Chapter 7Chapter 7

How Cells Release How Cells Release Chemical energyChemical energy

Overview of Overview of Carbohydrate Breakdown PathwayCarbohydrate Breakdown Pathway

Plants and all other photoautotrophs get Plants and all other photoautotrophs get energy from the sun, heterotrophs get by energy from the sun, heterotrophs get by eating plants and one anothereating plants and one another

ATP is a common energy currency that ATP is a common energy currency that drives metabolic reactions in cellsdrives metabolic reactions in cells

Pathways of Carbohydrate Pathways of Carbohydrate BreakdownBreakdown

Starts withStarts with glycolysis in the cytoplasmglycolysis in the cytoplasm–– Convert glucose and other sugars to pyruvateConvert glucose and other sugars to pyruvate

Comparison of main pathwaysComparison of main pathways

Fermentation Fermentation pathwaypathway

Aerobic Aerobic RespirationRespiration

Produces ATP under Produces ATP under anaerobic conditionsanaerobic conditions

Ends in cytoplasm, do Ends in cytoplasm, do not use oxygen, yields 2 not use oxygen, yields 2 ATP per molecule of ATP per molecule of glucoseglucose

Produces ATP by Produces ATP by using oxygenusing oxygen

Ends in mitochondria, Ends in mitochondria, uses oxygen, yields up uses oxygen, yields up to 36 ATP per glucose to 36 ATP per glucose moleculemolecule

Oxygen acts as the Oxygen acts as the final acceptor of final acceptor of electrons used during electrons used during these reactionsthese reactions

Overview of Aerobic RespirationOverview of Aerobic Respiration

Three main stages of aerobic respiration:Three main stages of aerobic respiration:1. Glycolysis1. Glycolysis

2. Krebs cycle2. Krebs cycle

3. Electron transfer phosphorylation3. Electron transfer phosphorylation

Summary equation:Summary equation:

CC66HH1212OO66 + 6O+ 6O22 →→ 6CO6CO22 + 6 H+ 6 H22OOglucose

Cytoplasm

GLYCOLYSIS

KrebsCycle ATP

ATP

6CO2

4 ATP(2net)

32 ATP

2 NADH

8 NADH, 2 FADH2

2 pyruvate

oxygen

2 ATP

glucose

ATPATP

2 ATP

Electron TransferPhosphorylation

Mitochondrion

The first stage, glycolysis, occurs in the cell’s cytoplasm. Enzymes convert a glucose molecule to 2 pyruvate for a net yield of 2 ATP. During the reactions, 2 NAD+ pick up electrons and hydrogen atoms, so 2 NADH form.

The second stage, the Krebs cycle and a few steps before it, occurs inside mitochondria. The 2 pyruvates are broken down to CO2, which leaves the cell. During the reactions, 8 NAD+ and 2 FAD pick up electrons and hydrogen atoms, so 8 NADH and 2 FADH2 form. 2 ATP also form.

The third and final stage, electron transfer phosphorylation, occurs inside mitochondria. 10 NADH and 2 FADH2 donate electrons and hydrogen ions at electron transfer chains.Electron flow through the chains sets up H+ gradients that drive ATP formation. Oxygen

Overview of aerobic respirationOverview of aerobic respiration

Glycolysis Glycolysis ––Glucose Breakdown StartsGlucose Breakdown Starts

First step First step –– Conversion of glucose to Conversion of glucose to pyruvatepyruvateEnzymes of glycolysis use two ATP to Enzymes of glycolysis use two ATP to convert one molecule of glucose to two convert one molecule of glucose to two molecules of threemolecules of three--carbon pyruvate carbon pyruvate

Reactions transfer electrons and hydrogen Reactions transfer electrons and hydrogen atoms to two NADatoms to two NAD++ (reduces to NADH) (reduces to NADH)

2 ATP is formed by substrate2 ATP is formed by substrate--level level phosphorylationphosphorylation

Products of GlycolysisProducts of Glycolysis

Net yield of glycolysis:Net yield of glycolysis:–– 2 pyruvate, 2 ATP, and 2 NADH per glucose2 pyruvate, 2 ATP, and 2 NADH per glucose

