A Plant’s dilemma!
“When to open stomata and photosynthesize – When to close stomata and conserve water!?”
Consider that the limiting factor to photosynthesis is the availability of CO2 in the atmosphere which
currently is present in very dilute concentrations of 0.039% whereas O2 is relatively concentrated at
20%.
A well hydrated plant would keep stomata open at daytime to allow ready
access to atmospheric CO2 while keeping stomata closed at nighttime in
order to conserve the CO2 generated by respiration. The large vacuoles of
plant cells could be considered botanical scuba tanks permitting the storage
of waste CO2 generated by nighttime respiration. O2 is never in limiting
supply; however, those same large vacuoles could overcome the problems of
laggard diffusion during gas exchange, by sequestering copious waste O2
generated during photosynthesis. Vacuoles can therefore provide a reserve
of CO2 for photosynthesis or a reserve of O2 for respiration as need be.
Waste not – want not!
So logically, a well hydrated plant’s default setting would be stomata open in daytime and closed at
nighttime. And often this is indeed the case. However, there are three unfortunate wrinkles
complicating this straightforward story:
1. Desiccation
2. Photorespiration
3. Nutritional (specifically Nitrogen) requirements
By definition, photosynthesis happens in light. However when conditions are most “light”, conditions
are also most “hot and dry”. Plants are most prone to water loss precisely when they open stomata for
gas exchange during the day.
Another unanticipated wrinkle is a relic of plant evolution called
Photorespiration. Let’s start with what is supposed to happen during
Photosynthesis. The start of the Calvin Cycle combines CO2 with the
phosphorylated 5-carbon sugar ribulose bisphosphate to produce two
molecules of 3-phosphoglycerate. This reaction is catalyzed by the
enzyme ribulose bisphosphate carboxylase oxygenase (RuBisCO).
From an enzymatic standpoint, the fixation of CO2 by RuBisCO is
very inefficient (a high Km – low affinity for substrate); however, whatever
RuBisCO lacks in efficiency, it makes up for in quantity! RuBisCO
can fairly claim to be the most abundant protein on earth accounting
(in some plants) for up to 50% of soluble leaf protein.
RuBisCO’s active site can actually bind either CO2 or O2. When photosynthetic algae first arose, the
early Earth’s atmosphere contained little, if any, oxygen. RuBisCO would have functioned very well
under these conditions. It was only later, when the concentration of oxygen in the atmosphere increased
considerably, did the competitive binding of O2 to RuBisCO’s active site pose a problem.
What would that problem be? When RuBisCO binds O2,
(instead of CO2) only one 3C molecule of PGA is produced
(instead of two) and a toxic 2C molecule called
Phosphoglycolate is produced. The plant must rid itself of
the Phosphoglycolate involving a complex network of
enzyme reactions that exchange metabolites between
chloroplasts, leaf peroxisomes and mitochondria. Since this
alternate pathway requires light and produces CO2, it is
called “Photorespiration”. However, no useful energy is
gained from Photorespiration.
Higher temperatures melt the tertiary structure of RuBisCO, rendering it less able to discriminate
between CO2 and O2. Meanwhile, warmer temperatures also decrease the solubility of CO2 which poses
a problem if CO2 is already in limiting supply. (Remember soda pop is more likely to fizz if it is warm
as opposed to cold). CO2 availability is further limited by the constraints of diffusion during gas
exchange. Meanwhile relative O2 concentrations accumulate in flagrant excess as a result of
Photosystem II located in the very same chloroplast as the Calvin Cycle’s RuBisCO. When the
concentration of CO2 drops below 0.01 percent, O2 will out-compete CO2 at RuBisCO’s active site, and
no net photosynthesis occurs.
To summarize: RuBisCO catalyzes two different reactions:
•adding CO2 to ribulose bisphosphate — the carboxylase activity during photosynthesis
•adding O2 to ribulose bisphosphate — the oxygenase activity during photorespiration
Which of these two reactions predominates would depend on the relative concentrations of O2 and CO2
where:
•high CO2, low O2 favors the carboxylase action during photosynthesis,
•high O2, low CO2 favors the oxygenase action during photorespiration
The light reactions of photosynthesis liberate oxygen and deplete carbon dioxide. Meanwhile, the
availability of soluble carbon dioxide is significantly decreased at higher solvent temperatures.
