We’ve already learned that plants need to leave their
stomata open during the daytime, for the light dependent
reactions.
• While it’s true, plants don’t need to take in CO2 for the light
dependent reactions, one of the by-products of noncyclic
photophosphorylation needs to be emitted from the leaf, and it
does so through its stomata. (O2 waste from photolysis)
 Oxygen build-up in plants is bad! It is a waste material that leads to
photorespiration
 At the same time, plants lose water through open stomata in the
process of transpiration
Plants also must take in CO2 in order for the light
independent reactions to occur. This CO2 goes on to help
create glucose.
How can plants in hot, and dry biomes possibly allow their
stomata to remain open, if it also promotes desiccation?
The most common enzyme on Earth, and indeed protein, is rubisco. It is
this enzyme that participates in the C-B cycle and catalyzes the fixation
of CO2 (rubisco catalyzes the merging of CO2 and RuBP to produce 12
PGA)
• Rubisco isn’t a particularly efficient molecule,
because it also wants to “fix” oxygen. When oxygen is
present, it becomes “fixed”, more readily than CO2 in a
process known as photorespiration.
• The products of fixed O2 are not particularly healthy
for the plant, and most assuredly don’t lead to the
formation of glucose, like the fixation of CO2 does.
 Instead, the products of RuBP and O2 are
stored in peroxisomes, which are found near the
chloroplast. The peroxisome breaks down the
products of photorespiration, and gets rid of the
by-products.
It is for this reason that scientists know that
there was little or no oxygen in Earth’s early
atmosphere, around the time of the first plants. If
oxygen had been plentiful, rubisco would not have
given plants a selective advantage.
Over Earth’s vast geologic history, the atmosphere has
contained both very high (time of dinosaurs >50%), and very
low (when life was first evolving-0%) levels of O2. Now, our
planet has approximately 21% O2 in our atmosphere.
Because of the very
low O2 levels when life
was first evolving,
plants using rubisco
didn’t struggle as
much with the issue of
photorespiration.
Some special plants
evolved an active CO2concentrating
mechanism that
increases the CO2
concentration near the
site of the
photosynthetic
reactions, suppressing
photorespiration.
Open stomata are needed for both light dependent, and
light independent reactions. Why?
ANS: In light dependent reactions, the stomata are
open to allow waste gas, O2 to exit the plant. In light
independent reactions, they remain open to allow CO2 into
the leaf, in order to create the carbohydrate glucose. Let’s
not forget, the water that is moved through the plant,
largely because of open stomata, is also needed in noncyclic photophosphorylation for its donation of electrons,
and hydrogen to NADPH formation during the process of
photolysis.
What does rubisco do when O2 is present in higher
concentrations than CO2?
ANS: It catalyzes the fixation of O2 instead of CO2,
in the process of photorespiration. The products of
photorespiration are not particularly good for the plant,
and they certainly don’t lead to the production of glucose.
C4 plants are so named because they form a four-carbon compound as the first
product of the dark reactions of photosynthesis known as oxaloacetic acid
(OAA), instead of the 3-carbon PGA formed in typical C3 photosynthesis.
Several thousand species in at least 19 families use the C4 pathway.
Agriculturally important C4 plants are sugarcane, corn, sorghum, and other
members of the grass family.
To understand the C4 pathway, it is really
important to fully understand the structures of
the leaf. Let’s review:
• Cuticle
• Stomata and guard
cells
• Palisade and spongy
parenchyma (mesophyll)
• Xylem and Phloem
• Bundle sheath cells
Notice how the bundle
sheath cells are far away
from the stomata, and spongy
parenchyma. This will become
important.
Where is most C3
photosynthesis occurring?
• Instead of being fixed
by rubisco to form PGA,
CO2 combines with PEP
(Phosphoenolpyruvic
acid) to form OAA
(oxaloacetic acid), using
the fixing enzyme PEP
carboxylase.
• OAA is then
converted to malate,
and that is shuttled to
the bundle sheath cells
through plasmodesmata.
• Here, malate is
converted to pyruvate
and CO2.
• The pyruvate moves
back to the mesophyll
cells where one ATP
is broken down to
form AMP (not ADP)
which is required to
convert the pyruvate
back to PEP (to help
continue the cycle)
• The CO2 goes on to generate glucose in the Calvin Cycle. The
overall effect of this process is to move CO2 from mesophyll cells
to the bundle sheath cells, in order to make photosynthesis more
efficient.
