OK, back to bikes. Everyone knows it’s a bear to get a singlespeed up a steep climb. But people who ride them regularly know their other weakness: going downhill. Not the steep, braking, hang-off-the-back downhills, but the gradual downhills, where you’re coasting at maybe 15-20 MPH, but if you pedaled, you could get up to maybe 25+ MPH. Only on a singlespeed, you can’t; you spin faster and faster and faster, but your spinning becomes less and less efficient.
The Problem With RuBisCO
One of the arguments used by creationist types against evolution is that nature works too perfectly, too efficiently, to be the product of natural selection fueled by random mutation. Here’s a great counter-argument: RuBisCO. RuBisCO, in many ways, sucks. It’s inefficient, it’s slow, and it’s “designed” for an environment that hasn’t existed in something like half a billion years.
So ideally, C3 Carbon fixation goes like this: RuBisCO facilitates a reaction between a big, honking molecule called Ribulose-1,5-biphosphate, one water molecule and one carbon dioxide molecule. Together they produce 2 molecules of glycerate 3-phosphate or 3PGA, which is that 3 carbon, “1st-step” product for which C3 is named, and to which I alluded in the last post.
RuBisCO is real good at binding with carbon when there’s not a lot of free oxygen around. Oxygen competes with carbon for the binding sites on the RuBisCO molecule. When this happens, instead of making 2 3PGA molecules, the RuBisCO fuels a reaction that produces 1 3PGA molecule, and 1 “problem molecule” (Phosphoglycolate.) To deal with the “problem molecule”, a plant has to undertake additional, resource-consuming steps to get the carbon to the right place, and this whole tedious, cumbersome process is called photorespiration.
Photorespiration works, but it works poorly and inefficiently compared to standard carboxylation. Sort of like… a singlespeeder spinning like crazy on a gentle downhill. The problem is exacerbated by heat, which excites the oxygen atoms and makes them more formidable competitors for the binding sites.
Back when RuBisCO evolved, half a billion years or so ago, this wasn’t a problem. CO2 densities in the atmosphere were far higher than they are today, and there was little free oxygen. But as plants photosynthesized and made more plants who photosynthesized and so on and so on, the atmosphere became filled with free oxygen, which then competed with the carbon in CO2 for RuBisCO binding sites.
So, the most common protein on the planet, the enzyme that ultimately produces pretty much everything we- and every other living thing- consume, is an out-of-date anachronism. RuBisCO is the Harley-Davidson ignition of the living world.
Over millions of years, the amount of oxygen in the Earth’s atmosphere has varied considerably, and about 25-30 million years ago, it reached levels not seen since the Carbinoferous* (~300 million years ago.) When it did, C3 plants- especially in the tropics- were photorespirating like crazy, and in response some of them evolved a new, third type of photosynthesis: C4.
*This topic- O2 levels in past ages- is way fascinating, and surprisingly unsettled. I started a tangent about it that spiraled way out of control, so I bagged it for now, but hope to return to the topic in a future post.
C4 works like this: CO2 is taken in through stomates during the day, but the carbon is initially fixed not to RuBisCO, but to another enzyme, PEP (which we met when we looked at CAM plants) which produces either a 4-carbon malic acid (like CAM) or the structurally similar oxaloacetic acid (also 4 carbon atoms to a molecule.) The 4-carbon acid is then transported from the leaf-surface mesophyll cells to bundle-sheath cells, which are located internally in the leaf, along the vascular bundles (leaf equivalents of veins.)
Tangent: The above description is way oversimplified. There are actually at least 3 different chemistries at work in C4 plants, but the overall idea is the same.
Nested Tangent: OK, even that is oversimplified. There are actually a total of 6 known C4 chemical pathways, but 3 of them only occur in photosynthetic bacteria or archaea*.
*No, I haven’t yet introduced archaea, and yes, they’re different from bacteria, but for this post all you need to know is that they’re both single-celled critters that you need a microscope to see.
If this sounds a lot like CAM, it should, because they’re using the same basic chemistry, the difference being that in CAM the 2 carbon fixation stages are separated in time (night vs. day) while in C4 they’re separated in location (mesophyll vs. bundle-sheath cells.) C4 plants have a distinctive and recognizable (to a botanist with a microscope) structure called the Kranz Anatomy. (diagram right)
Like CAM, C4 has evolved independently many times- at least 45! But it’s a much more recent invention, dating back maybe as far as 30M years, and really taking off only in the last 7-10M. And it’s far more limited in distribution; all C4 plants are angiosperms, they comprise ~7,500 species, or about 3% of all angiosperms. A classic- and oft-cited- example of a C4 plant here in Utah is Four-Wing Saltbush, Atriplex canescens. (pic left)
But Saltbush isn’t really a great example; C4 dicots are pretty rare. The vast majority of C4 plants are monocots, and more specifically, grasses. (See this post for explanation of monocots vs. dicots.) Tropical and sub-tropical grasslands around the world are C4-dominated, and what’s even more interesting is the correlation between C3/C4 grass distributions and seasonal precipitation.