Therefore,
•high light intensities and
•high temperatures (above ~ 30°C)
… favor the oxygenase reaction of Photorespiration over the carboxylase activity of regular
Photosynthesis.
In other words, if a plant is to survive the deleterious effects of photorespiration, it needs to avoid both
high light and temperature conditions or find other ways of storing CO2.while isolating O2 from
RuBisCO.
Let’s start with the simplest scenario. When water is abundant and temperatures are relatively low, a
plant’s life is pretty straight forward. No special adaptations are required. Plants that immediately bind
CO2 during photosynthesis are referred to as C3 Plants because the molecule 3-phosphoglycerate (see
diagram above) has a backbone comprised of three carbon atoms. C3 Plants open their stomata during
the daylight hours and as a result cannot survive intense light or heat.
“C4 plants” only open their stomata during cooler parts of the day. That means they require a store of
CO2 for photosynthesis when stomata are closed. “C4 plants” get their name by storing CO2 as a stable
product four-carbon organic compound, usually malate.
The details of the C4 cycle (as cut and pasted from John W. Kimball's excellent site)




After entering through stomata, CO2 diffuses into a mesophyll
cell.
o Being close to the leaf surface, these cells are exposed to
high levels of O2, but have no RUBISCO so cannot start
photorespiration (nor the dark reactions of the Calvin
cycle).
Instead the CO2 is inserted into a 3-carbon compound (C3)
called phosphoenolpyruvic acid (PEP) forming the 4-carbon
compound oxaloacetic acid (C4).
Oxaloacetic acid is converted into malic acid or aspartic acid
(both have 4 carbons), which is transported (by plasmodesmata)
into a bundle sheath cell. Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot
diffuse easily to them; often have thylakoids with reduced photosystem II complexes (the one
that produces O2).
o Both of these features keep oxygen levels low.
Here the 4-carbon compound is broken down into carbon dioxide, which enters the Calvin cycle
to form sugars and starch.
o pyruvic acid (C3), which is transported back to a mesophyll cell where it is converted
back into PEP.
In other words, the C4 pathway minimizes photorespiration by separating the so-called light and dark
reaction in different locations of the leaf, thereby isolating O2 from RuBisCO. There is a cost to this
strategy: every CO2 molecule has to be fixed twice, first by 4-carbon organic acid and second by
RuBisCO. As a result, the C4 pathway uses more energy than the C3 pathway. The C3 pathway requires
18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30
molecules of ATP. For tropical plants, this added energy debt is more than compensated by avoiding the
expenditure of over half of photosynthetic carbon to photorespiration.
The separation of the so-called light and dark reaction in different locations of C4 leaves explains the
separation of Palisade mesophyll cells from the radially oriented bundle sheath cells surrounding the
veins. This unique feature of C4 leaves is referred to as “Krantz (i.e. “wreath” in German) Anatomy”.
Krantz Anatomy
CAM Plants
(CAM stands for Crassulacean Acid Metabolism because it was first studied in members of the plant
family Crassulaceae.) CAM plants are also C4 plants but instead of segregating the so-called light and
dark reactions of photosynthesis in different locations of the leaf, these reactions occur instead at
different times. CAM Plants are unique by only opening their stomata at night, when at all!
CAM Details - (as cut and pasted from John W. Kimball's excellent site)
At night:

CAM plants take in CO2 through their open stomata (they
tend to have reduced numbers of them).
 The CO2 joins with PEP to form the 4-carbon oxaloacetic
acid.
 This is converted to 4-carbon malic acid that
accumulates during the night in the central vacuole of the cells.
In the morning,

the stomata close (thus conserving moisture as well as
reducing the inward diffusion of oxygen).
 The accumulated malic acid leaves the vacuole and is
broken down to release CO2.
