Inligting

8.0: Voorspel tot Fotosintese - Biologie

8.0: Voorspel tot Fotosintese - 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.

Die prosesse in alle organismes—van bakterieë tot mense—vereis energie. Om hierdie energie te kry, kry baie organismes toegang tot gestoorde energie deur te eet, dit wil sê deur ander organismes in te neem. Maar waar kom die gestoorde energie in voedsel se oorsprong? Al hierdie energie kan teruggevoer word na fotosintese.


IGCSE & GCSE Biology deur D. G. Mackean

Hier sal jy die antwoorde vind op die 'in-teks' vrae wat voorkom in IGCSE Biologie (2de uitgawe) en GCSE Biologie (3de uitgawe) deur D. G. Mackean, uitgegee deur Hodder Education, Londen, VK.

Hoofstuk 5. Fotosintese en voeding in plante

1. a koolstofdioksied, d water, e chlorofil, g lig.
2. Die wit dele van die blaar bevat nie chlorofil nie en dien as 'n kontrole.
3. Om 'n blaar te ontstysel beteken om aan die begin van die eksperiment stappe te doen om te verseker dat dit nie stysel bevat nie. Indien die blare reeds stysel bevat, kan nie gedemonstreer word dat 'n bepaalde proses nodig is vir die produksie van stysel nie.
4. 'n Sodakalk absorbeer koolstofdioksied en dus word die blaar van hierdie gas ontneem.
b Natriumwaterstofkarbonaat in oplossing ontbind om koolstofdioksied te vorm. Dit verseker dat die aanleg 'n voldoende toevoer van hierdie gas het.
c Die politeensak keer dat koolstofdioksied in die lug die plant bereik.
5. Deur damkruid te gebruik is dit moontlik om die gasborrels uit die blare te sien ontsnap. Die keuse van damkruid kan lei tot die bewering dat die produksie van suurstof in lig slegs in damkruid plaasgevind het.
6. Om 'n plant van water te ontneem sal lei tot verwelking en uiteindelik tot die dood van die plant, ongeag die rol van water in fotosintese.
7. Ja. Ontstysel word bereik deur 'n plant vir 2 of drie dae in die donker te laat sodat stysel van die blare verwyder kan word. Daar word aanvaar dat geen nuwe stysel in duisternis gevorm sal word nie.

1. Koolstofdioksied uit die lug. Water uit die grond.
2. a Die palissade selle. Hulle het die grootste aantal chloroplaste en is die naaste aan die ligbron.
b Die sponsagtige mesofilselle. Hulle het minder chloroplaste en is verder van die ligbron af.
c Die selle van die epidermis. Hulle het geen chloroplaste nie
3. a Die energie vir fotosintese kom van sonlig.
b Respirasie is die bron van energie vir alle ander lewende prosesse.


Sleutelkonsepte

  • Chlorofilbevattende organismes, soos groen plante, gebruik fotosintese (letterlik "maak met lig") om koolhidrate en suurstof uit water en koolstofdioksied te produseer, in die teenwoordigheid van lig.
  • Sommige bakterieë voer anoksigeniese fotosintese uit, wat ander elektronskenkers as water gebruik en nie suurstof produseer nie.
  • In ligreaksies absorbeer fotosisteme ligenergie om die elektronvervoerketting aan te dryf en ATP en NADPH te produseer.
  • ATP en NADPH dryf die koolstofbindende donkerreaksies, of Calvin-siklus, aan wat organiese molekules uit koolstofdioksied sintetiseer.
  • C4 fotosintese en krassulaatsuurmetabolisme (CAM) is plantaanpassings by droë omgewings.

Die sintese van chemiese verbindings met behulp van lig, veral die vervaardiging van organiese verbindings (hoofsaaklik koolhidrate) uit koolstofdioksied en 'n waterstofbron (soos water), meestal met gelyktydige vrystelling van suurstof, deur chlorofil- of bakteriochlorofilbevattende selle. Die term fotosintese word feitlik uitsluitlik gebruik om een ​​besonder belangrike natuurlike proses aan te dui: die gebruik van lig in die vervaardiging van organiese verbindings (hoofsaaklik sekere koolhidrate) uit anorganiese materiale deur chlorofil- of bakteriochlorofilbevattende selle (Fig. 1). Hierdie proses vereis 'n toevoer van energie in die vorm van lig omdat sy produkte baie meer chemiese energie as sy grondstowwe bevat. Dit word duidelik getoon deur die vrystelling van energie in die omgekeerde proses, naamlik die verbranding van organiese materiaal met suurstof, wat tydens respirasie plaasvind. Sien ook: Koolhidrate Chlorofil Energie metabolisme Plant asemhaling Asemhaling

Onder chlorofilbevattende plant- en algselle, sowel as in sianobakterieë (voorheen bekend as blougroen alge), behels fotosintese die oksidasie van water (H)2O) om suurstof te produseer (O2) molekules, wat dan in die omgewing vrygestel word. Dit word suurstoffotosintese genoem (Fig. 2). In teenstelling hiermee behels bakteriese fotosintese nie O2 evolusie (produksie). In hierdie geval, ander elektronskenkers, soos waterstofsulfied (H2S), word gebruik in plaas van H2O. Hierdie proses word anoksigeniese fotosintese genoem. Sien ook: Alge Sianobakterieë Suurstof Plant Plantsel Plantfisiologie Water

Omdat alle soorte fotosintese lig benodig, is fotosintetiese organismes oor die algemeen beperk tot die nou gebied van die Aarde naby die oppervlak wat sonlig ontvang. Die enigste bekende uitsonderings is die anoksigeniese fotosintetiese bakterieë wat naby diepsee hidrotermiese vents woon en wat die baie swak lig wat deur die warm vents uitgegee word, benut. Die ligenergie wat deur die pigmente van fotosinteserende selle geabsorbeer word, veral deur chlorofil- of bakteriochlorofilpigmente, word doeltreffend in gestoorde chemiese energie omgeskakel. Saam kombineer die twee aspekte van fotosintese - die omskakeling van anorganiese materiaal in organiese materiaal en die omskakeling van ligenergie in chemiese energie - om die fundamentele proses van lewe op Aarde te skep: dit is die uiteindelike bron van alle lewende materie en van byna al die energie van die lewe. Sien ook: Hidrotermiese ventilasie

Suurstoffotosintese

Die netto algehele chemiese reaksie van suurstoffotosintese (deur plante, alge en sianobakterieë) word in reaksie (1) getoon: (1)

waar <>2O> staan ​​vir 'n koolhidraat (suiker). Die fotochemiese reaksie in fotosintese behoort aan die tipe bekend as oksidasie-reduksie, met koolstofdioksied (CO2) wat as die oksidant (elektronontvanger) optree en water wat as die reduktant (elektronskenker) optree. Die unieke kenmerk van hierdie spesifieke oksidasie-reduksie is dat dit energeties ongunstig is, dit wil sê, dit omskep chemies stabiele materiale in chemies onstabiele produkte. Ligenergie word gebruik om hierdie "opdraande" reaksie moontlik te maak (Fig. 2). 'n Aansienlike deel van die ligenergie wat vir hierdie proses gebruik word, word as chemiese energie gestoor. Sien ook: Koolstofdioksied Oksidasie-vermindering