Pyruvate may: Pyruvate may: –– Enter fermentation pathways in cytoplasm Enter fermentation pathways in cytoplasm

–– Enter mitochondria and be broken down Enter mitochondria and be broken down further in aerobic respirationfurther in aerobic respiration

ATP

ATP

glucose

ADP

ADP

P

Pglucose–6–phosphate

fructose–1,6–bisphosphate

Glycolysis

ATP Requiring Steps

p

ATP

2 ADP

2 NAD+ + 2 Pi

2 PGA

NADH

2 PGAL

ATP

2 pyruvate

2 PEP

2 ADP

to second stage Net 2 ATP + 2 NADH

2 ATP producedby substrate-levelphosphorylation

2 ATP producedby substrate-levelphosphorylation

2 reduced coenzymes

ATP Generating steps

Second Stage of Aerobic Second Stage of Aerobic RespirationRespiration

Krebs CycleKrebs CycleBreak down of pyruvate to Carbon dioxideBreak down of pyruvate to Carbon dioxide

The second stage of aerobic respiration The second stage of aerobic respiration takes place in the inner compartment of takes place in the inner compartment of mitochondria mitochondria

It starts with acetylIt starts with acetyl--CoA formation and CoA formation and proceeds through the Krebs cycleproceeds through the Krebs cycle

outer mitochondrialcompartment (inbetween the twomembranes)

inner mitochondrialcompartment

inner membrane

outer membrane(next to cytoplasm)

KrebsCycle

2 acetyl–CoA Breakdown of 2 pyruvate to 6CO2 yields 2 ATP. Also, 10 coenzymes are reduced (8 NADH, 2 FADH2). The coenzymes carry hydrogenions and electrons to sites of the third stage of aerobic respiration.

INNER COMPARTMENT

OUTER COMPARTMENT

2 pyruvate(glycolysis)

glucose

b The second stage starts after membrane proteins transport pyruvate from the cytoplasm, across both mitochondrial membranes, to the innercompartment. Six carbon atoms enter these reactions (in two pyruvate), and six leave (in six CO2). Many coenzymes form.

a An inner membrane divides a mitochondrion’s interior into two compartments. The second and third stages of aerobic respiration take place at this membrane.

FADH2

NADH

ATPCO2

AcetylAcetyl--CoA FormationCoA Formation

Two pyruvates from glycolysis are Two pyruvates from glycolysis are converted to two acetylconverted to two acetyl--CoA CoA

Two COTwo CO22 leave the cellleave the cell

AcetylAcetyl--CoA enters the Krebs cycle CoA enters the Krebs cycle

Krebs CycleKrebs Cycle

In each turn of the Krebs cycle, one acetylIn each turn of the Krebs cycle, one acetyl--CoA is converted to two molecules of COCoA is converted to two molecules of CO22

After two cyclesAfter two cycles–– Two pyruvates are dismantledTwo pyruvates are dismantled

–– Glucose molecule that entered glycolysis is Glucose molecule that entered glycolysis is fully broken downfully broken down

Energy ProductsEnergy Products

Reactions transfer electrons and hydrogen Reactions transfer electrons and hydrogen atoms to NADatoms to NAD++ and FADand FAD–– Reduced to NADH and FADHReduced to NADH and FADH22

ATP forms by substrateATP forms by substrate--level level phosphorylationphosphorylation–– Direct transfer of a phosphate group from a Direct transfer of a phosphate group from a

reaction intermediate to ADPreaction intermediate to ADP

Net ResultsNet Results

Second stage of aerobic respiration results inSecond stage of aerobic respiration results in

Six COSix CO22, two ATP, eight NADH, and two FADH, two ATP, eight NADH, and two FADH22

for every two pyruvates for every two pyruvates

Adding the yield from glycolysis, the total isAdding the yield from glycolysis, the total is

–– Twelve reduced coenzymes and four ATP for Twelve reduced coenzymes and four ATP for each glucose moleculeeach glucose molecule