 The CO2 is taken up into the Calvin (C3) cycle.
In summary, when conditions are extremely dry, CAM plants simply close their stomata both night and
day. O2 produced during photosynthesis is recycled for respiration and CO2 produced during respiration
is similarly recycled for photosynthesis. Like our own planet, CAM plants represent a closed system in
terms of matter and an open system in terms of energy. Note the plant cannot grow while CAM-idling.
There are many variations of the C3/C4/CAM theme. For example, there seem to be three versions of
CAM: "obligate CAM plants” vs. "inducible CAM plants" and “CAM-idlers” aka "CAM-cycling".
There are also different versions of C4 - let’s leave all these details for later study in university.
Evolution – Some suggest that C4 plants emerged with a spike in atmospheric O2 levels 25-35 million
years ago. http://tinyurl.com/ce8av56
Some authorities make sense of a putative 25-35 million year time limit for C4 emergence by suggesting
C4 carbon fixation must have evolved on at least 45 independent occasions in different lineages of
plants, making C4 a prime example of convergent evolution. http://tinyurl.com/cegocy7
Such suggestions are reminiscent of Ernst Mayr’s outdated speculation that metazoan eyes may have
evolved independently on 40 different occasions! Evo-Devo comes to the rescue by invoking the
versatility of metazoan “molecular toolkits”. The metazoan Urbilateran probably had the molecular
toolkits for both ciliary and rhabdomeric photoreceptors, either of which was differentially lost in later
Eumetazoan lineages. http://tinyurl.com/ykkayn2 A similar story probably happened during plant
evolution. Emerging molecular clock analyses will undoubtedly move the emergence of C4 metabolism
back to an era much earlier than paleobotanists ever suspected.
Evolutionary analysis is further compounded by “horizontal” or “lateral” gene transfer!
http://tinyurl.com/d979bqk In other words, Darwin’s “Tree of Life” is beginning to resemble more a
“Cobweb of Life”. “Horizontal” or “Lateral” gene transfer is far more likely to occur in plants than in
animals by a variety of mechanisms such as “illegitimate pollination”.
Photorespiration may serve some purpose? It has been predicted that the increase in ambient CO2
concentrations predicted over the next 100 years may reduce the rate of photorespiration in most plants
by around 50%; thereby increasing ecosystem productivity and the sequestration of atmospheric CO2.
However, reducing photorespiration may not necessarily result in increased growth rates for plants.
Some research has suggested, for example, that photorespiration may be necessary for the assimilation
of nitrate from soil. (link)
Thus, a reduction in photorespiration either by genetic engineering or by increasing atmospheric CO2 due
to fossil fuel combustion may not be as beneficial to plants as some propose. Several physiological
processes may be responsible for linking photorespiration and nitrogen assimilation: one is that
photorespiration perhaps increases availability of NADH, which is required for the conversion of nitrate
to nitrite. Such speculation is confounded by the fact that certain nitrite transporters also transport
bicarbonate, and elevated CO2 has been shown to suppress nitrite transport into chloroplasts.
That said; Photorespiration also may still be used as a mechanism to dissipate excess energy at high
irradiance levels to prevent damage to plant cells.
Ecological considerations: Increasing atmospheric CO2 levels favor C3 Plants. However, increasing
global temperatures favor C4 Plants. Global or regional changes in climate have the potential to change
a predominantly C3 forest into a predominantly C4 grassland or vice versa. When such changes occur
quickly, ecological collapse and mass extinction are sure to follow. During the Younger Dryas of the
Late Pleistocene; rapid climatic oscillations accompanied planet-wide ecological collapse together with
mass extinctions that included Ice Age Mammoths, camels, llamas, and saber-tooth tigers.
