The fact that we photosynthesize Vitamin D by exposing our skin to the sun goes back to the very first phytoplankton that did this in the earth’s oceans 500m years ago and it is one of the few things that connect us to the world of plants

But the more mindblowing story about Photosynthesis is this. It's fair to say that it is the most important reaction for life on the planet. All life ultimately owes its existence to plants' ability to convert sunlight into sugars.

The TL;DR version of the reaction is that plants use a molecule called Chlorophyll to trap energy from sunlight to create a 5-carbon molecule that, through a series of complex steps, is converted into the 6-carbon molecule, Glucose.

And one of those complex steps involves plants taking Carbon dioxide from the air and using that Carbon atom to turn that 5-carbon molecule (called RuBP) into Glucose. Turns out that is a rather hard thing to do, and involves a broker enzyme with the cool name, RuBisCO

It also turns out that RuBisCO is rather terrible at its job. To put things in perspective, enzymes catalyze about a thousand reactions every second. I mean, that's the whole point of the enzyme - to help move reactions along. RuBisCO? 1-2 reactions per second. Yep. That bad.

So you might be wondering - Come on. Plants have had 600 million years to fine-tune this, so how on earth have they not evolutionary figured out a way to throw this guy out and replace him with someone more productive.

It turns out that we are being rather harsh with RuBisCO because the ultimate cause for its inefficiency happened way before land plants evolved. We have to go back to photosynthesizing bacteria 3 billion years ago. You see, back then, there wasn't much Oxygen in the atmosphere

And the invention of photosynthesis was so game-changing that it was the singular cause for pretty much all the oxygen we have in our atmosphere. And guess what, RuBisCO evolved in a low oxygen world.

And photosynthesis was so successful so fast that it couldn't hit the undo button on one small problem - RuBisCO works terribly in high oxygen environments, and ends up producing all manner of useless molecules the plant doesn't need.

And this, folks, is why plants take such a long time to go about their lives in general. Because they are Ferraris with one autorickshaw carburettor that has been welded to the assembly with no way to fix it.

Which makes me wonder. If evolution had somehow fixed RuBisCO's oxygen problem billions of years ago, would we have Ents on the planet today?

PS: This isn't to say that plants haven't tried to fix this problem. They have. There is something like C4 photosynthesis which is more efficient than the usual process, and those plants, not surprisingly, are things we eat a lot - Rice, Maize, Millets etc https://en.wikipedia.org/wiki/C4_carbon_fixation

PS 2: Apparently, the holy grail for plant scientists is to engineer faster photosynthesis into crop plants and get them to grow bigger faster, which could mean more food from smaller amounts of land using less fossil fuels, and if that happens, it will change the world.