Temporele fases van fotosintese

Fotosintese is 'n komplekse veelfase-proses wat uit byna honderd fisiese prosesse en chemiese reaksies bestaan. Om hierdie komplekse proses meer verstaanbaar te maak, is dit nuttig om dit in vier temporele stadiums te verdeel. Elke fase is rofweg gebaseer op die tydskaal waarin dit plaasvind. Hierdie fases is (1) fotonabsorpsie en energieoordragprosesse in antennas (of antennachlorofille, dit wil sê molekules wat ligkwanta versamel) (2) primêre elektronoordrag in fotochemiese reaksiesentrums (3) elektronvervoer en adenosientrifosfaat (ATP) vorming en (4) koolstofbinding en uitvoer van stabiele produkte (Fig. 2). Sien ook: Adenosientrifosfaat (ATP)

Sites van fotosintese

Die fotosintetiese proses in plantselle en alge vind plaas binne pigmentdraende subsellulêre organelle genaamd chloroplaste (wat selplastiede is) [Fig. 3]. In die blare van die hoër landplante is hierdie organelle gewoonlik plat ellipsoïede wat ongeveer 5 μm in deursnee en 2,3 μm in dikte meet. Tien tot 100 van hulle kan in die gemiddelde parenchiemsel van 'n blaar voorkom. Onder die elektronmikroskoop toon alle chloroplaste 'n gelaagde struktuur met afwisselende ligter en donkerder lae van ongeveer 0,01 μm dik. Hierdie lae is membrane, genoem tilakoïedmembrane (tilakoïed staan ​​vir 'n membraansak), wat proteïene bevat. Hierdie proteïene bind al die chlorofil. Die tilakoïedmembrane is die plekke van die eerste drie fases van fotosintese. By alge is die aantal en vorm van die chloroplaste baie meer veranderlik. Sien ook: Selplastiede Blaar Parenchiem Plant anatomie

Die fotochemiese apparaat is minder kompleks in sianobakterieë. Hierdie selle is prokariote en het dus nie 'n kern en ander organelle nie, insluitend chloroplaste en mitochondria. Die vroeë fases van fotosintese vind plaas op tilakoïedmembrane, wat dwarsdeur die binnekant van die sel strek. Sien ook: Prokariote

Twee fotosisteme

Twee fotochemiese gebeurtenisse werk saam om suurstoffotosintese uit te voer (Fig. 2). Eksperimente dui daarop dat plante twee pigmentstelsels bevat het. Een (genoem fotosisteem I, of PSI sensitiseringsreaksie I) bestaan ​​hoofsaaklik uit chlorofil a die ander (genoem fotosisteem II, of PSII-sensitiseringsreaksie II) is ook saamgestel uit chlorofil a, maar sluit die meeste van chlorofil in b of ander hulppigmente (insluitend die karotenoïede en die phycobilins). Doeltreffende fotosintese vereis die absorpsie van 'n gelyke aantal kwanta in PSI en in PSII. Dit verseker dat die opwekkingsenergie binne beide stelsels deur die antennastelsel geabsorbeer word en aan elke fotostelsel verdeel word, waar die energie die chemiese reaksies dryf. Die PSII-reaksie is die een wat die naaste met O2 evolusie. Die finale resultaat van hierdie stel reaksies is die oksidasie van water na O2 en die reduksie van 'n plastokinoon ('n oksidasie-reduksie katalisator). Huidige bewyse dui daarop dat lig wat deur die grootste deel van die bykomstige pigmente geabsorbeer word, uiteindelik na 'n spesiale chlorofil oorgedra word. a molekule in die PSII-reaksiesentrum, wat in 'n gunstige posisie is om as 'n energieval op te tree.

Die twee fotochemiese gebeurtenisse (PSI en PSII) vind plaas op vier groot proteïenkomplekse wat in die tilakoïedmembraan ingebed is (Fig. 4). Let daarop dat die membraan 'n inherente asimmetrie het deurdat die proteïenkomplekse op 'n bepaalde manier in die membraan georiënteer is. Hierdie oriëntasie is noodsaaklik vir die behoorlike funksionering van fotosintese.

Figuur 5 toon die Z-skema aan, wat die manier omskryf waarop die twee fotosisteme saamwerk om die elektronoordragreaksies wat by fotosintese betrokke is, uit te voer. Dit is 'n energetiese diagram, deurdat die energie van die komponent, wat gemeet word as die middelpunt redokspotensiaal Em, word op die gewys y as en die vordering van die reaksie word op die getoon x as. Die twee vertikale pyle in die diagram verteenwoordig energie-invoer na die stelsel as gevolg van fotonabsorpsie. Sien ook: Foton

Fotofosforilering

Wanneer verlig in die teenwoordigheid van adenosiendifosfaat (ADP) en anorganiese fosfaat (Pi), die binneste sitoplasmiese membrane van fotosintetiese bakterieë, sianobakteriese selle en chloroplaste van groen plante en alge gebruik ligenergie om adenosientrifosfaat (ATP) te sintetiseer. Ongeveer 42 kilojoules (kJ) omgeskakelde ligenergie in hierdie reaksie word in elke mol van die hoë-energie fosfaat, ATP, gestoor. Hierdie fotofosforilering word gepaard met energievrystellende stappe in fotosintese, soos die elektronvloei van PSII na PSI. Wanneer fosforilering geassosieer word met niesikliese elektronvloei vanaf H2O tot NADP + , dit word niesikliese fotofosforilering genoem. Daarbenewens, onder sekere toestande, kan elektrone in PSI, in plaas daarvan om na NADP + te gaan, terugkeer na 'n intermediêre (soos 'n sitochroom, plastokinoon of plastosianien) en sodoende die siklus sluit. Hierdie tipe sikliese elektronvloei, bemiddel deur bygevoegde kofaktore sowel as ADP en anorganiese fosfaat, lei ook tot die produksie van ATP en is sikliese fosforilering genoem, dit is bewys dat dit onder sekere eksperimentele toestande in vivo bestaan. Sien ook: Nikotinamied adenien dinukleotied (NAD)

Bykomstige antenna pigmente

Behalwe chlorofil a (wat in byna alle suurstoffotosintetiese organismes teenwoordig is), is daar ander chlorofille, insluitend chlorofil b in die groen alge en hoër plante. Daarbenewens, chlorofil c vervang chlorofil b in bruin alge, terwyl die meeste van die chlorofil a in die mariene sianobakterie Acaryochloris marina word deur chlorofil vervang d. Daar is ook nie-chlorofilagtige pigmente wat aan twee groepe behoort: die karotenoïede en die fikobiliene. Die karotenoïede (genoem vanweë hul ooreenkoms met die oranje pigment van wortels) is 'n veranderlike verskeidenheid pigmente wat in alle fotosintetiese hoër plante en alge voorkom. Die phycobilins, of plantaardige galpigmente, is chemies verwant aan dierlike galpigmente. Hulle is óf rooi (phycoerythrin) of blou (phycocyanin). Al hierdie pigmente word met spesifieke proteïene geassosieer om sogenaamde antenna-pigmentproteïene te vorm. Die strukture van baie van hierdie antennakomplekse is bekend, en hul absorpsiespektra is ontleed (Fig. 6). Sien ook: Karotenoïede Phycobilin