Biotechnology: Converting plants from C3 to C4 Given the advantages of C4 metabolism at higher
temperatures, a group of scientists from institutions around the world are working on the C4 Rice
Project to turn rice, a C3 plant, into a C4 plant. Rice is for more than half the world’s population its most
important staple food. Rice that is more efficient at converting sunlight into carbohydrate could
significantly augment global food security. Some suggest C4 rice could produce up to 50% more food
energy - and be able to do it with less water and nutrients. (link) (link)
Questions:
How did C3 Plants get their name? _______________________________________________________
C4 Plants? _________________________________________________________________________
What is/are the overall
function(s) of
photosystem I?
Are the compounds
listed here used or
produced in:
What is/are the overall
function(s) of
photosystem II?
Photosystem I?
What is/are the overall
function(s) of the Calvin
cycle?
Photosystem II?
The Calvin cycle?
Glucose
O2
H2O
CO2
ATP
ADP + Pi
NADPH
NADP+
C3
Draw simplified
diagrams of the
cross sections of a
leaf from a C3 ,a C4
and a CAM plant.
C4
CAM
C3 Plants have one kind of chloroplast whereas C4 Plants have two. Explain:
____________________________________________________________________________________
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____________________________________________________________________________________
Present-day C4 plants are generally concentrated in the tropics and subtropics (below latitudes of 45°).
Why would that be? ___________________________________________________________________
____________________________________________________________________________________
Some call photorespiration a “mistake” in the functioning of the plant cell? Explain the Pros & Cons to
that statement: _______________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
Rubisco is thought to have evolved when Earth had a reducing atmosphere. Does this help to explain the
so-called “mistake”? __________________________________________________________________
____________________________________________________________________________________
What makes C4 photosynthesis more efficient than C3 photosynthesis in tropical climates?
____________________________________________________________________________________
____________________________________________________________________________________
What makes C3 photosynthesis more efficient than C4 photosynthesis in temperate climates?
____________________________________________________________________________________
____________________________________________________________________________________
What makes CAM photosynthesis most efficient in desert climates?
____________________________________________________________________________________
____________________________________________________________________________________
CAM plants thrive in very high temperatures. High temperatures reduce the CO2 (g) solubility.
However, as temperatures increase, CAM plants’ abilities to store CO2 improve. Explain
____________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
The project to turn rice, a C3 plant, into a C4 plant is a bit of a gamble. Explain why this experiment is
being attempted: ______________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
… and what could go wrong? ___________________________________________________________
____________________________________________________________________________________
Define convergent evolution: ___________________________________________________________
____________________________________________________________________________________
The suggestion that C4 emerged ONLY 25-35 million years ago is pretty contentious! Why is there
disagreement? _______________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
For the sake of argument, let’s say that that C4 did indeed emerge ONLY 25-35 million years ago.
Without invoking repeated incidents of convergent evolution; provide another explanation how so many
different lineages of plants could have all acquired C4. _______________________________________
____________________________________________________________________________________
If you grow bean plants (C3) and corn plants (C4) in a sealed terrarium, one quickly dies even as the
other thrives. Explain these results: ______________________________________________________
____________________________________________________________________________________
______________________________________________________ (Hint: as a plant grows, carbohydrates are made)
Marine planktonic diatoms are responsible for up to 20% of primary production on earth, fixing more
than 10 billion tons of inorganic carbon each year. The existence of both the C3 and C4 pathways were
recently discovered in a marine diatom. In this unicellular organism, the two paths are kept separate by
having the C4 path in the cytosol, and the C3 path confined to the chloroplast. What are the evolutionary
implications of this discovery? _________________________________________________________
___________________________________________________________________________________
Diatoms are by definition aquatic organisms. Does that make their C4 pathway surprising? Explain
____________________________________________________________________________________
____________________________________________________________________________________
If the Greenhouse Effect continues unabated, will C4 diatoms thrive or perish? Explain
____________________________________________________________________________________
____________________________________________________________________________________
Essay Question / Research
The C4 Rice Project represents the acme of Genetically Modified Foods (GM foods) aka Biotech
Foods. GM foods are extremely controversial and are even sometimes referred to as “Frankenfood”.
Write an essay citing the Pros and Cons of GM foods and be sure to explain why you come down on one
side or the other on this contentious issue.
Please use proper citations and do not copy and paste from any source.