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 thee 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);
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)
… favour 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 3phosphoglycerate (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
o 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
o are deep in the leaf so atmospheric oxygen cannot diffuse easily to them;
o 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
o 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 socalled 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 4carbon 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,



cycle.
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)
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.
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.
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. 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! 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 favour C3 Plants. However,
increasing global temperatures favour 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.
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:
Glucose
O2
H2O
CO2
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?
Are the compounds
listed here used or
produced in:
Photosystem I?
Photosystem II?
The Calvin cycle?
C3
C4
CAM
ATP
ADP + Pi
NADPH
NADP+
Draw simplified
diagrams of the
cross sections of a
leaf from a C3 ,a C4
and a CAM plant.
C3 Plants have one kind of chloroplast whereas C4 Plants have two. Explain:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
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
______________________________________________________________________________
______________________________________________________________________________