THE LIGHT REACTIONS OF
PHOTOSYNTHESIS
These reactions begin with the absorption of light
energy by pigments, and end with the production
of stored chemical energy in the form of NADPH
and ATP. Now we will follow the NADPH and ATP
molecules as they enter the Calvin cycle. Their
stored energy will be used to make sugar from
carbon dioxide. These anabolic reactions are
endergonic (have a positive delta G), and
therefore require energy (from ATP and NADPH).
The basic relationship between the Calvin cycle
and the light reactions is summarized in this
figure.
These reactions are sometimes called the "dark reactions"
because they can occur in the dark (as long as ATP and
NADPH are available). All of the processes of photosynthesis
(light and dark reactions) occur within the chloroplast. By the
end of this tutorial you should have a basic working
understanding of:
Energetics
Before we begin our study of the Calvin cycle, let's look at
the energetics (total energy relations and transformations
of a system) of sugar synthesis.
We have already studied the catabolism (breakdown) of
glucose during cellular respiration. You know that the
process is exergonic and releases about 686 kcal of
energy. Thus, the delta G for the overall reaction is -686
kcal/mole.
glucose + O2 --> CO2 + H2O + ATP
If 686 kcal of energy per mole are released in the process
of respiration, then it follows that 686 kcal of energy
(minimum) are required to produce one mole of glucose.
(Remember the first law of thermodynamics?) The Calvin
cycle is the process by which glucose is made, and it
requires all of that energy. Where does the energy come
from? The light reactions of photosynthesis produce ATP,
which provides the Calvin cycle with the necessary energy.
In addition, the NADPH produced by the light reactions
provides the reducing power to put glucose together.
sunlight + CO2 + H2O --> O2 + glucose
Overview of the Calvin Cycle
The light reactions of photosynthesis
produce ATP and NADPH, which are
then used in glucose synthesis during
the Calvin cycle. As you should know
from studying the Krebs cycle,
metabolic cycles involve inputs and
outputs, but some molecules are
recycled to go full circle.
Figure 2
In the case of the Calvin Cycle, the input molecules are carbon dioxide, ATP, and NADPH. The output
molecules are sugar, ADP, NADP+, and inorganic phosphate (Pi). The recycled molecule is ribulose
bisphosphate (RuBP). Look at this figure and take a moment to locate these molecules.
Carbon Dioxide Fixation Yields Two,
3-Carbon Compounds
Calvin first saw a three-carbon
compound that was radioactively
labeled. This led him to conclude that
there was a two-carbon compound that
was binding to the carbon dioxide,
fixing it into an organic compound.
However, when he stopped the process
almost immediately after injecting
carbon dioxide, he found a six-carbon
compound. It turns out that carbon
dioxide is initially fixed (i.e., taken out
of the gas phase) by joining to a fivecarbon compound. The resulting sixcarbon compound is so unstable that it
very quickly breaks into two, threecarbon compounds.
Figure 2(Click image to enlarge)
Regeneration of G3P to RuBP
As shown in these two figures, the Krebs cycle and the Calvin
cycle have some general similarities. Remember, the Krebs
cycle regenerates oxaloacetate at the end of one cycle to
begin the next. In much the same way, the Calvin cycle
regenerates RuBP to begin the next cycle. For every three
carbon dioxide molecules that are fixed, three molecules of
RuBP were needed. Thus, at the end of the cycle there must
be three molecules of RuBP or the cycle would get out of
balance.
The three molecules of RuBP that began the cycle
had a total of three carbons multiplied by five
molecules, or 15 atoms of carbon. Three molecules
of carbon were then fixed for a surplus of three
carbons in the cycle. Those three carbons are
expelled from the cycle as one molecule of G3P.
The remaining 15 carbons are still in the form of
G3P. Therefore, they must be converted back to
RuBP to start the process again. More ATP, as well
as many steps involving enzymes, are necessary
to do this regeneration.
Although we commonly think about the Calvin
cycle occurring in this simple form, it is important
to keep in mind the complexity of the cell. Our
simplification of the Calvin cycle shows that a
given chloroplast only has one enzyme for each
step in the cycle. You need to remember that each
chloroplast has a multitude of each type of
enzyme. Thus, hundreds of molecules are going
through this cycle at once. Before proceeding to
the next page, click on the image of the Calvin
cycle and note where NADPH is first used. The
next question will ask what the consequences
would be if NADPH levels were to suddenly drop.
(In the absence of the reducing power of NADPH,
what intermediate of the Calvin Cycle would begin
to accumulate?)
Global Warming
Carbon dioxide is constantly being fixed into sugars (and
other macromolecules), which, in turn, are oxidized back into
CO2. This relationship, on a global scale, is termed the
carbon cycle. However, humans are burning fossil fuels at a
faster rate than plants can fix them back into sugars and
other carbon molecules. Therefore, the global carbon cycle is
out of balance. Or is it? Some scientists think that the current
rise in CO2 levels is part of a natural cycle (rising and falling
CO2 levels) that has been going on for millions of years. Thus,
there are cycles within cycles, each interacting with and
affecting the others. For more information on global warming,
visit these sites:
Enivironmental Protection Agency
National Oceanographic and Atmospheric Administration
Summary
We have finished our discussion of photosynthesis by
showing the major anabolic pathway that results in the
reduction of carbon dioxide to various sugars (including
glucose). The formation of sugars is energetically
unfavorable, as can be predicted from the positive ?G for
the formation of glucose from carbon dioxide and water.
The energy and reducing power for this process comes
from the light-dependent reactions, which were presented
in the last tutorial.
Basically, the Calvin cycle is a collection of enzymes that
work to introduce carbon dioxide into the anabolic pathway
involved in sugar production. Keep in mind, there are some
variations on how carbon is initially fixed (depending on
the species of photosynthetic organism), but they all have
the enzymes that are involved in the Calvin cycle.
Carbon dioxide enters the Calvin cycle by being added to
the 5-carbon compound called ribulose bisphosphate
(RuBP). This reaction, involving the enzyme rubisco (the
most abundant protein on Earth), results in a 6-carbon
compound that is very unstable and is rapidly split into
two, 3-carbon molecules. The resulting 3-carbon compound
is activated by phosphorylation (via ATP obtained from the
light-driven reactions of photosynthesis), then reduced by
NADPH (again, obtained from the light-driven reactions of
photosynthesis). After reduction, a 3-carbon molecule,
glyceraldehyde 3-phosphate (G3P), exists. G3P has two
fates. Some molecules go on to form more molecules of
RuBP (which requires additional ATP; again derived from
the light-driven photosynthetic reactions). Some molecules
exit the Calvin cycle to form sugars (including glucose).