Transcript produced in photosynthesis

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 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.
 Over Earth’s vast geologic history, the atmosphere has
contained both very high (time of dinosaurs >50%), and very
low (when life was still evolving-0%) levels of O2.
 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.
C4 plants are so named because they form a four-carbon compound as the first
product of the dark reactions of photosynthesis (OAA), instead of the 3carbon 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, and members of the
grass family.
• Instead of being fixed by rubisco to form
PGA, CO2 combines with PEP 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.
• 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 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
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
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.
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 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 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.
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.
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, this process 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, 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 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