Inligting

6.4: Glikolise - Biologie

6.4: Glikolise - Biologie


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Selfs eksergoniese, energie-vrystellende reaksies vereis 'n klein hoeveelheid aktiveringsenergie om voort te gaan. Oorweeg egter endergoniese reaksies, wat baie meer energie-insette vereis omdat hul produkte meer vrye energie het as hul reaktante. Binne die sel, waar kom energie vandaan om sulke reaksies aan te dryf? Die antwoord lê by 'n energievoorsienende molekule genaamd adenosientrifosfaat, of ATP. ATP is 'n klein, relatief eenvoudige molekule, maar binne sy bindings bevat die potensiaal vir 'n vinnige uitbarsting van energie wat ingespan kan word om sellulêre werk te verrig. Hierdie molekule kan beskou word as die primêre energie-geldeenheid van selle op dieselfde manier dat geld die geldeenheid is wat mense verruil vir dinge wat hulle nodig het. ATP word gebruik om die meerderheid van die energie-benodigde sellulêre reaksies aan te dryf.

ATP in lewende stelsels

'n Lewende sel kan nie aansienlike hoeveelhede vrye energie stoor nie. Oormaat vrye energie sal lei tot 'n toename in hitte in die sel, wat ensieme en ander proteïene sal denatureer, en sodoende die sel vernietig. 'n Sel moet eerder in staat wees om energie veilig te stoor en dit vry te stel vir gebruik net soos nodig. Lewende selle bereik dit met behulp van ATP, wat gebruik kan word om enige energiebehoefte van die sel te vul. Hoe? Dit funksioneer as 'n herlaaibare battery.

Wanneer ATP afgebreek word, gewoonlik deur die verwydering van sy terminale fosfaatgroep, word energie vrygestel. Hierdie energie word gebruik om werk deur die sel te doen, gewoonlik deur die binding van die vrygestelde fosfaat aan 'n ander molekule, en sodoende dit te aktiveer. Byvoorbeeld, in die meganiese werk van spiersametrekking, verskaf ATP energie om die kontraktiele spierproteïene te beweeg.

ATP-struktuur en -funksie

In die hart van ATP is 'n molekule adenosienmonofosfaat (AMP), wat saamgestel is uit 'n adenienmolekule wat aan beide 'n ribosemolekule en 'n enkele fosfaatgroep gebind is (Figuur 4.2.1). Ribose is 'n vyfkoolstofsuiker wat in RNA voorkom en AMP is een van die nukleotiede in RNA. Die byvoeging van 'n tweede fosfaatgroep tot hierdie kernmolekule lei tot adenosien difosfaat (ADP); die byvoeging van 'n derde fosfaatgroep vorm adenosien driefosfaat (ATP).

Die byvoeging van 'n fosfaatgroep tot 'n molekule vereis 'n hoë hoeveelheid energie en lei tot 'n hoë-energiebinding. Fosfaatgroepe is negatief gelaai en stoot mekaar dus af wanneer hulle in serie gerangskik is, soos in ADP en ATP. Hierdie afstoting maak die ADP- en ATP-molekules inherent onstabiel. Die vrystelling van een of twee fosfaatgroepe uit ATP, 'n proses wat hidrolise genoem word, stel energie vry.

Glikolise

Jy het gelees dat byna al die energie wat deur lewende dinge gebruik word, na hulle toe kom in die bindings van die suiker, glukose. Glikolise is die eerste stap in die afbreek van glukose om energie te onttrek vir selmetabolisme. Baie lewende organismes voer glikolise uit as deel van hul metabolisme. Glikolise vind plaas in die sitoplasma van die meeste prokariotiese en alle eukariotiese selle.

Glikolise begin met die ses-koolstof, ringvormige struktuur van 'n enkele glukose molekule en eindig met twee molekules van 'n drie-koolstof suiker genoem piruvaat. Glikolise bestaan ​​uit twee verskillende fases. In die eerste deel van die glikolise-baan word energie gebruik om aanpassings te maak sodat die ses-koolstof-suikermolekule eweredig in twee drie-koolstof-piruvaatmolekules verdeel kan word. In die tweede deel van glikolise word ATP en nikotinamied-adenien-dinukleotied (NADH) geproduseer (Figuur 4.4.2).

As die sel nie die piruvaatmolekules verder kan kataboliseer nie, sal dit slegs twee ATP-molekules van een molekule glukose oes. Byvoorbeeld, volwasse rooibloedselle van soogdiere is slegs in staat tot glikolise, wat hul enigste bron van ATP is. As glikolise onderbreek word, sal hierdie selle uiteindelik sterf.

Opsomming

ATP funksioneer as die energiegeldeenheid vir selle. Die struktuur van ATP is dié van 'n RNA-nukleotied met drie fosfaatgroepe aangeheg. Aangesien ATP vir energie gebruik word, word 'n fosfaatgroep losgemaak, en ADP word geproduseer. Energie afkomstig van glukose-katabolisme word gebruik om ADP in ATP te herlaai.

Glikolise is die eerste pad wat gebruik word in die afbreek van glukose om energie te onttrek. Omdat dit deur byna alle organismes op aarde gebruik word, moes dit vroeg in die geskiedenis van lewe ontwikkel het. Glikolise bestaan ​​uit twee dele: Die eerste deel berei die ses-koolstofring van glukose voor vir skeiding in twee driekoolstofsuikers. Energie van ATP word tydens hierdie stap in die molekule belê om die skeiding te bekragtig. Die tweede helfte van glikolise onttrek ATP en hoë-energie elektrone uit waterstofatome en heg dit aan NAD+. Twee ATP-molekules word in die eerste helfte belê en vier ATP-molekules word gedurende die tweede helfte gevorm. Dit lewer 'n netto wins van twee ATP-molekules per molekule glukose vir die sel.

Woordelys

ATP
(ook adenosientrifosfaat) die sel se energiegeldeenheid
glikolise
die proses om glukose in twee driekoolstofmolekules op te breek met die produksie van ATP en NADH

Stamselmetabolisme en dieet

Doel van hersiening: Dieet het 'n groot impak op gesondheid en lang lewe. Bewyse kom voor wat daarop dui dat dieet die metaboliese weë in weefselspesifieke stamselle beïnvloed om gesondheid en siekte te beïnvloed. Hier hersien ons die ooreenkomste en verskille in die metabolisme van stamselle uit verskeie weefsels, en beklemtoon die mitochondriale metaboliese kontrolepunt in stamselonderhoud en veroudering. Ons bespreek hoe dieet die voedingstofdetectie-metaboliese weë betrek en stamselonderhoud beïnvloed. Laastens ondersoek ons ​​die terapeutiese implikasies van dieet- en metaboliese regulering van stamselle.

Onlangse bevindings: Stamseloorgang van stilte na proliferasie word geassosieer met 'n metaboliese oorskakeling van glikolise na mitochondriale OXPHOS en die mitochondriale metaboliese kontrolepunt word krities beheer deur die voedingstofsensors SIRT2, SIRT3 en SIRT7 in hematopoietiese stamselle. Ingewande stamselhomeostase tydens veroudering en in reaksie op dieet is krities afhanklik van vetsuurmetabolisme en ketoonliggame en word beïnvloed deur die nis wat deur die voedingstofsensor mTOR bemiddel word.

Opsomming: Voedingstofwaarneming metaboliese weë reguleer krities stamselonderhoud tydens veroudering en in reaksie op dieet. Die toeligting van die molekulêre meganismes onderliggend aan dieet- en metaboliese regulering van stamselle bied nuwe insigte vir stamselbiologie en kan terapeuties geteiken word om stamselveroudering en weefseldegenerasie om te keer.

