Photosynthesis is the process by which plants capture the energy
of the sun to make carbohydrates.  But there are minor variations
in how plants do this, depending on the environment in which the plant
lives.  In this tutorial, we'll look at the differences between the three
types of photosynthesis: C3, C4 and
CAM.  Most plants - around 85% or so-
carry out C3 photosynthesis.  As we will see,
the name C3 has to do with the type of carbon molecule
produced during the process.  C3 photosynthesis occurs in the
mesophyll cells of the C3 plant, which are located just under the
surface of the leaves and other tissues.  Let's take a quick look at how
photosynthesis occurs in these cells.  During C3
photosynthesis, CO2 is taken up and transported directly
to the Calvin Cycle, where it is fixed into a 3 carbon molecule.
Thus the name C3.  If the stomata are
open, CO2 moves in, and oxygen moves out
of the cell.  But under hot or dry conditions when the stomata
are closed, oxygen accumulates which inhibits the amount of
C3 that can be made.  Some plants have evolved an
adaptation that allows them to be successful in hot, dry conditions.
These plants carry out C4 photosynthesis, instead of
C3 photosynthesis. During C4 photosynthesis,
CO2 moves into the mesophyll cell, and is immediately fixed
into a 4 carbon molecule.  Thus the name C4 photosynthesis.
But the anatomy of a C4 plant is different from
that of a C3 plant.  A somewhat simplified version of this process
is shown here.  In a C4 leaf, chloroplasts are located in
mesophyll cells, but they are also located in bundle sheaf cells
which surround the leaf vein.  CO2 moves from the
mesophyll cells, into the bundle sheaf cells, where the Calvin Cycle
occurs.  This shields the plant from the oxygen build-up
that would otherwise occur when the stomata are closed during dry conditions.
A second way of addressing the problem of oxygen
is to change the timing of photosynthesis.  This is called CAM
photosynthesis, and in may ways it is similar to C4 photosynthesis
except the timing of the process changes.  During CAM
photosynthesis, CO2 is taken up during the night when the stomata
are open, and is fixed into a 4 carbon molecule.
It is then stored until daylight as an intermediate molecule, which
gives the CAM cycle its name.  This adaptation allows
photosynthesis to occur in drier climates than in both C3
and C4 photosynthesis.  A great example of a CAM plant
is the cactus.  Let's do a quick review.
During C3 photosynthesis, CO2 is taken up
and transported directly to the Calvin cycle.  The C4
pathway alters the location of photosynthesis, and the CAM
pathway changes the timing.  Both of these adaptations allow
some plants to survive in environments that are less than ideal for the typical
C3 plant.