Coenzymes deliver electrons and hydrogen to Coenzymes deliver electrons and hydrogen to the third stage of reactionthe third stage of reaction

NADH

NADH

FADH2

ATP

KrebsCycle

KrebsCycle

NADH

NADH

coenzyme A

coenzyme A

acetyl–CoA

oxaloacetate citrate

pyruvate

Acetyl–CoAFormation

CO2

CO2NAD+

NAD+

NAD+

FAD

NAD+

CO2

ADP + P i

Third Stage: Aerobic RespirationThird Stage: Aerobic Respiration’’s s Big Energy PayoffBig Energy Payoff

Coenzymes deliver electrons and hydrogen ions Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner to electron transfer chains in the inner mitochondrial membrane mitochondrial membrane Energy released by electrons flowing through Energy released by electrons flowing through the transfer chains moves Hthe transfer chains moves H++ from the inner to from the inner to the outer compartmentthe outer compartmentHH++ gradient builds up across the inner gradient builds up across the inner membranemembraneHH++ ions flow by concentration gradient back to ions flow by concentration gradient back to the inner compartment through ATP synthasesthe inner compartment through ATP synthases

The Aerobic Part of Aerobic The Aerobic Part of Aerobic RespirationRespiration

At the end of electron transfer chain At the end of electron transfer chain oxygen and Hoxygen and H++ , forming water, forming water

Overall, aerobic respiration yields up to 36 Overall, aerobic respiration yields up to 36 ATP for each glucose moleculeATP for each glucose molecule

KrebsCycle

Glycolysis

glucose

youarehere

Electron TransferPhosphorylation

NADHFADH2

ATPADP + P i

H+

H+H+

H+H+H+

H+H+H+

H+ H+ H+

INNERMITOCHONDRIAL

MEMBRANE

OUTERCOMPARTMENT

INNERCOMPARTMENT

H2O

1/2 O2

H+

glucose

Glycolysis

2 pyruvate

KrebsCycle

Electron TransferPhosphorylation

(2 net)

2 NADH

ATP

2 FADH2

6 NADH

2 acetyl-CoA

32

ADP + P i

H+

ATP

2 NADH

2 NADH

ATP

ATP

H+ H+ H+ H+

2 NAD+

2 CO2

4 CO2

2

oxygen

INNER MITOCHONDRIALCOMPARTMENT

OUTER MITOCHONDRIALCOMPARTMENT

CYTOPLASM

water

2

Anaerobic Anaerobic EnergyEnergy--Releasing PathwaysReleasing Pathways

Fermentation PathwayFermentation PathwayBegins with glycolysis and ends in the Begins with glycolysis and ends in the cytoplasmcytoplasmDo not use oxygen or electron transfer Do not use oxygen or electron transfer chainschainsTwo ATP is formed from glycolysisTwo ATP is formed from glycolysisFinal steps do not produce ATP; only Final steps do not produce ATP; only regenerate oxidized NADregenerate oxidized NAD++ required for required for glycolysis to continueglycolysis to continue

FermentationPathway

Glycolysis

glucose

you arehere

Anaerobic PathwaysAnaerobic Pathways

Alcoholic fermentationAlcoholic fermentation

End product:End product: Ethyl alcohol (or ethanol)Ethyl alcohol (or ethanol)

Pyruvate(3 carbon) is broken downPyruvate(3 carbon) is broken down

Enzyme splits pyruvate into two Enzyme splits pyruvate into two -- carbon carbon acetaldehyde and carbon dioxideacetaldehyde and carbon dioxide

Alcoholic fermentationAlcoholic fermentation

Role of Yeast in fermentationRole of Yeast in fermentation

They are unicellular fungiThey are unicellular fungi

Sacchromyces cerevisiae Sacchromyces cerevisiae (Baker(Baker’’s yeast)s yeast)

Cells release carbon dioxide in Cells release carbon dioxide in fementation and dough expandsfementation and dough expands

Other strains are used in the production of Other strains are used in the production of winewine