Since bundle sheath cells rarely make contact with
intercellular spaces, very little oxygen reaches them,
decreasing the likelihood of photorespriation
C4 photosynthesis is called C4 because…
The first molecule produced is OAA (oxaloacetic
acid) which is a 4-carbon moleclue.
What happens when the OAA is converted into malate in
the mesophyll?
It is shuttled to the bundle sheath through through
the plasmodesmata, where it goes on to break down again
into CO2, and pyruvate.
What is the purpose of the plant moving the site of the
Calvin Cycle to the chloroplasts within the bundle sheath,
instead of the palisade or spongy parenchyma?
Bundle sheath cells are farther away from the spaces
within the plant cell that may contain oxygen, thereby
reducing the potential for photorespiration.
As soon as the CO2 is delivered to the bundle sheath cells,
rubisco begins the C-B cycle (C3 Photosynthesis).
Because so little O2 is present, little photorespiration takes
place, and photosynthesis is more efficient.
Because it is more efficient, the stomata
don’t have to remain open as long, which
decreases transpiration of H2O…
C4 plants are found in hot, dry
places where they possess an
advantage over typical C3 plants,
and are able to out-compete
them.
The advantage it gives them more than
makes up for the added energy required (1
ATP to AMP…two bonds are broken), for
the pyruvate to be converted back into PEP
which continues the cycle.
In ordinary C-3 plants which form a 3-carbon compound (PGA) during the initial
steps of the light independent reactions, photosynthesis in the leaf shuts down
without a sufficient supply of CO2.
C-4 plants have a competitive advantage during hot summer days because they
are able to carry on photosynthesis in the bundle sheaths where CO2 levels are
concentrated.
Weedy C-4 plants such as
Bermuda grass, spurges and
purslane grow rapidly during
hot summer days, while
photosynthesis and growth in
C-3 plants shuts down.
Close-up view of a purslane
leaf showing the prominent
green veins. The chloroplasts
are concentrated in bundle
sheath cells surrounding the
veins.
Both C4 and CAM are two
evolutionary solutions to the
problem of maintaining
photosynthesis with stomata
partially or completely closed on
hot, dry days.
However, it should be noted, that in all plants,
the Calvin cycle is used to make sugar from
carbon dioxide.
How exactly does C4 photosynthesis help a plant conserve
water?
CO2 is shuttled to an interior part of the plant leaf
called the bundle sheath cells in the form of malate (CO2
storage), which is far from any air spaces in spongy
parenchyma. Because CO2 is stored in these “malate”
molecules, stomata do not have to remain open as much.
This conserves water, as transpiration does not occur when
stomata are closed.
What process produces the carbohydrates in C4 plants?
Calvin-Benson Cycle.
What else happens, other than water conservation, when
stomata remain closed on hot, dry days?
Oxygen waste builds up, and photorespiration may
occur.
Some plants adapted to hot, arid regions have a different photosynthetic
mechanism called CAM photosynthesis.
CAM (Crassulacean Acid Metabolism) photosynthesis is found in cacti and
succulents, including the crassula family.
During the hot daylight hours
their stomata are tightly closed;
however they still carry on vital
photosynthesis as carbon dioxide
gas is converted into simple sugars.
How do they do it?
During the cooler hours of
darkness their stomata are open
and CO2 enters the leaf cells
where it combines with PEP
(phosphoenolpyruvate) to form 4carbon organic acids (malic and
isocitric acids).
The 4-carbon acids are stored in the vacuoles of photosynthetic cells in the
leaf. During the daylight hours the 4-carbon acids break down releasing CO2 for
the dark reactions (Calvin cycle) of photosynthesis inside the stroma of
chloroplasts.
The CO2 is converted into glucose through the Calvin-Benson cycle
with the help of ATP and NADPH, which were synthesized during the
light reactions of daylight in the grana of chloroplasts.
The adaptive advantage of
CAM photosynthesis is that
plants in arid regions can
keep their stomata
completely closed during the
daytime, thereby reducing
water loss from the leaves
through transpiration;
however, they can still carry
on photosynthesis with a
reserve supply of CO2 that
was trapped during the hours
of darkness in the vacuoles,
in the form of 4-carbon acids,
when the stomata were open.