Sleutelwoorde: SIRT2 SIRT3 SIRT7 kalorie beperking mTOR stamsel metabolisme.

Verklaring van belangebotsing

Botsing van belange Marine Barthez, Zehan Song, Chih Ling Wang en Danica Chen het geen botsing van belange nie.


Die beginmolekule vir glikolise is glukose, 'n eenvoudige en volop suiker wat in koolhidrate voorkom, wat die energie vir die meeste selle verskaf. Koolhidrate wat tydens fotosintese gesintetiseer word, dien as die hoofbergingsmolekules van sonenergie. Wanneer dit ingeneem word, word komplekse koolhidrate ensimaties gehidroliseer na monosakkariede, soos stysel na D(+)-glukose.

Die katabolisme van glukose is die primêre energiebron vir korttermynbehoeftes.

Die res van hierdie artikel sal fokus op die glikolitiese pad bekend as die Embden-Meyerhof-Parnas (EMP) pad, vernoem na sy ontdekkers, Gustav Embden, Otto Meyerhof en Jakub Karol Parnas. Die chemiese stappe van die pad word in die prent regs en via die video hieronder geïllustreer.


6.4: Glikolise - Biologie

DEEL II. HOEKSTENE: CHEMIE, SELLE EN METABOLISME

6.3. Die metaboliese weë van aërobiese sellulêre respirasie

Dit is 'n goeie idee om met die eenvoudigste beskrywing te begin en lae van begrip by te voeg terwyl jy na bykomende vlakke gaan. Daarom word hierdie bespreking van aërobiese sellulêre respirasie in twee vlakke verdeel:

1. 'n fundamentele beskrywing en

Vra jou instrukteur watter vlak vir jou kursus vereis word.

Fundamentele beskrywing

Glikolise is 'n reeks ensiembeheerde reaksies wat in die sitoplasma plaasvind. Tydens glikolise word energie van twee ATP-molekules by 'n 6-koolstof suikermolekule (glukose) gevoeg. Die byvoeging van hierdie energie maak sommige van die bindings van die glukosemolekule onstabiel, en die glukosemolekule word makliker afgebreek. Nadat die 6-koolstofglukose deur verskeie ensiembeheerde reaksies gegaan het, word die glukose afgebreek tot twee 3-koolstofmolekules bekend as gliseraldehied-3-fosfaat (ook bekend as PGA, of fosfogliseraldehied), wat addisionele reaksies ondergaan om pirodruivensuur te vorm ( CH3COCOOH).

Genoeg energie word deur hierdie reeks reaksies vrygestel om vier ATP-molekules te produseer. Omdat twee ATP-molekules gebruik is om die reaksie te begin en vier geproduseer is, is daar 'n netto wins van twee ATP's vanaf die glikolitiese pad (figuur 6.4). Tydens die proses van glikolise word sommige waterstowwe en hul elektrone verwyder van die organiese molekules wat verwerk word en deur die elektronoordragmolekule NAD + opgetel om NADH te vorm. Genoeg waterstowwe word tydens glikolise vrygestel om 2 NADH's te vorm. Die NADH met sy ekstra elektrone bevat 'n groot hoeveelheid potensiële energie, wat gebruik kan word om ATP in die elektronvervoerstelsel te maak. Die werk van die koënsiem NAD + is om hierdie energiebevattende elektrone en protone veilig na die elektronvervoerstelsel te vervoer. Sodra hulle hul elektrone laat val het, is die geoksideerde NAD + s beskikbaar om meer elektrone op te tel en die taak te herhaal.

FIGUUR 6.4. Glikolise: Fundamentele beskrywing

Glikolise is die biochemiese pad wat baie organismes gebruik om glukose te oksideer. Tydens hierdie volgorde van chemiese reaksies word die 6-koolstofmolekule van glukose geoksideer. As gevolg hiervan word pirodruivensuur geproduseer, elektrone word deur NAD + opgetel en ATP word geproduseer.

Fundamentele opsomming van een draai van glikolise

Die reeks reaksies bekend as die Krebs-siklus vind binne die mitochondria van selle plaas. Dit kry sy naam van sy ontdekker, Hans Krebs, en die feit dat die reeks reaksies begin en eindig met dieselfde molekule wat dit siklusse. Die Krebs-siklus staan ​​ook bekend as die sitroensuursiklus en die TriCarboxylic Acid-siklus (TCA). Die 3-koolstof pirodruivensuurmolekules wat deur glikolise vrygestel word, gaan die mitochondria binne. Dit word aangewend deur spesifieke ensieme wat gemaak word deur gebruik te maak van genetiese inligting wat gevind word op DNA wat binne die mitochondria (mDNA) geleë is. Een van hierdie koolstofstowwe word gestroop en die oorblywende 2-koolstoffragment word aan 'n molekule koënsiem A (CoA) geheg en word 'n verbinding genaamd asetiel-CoA. Koënsiem A word gemaak van pantetien (pantoteensuur), 'n vorm van vitamien B5. Asetiel-CoA is die molekule wat deur die Krebs-siklus beweeg. Wanneer die asetiel-CoA geproduseer word, word 2 waterstowwe aan NAD + geheg om NADH te vorm. Die koolstofatoom wat verwyder is, word as koolstofdioksied vrygestel.

Opsomming van veranderinge soos pirruviensuur na asetiel-KoA omgeskakel word

Tydens die Krebs-siklus (figuur 6.5) word die asetiel-KoA heeltemal geoksideer (m.a.w. die oorblywende waterstowwe en hul elektrone word verwyder). Die meeste van die elektrone word deur NAD + opgetel om NADH te vorm, maar op 'n stadium in die proses tel FAD elektrone op om FADH te vorm2. Ongeag watter elektrondraer gebruik word, word die elektrone na die elektronvervoerstelsel gestuur. Die oorblywende koolstof- en suurstofatome word gekombineer om CO te vorm2. Soos in glikolise, word genoeg energie vrygestel om 2 ATP-molekules te genereer. Aan die einde van die Krebs-siklus is die asetielgedeelte van die asetiel-CoA heeltemal afgebreek (geoksideer) na CO2. Die CoA is vrygestel en beskikbaar om weer gebruik te word. Die energie in die molekule is oorgedra na ATP, NADH of FADH2. Ook is van die energie vrygestel as hitte. Vir elk van die asetiel-KoA-molekules wat die Krebs-siklus binnegaan, 1 ATP, 3 NADH's en 1 FADH2 geproduseer word. As ons die NADH tel wat tydens glikolise geproduseer word, toe asetiel-CoA gevorm is, is daar 'n totaal van 4 NADH's vir elke pirodruivensuur wat 'n mitochondrion binnedring.

FIGUUR 6.5. Krebs-siklus: Fundamentele beskrywing

Die Krebs-siklus vind plaas in die mitochondria van selle om die oksidasie van glukose te voltooi. Tydens hierdie volgorde van chemiese reaksies word 'n pirodruivensuurmolekule wat uit glikolise vervaardig word van sy waterstof gestroop. Die waterstofstowwe word deur NAD + en FAD opgetel vir vervoer na die ETS. Die oorblywende atome word geherorganiseer in molekules van koolstofdioksied. Genoeg energie word tydens die Krebs-siklus vrygestel om 2 ATP's te vorm. Omdat 2 pirodruivensuurmolekules uit glikolise geproduseer is, moet die Krebs-siklus twee keer uitgevoer word om hul oksidasie te voltooi (een keer vir elke pirodruivensuur).

