Cellular Respiration - Principle Biology - Lecture Notes | BI 107, Study notes of Biology

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BI107 CHAP 9A^ Cellular Respiration

3 Stages of Cellular Respiration •^ 3 metabolic stages tocellular respiration –1) glycolysis 2) Krebscycle 3) electrontransport •^ Overview – 1

st^2

stages, glycolysis &Krebs cycle, arecatabolic pathwaysdecomposing glucose& other organic fuels

Overview cont.

-^ Glycolysis – occurs incytoplasm & breaksglucose into 2 moleculesof pyruvic acid •^ Krebs cycle – occurs inmitochondrial matrix &breaks down a derivativeof pyruvic acid to carbondioxide – so carbondioxide producedrepresents fragments ofoxidized organicmolecules

Overview cont.

-^ Some steps ofglycolysis & Krebscycle are redoxreactions wheredehydrogenaseenzymes transferelectrons fromsubstrates to NAD

+^ to

form NADH

Overview cont.

-^ Electron transport chainaccepts electrons frombreakdown products of 1

st^2

stages (via NADH) & passeselectrons from molecule tomolecule until at end of chainelectrons combine with oxygento form water • Energy released at each stepstored in a form mitochondriacan use to make ATP &process called oxidativephosphorylation since poweredby redox reactions that transferelectrons from food to oxygen

Overview cont.

-^ Electron transport &oxidative phosphorylationoccur in innermitochondrial membrane& produces 90% of ATPgenerated while 10% ofATP made by substratelevel phosphorylationwhere an enzymetransfers a phosphategroup from substratemolecule to ADP to formATP

Glycolysis

-^ 6 carbon glucosemolecule split into 23-carbon sugarswhich are oxidized &molecules rearrangedto form 2 molecules ofpyruvate (ionizedform of pyruvic acid) •^ 10 steps eachcatalyzed by aspecific enzyme

Glycolysis cont.

-^ Divided into 2 phases– energy investmentphase & energypayoff phase •^

energy investmentphase contains 1

st^5

steps – cell uses ATPto phosphorylate foodmolecules

Glycolysis cont.

-^ During energy payoffphase ATP producedby substrate levelphosphorylation &NAD+ reduced toNADH by oxidation offood with net energyyield per glucosemolecule – 2 ATP & 2NADH

Glycolysis cont.

-^ All carbons originally inglucose now in pyruvicacid & no CO

released 2

during glycolysis & no O

2

used so glycolysis occurswhether oxygen is or isnot present but if it ispresent then energystored in NADH can beconverted to ATP byelectron transport chain &oxidative phosphorylation& chemical energy inpyruvic acid extracted byKrebs cycle

Glycolysis cont.

-^ Glucose enters & isphosphorylated to

G-6-P

by hexokinase • G-6-P rearranged toisomer fructose-6-P byphosphogluco –isomerase • Fructose-6-P gets a 2

nd

phosphate from ATP dueto activity ofphosphofructokinaseforming fructose-1,6bisphosphate

Glycolysis cont.

-^ Fructose 1,6bisphosphate iscleaved into 2different 3-carbonsugars: G-3-P anddihydroacetone – Pand an isomeraseconverts reversibleinterchange

Glycolysis cont.

-^ In 1

st^ step of energy payoff phase G-3-Premoved by next enzyme,triose phosphatedehydrogenase forming1,3 bisphosphoglyceratewhich is acted upon byphosphoglycerokinase toform 3-Phosphoglycerategenerating ATP in theprocess

Glycolysis cont.

-^ Phosphate grouprearranged to form 2-phosphoglyceratewhich is acted on byenolase to formphosphoenolpyruvatewhich is acted on bypyruvate kinase toform pyruvate withformation of moreATP

Glycolysis cont.

-^ Kinase enzymes areinvolved in ATPformation or use •^ Dehydrogenases areused to removehydrogen ions andform NADH fromNAD+

Krebs Cycle

-^ If molecular oxygen ispresent the pyruvate,still containing most ofthe potential energy inthe glucose molecule,enters mitochondriawhere enzymes ofKrebs cycle completethe oxidation

Krebs cycle cont.

-^ Pyruvate 1

st^ converted to

acetyl coenzyme A oracetyl- CoA • Takes 3 reactions using amultienzyme complex

1)

pyruvate’s carboxyl groupis fully oxidized & isremoved as CO

. 2) 2 2

carbon fragment oxidizedto form acetate whileNADH is formed

Krebs cycle cont.

-^ 3) coenzyme A, asulfur containingderivative of a Bvitamin, is attaxhed toacetate making acetylgroup very reactive &acetyl-CoA entersKrebs cycle

Krebs cycle cont.

-^ Cycle has 8 steps eachcatalyzed by a specificenzyme •^ For each turn of cycle 2carbons enter as acetate& 2 different carbons exitas CO

2

-^ Acetyl CoA 1

st^ joined to

oxaloacetate to formcitrate then rest of cycleis used to reformoxaloacetate

Krebs cycle cont.

-^ For each acetate thatenters the cycle, 3NAD+ are reduced toNADH & in 1 stepelectrons aretransferred to FAD toform FADH2 – asimilar electron carrier •^ ATP is formed fromGTP by substratelevel phosphorylation

Electron transport

-^ Only 4 molecules of ATPproduced from glycolysisand the Krebs cycle withmost of energy extractedfrom food in the form ofhigh energy electroncarriers NADH andFADH2 which linkglycolysis & Krebs cycleto the machinery foroxidative phosphorylation

Electron transport – oxidative

phosphorylation

-^ Electron transport is acollection ofmolecules embeddedin the innermitochondrialmembrane withfolding of thismembrane providingspace for thousandsof copies of the chain

Electron transport – oxidative

phosphorylation cont.

-^ Most components areproteins which havetightly bound prostheticgroups – nonproteincomponents essential forcatalytic function •^ During electron transportthe prosthetic groups arereduced then oxidized asthey accept then donateelectrons

Electron transport – oxidative

phosphorylation cont.

-^ Electrons from NADHtransferred to 1

st

molecule a flavoproteinwith a flavinmononucleotideprosthetic group • Flavoprotein is oxidizedas it passes electrons tothe 2

nd^ component an iron-sulfur protein whichpasses electrons to theonly lipid componentcalled ubiquinone

Electron transport – oxidative

phosphorylation cont.

-^ Most of remainingelectron carriers areproteins calledcytochromes having aheme prosthetic group –4 organic ringssurrounding an iron atom •^ Several types ofcytochromes are presentwith the last beingcytochrome a

which 3

passes the electrons tomolecular oxygen