Koolstofdioksiedbinding

Die ligafhanklike omskakeling van stralingsenergie in chemiese energie as adenosientrifosfaat (ATP) en verminderde nikotinamied adenien dinukleotiedfosfaat (NADPH) dien as 'n voorspel tot die gebruik van hierdie verbindings vir die reduktiewe fiksasie van CO2 in organiese molekules. Sulke molekules, wat breedweg as fotosintete aangedui word, is gewoonlik (maar nie altyd nie) in die vorm van koolhidrate (byvoorbeeld glukose polimere of sukrose) en vorm die basis vir die voeding van alle lewende dinge, en dien ook as die beginmateriaal vir brandstof, vesel, veevoer, olie en ander verbindings wat deur mense gebruik word. Gesamentlik, die biochemiese prosesse waardeur CO2 geassimileer word in organiese molekules staan ​​bekend as die fotosintetiese donkerreaksies, wat so genoem word omdat lig nie nodig is nie (in teenstelling met die fotosintetiese ligreaksies). Veral CO2 fiksasie deur fotosintetiese organismes is 'n belangrike meganisme waardeur hierdie "kweekhuis" gasmolekule uit die atmosfeer verwyder word tydens koolstoffietsry op Aarde. Ongeveer 100 pentagram koolstof (1 pentagram is gelyk aan 10 9 metrieke ton) as CO2 word jaarliks ​​deur fotosintese in organiese molekules geassimileer (ongeveer die helfte van hierdie hoeveelheid word deur fotosintetiese mariene alge geassimileer).

C3 fotosintese

Die noodsaaklike besonderhede van C3 fotosintese kan gesien word in Figuur 7. Die hele siklus kan in drie fases geskei word—karboksilering, reduksie en regenerasie. Vir die doeleindes van begrip, is dit die maklikste om met drie molekules CO te begin2 omdat die kleinste intermediêre in die siklus uit drie koolstofstowwe bestaan. Tydens die aanvanklike karboksileringsfase het die drie molekules van CO2 word gekombineer met drie molekules van die vyfkoolstofverbinding ribulose 1,5-bisfosfaat (RuBP) in 'n reaksie wat deur die ensiem RuBP karboksilase/oksigenase (Rubisco) gekataliseer word om drie molekules van 'n intermediêre, onstabiele ensiemgebonde seskoolstofverbinding te vorm . Hierdie onstabiele molekules word verder gehidroliseer tot ses molekules van die driekoolstofverbinding fosfogliseriensuur (PGA). Hierdie produkte van die karboksileringsfase, dit wil sê die ses (drie-koolstof) PGA-molekules, word gefosforileer deur ses molekules ATP (wat ADP vrystel om vir fotofosforilering via die ligreaksies gebruik te word) om ses 1,3-bisfosfogliseraat te vorm (1) ,3-BP) molekules. Die resulterende verbindings word verminder (dit wil sê in die reduksiefase van die C3 siklus) deur die NADPH wat in die fotosintetiese ligreaksies gevorm word om ses molekules van die driekoolstofverbinding fosfogliseraldehied (PGAL) te vorm. PGAL word geïsomeriseer om nog 'n driekoolstofverbinding, dihidroksiesetoonfosfaat (DHAP) te vorm. PGAL (die aldehied) en DHAP (die ketoon) is energeties ekwivalente, gereduseerde verbindings en kan beskou word as die produkte van die reduktiewe fase van die C3 fotosintetiese siklus. PGAL en DHAP vorm saam die triose fosfaat (TP) poel van die chloroplast. Die chloroplast TP-poel is hoofsaaklik saamgestel uit PGAL (die isomerase wat verantwoordelik is vir PGAL:DHAP-interomsetting bevoordeel PGAL-vorming).

Die res van die C3 fotosintetiese siklus (die regenerasiefase) behels ensiematiese stappe wat regenerasie van RuBP, die aanvanklike karboksileringssubstraat, moontlik maak. Een molekule PGAL word beskikbaar gestel vir kombinasie met DHAP wat uit 'n tweede PGAL geïsomeriseer is (wat 'n tweede "draai" van die Calvin-Benson-Bassham-sikluswiel vereis) om 'n ses-koolstof suiker te vorm. Die ander vyf PGAL-molekules word deur 'n komplekse reeks ensiematiese reaksies herrangskik in drie molekules RuBP, wat weer met CO2 gekarboksileer kan word2 om die siklus voort te sit.

Daar moet kennis geneem word dat die ensiem RuBP karboksilase/oksigenase (Rubisco) [Fig. 8] wat CO2 in 'n organiese verbinding laat ook O2 om met RuBP te reageer—vandaar die "oksigenase" in die naam. Hierdie reaksie begin die proses genaamd fotorespirasie, wat lei tot die vrystelling van een voorheen geïnkorporeerde molekule CO22 vir elke twee molekules van O2 wat toegelaat word om te reageer. As gevolg van sy lae katalitiese doeltreffendheid, kan Rubisco tot die helfte van die oplosbare proteïen in C wees3 chloroplaste, en dit is waarskynlik die volopste proteïen wat in die natuur voorkom. Struktureel is Rubisco 'n groot en komplekse ensiem wat uit agt groot polipeptiedsubeenhede en agt klein subeenhede bestaan. Sien ook: Ensiem Fotorespirasie

Die netto produk van twee "draaie" van die siklus, dit wil sê 'n ses-koolstof suiker (G6P of F6P), word gevorm óf binne die chloroplast in 'n pad wat lei na stysel ('n polimeer van baie glukose molekules) of ekstern in die sitoplasma in 'n pad wat na sukrose lei (gekondenseer uit twee ses-koolstof suikers, glukose en fruktose). Hierdie verdeling van nuutgevormde fotosintaat lei tot twee afsonderlike poele stysel word in die fotosinteterende "bron" blaarselle gestoor, en sukrose is beskikbaar vir onmiddellike metaboliese vereistes binne die sel of vir uitvoer na "sinks", insluitend die ontwikkeling van voortplantingstrukture, wortels, of ander blare. Faktore binne die fotosinteserende sel, soos energiebehoeftes in verskillende kompartemente (mitochondria, sitoplasma en chloroplaste), saam met energiebehoeftes van die plant (byvoorbeeld verhoogde sinkbehoeftes tydens verskillende ontwikkelingstadia), en eksterne omgewingsfaktore (byvoorbeeld, ligintensiteit en -duur) reguleer uiteindelik die verdeling van die nuutgevormde fotosintetiese produk (PGAL) in stysel of sukrose. Sien ook: Plant metabolisme

C4 fotosintese

Aanvanklik het die C3 siklus was vermoedelik die enigste roete vir CO2 assimilasie, hoewel dit deur plantanatome erken is dat sommige vinnig groeiende plante (soos mielies, suikerriet en sorghum) 'n ongewone organisasie van die fotosintetiese weefsels in hul blare besit (Kranz-morfologie). Verdere werk het getoon dat plante met die Kranz-anatomie 'n bykomende CO gebruik het2 assimilasieroete, wat nou bekend staan ​​as die C4-dikarboksielsuurweg (Fig. 9). Koolstofdioksied gaan 'n mesofilsel binne, waar dit gekombineer word (in die vorm van bikarbonaat) met die driekoolstofverbinding fosfoenolpiruvaat (PEP) via die ensiem PEP-karboksilase om 'n vierkoolstofsuur, oksaloasetaat, te vorm, wat gereduseer word tot appelsuur of na asparaginsuur oorgedra. Die vierkoolstofsuur beweeg in bondelskede-selle in, waar die suur gedekarboksileer word en die CO2 herassimileer via die C3 siklus. Om die siklus te voltooi, beweeg die resulterende driekoolstofverbinding, pirodruivensuur, terug na die mesofilsel en word dit omskep in PEP (ten koste van 2 ATP-molekules) via die ensiem piruvaatfosfaatdikinase wat in die mesofilchloroplaste geleë is. Die netto effek van hierdie siklus is om die CO te verhoog2 konsentrasie rondom Rubisco, waardeur fotorespirasie verminder word via die mededingende oksigenase-aktiwiteit van hierdie ensiem.