Fundamentele opsomming van een draai van die Krebs-siklus

Die elektron-vervoerstelsel

Van die drie stappe van aërobiese sellulêre respirasie (glikolise, Krebs-siklus en elektronvervoerstelsel) genereer selle die grootste hoeveelheid ATP vanaf die elektronvervoerstelsel (figuur 6.6). Tydens hierdie stapsgewyse volgorde van oksidasie-reduksie reaksies, die energie van die NADH en FADH2 molekules wat in glikolise gegenereer word en die Krebs-siklus word gebruik om ATP te produseer. Ysterbevattende sitochroom (sito = selchroom = kleur) ensiemmolekules is op die membrane van die mitochondrion geleë. Die energieryke elektrone word van een sitochroom na 'n ander oorgedra (vervoer) en die energie word gebruik om protone (waterstofione) van die een kant van die membraan na die ander te pomp. Die gevolg hiervan is 'n hoër konsentrasie waterstofione aan die een kant van die membraan. Soos die konsentrasie waterstofione aan die een kant toeneem, bou 'n protongradiënt op. As gevolg van hierdie konsentrasiegradiënt, wanneer 'n membraankanaal oopgemaak word, vloei die protone terug na die kant vanwaar hulle gepomp is. Soos hulle deur die kanale beweeg, versnel 'n fosforilase-ensiem (ATP-sintetase, ook na verwys as ATPase) die vorming van 'n ATP-molekule deur 'n fosfaat aan 'n ADP-molekule te bind (fosforilering). Wanneer al die elektrone en waterstofione in berekening gebring word, word 'n totaal van 32 ATP's gevorm uit die elektrone en waterstowwe wat uit die oorspronklike glukosemolekule verwyder word. Die waterstowwe word dan aan suurstof gebind om water te vorm.

FIGUUR 6.6. Die elektron-vervoerstelsel: fundamentele beskrywing

Die elektronvervoerstelsel (ETS) staan ​​ook bekend as die sitochroomstelsel. Met behulp van ensieme word die elektrone deur 'n reeks oksidasie-reduksie-reaksies gevoer. Die energie wat die elektrone prysgee, word gebruik om protone (H + ) oor 'n membraan in die mitochondrion te pomp. Wanneer protone deur die membraan terugvloei, veroorsaak ensieme in die membraan die vorming van ATP. Die protone kombineer uiteindelik met die suurstof wat elektrone gekry het, en water word geproduseer.

Fundamentele opsomming van die elektron-vervoerstelsel

Die eerste fase van die sellulêre respirasieproses vind in die sitoplasma plaas. Hierdie eerste stap, bekend as glikolise, bestaan ​​uit die ensiematiese afbreek van 'n glukosemolekule sonder die gebruik van molekulêre suurstof. Omdat geen suurstof benodig word nie, word glikolise 'n anaërobiese proses genoem. Die glikolise-weg kan in twee algemene stelle reaksies verdeel word. Die eerste reaksies maak die glukosemolekule onstabiel, en latere oksidasie-reduksie-reaksies word gebruik om ATP te sintetiseer en waterstof vas te vang.

Omdat glukose 'n baie stabiele molekule is en nie outomaties sal afbreek om energie vry te stel nie, moet 'n bietjie energie by die glukosemolekule gevoeg word om glikolise te begin. In glikolise kry die aanvanklike glukosemolekule 'n fosfaat by om glukose-6-fosfaat te word, wat na fruktose-6-fosfaat omgeskakel word. Wanneer 'n tweede fosfaat bygevoeg word, fruktose-1,6-bisfosfaat (P—C6—P) gevorm word. Hierdie 6-koolstofmolekule is onstabiel en breek uitmekaar om twee 3-koolstof, gliseraldehied-3-fosfaatmolekules te vorm.

Elk van die twee gliseraldehied-3-fosfaatmolekules verkry 'n tweede fosfaat uit 'n fosfaatvoorraad wat normaalweg in die sitoplasma voorkom. Elke molekule het nou 2 fosfate geheg om 1,3-bisfosfogliseraat te vorm (P—C3—P). 'n Reeks reaksies volg, waarin energie vrygestel word deur chemiese bindings te breek wat die fosfate aan 1,3-bisfosfogliseraat hou. Die energie en die fosfate word gebruik om ATP te produseer. Aangesien daar twee 1,3-bisfosfogliseraatmolekules elk met 2 fosfate is, word 'n totaal van 4 ATP's geproduseer. Omdat 2 ATP's gebruik is om die proses te begin, het 'n netto opbrengs van 2 ATP's tot gevolg. Daarbenewens maak 4 waterstofatome los van die koolstofskelet en hul elektrone word na NAD + oorgedra om NADH te vorm, wat die elektrone na die elektronvervoerstelsel oordra. Die 3-koolstof pirodruivensuurmolekules wat oorbly, is die grondstof vir die Krebs-siklus. Omdat glikolise in die sitoplasma plaasvind en die Krebs-siklus binne mitochondria plaasvind, moet die pirodruivensuur die mitochondrion binnegaan voordat dit verder afgebreek kan word (figuur 6.7).

FIGUUR 6.7. Glikolise: Gedetailleerde beskrywing

Glikolise is 'n proses wat in die sitoplasma van selle plaasvind. Dit vereis nie die gebruik van suurstof nie, dus is dit 'n anaërobiese proses. Tydens die eerste paar stappe word fosfate vanaf ATP bygevoeg en uiteindelik word die 6-koolstofsuiker in twee 3-koolstofverbindings verdeel. Tydens die laaste stappe in die proses, aanvaar NAD + elektrone en waterstof om NADH te vorm. Daarbenewens word ATP geproduseer. Twee ATP's vorm vir elk van die 3-koolstofmolekules wat in glikolise verwerk word. Omdat daar twee 3-koolstofverbindings is, word 'n totaal van 4 ATP's gevorm. Omdat 2 ATP's egter gebruik is om die proses te begin, is daar 'n netto wins van 2 ATP's. Pirodruivensuur (piruvaat) word aan die einde van glikolise gelaat.

Opsomming van Gedetailleerde Beskrywing van Glikolise

Die proses van glikolise vind plaas in die sitoplasma van 'n sel, waar glukose (C6H12O6) voer 'n reeks reaksies in wat:

1. vereis die gebruik van 2 ATP's,

2. lei uiteindelik tot die vorming van 4 ATP's,

3. lei tot die vorming van 2 NADH's, en

4. lei tot die vorming van 2 molekules pirodruivensuur (CH3COCOOH).

Omdat 2 molekules ATP gebruik word om die proses te begin en 'n totaal van 4 ATP's gegenereer word, produseer elke glukosemolekule wat glikolise ondergaan 'n netto opbrengs van 2 ATP's.

Nadat piruvaat (piruviensuur) die mitochondrion binnedring, word dit eers deur 'n ensiem, saam met 'n molekule bekend as koënsiem A (CoA) (figuur 6.8) opgewerk. Dit lei tot drie beduidende produkte. Waterstofatome word verwyder en NADH word gevorm, 'n koolstof word verwyder en koolstofdioksied word gevorm, en 'n 2-koolstoffragment word gevorm, wat tydelik aan koënsiem A heg om asetielkoënsiem A te produseer. (Hierdie en daaropvolgende reaksies van die Krebs-siklus vind plaas in die vloeistof tussen die membrane van die mitochondrion.) Die asetielkoënsiem A betree die reeks reaksies bekend as die Krebs-siklus. Tydens die Krebs-siklus word die asetiel-CoA sistematies afgebreek. Sy waterstofatome word verwyder en die oorblywende koolstofstowwe word as koolstofdioksied vrygestel (Vooruitsigte 6.1).

