thefutureofamerica
thefutureofamerica t1_iuj3vnf wrote
Reply to comment by WritingTheRongs in ELI5 Why can’t plants absorb nitrogen from the air? by Cool-Boy57
Yes, Anabaena and Cyanothece are 2 different genera of cyanobacteria. I worked with other cyanobacteria for my PhD and my advisor's lab did a bunch of work on Cyanothece. They're really fascinating little creatures.
thefutureofamerica t1_iuj1jy1 wrote
I think this is the first time I’ve seen an ELI5 on the front page where all the answers are wrong!
Yes, atmospheric nitrogen takes a lot of energy to convert, but plants already provide all of that energy to their bacterial and fungal friends in the soil. So that’s not the reason.
The real issue is that photosynthesis and nitrogen fixation are incompatible because of oxygen. Photosynthesis makes oxygen molecules and oxygen molecules destroy the machinery that turns atmospheric nitrogen into usable forms.
There are organisms that do both photosynthesis and nitrogen fixation, but they don’t do it in the same place or at the same time. Some photosynthetic bacteria (like Anabaena) make special cells called heterocysts that fix nitrogen but don’t photosynthesize, while others (like Cyanothece) photosynthesize during the day, then fix nitrogen during the night.
When photosynthesis and nitrogen fixation were first evolving, this incompatibility wasn’t an issue because there wasn’t molecular oxygen in the atmosphere to any appreciable degree. Photosynthesis created it all. Once it had, created an environment that made nitrogen fixation much more challenging.
As u/writingtherongs pointed out, plants COULD have evolved a separate compartment to fix nitrogen in, but that just hasn’t happened in the last ~3 billion years. They’ve always relied on other organisms to provide their fixed nitrogen.
thefutureofamerica t1_iwoagvy wrote
Reply to Where do mitochondrial protons come from? by Infinite-Flow7945
Just like the electrons getting passed down the chain, they come from energy source molecules, like fats and sugars, and wind up getting passed on to oxygen at the end of the transport chain to form water.
The protons get passed through different chemical intermediates after they’ve been taken into the cell, but the overall equation C6H12O6 + 6O2 -> 6CO2 + 6H2O gives the idea. The H’s leave sugars and wind up in water.