C4 species dominate in grasslands where it’s hot, and also where the bulk of the precipitation occurs in the summer. A good example close to home is the contrast between grasses in Utah and Arizona. Here in Utah, the majority of our precip occurs in the winter months (as snow), and only 18% of our native grass species are C4.
But down in Arizona, most precip happens in the summer, in the form of monsoon rains, and a whopping 82% of native grasses are C4. (See map, left) Similar patterns happen all over the world; in Eastern Kenya 89% of the grasses are C4.
So at this point you may be saying, OK this is getting boring, what does this have to do with me? And the answer is- if you live in Utah and have a lawn- everything. Hairy Crabgrass, Digitaria sanguinalis, is a C4 grass, and in a typical roaster-oven Salt Lake Valley summer it will kick the wimpy European ass of your C3 “Kentucky” Bluegrass, Poa pratensis. What’s that? You have a sprinkler system set up in your yard that runs throughout the summer? Congratulations- you’ve introduced summer monsoon rains to your little grassland. (I talked about Crabgrass & C4 in this post last spring.)
Tangent: So here’s something cool I noticed over my Arizona weekend- I never saw a blade of Cheatgrass (pic right). Cheatgrass, which is all over the Great Basin and Northern Mojave, is a C3 grass, originally from the central Asian steppe. I assume that down in the Sonoran it does poorly compared to native C4 grasses.
The downside of C4, and why it hasn’t taken over the world, is that it’s got a lot of moving parts, and spends a fair amount of extra energy moving acids and enzymes around. And outside of its competitive tropical zone, that added machinery, effort and complexity hurts more than it helps… sort of like a 27-speed mountain bike in Lambert Park, where a singlespeed will do just fine.
More Cool Stuff About C4
An interesting side-benefit of C4 is because carbon delivery is optimized, the plant requires far less RuBisCO. As a result, C4 plants are poorer in key elements such as nitrogen and phosphorus, making their leaves less nutritious for herbivores, leading to the fascinating- but far from accepted- hypothesis that C4 evolved in part as a response to “herbivory” (yes, that’s a real word; it means predation by herbivores), the idea being that herbivores would generally eat nutritious C3 grasses before their C4 counterparts. Speaking of eating, corn and sugarcane- both of which are basically oversized grasses- are C4 plants.
Another not-quite-accepted hypothesis of C4 origins is the Nitrogen Hypothesis. Since C4 delivers only carbon (and not oxygen) to the RuBisCO in its bundle-sheath cells, it requires far less of that enzyme. Nitrogen is a necessary component of RuBisCO, and so it’s thought that C4 may have evolved in nitrogen-deficient environments. But the evidence for this idea is mixed.
Extra Detail: I haven’t included the molecular structure of RuBisCO in this post for the simple reason that it is a freaking monster molecule. But at its core is a single magnesium atom, held in place by 3 amino acids. Amino acids always include nitrogen.
Tangent: So you can probably tell that I’m really starting to get into organic chemistry, which is weird because I hated chemistry in high school and college. And what’s really fascinating about organic chemistry to a newbie like me (I never took an organic chem class, and I know this is like “duh, so what…” to readers who have, but hey it’s my blog) is that the same relative handful elements- carbon, hydrogen, oxygen, nitrogen, and bits of phosphorus and magnesium- are used over and over and over again. These same basic building blocks are used in a gazillion different combinations to make all the wonderful and varied living things in the world. Seriously, how amazing is that?
Nested tangent: Know what else is like that? Any song by the Ramones. Really, on your next road-trip, listen to “Ramones Mania” or one of their other anthologies. (Warning: set the cruise control first, or you will soon be driving at like 95 MPH.) These guys wrote like 200 songs, and they all use the same 4 chords. And each one of their songs sounds… remarkably like all the others. So I guess this is a really crappy example, but I was listening to them a bunch this weekend and really wanted to work them into this post…
Anywho, there’s one more difference between CAM and C4 that’s worth thinking about in mountain bike terms. CAM evolved to enable plants to survive where C3 plants couldn’t or could barely manage- just like a granny gear gets a mtn biker up a climb a singlespeed can’t. But C4 evolved in places where C3 plants could and did grow and reproduce; C4 just did it better, and out-competed them- just like a big-ring pedaler outpacing a singlespeeder on a gentle descent. The selection pressure that brought about CAM was survival; that which gave rise to C4 was competition.
Man, that was a great weekend. I can’t wait to get South again.