Die eerste stap in hierdie proses behels die asetiel-CoA. Die asetielgedeelte van die kompleks word oorgedra na 'n 4-koolstofverbinding genaamd oksaloasetaat (oksalasynsuur) en 'n nuwe 6-koolstof-sitraatmolekule (sitroensuur) word gevorm. Die koënsiem A word vrygestel om aan 'n ander reaksie met pirodruivensuur deel te neem. Hierdie nuutgevormde sitraat word in 'n reeks reaksies afgebreek, wat uiteindelik oksaloasetaat produseer, wat in die eerste stap van die siklus gebruik is (vandaar die name Krebs-siklus, sitroensuursiklus en trikarboksielsuursiklus). Die verbindings wat tydens hierdie siklus gevorm word, word ketosure genoem.

In die proses word elektrone verwyder en word saam met protone aan die koënsieme NAD + en FAD geheg. Die meeste raak geheg aan NAD + maar sommige raak geheg aan FAD. Soos die molekules deur die Krebs-siklus beweeg, word genoeg energie vrygestel om die sintese van 1 ATP-molekule moontlik te maak vir elke asetiel-CoA wat die siklus binnegaan. Die ATP word gevorm uit ADP en 'n fosfaat wat reeds in die mitochondria teenwoordig is. Vir elke piruvaatmolekule wat 'n mitochondrion binnegaan en deur die Krebs-siklus verwerk word, word 3 koolstofstowwe as 3 koolstofdioksiedmolekules vrygestel, 5 pare waterstofatome word verwyder en word aan NAD + of FAD geheg, en 1 ATP-molekule word gegenereer. Wanneer beide piruvaatmolekules deur die Krebs-siklus verwerk is, (1) is al die oorspronklike koolstofstowwe van die glukose in die atmosfeer vrygestel as 6 koolstofdioksiedmolekules (2) is al die waterstof wat oorspronklik op die glukose gevind is oorgedra na óf NAD + of FAD om NADH of FADH2 te vorm en (3) 2 ATP's is gevorm uit die byvoeging van fosfate by ADP's (hersien figuur 6.8).

FIGUUR 6.8. Krebs-siklus: Gedetailleerde beskrywings

Die Krebs-siklus vind binne die mitochondrion plaas. Piruvaat gaan die mitochondrion binne vanaf glikolise en word omgeskakel na 'n 2-koolstof fragment, wat aan koënsiem A geheg word tot vanaf asetiel-CoA. Met die hulp van CoA kombineer die 2-koolstoffragment (asetiel) met 4-koolstof oksaloasetaat om 'n 6-koolstof-sitraatmolekule te vorm. Deur 'n reeks reaksies in die Krebs-siklus word elektrone verwyder en deur NAD + en FAD opgetel om NADH en FADH te vorm2, wat na die elektronvervoerstelsel vervoer sal word. Koolstowwe word as koolstofdioksied verwyder. Genoeg energie word vrygestel dat 1 ATP gevorm word vir elke asetiel-CoA wat die siklus binnegaan.

Opsomming van gedetailleerde beskrywing van die eukariotiese Krebs-siklus

Die Krebs-siklus vind binne die mitochondria plaas. Vir elke asetiel-CoA-molekule wat die Krebs-siklus binnegaan:

1. Die drie koolstofstowwe van 'n piruvaat word omgeskakel na asetiel-CoA en vrygestel as koolstofdioksied (CO2). Een CO2 word eintlik vrygestel voordat asetiel-CoA gevorm word.

2. Vyf pare waterstof word aan waterstofdraers geheg om 4 NADH's en 1 FADH te word2. Een van die NADH's word vrygestel voordat asetiel-CoA die Krebs-siklus binnegaan.

Die elektron-vervoerstelsel

Die reeks reaksies waarin energie oorgedra word vanaf die elektrone en protone wat deur NADH en FADH gedra word2 staan ​​bekend as die elektronvervoerstelsel (ETS) (figuur 6.9). Dit is die finale stadium van aërobiese sellulêre respirasie en is toegewy aan die opwekking van ATP. Die reaksies waaruit die elektronvervoerstelsel bestaan, is 'n reeks oksidasie-reduksie-reaksies waarin die elektrone van een elektrondraermolekule na 'n ander oorgedra word totdat dit uiteindelik deur suurstofatome aanvaar word. Die negatief gelaaide suurstof kombineer met die waterstofione om water te vorm. Dit is hierdie stap wat die proses aërobies maak. Hou in gedagte dat potensiële energie toeneem wanneer dinge wat 'n afstootkrag ervaar saamgedruk word, soos om die derde fosfaat by 'n ADP-molekule te voeg. Potensiële energie neem ook toe wanneer dinge wat mekaar aantrek uitmekaar getrek word, soos in die skeiding van die protone van die elektrone.

Kom ons kyk nou net in 'n bietjie meer besonderhede na wat gebeur met die elektrone en protone wat deur NADH en FADH na die elektronvervoerstelsels gedra word2 en hoe hierdie aktiwiteite gebruik word om ATP te produseer. Die mitochondrion bestaan ​​uit twee membrane—'n buitenste, omsluitende membraan en 'n binneste, gevoude membraan. Die reaksies van die ETS word geassosieer met hierdie binnemembraan. Binne die struktuur van die membraan is verskeie ensiemkomplekse wat bepaalde dele van die ETS-reaksies uitvoer (hersien figuur 6.9). Die produksie van ATP's behels twee afsonderlike maar gekoppelde prosesse. Elektrone wat deur NADH gedra word, tree in reaksies in ensiemkompleks I in, waar hulle 'n mate van energie verloor en uiteindelik deur 'n koënsiem (koënsiem Q) opgetel word. Elektrone van FADH2 betree ensiemkompleks II en word ook uiteindelik oorgedra na koënsiem Q. Koënsiem Q dra die elektrone oor na ensiemkompleks III. In kompleks III verloor die elektrone addisionele energie en word na sitochroom c oorgedra, wat elektrone na ensiemkompleks IV oordra. In kompleks IV word die elektrone uiteindelik na suurstof oorgedra. Soos die elektrone energie verloor in kompleks I, kompleks III en kompleks IV, word bykomende protone in die intermembraanruimte ingepomp. Wanneer hierdie protone teen die konsentrasiegradiënt af vloei deur kanale in die membraan, is fosforilase-ensieme (ATPase) in die membraan in staat om die energie te gebruik om ATP te genereer.

FIGUUR 6.9. Die elektron-vervoerstelsel: gedetailleerde beskrywing

Die meeste van die ATP wat deur aërobiese sellulêre respirasie geproduseer word, kom van die ETS. NADH en FADH2 lewer elektrone aan die ensieme wat verantwoordelik is vir die ETS. Daar is verskeie proteïenkomplekse in die binneste membraan van die mitochondrion, wat elkeen verantwoordelik is vir 'n gedeelte van die reaksies wat ATP lewer. Die energie van elektrone word in klein hoeveelhede prysgegee en gebruik om protone in die intermembraanruimte in te pomp. Wanneer hierdie protone deur porieë in die membraan terugvloei, produseer ATPase ATP. Die elektrone word uiteindelik na suurstof oorgedra en die negatief gelaaide suurstofione aanvaar protone om water te vorm.

