La fotosintesi clorofilliana e i segreti del rapporto tra piante e luce solare.
di
Luca Longo
12 febbraio 2021
4 min di lettura
di
Luca Longo
12 febbraio 2021
4 min di lettura
Sappiamo che l’eccesso di anidride carbonica (CO2) in atmosfera è la principale causa del cambiamento climatico. Abbiamo capito che la Terra si sta surriscaldando perché nell’ultimo secolo o due abbiamo bruciato un sacco di combustibili fossili che avevano impiegato centinaia di milioni di anni per formarsi ed accumularsi nelle viscere del nostro pianeta.
Per fortuna, sulla superficie della Terra esistono i cacciatori di CO2 che passano la loro vita a catturare senza pietà l’anidride carbonica. La spaccano, liberano parte dell’ossigeno e rimontano insieme quello che resta, un po’ d’acqua e un pizzico di altre sostanze per produrre semi, frutti, foglie e altre cose buone. Le piante – i nostri cacciatori di CO2– da circa due miliardi e mezzo di anni sanno realizzare una cosa che tutti gli animali presenti sulla Terra non hanno ancora imparato a fare. Compresi noi che negli ultimi tempi (geologici) stiamo dominando il Pianeta. Per comprendere questo processo abbiamo bisogno di partire dalla definizione di fotosintesi clorofilliana.
Lafotosintesi clorofilliana è un meccanismo piuttosto complicato che permette a minuscoli organelli specializzati presenti sulle foglie –i cloroplasti– di convertire la luce solare in energia. I fotoni provenienti dal Sole che riescono a superare la barriera dell’atmosfera hanno una lunghezza d’onda compresa fra i 380 e i 750 nanometri (nm; un nanometro è un miliardesimo di metro). La regione dello spettro solare che le piante riescono a sfruttare per la fotosintesi va dai 400 nm (blu) ai 700 nm (rosso).
A differenza degli animali, le piante utilizzano i raggi solari della giusta lunghezza d’onda per eccitare alcuni elettroni in speciali molecole. L’energia del Sole convertita in energia di eccitazione di questi elettroni subisce numerose altre trasformazioni a catena ed alla fine permette di produrre carboidrati. Questi carboidrati possono essere zuccheri, amidi, cellulosa, lignina e glicogeno.
In pratica, le piante sfruttano il Sole per costruire ordine dal disordine. Convertono sostanze disordinate (l’anidride carbonica dispersa in atmosfera, l’acqua e una manciata di altre sostanze elementari sparse nel terreno) in strutture altamente ordinate e in grado di conservare un sacco di energia all’interno dei loro legami chimici. Grazie a questa gentilezza da parte delle piante, quando ne abbiamo bisogno possiamo bruciare l’energia contenuta negli zuccheri e negli amidi per fare i dolci e la pastasciutta o bruciare cellulosa e lignina per costruire oggetti di legno o per scaldarci davanti al fuoco. Ovviamente, quando abbiamo voglia di una bistecca, di uova o di latte, possiamo anche usare queste scorte concentrate di energia per allevare animali. E non dimentichiamo che il sottoprodotto di tutto questo lavorio delle piante è proprio l’ossigeno che ci permette di respirare!
Tutte le piante –dal trifoglio alla quercia– per fare la fotosintesi usano due molecole speciali in grado di interagire con la luce: la Clorofilla A e la Clorofilla B. Le alghe verdi funzionano quasi nello stesso modo, mentre le alghe rosse usano –insieme all’onnipresente A– anche la Clorofilla D al posto della B. Infine le alghe brune preferiscono la Clorofilla C. Queste differenti molecole sono costruite in modo da lavorare al meglio quando vengono colpite da raggi di luce di un colore preciso, cioè da onde elettromagnetiche di un certo intervallo di lunghezze d’onda.
In particolare, la A assorbe la luce attorno a 435 nm (blu violetto) e 670-680 nm (rosso). Invece, la B preferisce lavorare con le lunghezze d’onda 480 nm (blu) e 650 nm (arancione). Oltre alle clorofille, anche i carotenoidi e i licopeni danno il proprio contributo all’assorbimento della luce visibile –soprattutto nella zona del verde–, i primi sono i responsabili dell’arancione delle carote mentre i secondi fanno rossi i pomodori. Quindi, tenendo conto di tutte le componenti, le piante assorbono la luce viola-blu e rosso-arancione, mentre non sono in grado di utilizzare la luce nelle lunghezze d’onda del giallo e del verde. Tutti i raggi di luce giallo-verdi, quindi, vengono riflessi dalle foglie.
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