Saturday, September 3, 2011

I C4...there I am!

Sorry for the bad pun!

It's the hottest and sunniest time of the year.  That means the C4 plants are about to lap the C3 plants in the race on the farm.  C4 plants are ones that have evolved an extra step to their photosynthesis process that makes them more efficient than other plants, especially during the heat of summer.  Most plants are C3 plants, but crabgrass, sugar cane, corn, sorghum and many other plants are C4 plants.  C4 plants comprise only about 3% of flowering plants, but they do 25% of the photosynthesis that occurs on land*.  Unfortunately, the only C4 plants on the farm are weeds, and the worst of the worst is crabgrass!!!!!

My nemesis.  Notice the star shape of the first few crabgrass stems.
Now brace yourself.  We're going to have to wade into some technicalities of photosynthesis.  Trust's way cool, as the young kids say.  It is imperative that you understand the tricks that C4 plants can do, because if you ever have to spend an entire day weeding crabgrass, you'll want something to think about.

First though, C3.  Think back to 9th grade.  Remember photosynthesis?  It's that thing that plants do to make their food.  The idea is that plants capture the energy that the sun is giving off, and then they use that energy to knit carbons, hydrogens and oxygens together to make sugar.  As we all know, sugar is a basic food.  In plants, it stores the sun's energy in chemical form until the plant needs it.  Sugar is also the basic molecular building block for a lot of the structural molecules plants use to build themselves.  In fact, all the food on our entire planet originated from photosynthesis (even meat because cows get their energy from grass).  Also, all the oxygen in the air on Earth was made as a byproduct of photosynthesis.  So it's kind of important.

Now to the name C3.  The C refers to carbon, which is the most important atom involved in photosynthesis.  Plants collect carbons form the air in the form of carbon dioxide, which they use to make sugar.  The 3 refers to the size of the molecule that the plant uses to 'trap' the carbon dioxide.  In C3 plants, when a carbon dioxide molecule (which has one carbon atom) is caught by the plant, it joins with the plant's carbons to make a three-carbon molecule (called 3-phosphoglyceric acid, or PGA).  Eventually, when a plant has collected six carbon dioxide molecules, it has enough carbons to make one sugar molecule.  In a single teaspoon of plant sugar, there are approximately 1.7 x 10^22 molecules of sugar, so plants do a LOT of photosynthesis.

But, you can't understand the C4s' advantage until you understand the last piece of this photosynthesis puzzle: RUBISCO.  I'm not just typing in all caps because I'm excited about RUBISCO; it's also an acronym.  RUBISCO stands for ribulose 1,5 bisphosphate carboxylase oxygenase, which is a handy phrase to work into almost any conversation.  RUBISCO is the molecular machine, or enzyme, that allows the plant to 'grab' CO2 out of the air and bond it to make the three carbon molecule.  The carboxylase part of the name refers to its ability to bond carbon.  Unfortunately for plants, RUBISCO can also bond oxygen (that's the oxygenase part of its name).  It's unusual for enzymes to be able to bond two different substrates - usually they have one job only.  For RUBISCO, it's a problem to be able to bond two things, because every time the RUBISCO bonds to oxygen instead of carbon dioxide, it costs the plant energy instead of gaining energy like photosynthesis is supposed to do.  In the summer, when plants are photosynthesizing fastest (and producing oxygen, remember), there is even more oxygen around the plant.  So plants are not able to photosynthesize to their full potential because their RUBISCO is wasting more time with oxygen.  Scientists think the RUBISCO problem exists because RUBISCO evolved in plants' ancestors before there was any oxygen in the air to worry about.
It all used to look like the right side before I pulled out the crabgrass.
C4 plants have evolved more recently, and they have solved the RUBISCO paradox.  As you can see in the pictures above and below, the C4 crabgrass is kicking the butts of the C3 beets growing in identical conditions. 
Unweeded beets on the left, free beets on the right.
 Crabgrass, like all other C4 plants, bonds carbon dioxide to make a....drum roll please.....FOUR carbon molecule!!!  Crabgrass has a stand-in molecule that doesn't use RUBISCO to trap carbon dioxide from the air.  This C4 molecule exists near the surfaces of the plants that are exposed to air, where carbon dioxide (and oxygen) is.  It can ONLY trap carbon dioxide and not oxygen.  Once it traps a carbon dioxide, the four-carbon molecule, called oxaloacetic acid, travels deep into the plant away from air.  All the plant's RUBISCO is stored near the center of the plant's leaves and stems, and the oxaloacetic acid drops off the trapped carbon to RUBISCO, where it goes through the rest of photosynthesis like normal.  So you have all of the photosynthesis with none of the waste from exposure to oxygen.  And thus the crabgrass flourishes in the middle of summer when photosynthesis is happening at its fastest.
* Most of the photosynthesis that occurs on earth is done by algae in the ocean.

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