Altesaam 12 pare elektrone en waterstof word vanaf glikolise en die Krebs-siklus na die ETS vervoer vir elke glukose wat die proses binnegaan. In eukariotiese organismes kan die pare elektrone soos volg verantwoord word: 2 pare word deur NADH gedra en is tydens glikolise buite die mitochondrion gegenereer, 8 pare word as NADH gedra en is binne die mitochondrion gegenereer, en 2 pare word deur FADH gedra.2 en is binne die mitochondrion gegenereer.

• Vir elk van die 8 NADH's wat binne die mitochondrion gegenereer word, word genoeg energie vrygestel om 3 ATP-molekules te produseer. Daarom word 24 ATP's vrygestel van hierdie elektrone wat deur NADH gedra word.

• In eukariotiese selle word die elektrone wat tydens glikolise vrygestel word deur NADH gedra en omgeskakel na 2 FADH2 om hulle in die mitochondria in te skuif. Sodra hulle binne die mitochondria is, volg hulle dieselfde pad as die ander 2 FADH2s uit die Krebs-siklus.

Die elektrone wat deur FADH gedra word2 is laer in energie. Wanneer hierdie elektrone deur die reeks oksidasie-reduksiereaksies gaan, stel hulle genoeg energie vry om 'n totaal van 8 ATP's te produseer. Daarom word 'n totaal van 32 ATP's geproduseer uit die waterstofelektrone wat die ETS binnedring.

Ten slotte, 'n volledige boekhouding van al die ATP's wat tydens al drie dele van aërobiese sellulêre respirasie geproduseer word, lei tot 'n totaal van 36 ATP's: 32 van die ETS, 2 van glikolise en 2 van die Krebs-siklus.

Opsomming van gedetailleerde beskrywing van die eukariotiese elektron-vervoerstelsel

Die elektronvervoerstelsel vind binne die mitochondrion plaas, waar:

1. Suurstof word opgebruik aangesien die suurstofatome waterstowwe van NADH en FADH aanvaar2 water vorm (H2O).

2. NAD + en FAD word vrygestel om weer gebruik te word.

3. Twee-en-dertig ATP's word geproduseer.

Wat gebeur as jy alkohol drink

Etielalkohol (CH3CH2OH) is 'n 2-koolstof organiese verbinding met 'n enkele alkoholiese funksionele groep. Omdat dit oplosbaar is in water, word dit maklik in die bloedstroom opgeneem. Nadat 'n alkoholiese drank die liggaam binnegekom het, word dit vinnig deur die bloedsomloopstelsel deur die liggaam versprei en die brein binnedring. Die meerderheid van die alkohol word uit die maag (20%) en dunderm (80%) opgeneem. Hoe meer 'n persoon drink, hoe hoër is die bloedalkoholvlak. Hoe vinnig alkohol geabsorbeer word, hang van verskeie faktore af.

1. Voedsel in die maag vertraag absorpsie.

2. Strawwe fisiese oefening verminder absorpsie.

3. Dwelms (bv. nikotien, dagga en ginseng) verhoog absorpsie.

Negentig persent van etielalkohol word in mitochondria geoksideer na asetaat (CH3CH2OH + NAD + → CH3CHO + NADH). Die asetaat word dan omgeskakel na asetiel-CoA wat die Krebs-siklus binnegaan waar ATP geproduseer word. Alkohol is hoog in kalorieë (1g = 7 000 kalorieë, of 7 voedselkalorieë). ’n Standaardglas wyn het sowat 15 g alkohol en sowat 100 kilokalorieë. Die 10% wat nie gemetaboliseer word nie, word in sweet of urine uitgeskei, of in asem afgegee. Dit neem die lewer een uur om een ​​eenheid alkohol te hanteer. 'n Eenheid alkohol is:

• 250 ml (1/2 pint) gewone sterktebier/piler.

• Een glas (125 ml/4 fl oz) wyn.

• 47 ml/1,5 onse sjerrie/vermout.

As alkohol vinniger verbruik word as wat die lewer dit kan afbreek, styg die bloedalkoholvlak. Dit veroorsaak 'n aanvanklike gevoel van warmte en lighoofdigheid. Alkohol is egter 'n depressant, dit wil sê dit verminder die aktiwiteit van die senuweestelsel. Aanvanklik kan dit kringe in die brein inhibeer wat normaalweg 'n persoon se optrede inhibeer. Dit lei gewoonlik daartoe dat 'n persoon meer spraaksaam en aktief word - ongeremd. Soos die alkohol se effek egter voortduur, kan ander veranderinge plaasvind. Dit sluit in verhoogde aggressie, verlies aan geheue en verlies aan motoriese beheer.

Langtermyn, oormatige gebruik van alkohol kan skade aan die lewer veroorsaak, wat lei tot die ontwikkeling van 'n vetterige lewer, alkoholiese hepatitis en alkoholiese sirrose. Dit kan ook inmeng met die niere se regulering van water, natrium, kalium, kalsium en fosfaat en met die nier se vermoë om 'n behoorlike suur-basis balans te handhaaf, en hormone te produseer. Dit veroorsaak ook lae bloedsuikervlakke, dehidrasie, hoë bloeddruk, beroertes, hartsiektes, geboortedefekte, osteoporose en sekere kankers.

Drink alkohol in moderering hou wel sekere gesondheidsvoordele in as die drank antioksidante bevat (byvoorbeeld rooiwyne en donker bier). Die antioksidante in rooiwyn (polifenole) blyk die negatiewe effek van chemikalieë genaamd vrye radikale wat tydens metabolisme vrygestel word, teë te werk. Dit is bekend dat vrye radikale selkomponente vernietig en mutasies veroorsaak, skade wat kan lei tot hartsiektes en kankers. Antioksidante beskerm teen hierdie soort skade deur vrye radikale vas te vang.

6. Vir glikolise, die Krebs-siklus en die elektronvervoerstelsel, lys twee molekules wat inkom en twee wat elke pad verlaat.

7. Hoe is elk van die volgende betrokke by aërobiese sellulêre respirasie: NAD + , pirodruivensuur, suurstof en ATP?

As jy die kopiereghouer is van enige materiaal wat op ons werf vervat is en van plan is om dit te verwyder, kontak asseblief ons werfadministrateur vir goedkeuring.


Toepassing van metaboliese beheeranalise op die studie van toksiese effekte van koper in spierglikolise

Eksperimentele en modelstudies is uitgevoer om die effekte van Cu2+ op die aktiwiteite van individuele glikolitiese ensieme en op die vloed en interne metabolietkonsentrasies van die boonste deel van glikolise in muisspierekstrakte te karakteriseer. Cu2+ het die triosefosfaatproduksie vanaf glukose aansienlik geïnhibeer met 'n IC50 van ongeveer 6.0 mikroM. Met 'n soortgelyke verlenging het Cu2+ heksokinase en fosfofruktokinase geïnhibeer, met 'n IC50 van 6.2 mikroM en 6.4 mikroM onderskeidelik, terwyl die effekte op die aktiwiteite van aldolase, fosfoglukose-isomerase en die interne metabolietvlakke nie betekenisvol was nie. Fluksbeheerkoëffisiënte en vloedreaksiekoëffisiënte is bepaal in die teenwoordigheid van koperkonsentrasies tussen 0 en 10 mikroM. Dieselfde waardes van vloedbeheerkoëffisiënte vir heksokinase en vir fosfofruktokinase (0.8 en 0.2 onderskeidelik) is gevind in afwesigheid en in teenwoordigheid van koper. By Cu2+ gelyk aan die vloed IC50 was die responskoëffisiënt -1. Die elastisiteitskoëffisiënte vir heksokinase en fosfofruktokinase by Cu2+ gelyk aan die IC50 was ook -1. 'n Wiskundige model is gebruik om die effek van koper op glikolise onder verskillende toestande te ontleed deur eksperimentele kinetiese parameters en tempovergelykings vir ensiematiese reaksies van die boonste gedeelte van glikolise te gebruik.