The tropical strangler Clusia rosea exhibits CAM photosynthesis. This
unusual tree starts out as an epiphyte on other trees and then
completely envelops and shades out its host.
What is the basic difference between CAM
and C4 photosynthesis?
C4 stores it’s CO2 molecules in the form
of malate in the bundle sheath cells. CAM
stores CO2 in the form of malic and isocitric
acid within vacuoles. In CAM
photosynthesis, all CO2 can be taken in
through the stomata during the night time
hours, so they can remain closed all day long.
Typical, or C3 photosynthesis is carried out by most plants growing in
areas with sufficient water. In this type of photosynthesis, an enzyme
called RuBP carboxylase grabs CO2 in the light independent reactions
of photosynthesis. This works fine as long as there is plenty of carbon
dioxide and relatively little oxygen.
If there is too much oxygen, RuBP carboxylase will grab that instead of
the CO2, and a process called photorespiration will occur.
Photorespiration does not help build up any sugars, so if
photorespiration occurs, growth stops. C3 plants will wither.
Normally, oxygen (produced in photosynthesis) exits the plant through
the stomata; however, if there isn't enough water available (as would
happen under bright, hot, sunny conditions), excess oxygen may build up
and trigger photorespiration, because the stomata close to conserve
water.
If water is present, however, C3 photosynthesis is very efficient
because both the light reactions and Calvin Cycle can occur
simultaneously in the same cell, and almost all of the cells in the leaf
will be producing sugars.
C4 photosynthesis differs from C3 in 2 key ways. First, instead of RuBP
carboxylase, a different enzyme, PEP carboxylase, is used to grab
CO2. The PEP carboxylase is less likely to bind to oxygen, thus
photorespiration is less likely to occur, a decided advantage under hot,
dry conditions where water may be scarce and the stomata remain
closed for long periods, trapping oxygen in the plant.
This process is relatively inefficient, but if
water is in short supply the inefficient C 4
route is still better than the C3 route with
photorespiration. Also, since there are
fewer cells involved in making sugars (bundlesheath), fewer sugars can be made.
Thus desert plants can survive the dry
conditions, but at the cost of rapid
growth. Desert plants are often very slow to
grow, and this is one of the reasons they
invest so much energy in defensive
structures (spines) and bitter chemicals
C3 is best under moist conditions, C4 under warm, sunny, dry
conditions, CAM under desert conditions
Characteristics of Photosynthesis in C3, C4 and CAM plants
Characteristic
C3 Plant
C4 Plant
CAM Plant
Yes
Little
none
15-25o C
30-40o C
--------------------
Rubisco present
Yes
Yes
Yes
PEP Carboxylase present
No
Yes
Yes
Initial CO2 fixation
directly into
Calvin Cycle
via Rubisco
into OAA via PEP
carboxylase, then to
malic acid which
moves from mesophyll
cell to bundle sheath
cell and then releases
CO2.
into OAA via PEP carboxylase,
then to malic acid which
moves into vacuole (during
night). CO2 released from
malate during day.
Secondary CO2 fixation
----------------
In bundle sheath cell
using Rubisco
In “mesophyll”* cell using
Rubisco – in morning
Site of Calvin cycle
mesophyll cells
bundle sheath cells
“mesophyll” cells
Site of Light Reactions
mesophyll cells
mostly in mesophyll
cells
“mesophyll” cells
Photorespiration
Lower temp limit for
photorespiration
Why is C3 photosynthesis selected for on the planet today,
over CAM and C4 photosynthesis?
Because in the presence of water, light dependent
and independent reactions can occur simultaneously, and
nearly all the cells in the leaf can be producing sugars. In
optimum circumstances, it is by far, the most efficient
producer of glucose.
Which enzyme fixes CO2 in CAM and C4 photosynthesis,
instead of Rubisco, and why is this an advantage?
PEP Carboxylase; because it won’t fix oxygen, and
photorespiration will not occur as a result.
Where in the leaf do each of the following first fix carbon
dioxide? C3; C4; CAM
C3: mesophyll; palisade parenchyma primarily
C4: bundle sheath cells
CAM: mesophyll, CO2 stored in vacuoles as acid