C4 metabolisme word in drie tipes geklassifiseer, afhangende van die primêre dekarboksileringsreaksie wat met die vierkoolstofsuur in die bondelskede-selle gebruik word. Die meerderheid van C4 spesies (voorbeeld deur suikerriet, mielies, krabgras en sorghum) behoort aan tipe 1 en gebruik NADP-appelsiekte ensiem (NADP-ME) vir dekarboksilering. NAD-appelagtige ensiem (NAD-ME) C4 plante behoort aan tipe 2 en sluit in Amaranthus, Atripleks, gierst, varkkruid en postelein in teenstelling, C4 plante wat as tipe 3 gekategoriseer word, gebruik fosfoenolpiruvaatkarboksikinase (PCK) vir dekarboksilering en sluit in Paniek grasse.

Crassulacean suur metabolisme (CAM) fotosintese

Onder droë en woestyntoestande, waar grondwater 'n tekort is, kan transpirasie gedurende die dag, wanneer temperature hoog is en humiditeit laag is, die plant vinnig van water uitput, wat tot uitdroging en dood lei. Deur huidmondjies gedurende die dag toe te hou, kan water egter bewaar word, die opname van CO2, wat geheel en al deur die huidmondjies voorkom, word voorkom. Daarom het baie woestynplante (insluitend dié in die Crassulaceae-, Cactaceae- en Euphorbiaceae-families) ontwikkel, klaarblyklik onafhanklik van C4 plante, 'n soortgelyke strategie om CO te konsentreer en te assimileer2 waardeur die CO2 word in die nag ingeneem wanneer die huidmondjies in die algemeen oopgaan, is waterverlies laag as gevolg van die verlaagde temperature en dienooreenkomstig hoër humiditeite. Die biochemiese begrip van die meganismes betrokke by hierdie proses is vir die eerste keer bestudeer in plante van die Crassulaceae-familie, dus word die proses Crassulacean-suurmetabolisme (CAM) genoem. Sien ook: Plant-water verhoudings

In teenstelling met C4 fotosintese, waar twee seltipes gewoonlik saamwerk, vind die hele CAM-proses binne 'n individuele sel plaas, die skeiding van C4 en C3 is dus tydelik eerder as ruimtelik. In die nag, CO2 kombineer met PEP deur die werking van PEP-karboksilase, wat lei tot die vorming van oksaloasynsuur en die omskakeling daarvan in appelsuur. Die PEP word gevorm uit stysel of suiker via die glikolitiese roete van asemhaling. Daar is dus 'n daaglikse wederkerige verhouding tussen stysel ('n bergingsproduk van C3 fotosintese) en die ophoping van appelsuur (die terminale produk van nag CO2 assimilasie) [Fig. 10].

Bakteriese fotosintese

Sekere bakterieë het die vermoë om fotosintese uit te voer. 'n Algemene vergelyking vir bakteriese fotosintese word in reaksie (2) getoon: (2)

waar A enige van 'n aantal reduktante verteenwoordig, mees algemeen S (swael). Omdat fotosintetiese bakterieë nie water as die waterstofskenker kan gebruik nie, is hulle nie in staat om suurstof te ontwikkel nie. Hulle word dus anoksigeniese fotosintetiese bakterieë genoem. (Let daarop dat die prokariotiese sianobakterieë uitgesluit word omdat hul fotosintetiese sisteem baie ooreenstem met dié wat in eukariotiese alge en hoër plante voorkom.) Anoksigeniese fotosintetiese bakterieë kan in vier hoofgroepe geklassifiseer word: (1) Proteobakterieë, insluitend nieswaelpers bakterieë (Rhodospirillaceae) bakterieë (Chromatiaceae) (2) groen swael bakterieë (Chlorobiaceae) (3) groen gly bakterieë (Chloroflexi) en (4) Heliobacteria (Heliobacteriaceae). Soos plante, alge en sianobakterieë, is anoksigeniese fotosintetiese bakterieë in staat tot fotofosforilering, wat die produksie van adenosientrifosfaat (ATP) uit adenosiendifosfaat (ADP) en anorganiese fosfaat (P) is.i) lig as die primêre energiebron gebruik.

Fotosintetiese bakterieë het nie gespesialiseerde organelle soos die chloroplaste van groen plante nie. Elektronmikrofoto's van sekere fotosintetiese bakterieë toon klein sferiese sakkies, met dubbellaagse mure, as gevolg van invaginasies wat stapels membrane vorm. Ander fotosintetiese bakterieë het invaginasies wat tilakoïede vorm. Hierdie intrasitoplasmiese membrane, wat dikwels chromatofore genoem word, bevat die fotosintetiese apparaat en kan maklik geïsoleer word deur meganiese ontwrigting van bakterieë gevolg deur differensiële sentrifugering. Geïsoleerde chromatofore word dikwels gebruik vir biochemiese en biofisiese studies van bakteriese fotosintese. Sien ook: Chromatofore

Die pigment bakteriochlorofil (BChl) is 'n noodsaaklike komponent vir bakteriese fotosintese. Daar is gespesialiseerde BChl-molekules in bakterieë wat betrokke is by die primêre chemiese reaksies van fotosintese. Benewens hierdie gespesialiseerde molekules, is daar 40–50 BChl-molekules waarna verwys word as antennapigmente, wie se enigste funksie is om ligenergie te oes en dit na reaksiesentrummolekules oor te dra. Dit is soortgelyk aan die fotosintetiese eenheid van plante, alge en sianobakterieë. Elke reaksiesentrum bevat 'n spesiale paar (dimeer) BChl-molekules wat in chemiese reaksies betrokke raak nadat hulle die geabsorbeerde ligenergie vasvang. Hulle word ook die energievalle van bakteriese fotosintese genoem. Sien ook: Bakteriële fisiologie en metabolisme


28.1: Voorspel tot Fotochemie

  • Bygedra deur John D. Roberts en Marjorie C. Caserio
  • Professore (Chemie) by California Institute of Technology

Die rol van lig om chemiese verandering te bewerkstellig word al vir baie jare erken. Die verband tussen sonenergie en die biosintese van plantkoolhidrate uit koolstofdioksied en water was inderdaad teen die vroeë 1800's bekend. Tog was organiese fotochemie stadig om te ontwikkel as 'n goed verstaanbare en hanteerbare wetenskap. Vordering het eers vinnig geword ná die ontwikkeling van spektroskopie en spektroskopiese tegnieke vir struktuurbepaling en die opsporing van verbygaande spesies. Om hierdie rede was fotochemie vir baie jare die domein van fisiese en teoretiese chemici. Hul werk het die grondslag gelê vir moderne organiese fotochemie, wat die aard van opgewonde elektroniese toestande van molekules korreleer met die reaksies wat hulle ondergaan.

