We covered the topic of photosynthesis with reasonable depth in the biochemistry series,
and we’ve mentioned it quite a bit in this botany series as well. Given that
this series focuses largely on the diversity of plants, we need to revisit photosynthesis
for a moment. But this time we are not going to focus quite as much on the general mechanism.
Instead, we must outline the major differences between the three different metabolic pathways
that lead to carbon fixation in angiosperms. If this term is unfamiliar, carbon fixation refers to
the process by which carbon dioxide is converted into more biologically useful organic compounds
within living organisms. Again, we already have a general understanding of this process,
but there are some key differences between certain groups of plants that must now be elucidated.
In most plants, initial fixation of carbon occurs when the enzyme rubisco adds CO2 to RuBP. These
plants are known as C3 plants because the first organic compound produced is the three-carbon
compound 3-PGA. C3 carbon fixation is the most common of the three pathways we will cover, so
because it is the most frequently studied method of photosynthesis, it is the method explained in
detail in the photosynthesis tutorial. The plants that utilize this process include soybeans, oats,
wheat, and rice. One aspect of this method of photosynthesis which makes it difficult for plants
in dry environments is that when the stomata close to reduce water loss, O2 builds up and rubisco
will add O2 instead of CO2 to RuBP. A two-carbon product of this reaction is then broken down to
CO2 and H2O in a process called photorespiration. Photorespiration yields no sugar and produces no
ATP. Instead it can actually burn through carbon previously fixed by the Calvin Cycle.
So C3 photosynthesis, while the most widespread of all forms of photosynthesis, is actually not very
efficient in arid environments. Plants that have adapted to life in hot, dry places have actually
evolved alternate methods of carbon fixation that save water without shutting down photosynthesis.
One such method, developed by C4 plants like corn and sugarcane, involves preceding the
Calvin cycle by first fixing CO2 into a four-carbon compound. Like C3 plants,
C4 plants will close their stomata to conserve water loss. However, unlike C3 plants,
C4 plants will continue to make sugars using photosynthesis instead of entering
photorespiration. They do this by initially fixing carbon dioxide in the mesophyll cells
via the enzyme PEP carboxylase, which reacts the three carbon phosphoenolpyruvate, or PEP,
with CO2 to form the four carbon oxaloacetic acid, or OAA. The resulting four carbon compound
within the mesophyll then cycles into the bundle-sheath cells and releases CO2, meaning
the levels of carbon there will always remain high enough for the Calvin cycle to make sugars.
It’s important to note that the utilization of C4 carbon fixation
does not indicate common ancestry. About 3% of land plants use C4 carbon fixation, most of which
are monocots. However, C4 carbon fixation has evolved independently at least 20 times
in the grass family alone, and is thus an excellent example of convergent evolution.
Another alternate method of carbon fixation developed by plants like pineapples and aloe
in arid environments is called crassulacean acid metabolism, or CAM photosynthesis. Plants
that use CAM photosynthesis are especially adapted to extremely arid environments.
Unlike most C3 and C4 plants, CAM plants conserve water by opening their stomata
and admitting CO2 only at night. When the CO2 enters the leaves, it is fixed into a four-carbon
compound through a PEP reaction similar to the C4 plants. However, unlike C4 plants,
in CAM plants, the resulting organic acids are stored in vacuoles for later use. Then during the
day, the stomata close to conserve water and the carbon dioxide-storing organic acids are
released from the vacuoles of the mesophyll cells into the Calvin cycle. This keeps photosynthesis
going in CAM plants during the day even though the leaf’s stomata are closed. Like C4 carbon fixation
CAM photosynthesis evolved convergently several times in different plant clades, though it is
more common in the epiphytes, such as orchids and bromeliads, and succulents, like cacti.
To summarize, C3 photosynthesis is the most common form of photosynthesis in plants,
but it is not well-adapted to hot, dry environments. In C4 photosynthesis,
carbon fixation and the Calvin cycle occur in different types of cells,
while in CAM photosynthesis, these processes occur in the same cells but at different times.
These two pathways are two different evolutionary solutions aimed at balancing the competing
priorities of maintaining photosynthesis and conserving water loss during hot, dry days.
And with that we understand a bit about the three known forms of photosynthesis in plants.