Download Reactions

Photosynthesis
The carbon reactions
(Dark Reactions)
Chapter 11
Overall Perspective
• Light reactions:
– Harvest light energy
– Convert light energy to
chemical energy
• Carbon (Dark) Reactions:
– Expend chemical energy
– Fix Carbon [convert CO2 to
organic form]
At the end of the light reactions
• The reaction of the light reaction is:
– CO2 +H2O
(CHOH) + O2
• Recent estimates indicate that about 200
billion tones of CO2 (Mr = 44) are converted
to biomass each year
– 40 % of this is from marine phytoplankton
– The bulk of the carbon is incorporated into
organic compounds by the carbon reducing
reactions (dark reactions) of photosynthesis
At the end of the light reactions
• The reactions catalyzing the
reduction of CO2 to
carbohydrates are coupled
to the consumption of
NADPH and ATP by enzymes
found in the stroma
– fluid environment
• These reactions were
thought to be independent
of the light reactions
– So the name “dark reactions”
stuck
• However, these chemical
reactions are regulated by
light
– So are called the “carbon
reactions” of photosynthesis
Overview of the carbon reactions
• The Calvin cycle:
• Stage 1:
– CO2 accepted by Ribulose1,5-bisphosphate.
– This undergoes
carboxylation
• Has a carboxyl group
(-COOH) attached to it
– At the end of stage 1, CO2
covalently linked to a
carbon skeleton forming 2
3-phosphycerate
molecules.
Carboxylation: The first step is
the most important
• Step 1: The enzyme RUBISCO (Ribulose bis-phosphate carboxylase
oxygenase) carries out this conversion
• Rubisco accounts for 40% of the protein content of chloroplasts
– is likely the most abundant protein on Earth
• Rubisco is, in fact, very inefficient, and that a mechanism has
evolved to deal with this handicap
Overview of the carbon reactions
• The Calvin cycle:
• Stage 2:
– Each of the two 3-phosphycerate
molecules are altered.
– First phosphorylated through the
use of the 3 ATPs generated
during the light reaction.
– Then reduced through the use of
the 2 NADPHs generated during
the light reaction.
– Forms a carbohydrate
• glyceraldehyde-3-phosphate
3-phosphycerate molecules are
altered
• First phosphorylated through the use of the 3 ATP molecules generated
during the light reaction
– Forms 1,3-bisphosphoglycerate
• Then reduced through the use of the 2 NADPH molecules generated
during the light reaction
– Forms glyceraldehyde-3-phosphate
• Note the formation of triose phosphate
Overview of the carbon reactions
• The Calvin cycle:
• Stage 3:
– Regeneration of Ribulose-1,5bisphosphate.
– This requires the coordinated
action of eight reaction steps
• And thus eight specific enzymes
– Three molecules of Ribulose-1,5bisphosphate are formed from
the reshuffling of carbon atoms
from triose phosphate.
Regeneration of
Ribulose-1,5-bisphosphate
• The Calvin cycle reactions regenerate the
biochemical intermediates needed for operation
• More importantly, the cycle is Autocatalytic
– Rate of operation can be enhanced by increasing the
concentration of the intermediates in the cycle
• So, Calvin cycle has the metabolically desirable of
producing more substrate than is consumed
– Works as long as the produced triose phosphate is NOT
diverted elsewhere (as in times of stress or disease)
Overview of the carbon reactions
• The Calvin cycle:
• The cycle runs six times:
– Each time incorporating a
new carbon . Those six
carbon dioxides are reduced
to glucose:
– Glucose can now serve as a
building block to make:
• polysaccharides
• other monosaccharides
• fats
• amino acids
• nucleotides
Only one-sixth of the triose phosphate
is used for polysaccharide production
• Synthesis of polysaccharides, such as starch and
sucrose, provide a sink
– Ensures an adequate flow of carbon atoms through the
cycle IF CO2 is constantly available
• During a steady rate of photosynthesis 5/6 of the
triose phosphates are used for the regeneration of
Ribulose-1,5-bisphosphate
• 1/6 is transported to the cytosol for the synthesis
of sucrose or other metabolites that are converted
to starch in the chloroplast
Regulation of the Calvin cycle
• The high energy efficiency of the Calvin
cycle indicates that some form of regulation
ensures that all intermediates in cycle:
– Are present at adequate concentrations
– The cycle is turned off when it is not needed
in the dark
• Remember:
– These are the “carbon reactions”, NOT the “dark
reactions”
• Many factors regulate the Calvin cycle
Regulation of the Calvin cycle
• 1: The pH of the stroma increases as
protons are pumped out of it through the
membrane assembly of the light reactions.
