Let’s put this in perspective for a second: if you want to understand how any living thing gets energy, you have to start with the primary producers in a system. For us terrestrial beings, that means organisms that can photosynthesize (green plants, algae) — and when you get down to it, what it really means is that you’re alive because a leaf can literally take energy from the sun and make it into food. All energy in a system — all the energy that supports other (consumer) forms of life — starts there. When I teach this, I tend to bounce around like a kindergartener eagerly explaining the shape of the earth, hoping my students will be as awestruck by this fact as I am. Sometimes, it even works.
(Image: the northern mixed prairie, Saskatchewan)
Sometimes I like thinking about the endosymbiont hypothesis. It’s the idea that important cellular components, in particular the mitochondria (POWERHOUSE OF THE CELL) and chloroplasts (the organelle that facilitates photosynthesis), were originally free living bacteria that formed a symbiotic relationship with early prokaryotic cells. Eukaryotic life may only have evolved because of that emergent relationship. Sometimes it’s nice to think about the idea that life (literally, life) is incidental.
(Image: visible chloroplasts in P. ellipticum)
If you think back to high school and/or college biology — do you guys remember how there are multiple kinds of photosynthesis? For example, some desert plants undergo CAM photosynthesis, which allows them to collect CO2 at night but shut down their stomata (pores, basically) during the day so they don’t lose water when it’s hot and dry. Or C3 vs. C4 photosynthesis — the C4 pathway evolved in plants that live in hot, high light intensity, dry climates to minimize inefficiencies. What kinds of inefficiencies? Well! So first of all, remember how the basic equation for photosynthesis is:
CO2 + H2O + light ——> C6H12O6+ O2
(Yes, that equation is unbalanced, because I feel like I’m asking a lot already without putting on more numbers. Please just go with me!)
So — carbon dioxide and water and light yields sugar and oxygen. (Incidentally, look at where the carbon is coming from. The entire mass of a plant basically comes from the air. If that doesn’t blow your mind I don’t know what to tell you.)
There is an important enzyme called RuBisCo (remember that guy?) necessary for fixing that carbon. RuBisCo, though, is very bad at its job. Like, sometimes it gets overwhelmed and is like OMG I CAN’T and just starts grabbing oxygen instead of carbon dioxide. The plant then has to deal with the products of that reaction, which is exhausting, inefficient, and sometimes literally dangerous to the plant. C4 photosynthesis evolved because RuBisCo tends to do that more in hot climates. I have a joke with a friend about how on bad days, we empathize strongly with RuBisCo, but really, none of us is that bad. RuBisCo is such a fuckup that literally an entire class of plants was, evolutionarily speaking, like ok, we need to address this situation because it’s really out of control. Suddenly leaving my dry erase markers at home doesn’t seem like such a big deal. I don’t know about you guys, but I’ve never done so bad a job at something that an entire class of organisms responsively evolved.
RuBisCo is also a great example of how evolution doesn’t drive any particular direction, and it only works on what’s available. RuBisCo is a fuckup, but nothing better has ever come along, so here it is, maintained. Natural selection only goes as far as what’s already around, or else our knees might go the other direction.
Anyway. These are the things I think about on a daily basis.
ETA: Photosynthesis is literally so cool and important that I managed to write several paragraphs about how it drives life without mentioning that it also maintains oxygen levels...like, for us to breathe. So, that’s important too.