Afgesien van die ongeëwenaarde belangrikheid van fotosintese, het fotochemiese reaksies 'n groot impak op biologie en tegnologie, beide goed en sleg. Visie by alle diere word deur fotochemiese reaksies veroorsaak. Die vernietigende uitwerking van ultraviolet bestraling op alle vorme van lewe kan herlei word na fotochemiese reaksies wat sellulêre DNA verander, en die skadelike effekte van oormatige blootstelling aan sonlig en die gevolglike voorkoms van velkanker is goed gevestig. Die tegniese toepassings van fotochemie is veelvuldig. Die kleurstofbedryf is gebaseer op die feit dat baie organiese verbindings bepaalde golflengtes van sigbare lig absorbeer, en die soeke na beter kleurstowwe en pigmente rondom die draai van hierdie eeu was grootliks verantwoordelik vir die ontwikkeling van sintetiese organiese chemie. Kleurstofchemie het gehelp om die verband tussen chemiese struktuur en kleur vas te stel, wat ook belangrik is in kleurdrukwerk en kleurfotografie. Ons dek hierdie belangrike toepassings van fotochemie slegs kortliks in hierdie hoofstuk, maar ons hoop om 'n mate van begrip van die betrokke grondbeginsels oor te dra.

Die meeste fotochemiese reaksies kan in drie fases geag word:

1. Absorpsie van elektromagnetiese straling om elektronies opgewonde toestande te produseer.
2. Primêre fotochemiese reaksies wat opgewonde elektroniese toestande insluit.
3. Sekondêre of donker reaksies waardeur die produkte van die primêre fotochemiese reaksies omgeskakel word na stabiele produkte.

Ons sal begin met 'n nader kyk na elektroniese opwekking, waarvan sommige aspekte in Afdeling 9-9 bespreek is. Omdat die oordrag van elektroniese energie van een molekule na 'n ander 'n basiese proses in fotochemie is, sal ons energie-oordrag ook bespreek voordat ons 'n oorsig gee van verteenwoordigende fotochemiese reaksies. Die nou verwante verskynsels van chemiluminessensie en bioluminessensie sal dan beskryf word. Laastens sal daar 'n bespreking wees van verskeie belangrike toepassings van fotochemie.


Stysel Toets Eksperiment

Wat jy nodig het:

Toets vir stysel in plante:

  1. Plaas een van die plante in 'n donker kamer vir 24 uur plaas die ander een op 'n sonnige vensterbank.
  2. Wag 24 uur.
  3. Vul die beker of fles met etielalkohol.
  4. Plaas die beker of fles in 'n kastrol vol water.
  5. Verhit die pan totdat die etielalkohol begin kook.
  6. Verwyder van die hitte.
  7. Doop elk van die blare in die warm water vir 60 sekondes, met 'n pincet.
  8. Gooi die blare vir twee minute in die beker of fles etielalkohol (of totdat hulle amper wit word).
  9. Plaas hulle elkeen in 'n vlak skottel.
  10. Bedek die blare met 'n bietjie jodiumoplossing en kyk.

Wat het gebeur:

Die warm water maak die blaar dood en die alkohol breek die chlorofil af en neem die groen kleur uit die blaar. Wanneer jy jodium op die blare sit, sal een van hulle blou-swart word en die ander sal 'n rooibruin wees. Jodium is 'n aanduiding wat blou-swart word in die teenwoordigheid van stysel. Die blaar wat in die lig was, word blou-swart, wat wys dat die blaar fotosintese uitvoer en stysel produseer.

Probeer die toets weer met 'n bont blaar (een met beide groen en wit) wat in die sonlig was. 'n Blaar het chlorofil nodig om fotosintese uit te voer - gebaseer op daardie inligting, waar op die bont blaar dink jy sal jy stysel vind?


Sikliese fotofosforilering

Sikliese fotofosforilering is 'n alternatiewe meganisme van elektronvervoer in die tilakoïedmembraan, en dit gebruik slegs PSI. Hierdie elektronvervoerketting is siklies: elektrone in PSI word foto-geaktiveer en aan ferredoksien geskenk, hulle word dan na die sitochroom b oorgedra.6-f-kompleks (in plaas van Fd NADP-reduktase) en uiteindelik reis hulle terug na PSI via 'n rekenaar. Hierdie elektron vervoerketting genereer 'n waterstof elektrochemiese gradiënt daarom vind ATP sintese plaas. Anders as lineêre fotofosforilering, genereer sikliese fotofosforilering nie NADPH of maak suurstof vry nie. Chloroplaste gebruik beide lineêre en sikliese fotofosforilering om die relatiewe vlakke van NADPH en ATP te verander.


8.0: Voorspel tot Fotosintese - Biologie

Fotosintese en die Reef Akwarium,
Deel I: Koolstofbronne

Fotosintese is die proses waardeur organismes ligenergie inneem en dit in bruikbare chemiese energie omskakel. Dit is 'n uiters belangrike proses in die meeste rif-akwariums, maar een waaraan die meeste akwaristen min aandag gee, afgesien van die erkende belangrikheid van toepaslike beligting. Hierdie artikel is die eerste in 'n reeks wat kyk na fotosintese in rif akwariums vanuit 'n chemiese perspektief. Sulke chemiese kwessies sluit byvoorbeeld in hoe organismes die grondstowwe vir fotosintese kry, of akwariste dit moet "aanvul", hoe organismes die "afval" produkte van fotosintese uitskakel, wat is die chemiese implikasies van te veel of te min lig, hoe verkalking in korale en mossels hou verband met fotosintetiese doeltreffendheid, wat die biochemiese masjinerie is om lig te versamel en dit in energie om te skakel, en hoe organismes hierdie prosesse in verhouding tot hul natuurlike habitatte ontwikkel het.

Die antwoorde op hierdie vrae kan 'n belangrike invloed hê op boerderypraktyke op maniere wat rifakwariste dalk nie oorweeg het nie. Onderwerpe wat in hierdie artikel behandel word, sluit veral in of die pH of alkaliniteit van 'n akwarium of 'n refugium die tempo van fotosintese kan beïnvloed, en of akwariumkundiges die beskikbaarheid van koolstofdioksied vir fotosinteserende organismes moet oorweeg.

Die mees vereenvoudigde chemiese vergelyking wat fotosintese beskryf, is:

koolstofdioksied + water + ligte koolhidraat plus suurstof

Hierdie artikel handel hoofsaaklik oor die eerste reaktant in hierdie vergelyking, koolstofdioksied. Die prosesse wat lei tot die opname van koolstofdioksied deur fotosinteserende mariene organismes is 'n aktiewe navorsingsgebied, met die meeste van die relevante publikasies op hierdie gebied wat eers in die afgelope vyf jaar vrygestel is. Dit blyk dat die simbiotiese dinoflagellate (zooxanthellae) binne korale en mossels 1 'n spesiale geval is in terme van koolstofdioksied-verkryging as gevolg van die omliggende gasheerdier, sowel as die aansienlike hoeveelheid verkalking wat in dieselfde organisme plaasvind. Omdat fotosintese en verkalking chemies onderling verwant kan wees, sal die spesiale aspekte van fotosintese in simbiotiese en verkalkende organismes in 'n toekomstige artikel uiteengesit word.

Freshwater aquarists caring for brightly-lit planted aquaria have long known the importance of CO2, and often add carbon dioxide directly to the aquarium water in one way or another to supply those tanks' substantial need for this material. Reef aquarists, on the other hand, might have just as much or more photosynthesis taking place, but rarely worry about adding carbon dioxide. Hoekom? That's one of the topics to be detailed in subsequent sections of this article. The answer is not that seawater contains more CO2 than does freshwater, but rather that seawater contains other chemicals that can, in some cases, be used to supply carbon dioxide.