– The enzymes of the Calvin Cycle function
better at this higher pH.
• 2: The reactions of the Calvin cycle have to
stop when they run out of substrate
– as photosynthesis stops, there is no more ATP
or NADPH in the stroma for the dark
reactions to take place.
Regulation of the Calvin cycle
• 3: The light reactions increase the
permeability of the stromal membrane to
required cofactors
– Mg ions are required for the Calvin Cycle.
• 4: Several enzymes of the Calvin Cycle are
activated by the breaking of disulphide
bridges of enzymes involved in the working
of the cycle.
– the activity of the light reactions is
communicated to the dark reactions by an
enzyme intermediate
When conditions are
not optimum
Photorespiration
• Occurs when the CO2 levels inside a leaf become
low
– This happens on hot dry days when a plant is forced
to close its stomata to prevent excess water loss
• If the plant continues to attempt to fix CO2 when
its stomata are closed
– CO2 will get used up and the O2 ratio in the leaf will
increase relative to CO2 concentrations
• When the CO2 levels inside the leaf drop to around
50 ppm,
– Rubisco starts to combine O2 with Ribulose-1,5bisphosphate instead of CO2
Photorespiration
• Instead of producing 2 3C
PGA molecules, only one
molecule of PGA is produced
and a toxic 2C molecule called
phosphoglycolate is produced
• The plant must get rid of the
phosphoglycolate
• The plant immediately gets
rid of the phosphate group
– converting the molecule
to glycolic acid
Photorespiration
• The glycolic acid is then
transported to the
peroxisome and there
converted to glycine
– Peroxisomes are
ubiquitous organelles that
function to rid cells of
toxic substances
• The glycine (4 carbons) is
then transported into a
mitochondria where it is
converted into serine (3
carbons)
– Releases CO2
Photorespiration
• The serine is then used to
make other organic molecules
• All these conversions cost
the plant energy and results
in the net lost of CO2 from
the plant
• 75% of the carbon lost during
the oxygenation of Rubisco is
recovered during
photorespiration and is
returned to the Calvin cycle
The C4 Carbon
cycle
The C4 carbon Cycle
• The C4 carbon Cycle occurs in 16 families of
both monocots and dicots.
–
–
–
–
Corn
Millet
Sugarcane
Maize
• There are three variations of the basic C4
carbon Cycle
– Due to the different four carbon molecule used
The C4 carbon Cycle
• This is a biochemical pathway
that prevents photorespiration
• C4 leaves have TWO chloroplast
containing cells
– Mesophyll cells
– Bundle sheath (deep
in the leaf so
atmospheric oxygen cannot diffuse easily
to them)
• C3 plants only have Mesophyll cells
• Operation of the C4 cycle requires the
coordinated effort of both cell types
– No mesophyll cells is more than
three cells away from a bundle
sheath cells
• Many plasmodesmata for
communication
The C4 carbon Cycle
• Four stages:
• Stage 1:
• In Mesophyll cell
– Fixation of CO2 by the
carboxylation of phosphenolpyruvate (primary acceptor
molecule)
– forms a C4 acid molecule
– Malic acid and/or aspartate
• Stage 2:
– Transport of the C4 acid
molecule to the bundle sheath
cell
The C4 carbon Cycle
• Stage 3:
– Decarboxylation of the C4
acid molecule (in bundle sheath)
– Makes a C3 acid molecule
– This generates CO2
– This CO2 is reduced to
carbohydrate by the Calvin
cycle
• Stage 4:
– The C3 acid molecule (pryuvate)
is transported back to
mesophyll cells
– Regeneration of phosphenolpyruvate
The C4 carbon Cycle
• Regeneration of phosphenol-pyruvate consumes two high
energy bonds from ATP
• Movement between cells is by diffusion via plasmodesmata
• Movement within cells is regulated by concentration
gradients
• This system generates a higher CO2 conc in bundle sheath
cells than would occur by equilibrium with the atmosphere
– Prevents photorespiration!!!!!!!!!!