Alcoholic FermentationAlcoholic Fermentation

Anaerobic PathwaysAnaerobic Pathways

Lactate fermentationLactate fermentation

End product:End product: LactateLactate

Conversion of pyruvate into three carbon Conversion of pyruvate into three carbon lactate (lactic acid)lactate (lactic acid)

Fermenters such as Fermenters such as lactobacillus lactobacillus acidophillusacidophillus can ferment milk, butter milk, can ferment milk, butter milk, cheese, yogurtcheese, yogurt

glucoseGlycolysis

2

pyruvate

NADH

2 NAD+

2ATP

ATP4

NADH

2 NAD+

2

LactateFermentation

lactate

The TwitchersThe Twitchers

SlowSlow--twitch and fasttwitch and fast--twitch skeletal muscle twitch skeletal muscle fibers can support different activity levels fibers can support different activity levels

Aerobic respiration and lactate Aerobic respiration and lactate fermentation proceed in different fibers of fermentation proceed in different fibers of musclesmuscles

These pathways yeilds ATP for musclesThese pathways yeilds ATP for muscles

Alternative Energy SourcesAlternative Energy Sources

Complex Carbohydrate break downComplex Carbohydrate break downThey are broken down into simple sugars like They are broken down into simple sugars like glucoseglucoseGlucose gets converted to glucoseGlucose gets converted to glucose--66--phosphate.phosphate.If the body doesnIf the body doesn’’t need glucose for t need glucose for energy,glucoseenergy,glucose--66--phosphate can be converted phosphate can be converted to glycogen for storageto glycogen for storageWhen blood sugar drops, glycogen is converted When blood sugar drops, glycogen is converted to glucoseto glucose--66--phosphate and and enters the phosphate and and enters the glycolysis pathwayglycolysis pathway

Alternative Energy Sources;Alternative Energy Sources;Energy from FatsEnergy from Fats

Most fat in the body are triglyceridesMost fat in the body are triglyceridesEnzymes convert fat into glycerol and fatty Enzymes convert fat into glycerol and fatty acidacidGlycerol is converted into PGAL, an Glycerol is converted into PGAL, an intermediate of glycolysisintermediate of glycolysisThe carbon back bones of the fatty acid The carbon back bones of the fatty acid tail is broken apart, and fragments are tail is broken apart, and fragments are converted into acetyl CoA, which can enter converted into acetyl CoA, which can enter the Krebs cyclethe Krebs cycle

Energy from ProteinsEnergy from Proteins

Enzymes split proteins into amino acid Enzymes split proteins into amino acid subunitssubunits

The amino group is removed and The amino group is removed and becomes ammonia, then ureabecomes ammonia, then urea

Urea is excretedUrea is excreted

Carbon Back bones can enter at several Carbon Back bones can enter at several different points of Krebs cycledifferent points of Krebs cycle

FOOD

fats COMPLEX CARBOHYDRATES PROTEINS

glucose, other simple sugars amino acids

Glycolysis

glycerolfatty acids

pyruvate

acetyl-coA

NADH

KrebsCycle

NADH, FADH2

PGALacetyl-coA

oxaloacetateor anotherintermediateof the Krebs

Electron TransferPhosphorylation

LifeLife’’s Unitys Unity

Photosynthesis and aerobic respiration are Photosynthesis and aerobic respiration are interconnected on a global scale interconnected on a global scale

In its organization, diversity, and continuity In its organization, diversity, and continuity through generations, life shows unity at through generations, life shows unity at the bioenergetic and molecular levelsthe bioenergetic and molecular levels

Driven by energy input from the sun, electrons and hydrogen are used to form ATP. ATP energy drives the synthesis of glucose from hydrogen, electrons (delivered by coenzymes), and carbon dioxide’s atoms.

Photosynthesis Aerobic Respiration

Energy input from two ATPinitiates three stages ofreactions. Many ATP form during thecomplete breakdown ofglucose to carbon dioxideand water.

energy out (heat)

chemical energy in many ATP available to drive nearly all cellular tasks

carbondioxide,water

glucose(storedchemicalenergy)

oxygen

energy out (heat)

sunlightenergy in