6.4: Glikolise - Biologie

In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. As suurstof beskikbaar is, sal aërobiese respirasie vorentoe gaan. In mitochondria, pyruvate will be transformed into a two-carbon acetyl group (by removing a molecule of carbon dioxide) that will be picked up by a carrier compound called coenzyme A (CoA), which is made from vitamin B. The resulting compound is called acetyl CoA . ([link]). This set of reactions is referred to as the transition reaction, as it happens during pyruvate transport into the mitochondria. The major function of acetyl CoA is to deliver the acetyl group (2 carbon fragment) derived from pyruvate to the next pathway in glucose catabolism, which is the citric acid/Kreb's cycle. Note that during the transition reaction, each pyruvate/pyruvic acid molecule loses one carbon as carbon dioxide and one molecule of NADH is produced. Therefore, a total of two molecules of carbon dioxide and two molecules of NADH are produced per glucose that started glycolysis.

During the transition reaction, pyruvate is converted into acetyl-CoA before entering the citric acid/Kreb's cycle.

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle (also called the Kreb's cycle) in eukaryotic cells takes place in the matrix of the mitochondria. Anders as glikolise, is die sitroensuursiklus 'n geslote lus: Die laaste deel van die pad regenereer die verbinding wat in die eerste stap gebruik is. The eight steps of the cycle are a series of chemical reactions that produces two carbon dioxide molecules, one ATP molecule (or an equivalent), and reduced forms (NADH and FADH2) of NAD + and FAD + , important coenzymes in the cell. Part of this is considered an aerobic pathway (oxygen-requiring) because the NADH and FADH2 geproduseer moet hul elektrone na die volgende pad in die sisteem oordra, wat suurstof sal gebruik. If oxygen is not present, this transfer does not occur. Note that per glucose that started glycolysis, processing of the two pyruvate/pyruvic acid molecules in the citric acid cycle will result in the production of a total of six NADH, two FADH2, and two ATP. Also note that at this point, a total of six molecules of carbon dioxide have been released, which accounts for the six carbons in the starting glucose molecule. The high-energy NADH and FADH2 will be used in the last stage of aerobic respiration to produce additional ATP molecules.

Electron Transport Chain/Oxidative Phosphorylation

You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Die meeste van die ATP wat tydens die aërobiese katabolisme van glukose gegenereer word, word egter nie direk vanaf hierdie weë gegenereer nie. Rather, it derives from a process that begins with passing electrons through a series of chemical reactions to a final electron acceptor, oxygen. These reactions take place in specialized protein complexes located in the inner membrane of the mitochondria. The energy of the electrons is harvested and used to generate a electrochemical gradient of hydrogen ions across the inner mitochondrial membrane. The potential energy of this gradient is used to generate ATP by providing the energy to add phosphate groups to ADP molecules. The entirety of this process is called oxidative phosphorylation , as oxygen is required as the terminal electron acceptor and phosphate groups are added to ADP molecules.

The electron transport chain ([link]a) is the last component of aerobic respiration and is the only part of metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants for this purpose. In animals, oxygen enters the body through the respiratory system. Electron transport is a series of chemical reactions that resembles a bucket brigade in that electrons are passed rapidly from one component to the next, to the endpoint of the chain where oxygen is the final electron acceptor and water is produced. There are four complexes composed of proteins, labeled I through IV in [link]c, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain . The electron transport chain is present in multiple copies in the inner mitochondrial membrane of eukaryotes and in the plasma membrane of prokaryotes. In each transfer of an electron through the electron transport chain, the electron loses energy, but with some transfers, the energy is stored as potential energy by using it to pump hydrogen ions across the inner mitochondrial membrane into the intermembrane space, creating an electrochemical gradient.

(a) The electron transport chain is a set of molecules that supports a series of oxidation-reduction reactions. (b) ATP synthase is a complex, molecular machine that uses an H + gradient to regenerate ATP from ADP. (c) An overview of the entire process.

Electrons from NADH and FADH2 are passed to protein complexes in the electron transport chain. As they are passed from one complex to another (there are a total of four), the electrons lose energy, and some of that energy is used to pump hydrogen ions from the mitochondrial matrix into the intermembrane space. In the fourth protein complex, the electrons are accepted by oxygen, the terminal acceptor. The oxygen with its extra electrons then combines with two hydrogen ions, further enhancing the electrochemical gradient, to form water. If there were no oxygen present in the mitochondrion, the electrons could not be removed from the system, and the entire electron transport chain would back up and stop. The mitochondria would be unable to generate new ATP in this way, and the cell would ultimately die from lack of energy. This is the reason we must breathe to draw in new oxygen.

In the electron transport chain, the free energy from the series of reactions just described is used to pump hydrogen ions (via active transport) across the membrane. The uneven distribution of H + ions across the membrane establishes an electrochemical gradient, owing to the H + ions’ positive charge and their higher concentration on one side of the membrane.

Hydrogen ions diffuse through the inner membrane through a membrane protein called ATP synthase ([link]b). This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient from the intermembrane space, where there are many mutually repelling hydrogen ions to the matrix, where there are few. The turning of the parts of this molecular machine regenerate ATP from ADP and phosphate.

Chemiosmosis ([link]c) is used to generate 90 percent of the ATP made during aerobic glucose catabolism. The result of the reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. Hierdie atome was oorspronklik deel van 'n glukosemolekule. At the end of the electron transport system, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen ions attract hydrogen ions (protons) from the surrounding medium, and water is formed.

ATP Yield

Die aantal ATP-molekules wat deur die katabolisme van glukose gegenereer word, verskil. In general, processing of each NADH yields approximately 3 ATP and each FADH2 yields approximately 2 ATP. Overall, a total of 10 NADH and 2 FADH2 were produced in glycolysis, transition reaction, and the citric acid cycle per glucose molecule. This results in the production of approximately 34 ATP. Remember, that two additional ATP were produced directly in both glycolysis and the citric acid cycle, resulting in a total yield of 38 ATP per glucose. This represents an efficiency of approximately 35%, with the remaining energy potential lost as heat or other products.

Mitochondrial Disease Physician What happens when the critical reactions of cellular respiration do not proceed correctly? Mitochondrial diseases are genetic disorders of metabolism. Mitochondrial disorders can arise from mutations in nuclear or mitochondrial DNA, and they result in the production of less energy than is normal in body cells. Symptoms of mitochondrial diseases can include muscle weakness, lack of coordination, stroke-like episodes, and loss of vision and hearing. Most affected people are diagnosed in childhood, although there are some adult-onset diseases. Identifying and treating mitochondrial disorders is a specialized medical field. The educational preparation for this profession requires a college education, followed by medical school with a specialization in medical genetics. Medical geneticists can be board certified by the American Board of Medical Genetics and go on to become associated with professional organizations devoted to the study of mitochondrial disease, such as the Mitochondrial Medicine Society and the Society for Inherited Metabolic Disease.

Afdeling Opsomming

The citric acid cycle is a series of chemical reactions that removes high-energy electrons and uses them in the electron transport chain to generate ATP. One molecule of ATP (or an equivalent) is produced per each turn of the cycle.

