The Calvin cycle: formation of sugar from CO2

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The Calvin cycle formation of sugar from
CO2 What are the phases of the carbon
cycle? How does carbon fixation vary in
different climates?
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Calvin cycle takes place in chloroplast
stroma in plants and algae ATP and NADPH
accumulate there CO2 enters leaf through
stomata, diffuses to stroma
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• Carbon fixation
• CO2 is incorporated into ribulose-1,5-
• bisphosphate
• water is incorporated
• 2 3-phosphoglycerate molecules are formed
• immediately
• Enzyme RuBP carboxylase (rubisco)

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II. 3-PGA is reduced to form glyceraldehyde 3-P
(G3P from NADPH) 3 CO2 molecules are fed into
the pathway 6 G3P are produced 6 ATP and 6
NADPH are used One G3P is used to produce sugar.
The others are used to regenerate RuBP
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III. Regeneration of RuBP 5 G3P are rearranged
to produce 3 RuBP (necessary to keep Calvin
cycle going) 3 ATP molecules are hydrolyzed The
cycle is complete and ready for uptake of more
CO2
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3 CO2 9 ATP 6 NADPH 5 H2O? G3P 9 ADP
6 NADP 8 Pi G3P can be interconverted with
DHAP triose phosphates (intermediate in which
pathway?) also used for sucrose synthesis in
cytosol, starch in stroma Triose phosphates are
transported out of the chloroplast by a
phosphate translocator Antiport triose
phosphates out, Pi in (from cytosol to stroma)
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Calvin cycle does not require light directly, But
light is necessary to generate ATP, NADPH How is
this regulated? No rubisco in nonphotosynthetic
tissue Rubisco and other enzymes require high pH
and magensium ions for activity What happens to
pH of stroma when light- dependent reactions are
going on?
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As protons are pumped INTO the thylakoid
space, magnesium ions are pumped into
STROMA (charge neutrality) In the dark, this
doesnt happen Rubisco and other enzymes are not
as active Therefore, no Calvin cycle
activity Activation of Calvin cycle inactivates
phospho- fructokinase (therefore no glycolysis)
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What happens to the triose phopshates (G3P,
DHAP) They can remain in the chloroplast
stroma or be translocated to the cytosol Either
way, they are condensed to form a 6-carbon sugar
(fructose 6-P) eventually yields
glucose-1-P What happens next depends of the
location of the reaction
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In cytosol, sucrose is formed (and
dephosphory- lated) disaccharide of glucose
and fructose Exported from leaves by vascular
bundles Energy and carbon source for
nonphotosynthetic tissue
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Chloroplast stroma starch biosynthesis Only
occurs if plant has sufficient energy and carbon
sources Many other pathways occur in
chloroplasts Fatty acid, chlorophyll, carotenoid
synthesis Nitrate reduction?ammonia ?
synthesis of amino acids, nucleotides Sulfate
reduction ? sulfides (which biological molecules
contain sulfur?)
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Rubisco adds CO2 to RuBP as first step
in Calvin cycle It can also add oxygen instead
of CO2 (oxygenase) Molecule splits, but
products cannot be fed into Calvin cycle (3-PGA
and phospho- glycolate) No apparent use for
these molecules Rubisco has lower affinity for
oxygen than CO2, but there is much more oxygen
in the atmosphere
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Process appears to be wasteful, and
problem- atic in hot, dry climates CO2 is less
soluble than O2 when hot Stomates close to
conserve water CO2 cant get in, O2 cant get
out Water photolysis (oxygen generation)
increases in sunny conditions How do plants
deal with this?
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• Glycolate pathway
• in peroxisomes and mitochondria of plant cells
• (special peroxisome leaf peroxisome)

Summary phosphoglycolate is dephosphorylated, tr
ansported to leaf peroxisome converted to
glycine, transported to mitochondrion In
mitochondrion, converted to serine (CO2 given
off) Serine transported back to peroxisome
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In peroxisome, serine is converted to
glycerate transported back to chloroplast Phosp
horylated? 3-PGA (Calvin cycle intermediate PHOT
ORESPIRATION- uses up oxygen, but yields no ATP
and no food Expensive, but helps recover carbon
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II. C4 pathway (Hatch-Slack pathway) C4 plants
are adapted to hot, dry climates two kinds of
photosynthetic cells Helps concentrate CO2 in
bundle sheath cells Carbon-fixing enzyme is PEP
carboxylase (much higher affinity for CO2 than
rubisco)
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Note additional ATP requirement
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At high temperature (gt30oC) C4 plants are much
more efficient than C3 plants Less
photorespiration More available CO2 PEP
carboxylase is better at scavenging
scarce CO2 Commercially important C4 plants
maize, sugar cane (1 of species are
C4) Commercially important C3 plants cereals
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What about extremely dry climates? Stomata open
only at night, so CO2 is taken up in the
dark Crassulacean acid metabolism (CAM
plants) about 4 of plant species succulents,
cacti, orchids
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At night, CO2 is taken up and stored as
malate formed in mesophyll cells, stored in
vacuoles In daylight, malate diffuses into
cytosol and is decarboxylated CO2 diffuses into
chloroplast stroma? Calvin cycle
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Some CAM plants can idle- stomata are closed
all the time CO2 is cycled from respiration to
photosynthesis no loss of water no growth
drought survival mechanism
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Perspective sunlight is the fundamental
source of energy on Earth Water is the source
of electrons action of two photosystems
transfers them to NADP proton motive forces
drives ATP synthesis ATP and NADPH are used to
fix and reduce CO2. Some is stored or used for
energy, some is used to regenerate Calvin
cycle. 3 CO2, 9 ATP, 6 NADPH are required to
produce one triose phosphate
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Cyclic, noncyclic electron flow pathways
insure necessary balance of ATP and NADPH
for this and other metabolic pathways Photorespi
ration may be an evolutionary relic but plants
have evolved diverse solutions to deal with
innate inefficiencies in the critical enzyme,
rubisco Another example that water is critical
to life!