In our course of study, we looked at cellular respiration first, because it is a simpler process than photosynthesis. We've now covered the light reactions of photosynthesis, and what's left are reactions that don't involve light, but do involve the use of ATP -- a kind of reverse of the Krebs cycle. We can now put together concepts we learned in cellular respiration with the details of plant electron transport and chemiosmosis, and see how the same types of reactions can produce very different results when they occur in chloroplasts instead of mitochondria.
As a result of photosynthesis, our plant cell has ATP and NADPH available, and along with the raw materials CO2 and H2O (which it pulls from its environment), it can make sugars and starches according to the reverse of the cellular respiration chemical reaction:
6 CO2 + 6 H2O → C6H12O6 + 9 O2
(Note that this reaction is balanced: there are 6 carbons, 12 hydrogen, and 18 oxygen atoms before and after the reaction takes place.)
This reaction takes place in the stroma or fluid between the grana stacks of thylakoids found in chloroplasts in the mesophyll cells in the leaf. Here reactions "fix" the carbon from the CO2 to the water molecules. The term "fixing" an element comes from alchemists and early chemists, who used it to describe the transformation of the natural form in which an element to a new form where it is affixed or attached to a different set of elements. In this case, the reactions of the Calvin cycle split carbon from oxygen and attach it to other oxygen and hydrogen atoms in a new arrangement. Because these reactions don't use light as an energy source, and often take place during the night cycle of the plant, they are called dark reactions.
Once the new molecules are available, the plant cells can arrange the component groups it will need for glucose molecules through a series of reactions called the Calvin cycle. As in the Krebs cycle, we start with a set of molecules, add some raw material molecules, spin off some product molecules, and wind up with a set of molecules that look like our original set -- that's how we get the "cycle" description for these reactions. The molecules at the end of the reaction series have the same formulae and structures as the molecules we started with, but they aren't necessarily composed of the same atoms. Some of the atoms we started with are spun off as part of the products, and replaced from the "raw materials" atoms added at the start of the cycle.
|2||The unstable compound breaks down into 2 3-PGA (phosphoglycerate)|
|3||Lots of ATP is used to convert the PGA to PGAL or G3P (glyceraldehyde-3-phosphate). Some of this becomes sugar.|
|4||Some G3P is rearranged into more RuBP in reactions requiring more ATP; the rest becomes glucose|
Notice all that ATP being used up? That's energy being stored in the bonds of the RuBP and the glucose or other carbohydrates; it can be released by aerobic respiration.
Study the Calvin cycle diagram at Michigan State University's biology site.
Another source of good information about the Calvin cycle is John Kyrk's dark reactions page, which shows you haw the molecules combine and transform during each reaction. This presentation is especially good at showing you have the atoms change positions, so while we start and end with RuBP, the components of the molecule change. It also shows you how the G3P (PGAL) product can be transformed into different important organic substances for use in the cell.
Notice also that Calvin cycle doesn't actually produce glucose! After several cycles through the reduction phase, we have several molecules of G3P. We've seen this molecule before -- in the cellular respiration glycolysis process, as a step on the way to pyruvate. In that process, ATP is required to rearrange glucose into fructose phosphate compounds, and then into G3P molecules. It's important to note the use of ATP in the glycolysis process: it means that rearranging glucose to fructose and adding the phosphates to make G3P are endergonic reactions. The reverse reactions are therefore exergonic....and spontaneous. In other words, if we have two G3P molecules near one another, they will release energy as they combine to form fructose diphosphate, break off the phosphates, and form glucose. So once we have G3P, it will readily form storage sugars and starches, without any additional energy required.
Because the Calvin cycle creates a three-carbon product, it is called the C3 pathway. Certain plants carry out their carbon fixation process in mesophyll cell using the PEP carboxylase enzyme to make a 4-carbon compound, oxaloacetate, from CO2. Consuming NADPH energy on the way, the oxaloacetate is transformed into malate, which is transferred to an internal bundle sheath cell, away from the light. Here, more NADHP releases a CO2 molecule from the malate (which becomes pyruvate!) into a Calvin cycle process. The pyruvate undergoes its own transformation with some help from ATP energy back to oxaloacetate.
While less energy efficient than direct use of CO2, this process captures CO2 from environment when it occurs in concentrations too low to support the Calvin cycle directly in mesophyll, and concentrates the CO2 in the bundle sheath cells, so that the Calvin cycle can at least take place there. A plant doesn't need to open its stomata as often or as long to capture CO2, which makes it possible to keep water from evaporating through the stomata. More efficient use of the energy from photosynthesis means the plant can grow faster than plants with only C3 paths. However, for the plant to gain any advantage, it has to be in a high-intensity light environment, so only plants that grow in the full sun in temperate and tropical areas can take advantage of the C4 pathway.
Here is simple diagram of the C4 Pathway, and a more detailed one from the C4 Fixation article in the Wikipedia (especially for those of you who love molecule diagrams). Note that here it is called the Hatch-Slack Pathway, after its discoverers, M. D. Hatch and C. R. Slack.
Plants which use both pathways can survive higher temperatures, light intensities, and lower humidity than plants which use only the Calvin cycle; they are also very efficient at storing sugar and using it for growth. These two-path plants include sugar cane, corn, and crabgrass.
The crassulacean acid metabolism pathway, or CAM, is another process is used to fix CO2 in plants; because it does not require light, plants use this cycle at night to store food. In a C4 plant, the C4 and Calvin pathways occur at the same time in different locations. In a CAM plant, the two pathways occur in the same cell, but at different times. The two reactions together reduce water loss, so CAM plants are often desert plants.
|Process||No reaction center; CO2 from air goes directly to Calvin cycle||Reaction center for CO2 fixation in bundle sheath cells,
separate from Calvin cycle cells
|Reaction center for CO2 fixation and Calvin cycle colocated in the same cell|
|Enzymes||ribulosodiphosphatcarboxylase (RuDP carboxylase)||phosphoenolpyrvatcarboxylase (PEP carboxylase)||phosphoenolpyrvatcarboxylase kinase (PEP-C kinase)|
|Product||CO2 → phosphoglycerate, a 3-carbon molecule||CO2 → oxalacetate → malate||CO2 → oxalacetate → malate|
|Example Plants||Wheat, barley, potatoes, sugar beets||Grasses, corn, sugar cane||Cactus, orchids, bromeliads|
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