The electron transport chain is the portion of aerobic respiration that uses free oxygen as the final electron acceptor for electrons removed from the intermediate compounds in glucose catabolism. The electrons are passed through a series of chemical reactions, with a small amount of free energy used at three points to transport hydrogen ions across the membrane. This contributes to the gradient used in chemiosmosis. As the electrons are passed from NADH or FADH2 down the electron transport chain, they lose energy. The products of the electron transport chain are water and ATP. A number of intermediate compounds can be diverted into the anabolism of other biochemical molecules, such as nucleic acids, non-essential amino acids, sugars, and lipids. These same molecules, except nucleic acids, can serve as energy sources for the glucose pathway.


The integration and control of metabolism

Phosphofructokinase (PFK)

Muscle phosphofructokinase is a classical example of an allosteric enzyme. It catalyses the rate-limiting step for glycolysis and is allosterically controlled by ATP and certain other ligands including AMP. Although small amounts of ATP are essential for the phosphofructokinase reaction ( page 226 ), at high concentrations ATP binds to a special inhibitory site and changes the shape of the substrate concentration curve from hyperbolic to sigmoidal ( Figure 23.5 ). As a result large changes in activity occur within the limited range of concentrations of fructose 6-phosphate present in muscle. The inhibitory effect of ATP is enhanced by a build-up of citrate which, like that of ATP, signals that plenty of energy is available and that it is not necessary to break down any more glucose. In resting muscles which are oxidizing fatty acids the levels of ATP and of citrate are inhibitory to phosphofructokinase so that glycolysis is reduced and glucose is conserved. Conversely phosphofructokinase is activated by ADP and AMP which serve to indicate that more energy is required. The changes in intracellular ATP and AMP concentrations that occur in anoxia are sufficient to account for the activation of phosphofructokinase and increased rate of glycolysis observed in such conditions. An even more potent allosteric activator of phosphofructokinase has recently been discovered. Dit is fructose 2,6-bisphosphate which also acts as an allosteric inhibitor of fructose 1,6-bisphosphatase the enzyme responsible for directing fructose 1,6-bisphosphate towards glycogenesis. Thus high concentrations of this metabolite inhibit glycogenesis at the same time as they promote glycolysis.

Figure 23.5 . The effect of (a) low and (b) high concentrations of ATP on phosphofructokinase


Biology Homework Chapter 6: Cellular Respiration and Glycolysis

Textbook assignment: Chapter 6: How Cells Harvest Energy, sections 1-8.

Study Notes
  • 6.1 The two key energy reactions of organic matter are photosynthesis, which captures light energy and stores it as chemical energy, and cellular respiration, which releases chemical energy to do work and generate heat.
  • 6.2 Distinguish between breathing (drawing air into the lungs to exchange oxygen and carbon dioxide), and cellular respiration, the combustion of oxygen (supplied by breathing respiration) within the mitochondria of the cell to produce ATP.
  • 6.3 Review ATP molecules from 5.4 if necessary. Cells use some ATP to support the respiration process, but produce more than they use. Although some heat is lost in the process (as it must be, according to the second law of thermodynamics), the process is more efficient that most mechanical ones engineered by humans.
  • 6.4 All human activity depends on ATP its a source of energy different activities "burn" this energy at different rates.
  • 6.5 Cellular respiration is a chemical reaction where the atoms in sugar and oxygen molecules are rearranged through a series of enzyme controlled steps. At each step, electrons wind up in a state of lowered stored potential energy. The released energy is stored by phosphorylation in ATP's bonds

Since electrons change their allegiance from one nucleus to another, these reactions are oxidation-reduction reactions. The loss of electrons is called oxidation, because this phenomenon was originally recognized in reactions where oxygen combined with some atoms or molecules. The gain of electrons is called reduction, because the atoms gaining the electrons now have reduced reactivity. (Since their electron orbitals are now full, they have further no tendency to combine with other atoms.) The electrons are carried from one atom to another by NAD+, a molecule that acts as a catalyst (something that enables a reaction but does not itself change with the reaction).

The last part of the reaction involves NADH releasing electrons to the membrane of the mitochondria, where the electron is passed from one receptor molecule to another and finally inside the cell to an oxygen molecule. Each receptor molecule takes some energy from the electron.

Web Lecture

Read the following weblecture before chat: Cellular Respiration I

Take notes on any questions you have, and be prepared to discuss the lecture in chat.

Perform the study activity below:

  • Use the BioCoach activity Cellular Respiration. Focus on Concepts 1: Overview of Respiration and Concept 2 Glycolysis.
  • Optional Website: Here's another overview of cellular respiration that may help you recognize where each phase takes place. Click on the labels for explanations of the terms.

Chat Preparation Activities

  • Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer voor chat starts for this session.
  • Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

Chapter Quiz

Lab Work

© 2005 - 2021 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.


Inhoud

Aerobic respiration requires oxygen (O2) in order to create ATP. Although carbohydrates, fats, and proteins are consumed as reactants, aerobic respiration is the preferred method of pyruvate breakdown in glycolysis, and requires pyruvate to the mitochondria in order to be fully oxidized by the citric acid cycle. The products of this process are carbon dioxide and water, and the energy transferred is used to break bonds in ADP to add a third phosphate group to form ATP (adenosine triphosphate), by substrate-level phosphorylation, NADH and FADH2

Simplified reaction: C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + heat
ΔG = −2880 kJ per mol of C6H12O6

The negative ΔG indicates that the reaction can occur spontaneously.

The potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen and protons (hydrogen) as the "terminal electron acceptors". [1] Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. The energy of O2 [1] released is used to create a chemiosmotic potential by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system). [4] However, this maximum yield is never quite reached because of losses due to leaky membranes as well as the cost of moving pyruvate and ADP into the mitochondrial matrix, and current estimates range around 29 to 30 ATP per glucose. [4]

Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism (which yields 2 molecules ATP per 1 molecule glucose) because the double bond in O2 is of higher energy than other double bonds or pairs of single bonds in other common molecules in the biosphere. [3] However, some anaerobic organisms, such as methanogens are able to continue with anaerobic respiration, yielding more ATP by using other inorganic molecules (not oxygen) of high energy as final electron acceptors in the electron transport chain. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post-glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.

Glycolysis

Glycolysis is a metabolic pathway that takes place in the cytosol of cells in all living organisms. Glycolysis can be literally translated as "sugar splitting", [5] and occurs with or without the presence of oxygen. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced, however, two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme aldolase. During the pay-off phase of glycolysis, four phosphate groups are transferred to ADP by substrate-level phosphorylation to make four ATP, and two NADH are produced when the pyruvate is oxidized. The overall reaction can be expressed this way:

Glucose + 2 NAD + + 2 Pi + 2 ADP → 2 pyruvate + 2 H + + 2 NADH + 2 ATP + 2 H + + 2 H2O + energy

Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. Glycogen can be converted into glucose 6-phosphate as well with the help of glycogen phosphorylase. During energy metabolism, glucose 6-phosphate becomes fructose 6-phosphate. An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-bisphosphate by the help of phosphofructokinase. Fructose 1,6-biphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.

Oxidative decarboxylation of pyruvate

Pyruvate is oxidized to acetyl-CoA and CO2 by the pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria of eukaryotic cells and in the cytosol of prokaryotes. In the conversion of pyruvate to acetyl-CoA, one molecule of NADH and one molecule of CO2 gevorm word.

Citric acid cycle

This is also called the Krebs siklus of die tricarboxylic acid cycle. When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, aerobic or anaerobic respiration can occur. [6] When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and is oxidized to CO2 while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two low-energy waste products, H2O en CO2, are created during this cycle.

The citric acid cycle is an 8-step process involving 18 different enzymes and co-enzymes. [6] During the cycle, acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) yields citrate (6 carbons), which is rearranged to a more reactive form called isocitrate (6 carbons). Isocitrate is modified to become α-ketoglutarate (5 carbons), succinyl-CoA, succinate, fumarate, malate, and, finally, oxaloacetate.