The C4 carbon Cycle
• The net effect of the C4 carbon Cycle is to convert
a dilute CO2 solution in the mesophyll into a
concentrated solution in the bundle sheath cells
– This requires more energy than C3 carbon plants
• BUT – This energy requirement is constant no
matter what the environmental conditions
• Allows more efficient photosynthesis in hotter
conditions
Crassulacean Acid
Metabolism
(CAM Plants)
CAM Plants
• The CAM mechanism enables plants to
improve water efficiency
– CAM plant
• Loses 50 – 100 g water for every gram of CO2 gained
– C4 plant
• Loses 250 – 300 g water for every gram of CO2 gained
– C3 plant
• Loses 400 – 500 g water for every gram of CO2 gained
• Similar to C4 cycle
– In CAM plants formation of the C4 acid is both
temporally and spatially separated
CAM Plants
• At night:
• Stomata only open at night
when it is cool
• CO2 is captured by
phosphenol-pyruvate
carboxylase in the cytosol –
leaves become acidic
• The malic acid formed is
stored in the vacuole
– Amount of malic acid formed
is equal to the amount of
CO2 taken in
CAM Plants
• During the day:
• Stomata close, preventing water loss,
and further uptake of CO2
• Malic acid is transported to the
chloroplast and decarboxylated to
release CO2
• This enters the Calvin cycle as it can
not escape the leaf
– Pyruvate is converted to starch in the
chloroplast – regenerates carbon
acceptor
Phosphorylation regulates phosphenolpyruvate (PEP) carboxylase
• CAM and C4 plants require a
separation of the initial
carboxylation from the
following de-carboxylation
• Diuranal regulation is used
• IN CAM PLANTS:-
• Phosphorylation of the serine
residue of phosphenol-pyruvate
(PEP) carboxylase (Ser-OP) yields
a form of the enzyme which is
active at night
– This is relatively insensitive
to malic acid
Photophorylation regulates phosphenolpyruvate (PEP)carboxylase
• During the day:
• De-Phosphorylation of the
serine (ser-OH) gives a form
of the enzyme which is
inhibited by malic acid
• THIS IS THE OPPOSITE
WAY AROUND FOR C4
PLANTS!
Summary
• The reduction of CO2 to carbohydrate via
photosynthesis is coupled to the consumption of
ATP and NADPH
• CO2 is reduced via the Calvin cycle
– Takes place in the stroma (soluble phase)
• CO2 and water combine with Ribulose-1,5bisphosphate in the following reaction
– CO2 +H2O
(CHOH) + O2
• Regeneration of the carrier is required for the
cycle to continue
Summary
• The Calvin cycle requires the joint action of
several light-dependant systems
– Changes in ions (Mg+ and H+)
– Changes in effector metabolites (enzyme
substrates)
– Changes in protein-mediated systems (rubisco
activase)
• Rubisco can also act as an oxygenase
– The carboxylation & oxygenation reactions
take place at the active sites of rubisco.
Summary
• C4 and CAM plants Prevent photorespiration!!!!!
• C4 leaves have TWO chloroplast containing cells
– Mesophyll cells
– Bundle sheath
• CAM Plants drastically reduce water lass
– CAM plant
• Loses 50 – 100 g water for every gram of CO2 gained
– C4 plant
• Loses 250 – 300 g water for every gram of CO2 gained
– C3 plant
• Loses 400 – 500 g water for every gram of CO2 gained
Any
Questions?