The contents of this article are:

M any organisms in a reef aquarium rely on photosynthesis to survive. These include diatoms, green hair algae, cyanobacteria, macroalgae, Tridacna clams and most corals and anemones that aquarists maintain. In the case of clams, corals and anemones, this photosynthesis is actually carried out by symbiotic organisms (zooxanthallae) that live within the tissue of the host animal. In every case, however, the cells that photosynthesize need to incorporate carbon dioxide somehow, and they excrete oxygen.

Sometimes obtaining adequate carbon dioxide is easy for photosynthesizing organisms, and sometimes it is difficult, requiring them to develop special mechanisms to obtain it rapidly enough. In order to understand how this happens in a reef aquarium, it is first necessary to understand what happens to carbon dioxide when it dissolves into seawater.

Carbon Dioxide in Seawater

C arbon dioxide is an interesting molecule. When it dissolves into water it can take a number of different forms. Even the rate at which it can move between some of these forms impacts how organisms must develop special mechanisms to be able to take up enough during rapid photosynthesis.

Carbon dioxide is present at about 350 ppm in normal air. It was lower in the past, and has been steadily rising for the past 100 years or so, largely due to the burning of fossil fuels. A liter of air weighs about 1.3 grams, so at 350 ppm carbon dioxide, that liter of air contains about 0.00046 grams (0.5 mg) of carbon dioxide. This very low amount, coupled with the kinetic issues (i.e., the slowness) of carbon dioxide's entry into seawater, explains why it is often difficult to keep reef aquarium water aerated enough to keep the pH from rising when processes such as photosynthesis or the addition of limewater consume carbon dioxide.

When a gas phase carbon dioxide molecule enters water, it is initially hydrated to carbonic acid:

That hydration process is surprisingly slow because it's an actual chemical reaction, as shown schematically below:

The time for half of the CO2 molecules added to water to hydrate is on the order of 23 seconds. That rate is slow enough that many organisms have developed enzymes to speed it up. Carbonic anhydrase, for example, catalyzes the hydration and the reverse reaction (dehydration) to allow organisms to process carbon dioxide more rapidly. It is used by a wide array of organisms, from algae to people. In people, it is important in allowing carbon dioxide gas to be expelled from the lungs. Without it, the carbonic acid in the lung tissues would not convert rapidly enough to gaseous CO2 to permit it to be adequately expelled by breathing.

The carbonic acid that is formed when carbon dioxide hydrates can then very quickly equilibrate into the water's carbonate buffer system, converting into both bicarbonate and carbonate by releasing protons (H + ):

The conversions between carbonic acid, bicarbonate and carbonate are much faster than the hydration of carbon dioxide and for most purposes can be considered instantaneous. Consequently, carbonic acid, bicarbonate and carbonate are in equilibrium with each other at any given point in time. The primary factor that determines the relative amount of each species at equilibrium in seawater is the pH, with a small temperature effect as well.

In order to assess whether an organism requiring CO2 could benefit from any of the forms besides CO2 itself, it is useful to understand how much of each is present in seawater. Seawater contains about 670 times more unhydrated carbon dioxide than the hydrated version (carbonic acid). At most pH values attained in a reef aquarium, however, bicarbonate is far more prevalent than carbon dioxide.

Using the known pKa values for carbonic acid and bicarbonate in seawater, we can proceed to determine exactly how much of each form is present in seawater as a function of pH. The relevant chemical equations and pKa values are:

These pKa values imply that seawater at pH 5.85 contains equal concentrations of carbon dioxide and bicarbonate, and that seawater at pH 8.92 contains equal concentrations of bicarbonate and carbonate. Figure 1 shows data calculated for all three species as a function of pH in seawater. From this graph, it is clear that if getting carbon dioxide itself is limiting at pH 8.2, it might be more efficient to get it from bicarbonate because so much more is present. In fact, roughly 200 times more bicarbonate than carbon dioxide is present in seawater at pH 8.2. In most reef aquaria the bicarbonate is present at between 2 and 4 mM (millimolar = meq/L), or about 122 to 244 mg/L bicarbonate. For comparison, carbon dioxide is much lower, on the order of 0.01 mM (0.5) mg/L at pH 8.2. Interestingly, that value of 0.5 mg/L for carbon dioxide in seawater is almost exactly the same as the concentration of carbon dioxide in air.

Obtaining Carbon Dioxide as Carbon Dioxide: Passive Uptake

C arbon dioxide is able to cross cell membranes because it is a small uncharged molecule with reasonable solubility in organic materials. Consequently, organisms that take up carbon dioxide can do so passively (without spending any energy) and with no special mechanisms (such as proteins designed to speed up that process). Many marine algae and other organisms take up some measurable portion of the carbon dioxide that they incorporate during photosynthesis by this process.

In most cases, however, this process can account for only a portion of the demand for carbon dioxide. The rate at which carbon dioxide is used by rapidly photosynthesizing organisms is fast enough that organisms can deplete the carbon dioxide in the surrounding seawater faster than it can be replaced by diffusion and other transport mechanisms through the seawater. The depletion is readily observed by the pH in the near surface regions of these organisms, where the pH rises due to carbon dioxide loss. For this reason many marine organisms have developed other means of obtaining carbon dioxide, including processes involving bicarbonate. 2

Freshwater algae, on the other hand, can sometimes obtain all of their required carbon dioxide by passive uptake. 3 While a review of such literature is unnecessary in this article, I'll give one example. The freshwater chrysophyte alga, Mallomonas papillosa, has been shown to have none of the more sophisticated mechanisms for carbon dioxide uptake that are described later in this article, and it relies on simple passive uptake. For this reason it has been shown to photosynthesize most effectively where carbon dioxide concentrations are high, at pH 5-7. 4

Obtaining Carbon Dioxide: Concentrating Mechanisms

A s mentioned above, few marine organisms have been shown to rely solely on passive carbon dioxide uptake, but the carbon dioxide concentrating mechanisms are often unknown. As stated in a review article 5 in 2005, marine diatoms fix more than 10 billion tons of carbon by photosynthesis each year, but "there are still a number of fundamental unresolved aspects of inorganic carbon assimilation by marine diatoms. It is not clear how the carbon-concentrating mechanism functions."

Obtaining Carbon Dioxide as Carbon Dioxide: Active Transport

C arbon dioxide can be actively transported across cell membranes by protein transporters. This process does not solve the problem of low levels of available carbon dioxide in the surrounding seawater, but it can ensure that uptake itself is not a limiting factor, and may be especially useful in environments where carbon dioxide is plentiful (implying low pH environments in seawater).

The two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein, demonstrate active uptake of carbon dioxide (or carbonic acid), but not bicarbonate. 6 Because this mechanism is fundamentally limited in its effectiveness, it has been speculated that these organisms may be CO2-limited in their natural environment. 7

Two marine haptophytes, Isochrysis galbana Parke and Dicrateria inornata Parke, demonstrate active uptake of both carbon dioxide (or carbonic acid) and bicarbonate (described below). 6,8

The marine diatom Skeletonema costatum 9 has been shown to have little capability of using bicarbonate to obtain carbon dioxide. It does, however, show active uptake mechanisms for carbon dioxide, and this capability depends on light levels. In higher light levels, the diatom shows higher affinity for carbon dioxide. This capability can be attained within two hours of exposure to high light, and slowly fades over a period of about 10 hours when returned to low light levels (where less carbon dioxide uptake is required). Presumably, the organism is producing a carbon dioxide transport protein when light levels are high and carbon dioxide is needed in large amounts, and it halts that production (allowing the transporters to slowly decline in population) when they are not needed. High ambient levels of carbon dioxide also repress the expression of its high affinity for carbon dioxide uptake. Apparently, this diatom spends the energy to take up carbon dioxide actively only when it is actually necessary to do so, and relies on diffusion when it can.