The net gain from one cycle is 3 NADH and 1 FADH2 as hydrogen- (proton plus electron)-carrying compounds and 1 high-energy GTP, which may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH2, and 2 ATP.

Oksidatiewe fosforilering

In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the boundary of the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. The electron transfer is driven by the chemical energy of exogenous oxygen [1] and, with the addition of two protons, water is formed.

The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation.

Stap coenzyme yield ATP yield Source of ATP
Glycolysis preparatory phase −2 Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm.
Glycolysis pay-off phase 4 Substrate-level phosphorylation
2 NADH 3 or 5 Oxidative phosphorylation : Each NADH produces net 1.5 ATP (instead of usual 2.5) due to NADH transport over the mitochondrial membrane
Oxidative decarboxylation of pyruvate 2 NADH 5 Oksidatiewe fosforilering
Krebs siklus 2 Substrate-level phosphorylation
6 NADH 15 Oksidatiewe fosforilering
2 FADH2 3 Oksidatiewe fosforilering
Total yield 30 or 32 ATP From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes.

Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized because of losses such as the cost of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria. All are actively transported using carriers that utilize the stored energy in the proton electrochemical gradient.

  • Pyruvate is taken up by a specific, low Km transporter to bring it into the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex.
  • Die phosphate carrier (PiC) mediates the electroneutral exchange (antiport) of phosphate (H2PO4 − Pi) for OH − or symport of phosphate and protons (H + ) across the inner membrane, and the driving force for moving phosphate ions into the mitochondria is the proton motive force.
  • Die ATP-ADP translocase (also called adenine nucleotide translocase, ANT) is an antiporter and exchanges ADP and ATP across the inner membrane. The driving force is due to the ATP (−4) having a more negative charge than the ADP (−3), and thus it dissipates some of the electrical component of the proton electrochemical gradient.

The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H + are needed to make 1 ATP. Obviously, this reduces the theoretical efficiency of the whole process and the likely maximum is closer to 28–30 ATP molecules. [4] In practice the efficiency may be even lower because the inner membrane of the mitochondria is slightly leaky to protons. [7] Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain and ATP synthesis. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in brown fat thermogenesis of newborn and hibernating mammals.

According to some newer sources, the ATP yield during aerobic respiration is not 36–38, but only about 30–32 ATP molecules / 1 molecule of glucose [8] , because:

  • ATP : NADH+H + and ATP : FADH2 ratios during the oxidative phosphorylation appear to be not 3 and 2, but 2.5 and 1.5 respectively. Unlike in the substrate-level phosphorylation, the stoichiometry here is difficult to establish.
      produces 1 ATP / 3 H + . However the exchange of matrix ATP for cytosolic ADP and Pi (antiport with OH − or symport with H + ) mediated by ATP–ADP translocase and phosphate carrier consumes 1 H + / 1 ATP as a result of regeneration of the transmembrane potential changed during this transfer, so the net ratio is 1 ATP : 4 H + .
  • The mitochondrial electron transport chainproton pump transfers across the inner membrane 10 H + / 1 NADH+H + (4 + 2 + 4) or 6 H + / 1 FADH2 (2 + 4).
    • ATP : NADH+H + coming from glycolysis ratio during the oxidative phosphorylation is
      • 1.5, as for FADH2, if hydrogen atoms (2H + +2e − ) are transferred from cytosolic NADH+H + to mitochondrial FAD by the glycerol phosphate shuttle located in the inner mitochondrial membrane.
      • 2.5 in case of malate-aspartate shuttle transferring hydrogen atoms from cytosolic NADH+H + to mitochondrial NAD +

      So finally we have, per molecule of glucose

        : 2 ATP from glycolysis + 2 ATP (directly GTP) from Krebs cycle
        • 2 NADH+H + from glycolysis: 2 × 1.5 ATP (if glycerol phosphate shuttle transfers hydrogen atoms) or 2 × 2.5 ATP (malate-aspartate shuttle)
        • 2 NADH+H + from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle: 8 × 2.5 ATP
        • 2 FADH2 from the Krebs cycle: 2 × 1.5 ATP

        Altogether this gives 4 + 3 (or 5) + 20 + 3 = 30 (or 32) ATP per molecule of glucose

        These figures may still require further tweaking as new structural details become available. The above value of 3 H+/ATP for the synthase assumes that the synthase translocates 9 protons, and produces 3 ATP, per rotation. The number of protons depends on the number of c subunits in the Fo c-ring, and it is now known that this is 10 in yeast Fo [9] and 8 for vertebrates. [10] Including one H+ for the transport reactions, this means that synthesis of one ATP requires 1+10/3=4.33 protons in yeast and 1+8/3 = 3.67 in vertebrates. This would imply that in human mitochondria the 10 protons from oxidizing NADH would produce 2.72 ATP (instead of 2.5) and the 6 protons from oxidizing succinate or ubiquinol would produce 1.64 ATP (instead of 1.5). This is consistent with experimental results within the margin of error described in a recent review. [11]

        The total ATP yield in ethanol or lactic acid fermentation is only 2 molecules coming from glycolysis, because pyruvate is not transferred to the mitochondrion and finally oxidized to the carbon dioxide (CO2), but reduced to ethanol or lactic acid in the cytoplasm. [8]

        Without oxygen, pyruvate (pyruvic acid) is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD + so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD + for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD + regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD + attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does not require oxygen.

        Fermentation is less efficient at using the energy from glucose: only 2 ATP are produced per glucose, compared to the 38 ATP per glucose nominally produced by aerobic respiration. This is because most of the energy of aerobic respiration derives from O2 with its relatively weak, high-energy double bond. [3] [1] Glycolytic ATP, however, is created more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. For multicellular organisms, during short bursts of strenuous activity, muscle cells use fermentation to supplement the ATP production from the slower aerobic respiration, so fermentation may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting.

        Cellular respiration is the process by which biological fuels are oxidised in the presence of a high-energy inorganic electron acceptor (such as oxygen [1] ) to produce large amounts of energy, to drive the bulk production of ATP.

        Anaerobic respiration is used by some microorganisms in which neither oxygen (aerobic respiration) nor pyruvate derivatives (fermentation) is the high-energy final electron acceptor. Rather, an inorganic acceptor such as sulfate (SO42-), nitrate (NO3–), or sulfur (S) is used. [12] Such organisms are typically found in unusual places such as underwater caves or near hydrothermal vents at the bottom of the ocean.

        In July 2019, a scientific study of Kidd Mine in Canada discovered sulfur-breathing organisms which live 7900 feet below the surface, and which breathe sulfur in order to survive. These organisms are also remarkable due to consuming minerals such as pyrite as their food source. [13] [14] [15]


        ACKNOWLEDGMENTS

        I would like to thank P. Bhargava, S. H. Snyder, P. A. Calabresi, P. M. Kim, E. Abramson, A. Snowman, M. Singh, B. Paul, L. Hester, V. Miller, R. Barrow, L. Albacarys, S. McTeer, S. Reese, V. Putluri, N. Putluri, M. D. Smith, and A. Hwang for their support and collaboration. M. K. receives funding from the Conrad N. Hilton Foundation (Marilyn Hilton Bridging Award for Physician Scientists, 17316), Race to Erase MS (Young Investigator Award, 90079114), and National Institute of Neurological Disorders and Stroke (K08NS104266).


        Kyk die video: Glikoliza (Oktober 2022).