is ready to accept another acetate groupProduces 2 ATP

Citric Acid CycleCitric Acid Cycle

Electron Transport Chain Electron Transport Chain The electron transport chain produces the ATP◦ The electrons form the oxidation of

NADH and FADH2 pass from one carrier to the next◦ Driven by the H+ gradient◦ The final electron acceptor is O2

◦ ATP synthase changes ADP into ATP◦ Produces 32 ATP

Electron Transport Chain Electron Transport Chain

Cellular RespirationCellular Respiration

Anaerobic RespirationAnaerobic Respiration

Absence of oxygenAlso known as fermentationRespiration stops with glycolysisOccurs in the cytosolOnly produces 2 ATP per glucoseTwo types:◦ Lactic acid fermentation◦ Alcoholic fermentation

Lactic Acid FermentationLactic Acid Fermentation

Pyruvate is the electron acceptorLactate is the productOccurs in muscle tissues of vertebrates when not enough O2 is available◦ Provides short bursts of energy, but also creates lactate, which makes the muscles sore and achy

Alcoholic FermentationAlcoholic Fermentation

Yeast cells and some plants when O2 is not availablePyruvate is converted into ethanol

Cell FitnessCell Fitness

The interaction between the molecules absorbed and produced by cells allows them to survive and to complete their required functions◦ Maintain homeostasis

If the levels of these molecules are not correct, the cell will not be able to function as needed

ap bio unit 3 2

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