Obtaining Carbon Dioxide from Bicarbonate: Carbonic Anhydrase

I f an organism is to obtain carbon dioxide from bicarbonate, several potential processes are available, and different organisms take different approaches. In many cases, the exact mechanisms have not been established. It is much easier to show that bicarbonate is a source of carbon dioxide for marine organisms than to show exactly how they take it up. A bicarbonate ion, being charged and insoluble in organic phases, cannot readily diffuse across cell membranes, so other mechanisms are needed.

Such uncertainty of mechanism is the case for Ulva lactuca, byvoorbeeld. It has been shown to be able to photosynthesize when out of the water (say, exposed at low tide), taking up carbon dioxide directly, and also when in the water, taking up bicarbonate. 10 But the exact mechanism of using bicarbonate to obtain carbon dioxide isn't known in this species.

One common way to use bicarbonate is for the cells exposed to the seawater to use extracellular carbonic anhydrase on their surfaces. As mentioned above, the enzyme carbonic anhydrase catalyzes the hydration and dehydration of carbon dioxide and carbonic acid, respectively. These organisms present this enzyme to the bicarbonate-rich seawater surrounding them. Because the bicarbonate is naturally in rapid equilibrium with carbonic acid, and the carbonic anhydrase keeps the carbonic acid in rapid equilibrium with unhydrated carbon dioxide, the bicarbonate is used as a ready pool to supply carbon dioxide to passively cross cell membranes and be taken up (shown schematically below).

The agarophyte Gracilaria lemaneiformis 11 has been shown to take up carbon in this fashion. It has carbonic anhydrase both inside the organism and out. Inhibiting either of these types of carbonic anhydrase greatly decreases photosynthesis, but adding an anion transport inhibitor does not. Adding TRIS buffer to the extracellular fluid (seawater) also has no effect (the purpose of which is discussed in the following section relating to proton pumping as a possible mechanism).

Photosynthesis in this organism is greatly reduced as the pH is raised (73% reduction when going from pH 8.0 to 9.0), presumably because the bicarbonate's propensity to form carbonic acid is reduced at higher pH.

The brown alga, Hizikia fusiforme (Sargassaceae), 12 from the South China Sea, has also been shown to exhibit carbonic anhydrase activity, both inside and out, and has been shown to be incapable of actively and directly transporting bicarbonate. Consequently, its carbon dioxide concentration likely operates by the mechanism shown above.

Two species of marine prymnesiophytes (Dicrateria inornata en Ochrosphaera neapolitana) 13 have been shown, through the use of various carbonic anhydrase inhibitors, to use extracellular carbonic anhydrase to collect carbon dioxide from ambient bicarbonate. They also employ an energy dependent process for taking up carbon dioxide itself. Growth in high carbon dioxide environments represses the expression of carbonic anhydrase active in these species, but does not reduce the active uptake of carbon dioxide.

Obtaining Carbon Dioxide from Bicarbonate: Direct Uptake

A n alternative way to obtain carbon dioxide via seawater bicarbonate is to take up the bicarbonate through protein transport mechanisms across the cell membranes, and then once inside the cells where it is needed, carbonic anhydrase converts it into carbon dioxide and hydroxide ion. The hydroxide is then pumped out, or H + is pumped in, to achieve pH balance.

Transporting ions across cell membranes using protein transporters is a widespread mechanism whereby organisms can get needed ions across a membrane through which they do not normally diffuse. Some of these are active transporters, using chemical energy to "pull" ions out of the extracellular fluid (our push them out, as necessary), and other transporters simply allow specific ions to pass though from high concentration on one side to lower concentration on the other side.

The marine red alga Gracilaria conferta has been shown to have an active bicarbonate uptake mechanism. 14 Three marine bloom-forming (red tide) dinoflagellates, Prorocentrum minimum, Heterocapsa triquetra en Ceratium lineatum, 15 have been shown to take up bicarbonate directly. They show little carbonic anhydrase activity, yet bicarbonate accounts for approximately 80% of the carbon dioxide they use in photosynthesis. It is believed that these dinoflagellates are not carbon limited in photosynthesis due to their efficient direct bicarbonate uptake mechanisms.

The marine diatom Phaeodactylum tricornutum 16 was found not only to have an active bicarbonate uptake mechanism, but the researchers further identified at least two different mechanisms. In particular, they showed that part of the uptake depended on the presence of extracellular potassium, and this part of the total carbon dioxide uptake was eliminated when potassium was missing from the medium. A second direct bicarbonate uptake mechanism was independent of potassium, indicating the presence of at least two different pathways for transporting bicarbonate into this organism.

Obtaining Carbon Dioxide from Bicarbonate: Proton Pumping

A nother way to obtain carbon dioxide via seawater bicarbonate is to pump H + out of the cells into the extracellular fluid (seawater near the cells) or into a special cavity where bicarbonate is present. 17 This low pH causes the bicarbonate to become protonated to become carbonic acid. The carbonic acid can then transform into carbon dioxide, and pass across the cell membranes.

The seagrass Zostera noltii Hornem 18 has been shown, for example, to use proton pumping to gather bicarbonate in the form of carbonic acid from the water. It contains no extracellular carbonic anhydrase, but rather uses ATP (adenosine triphosphate, the fundamental currency of chemical energy in most organisms) to drive the export of H + . Evidence for this mechanism is found by adding a buffer to the seawater (TRIS) without changing the pH. This buffer keeps the pH near the cell surface constant, counteracting the beneficial effect of the proton pumping in lowering pH and converting bicarbonate into carbonic acid. The simple presence of a non-absorbed buffer in the water can decrease the rate of photosynthesis in this organism by almost 80%.

Interestingly, those seagrass specimens acclimated to high light (where high rates of photosynthesis and consequent uptake of bicarbonate would be highest) showed the greatest ability to actively take up bicarbonate. In high light experiments, these previously high light-acclimated specimens were shown to be only light limited, while the shade-acclimated organisms were both light and carbon limited when put into high light. 18 Other seagrass species (e.g., Z. mulleri en Z. marina) have been shown to have external carbonic anhydrase, and so may have different uptake mechanisms. 18

Photosynthesis of Macroalgae as a Function of pH

O ne of the side effects of the necessity of taking up carbon dioxide to photosynthesize is that pH may affect the rate of photosynthesis, because the amount of carbon dioxide (as CO2 or H2CO3) in the water varies with pH. Assuming constant carbonate alkalinity, the effect is quite strong. A drop of 0.3 pH units implies a doubling of the carbon dioxide concentration. A reef aquarium at pH 8.5, for example, has one fourth the carbon dioxide of a reef aquarium at pH 7.9, assuming the carbonate alkalinity is the same.

Aquarists may rightly wonder whether organisms are able to photosynthesize efficiently as the pH is raised. The answer is mixed. Some can and some cannot. Those organisms that rely solely on carbon dioxide may not. Those that rely on both carbon dioxide and bicarbonate have a better chance of retaining efficiency at higher pH because a much larger amount of bicarbonate is present, and it does not change as rapidly with pH over the range of interest to aquarists.

