Ah…. I feel wonderfully refreshed after yesterday’s rant, and ready to tackle some chemistry! Unfortunately this post turned out a little bigger than I thought (like pretty much all my posts) so I’ve split it into 2. Today I’ll talk about singlespeed climbing, granny-gears and CAM photosynthesis, and next post I’ll talk about gentle fast descents, big-ring pedaling, and C4 photosynthesis.
The most common form of photosynthesis, C3, has a few more moving parts than a singlespeed, but has a similarly simple overall formula: 6H2O + 6CO2 -> C6H12O6 +6O2, which means is that for every six water molecules and six carbon dioxide molecules taken in, C3 photosynthesis produces one big sugar molecule and six oxygen molecules.
That’s the overall formula anyway. In truth, it’s a bit of an oversimplification; photosynthesis (of all types) occurs in multiple steps, in which all kinds of cool chemistry going on. But for this post, and to understand the key differences between the 3 different types of photosynthesis, you only have to understand the first step: carbon fixation, or carboxylation. Carbon fixation is the process by which the plant takes in CO2 and gets the carbon out of it. The end result of this first step is the compound 3-Phosphoglyceric acid, or PGA for short (diagram right). Every molecule of PGA has 3 carbon atoms, and that’s why this “standard” type of photosynthesis is called C3.
Tangent: I’ve decided that the molecular diagrams on Wikipedia suck. So starting with this post, I’m doing my own. And I have to say, they’ve turned out awesome. Check it out. Here’s the Wikipedia molecular diagram for Oxaloacetic acid, a compound I’ll be talking about in the next post, and whose formula = C4H4O5.
Where are the other 2 hydrogen atoms? And what about the 4 carbon atoms? Oh, I get it- they’re implied, at the cornices, because it’s like way too much effort to put in a couple of “C”s. Are you kidding me? My 7 year-old (either one) could draw a more informative molecular diagram than this. Now here’s mine:
Isn’t that nicer to look at? Everything shown, the right number of bonds between every atom… Dad would be proud. I am telling you, Wikipedia should use my graphics.
The key enzyme in carbon fixation is Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO for short, and it is the most common protein in the world. In the next post we’ll talk more about RuBisCO, and specifically, how it kind of sucks at its job, but let’s leave that aside for the moment.
Something like 95% of all plants use C3. The trees on your street, the grass in your yard (but not crabgrass), the flowers in your garden, and the PLTs and Aspens throughout the Wasatch- all of them use C3. It’s simple, it’s reliable, it works. Like a singlespeed.
But even the most ardent singlespeeding evangelist will admit that there are conditions when a singlespeed is not efficient, the most obvious being on extremely steep climbs. A cyclist who cannot pedal his/her singlespeed up a hill must either get off and walk, or… ride the hill with a geared bike.
Similarly, there are conditions under which C3 is not efficient, and a great example in the desert is when it’s really hot and dry. C3 plants must have their stomates (pores) open to photosynthesize, and during a Sonoran summer day, the cost of water-loss through open pores is too great to keep them open. So C3 plants in the desert- just like singlespeeders on steep climbs- have 2 options.
The first is the equivalent of getting off and walking- or just not riding- and that’s Not Photosynthesizing. A great example of a C3 plant which adopts this strategy is Brittlebush, Encilia farinosa (pic left). Brittlebush was all over the place this weekend, a small bush with fairly lush-looking leaves, and a few were even beginning to flower. But by May or June those leaves will shrivel up and/or drop off, and Brittlebush will “park it” for the summer before kicking it back up in the Fall.
The second option is to modify the C3 process, and this is what cacti and agaves (pic right) have done. They’ve developed an alternate photosynthetic process that fixes carbon at night, and then produces sugars during the day. This type of photosynthesis is called Crassulacean Acid Metabolism (terrible name) or CAM for short, and it works like this:
At night CAM plants have their pores open and intake CO2. The carbon is fixed by an enzyme- not RuBisCO, but a different enzyme, called phosphoenolpyruvate-carboxylase, or PEPC for short- into malic acid, which is a 4-carbon compound. The malic acid is then stored in little organelles called vacuoles until sunrise.
At dawn, 2 things happen. First, the stomates close. Second the malic acid is broken down (by yet another enzyme- PEP-carboxykinase. Hey, I didn’t name them…), “un-fixing” the carbon (a process called “decarboxylation”) and the freed carbon is then fixed again by RuBisCO in the traditional C3 process.
Extra Detail: When the CO2 is taken out of the malic acid, the leftover stuff is a compound called pyruvic acid which gets combined with some other stuff to make more PEPcase…
In other words, CAM is just C3 with a whole bunch of extra front-end mechanical hijinx going on to change how carbon is delivered to the C3 process… just like a gear-bike is just a singlespeed with a bunch of extra mechanical whatziz going on to change how power is delivered to the drive-train.
CAM has been around for somewhere between 40 and 100+ million years (maybe more- I’ve found wildly varying figures) and has evolved at least dozens, probably hundreds, of times independently, all over the world. And not just in angiosperms, but also in cycads, at least one gnetophyte*, and even in lycophytes**.
*Long-time readers may wonder if this is my favorite gnetophyte and common Western shrub, Mormon Tea. Unfortunately no; Ephedra is strictly C3. The known CAM gnetophyte is the bizarre Welwitschia mirablis (pic right) of the Namibian desert, which employs a variant of CAM called “CAM-cycling” in which the stomates are still open during the day.
** I recognize that I keep referring to lycophytes without having properly introduced them. What can I say- it’s on my “to-blog” list…
Examples of CAM plants include not only cacti and agave, but also yuccas, pineapple, and some ferns, orchids and even grapes.
But the CAM analogy with a low-geared mountain bike doesn’t stop there. Climbing a steep hill in a granny-gear has an obvious disadvantage: you go slow. And CAM is like a mountain bike that’s always in granny-gear; it never grows fast. CAM is limited by the carbon storage capacity of the vacuoles, and as a result CAM plants, like cacti, tend to be notoriously slow growers. Outside of their environmental niche they compete poorly with C3 plants, which is why you don’t find CAM plants in places like New England or the Pacific Northwest.*
*There’s a notable exception that I’m totally breezing over- aquatic CAM plants- but that’s a whole other deal…
Next Up: Downhill on a Singlespeed & C4 Photosynthesis