Månadens biologifakta: December

Dr. Westerlund pelargon som väldigt tydligt är påverkad av fototropism.
December var en så himla hektisk månad, så jag hann inte skriva något månadens biologifakta (de här inläggen är helt klart de som tar längst tid att skriva). Så här kommer det lite försenat, och så får det bli två stycken den här månaden helt enkelt! Den här månaden ska vi vända oss bort från olika djurs könsorgan och sexuella beteenden och kolla på växter istället och hur det kommer sig att de växer så att de sträcker sig mot ljuset. Varför växterna vill sträcka sig mot ljuset är kanske självklart, växter behöver solljus för att kunna utföra fotosyntes som är livsnödvändig för dem. Men hur går det till? Hur vet de var ljuset är?
Den här processen kallas för fototropism, vilket kommer efter grekiskans ord för ljus (foto) och vändning (trope). Jag har skrivit lite tidigare om växthormoner och hur de har en inverkan på sticklingars utveckling och apikal dominans, och även när det gäller fototropism så är det växthormoner som har en central roll. I den här processen är det hormonet auxin som är det viktiga. Hormonet har flera olika funktioner, tex får det växten att växa på höjden istället för att utveckla sidoskott vid apikal dominans. När det gäller fototropism är det en funktion hos auxin som är särskilt viktig, och det är att den påverkar cellers elongering. Alltså att cellerna blir längre. 
På första bilden så är ljuset jämt fördelat över växten, och auxin (de rosa prickarna) är också jämt fördelade i toppen. Men vid assymetriskt ljus, alltså när en sida belyses mer än den andra så kommer auxin-molekylerna att förflyttas till den skuggade sidan (bild 2). Exakt varför de flyttas tror jag inte att man vet säkert än, men det finns lite olika teorier. En är bland annat att ljuset deaktiverar auxinet på den ljusa sidan, och att det därför bara finns aktivt auxin på skuggsidan. En annan teori är att det blir ett slags flöde av auxin från ljussidan till skuggsidan. Men det man vet säkert är i alla fall att skuggsidan får en större andel auxin än ljussidan, exakt hur det går till kanske vi vet om några år. 
Som jag nämnde tidigare så har auxinet en elongerande effekt, cellerna på skuggsidan av växten blir alltså längre. Det sker genom att auxinet aktiverar protonpumpar, vilket leder till minskat pH i växtcellen. Den surare miljön mjukar upp cellväggen vilket gör det lättare för cellen att expandera. På bild 3 och 4 ser man hur cellerna blir längre på den skuggade sidan. Effekten blir då att växten växer i riktning mot ljuset.
Hur känner växter av var ljuset är då? De har olika sorters ljusreceptorer som kan fånga upp olika typer av våglängder, exempelvis finns fototropiner som fångar upp blått ljus och som tros vara det som styr fototropism i (åtminstone) en del sorts växter. 
Det finns också något som heter negativ fototropism, som betyder att växten växer mot mörker istället. Det kan bland annat ses i rotskott, som inte vill ha något ljus (här sker ingen fotosyntes). Rotskotten påverkas också av gravitropism, och det innebär helt enkelt att de känner av gravitationen och vet vad som är upp och vad som är ner. Så även om du skulle sätta en planta upp och ner så skulle roten börja leta sig neråt och skotten uppåt. Lite coolt tycker jag! 

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