Table 1 shows the response of a variety of macroalgae in terms of their ability to photosynthesize at pH 8.1 and 8.7. In seawater with constant carbonate alkalinity, there is 20% as much carbon dioxide at pH 8.7 as at pH 8.1, so an organism relying on carbon dioxide alone might experience a large drop in photosynthetic rate over this range. Clearly, the response varies with species. Chaetomorpha aerea, in particular, may be of substantial interest to aquarists. It is not necessarily the exact species that many grow in refugia (which is unidentified as far as I can tell), but this species of Chaetomorpha shows a 25% drop in photosynthesis when exposed to the higher pH. That drop is not as large as some other species, but may still be important, and it is more than many other species of macroalgae.

Of course, the photosynthesis rate does not necessarily translate to growth rates. If other nutrients are limiting growth (nitrogen, phosphorus, iron, etc.), then it may not matter if the rate of photosynthesis is reduced at higher pH. But because these nutrients are often present in surplus in reef aquaria, it may well be that carbon uptake is limiting in some cases, and in those cases aquarists might benefit from ensuring that the pH is not too high.


8.0: Prelude to Photosynthesis - Biology


I. Requirements of Life


A. Sun.
The ultimate source of energy for virtually all organisms on earth (with the exception of deep sea vent communities). As an aside, the sun even powers our human activities via fossil fuels which were are the byproducts of long-dead plants.


B. Plants are solar energy conversion factories

C. Plants and animals are powered by the products of photosynthesis.

D. Life and Thermodynamics.
According to the First Law of Thermodynamics - energy can be converted from one form to another Thus life is a process of thermodynamic energy conversions. Plants convert solar energy into chemical energy - they are about 2% efficient. They produce about 200 billion tons carbon fixed annually. Consumers convert the chemical energy in plants to additional forms of chemical energy.

E. Producers and Consumers
Autotrophs, like plants are producers heterotrophs like animals are consumers. Gross primary productivity refers to the total amount of photosynthesis by plants. Net primary productivity = gross - energy used for maintainance. Secondary productivity - amount of energy in consumers.

A. Energy flow is unidirectional - it never cycles!


B. Passage of energy flows from: sun → producers → consumers → decomposers


C. Some general notes about energy flow:

D. D etritivores (earthworms, insects) eat dead/decayed materials (larger), decomposers, like fungi and bacteria, eat smaller stuff

E. Each biotic transfer is termed a trophic (feeding) level

F. Food chain - linear sequence of biotic feeding interactions. i.e., plant → mouse → hawk. Uncommon in nature

G. Food web: complex, intertwined food chains. More stable, because of more links (a chain is only as strong as its weakest link) and

H. Energy is lost in the transfer of food from one trophic level to the next. The energy is lost primarily as heat.

A. 10% rule of thumb - as a very rough approximation, approximately 10% of the energy entering one level passes to the next.

B. Predicted by the 2nd Law of Thermodynamics - no energy conversion is 100% efficient, or in other words, all systems tend to a state of greatest entropy - randomness

C. Loss occurs as: indigestible parts inability to harvest entire trophic level metabolism (heat)

D. Pyramid model of energy flow

V. Lessons from the energy pyramid and 2nd Law

A. Sets a limit on the number of trophic levels in a food chain.
The maximum number is usually five - there is not enough energy at higher levels to sustain a species.

B. Energy efficiency increases as you go up the food chain.
For example, Golley calculated the energy efficiency of a small food chain of plant mice weasel. All of the plants, mice and weasels in an old field were collected and their total amount of energy was measured. Biomass is one way of estimating energy levels. Another more precise method is to "burn" the material in a calorimeter and note the heat output. In any case, the total energy available to each level was then known and could be calculated based on the following equation: energy e fficiency = energy assimilated/energy available x 100 . Check out the data in Table 1.


C . Size of organisms.
Think about some big creatures. What comes to mind? Redwood trees, whales, elephants, brontosaurus (apatosaurus). Where do they feed? At or near the bottom of the energy pyramid where energy is plentiful. Also note that many of the larger animals are aquatic or semi-aquatic. They take advantage of the buoyancy of water to help them move and minimize energy requirements.0


D. Principle of food size.
An individual must be large enough to capture and ingest its prey. Thus, consumers in each level: (1) tend to get bigger, OR (2) have functional modifications to make them functionally bigger (d.w.s., teeth, claws, hunt in packs). BUT, can't get too large because there is not enough energy to support and maintain large carnivores.


But, what about T. rex, the largest carnivore to ever have roamed on earth? Some speculate that T. rex was not the ferocious active hunter that s/he was portrayed to be in Jurassic Park rather, T. rex may have spent lots of time sleeping and eating "relatively" easy prey to catch (diseased, crippled, young) and/or carrion.


E. Human cultural evolution.
Humans have moved down the energy pyramid. Initially humans were hunter/gatherers eating lots of meat (high on the pyramid). Numbers were small. Then, humans learned to herd animals (herbivores), thus feeding lower on the chain and insuring a more predictable food supply. The development of agriculture permited large increases in human population size.

VI. Energy and the future
Will there be enough energy as our population increases? Probably, but we will almost surely:

VII. Biological Magnification.
Minute quantities of pesticides released into the environment get concentrated in organisms at higher trophic levels. This process occurs because as you go up the trophic pyramid, biomass decreases while fat-soluble pesticide levels remain about the same. Thus, you have more pesticide per unit biomass at higher levels. Fish consumption warnings in Minnesota are one sign of this phenomenon. Table 2 provides a model for how biological magnification works.


Chloroplasts, the photosynthetic units of green plants

The process of plant photosynthesis takes place entirely within the chloroplasts. Detailed studies of the role of these organelles date from the work of British biochemist Robert Hill. About 1940 Hill discovered that green particles obtained from broken cells could produce oxygen from water in the presence of light and a chemical compound, such as ferric oxalate, able to serve as an electron acceptor. This process is known as the Hill reaction. During the 1950s Daniel Arnon and other American biochemists prepared plant cell fragments in which not only the Hill reaction but also the synthesis of the energy-storage compound ATP occurred. In addition, the coenzyme NADP was used as the final acceptor of electrons, replacing the nonphysiological electron acceptors used by Hill. His procedures were refined further so that small individual pieces of isolated chloroplast membranes, or lamellae, could perform the Hill reaction. These small pieces of lamellae were then fragmented into pieces so small that they performed only the light reactions of the photosynthetic process. It is now possible also to isolate the entire chloroplast so that it can carry out the complete process of photosynthesis, from light absorption, oxygen formation, and the reduction of carbon dioxide to the formation of glucose and other products.


Develop an experiment to test how the level of CO2 affects the rate of photosynthesis. Construct a data table in the space below that shows the data you collected. Make sure to include information such as the color of light, light intensity, level of CO2 and the amount of bubbles produced. (Use the previous experiments as a guide)

Data Table

2. Based on the data, what CO2 level results in the fastest rate of photosynthesis? Propose an explanation for these results.

ANALYSIS

4. Based on the simulation experiments, what factors can affect the rate of photosynthesis in a plant? Hoe weet jy?

5. Write the equation for photosynthesis (use your book or online resources if you don't know it).

6. What are the bubbles you are measuring in this lab? Why do the bubbles tell you how fast photosynthesis is occurring?

7. Why is it important that you keep two variables constant (such as light level and color) while you're testing how a third variable (CO2 Level) affects photosynthesis?

8. What settings can you put the simulator on to get the MAXIMUM rate of photosynthesis?


Kyk die video: Photosynthesis: Crash Course Biology #8 (Oktober 2022).