Download - Lecture 9 Photosynthesis -II Chapter 8
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CalvinCycle
Also referred to as C3 photosynthesis –
Ribulose 1,5-bisphosphate (RuBP is carbo!ylated to for" 3-
phospho#lycerate (3-P$A via ribulose 1,5-bisphosphatecarbo!ylase (rubisco activity%
&i!ation of one "olecule of C' re)uires t*o "olecules of+AP and three of A.P%
.hree "olecule of C' yield a net synthesis of one "olecule of#lyceraldehyde 3-phosphate ($3P%
/ost (5 out of 0 "olecules of the $3P produced is used tore#enerate RuBP so the process can continue (Calvin cycle%
.he 1 out of 0 "olecules of the triose phosphates not
recycled often condense to for" he!ose phosphates, *hichulti"ately yield sucrose, starch and cellulose%
.he su#ars produced durin# carbon "etabolis" yield carbons2eletons that can be used for other "etabolic reactions li2ethe production of a"ino acids and lipids%
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•Rubisco also has an o!y#enaseactivity that uses "olecularo!y#en as a substrate instead ofC'%
•Rubisco catalyes the reaction of' *ith RuBP, producin# 3-P$A
and the t*o-carbon "olecule, -phospho#lycolate%
Rubisco
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The Calvin cycle begins when acarbon atom from a CO2molecule is added to a five-carbon molecule (the startingmaterial). The resulting six-carbon molecule is unstable andimmediately splits into three-carbon molecules.
Then through a series of reactionsenergy from !T" and hydrogensfrom #!$"% (the products of thelight-dependent reactions) areadded to the three-carbonmolecules. The now-reduced three-carbon molecules either combine to
ma&e glucose or are used to ma&eother molecules.
'ost of the reduced three-carbon molecules are used toregenerate the five-carbonstarting material thuscompleting the cycle.
Three "hases of the Calvin Cycle
2
CO2
"
*
+u,"(tarting material)
"
lucose
#!$"%
!T"
!T"
"
/ lyceraldehyde
-phosphate
"
"
+u,"(tarting material)
/-phosphoglycerate
/
/-phosphoglycerate
/
lyceraldehyde
-phosphate
lyceraldehyde
-phosphate
Carboxylation +eduction +egeneration
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-phospho-D-glycerateD-glyceraldehyde--phosphate
Three-carbon 'olecules in the Calvin Cycle
http://en.wikipedia.org/wiki/Image:D-glyceraldehyde-3-phosphate_wpmp.pnghttp://upload.wikimedia.org/wikipedia/en/0/0b/3-phospho-D-glycerate_wpmp.png
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PhotorespirationRubisco has 2 enzymatic activities
Carboxylation Addition of CO2 to RuBP
Favored under normal conditions
PhotorespirationOxidation of RuBP by the addition of O2
Favored hen stoma are closed in hot conditionsCreates lo!CO2 and hi"h!O2
CO2 and O2 compete for the active site on
RuBP
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PhotorespirationPhotorespiration occurs *hen the C' levels inside a
leaf beco"e lo*%.his situation happens on hot dry days *hen a plant
is forced to close its sto"ata to prevent e!cess *aterloss%
4f the plant continues to atte"pt to ! C'
*hen its
sto"ata are closed, the C' *ill #et used up and the
' ratio in the leaf *ill increase relative to C'concentrations%
6hen the C' levels inside the leaf drop to around 57
pp", Rubisco starts to co"bine ' *ith RuBP insteadof C'%
.he net result of this activity is that instead ofproducin# t*o P$As, only one "olecule of P$A isproduced and a to!ic C "olecule called
phospho#lycolate is produced%
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Photorespiration• .he plant "ust #et rid of the phospho#lycolate%
• &irst it i""ediately #ets rid of the phosphate#roup, convertin# the "olecule to #lycolic acid%
• .he #lycolic acid is then transported to thepero!iso"e and there converted to #lycine%
• .he #lycine is then transported into a"itochondrion *here it is converted into serine%
• .he serine is then used to "a2e other or#anic
"olecules%
• All these conversions cost the plant ener#y andresults in the net lost of C' fro" the plant%
• .o prevent this process, t*o specialiedbioche"ical additions have been evolved in the
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+ubisco as Oxygenase
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•C9 plants capture carbon dio!ide usin# an
eny"e called P:P carbo!ylase that addscarbon dio!ide to the three carbon "oleculephosphoenolpyruvate (P:P creatin# the 9-carbon "olecule o!aloacetic acid%
•Plants *ithout this eny"e are called C3
plants because the pri"ary carbo!ylation
reaction produces 3-P$A directly in the CalvinCycle%
•6hen o!y#en levels rise in the leaf, C9 plants
reverse the reaction to release C' thuspreventin# photorespiration%
•By preventin# photorespiration, C9 plants can
produce "ore su#ar than C3 plants in
C9 Plants
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C# plantsCorn$ su"arcane$ sor"hum$ and a number of
other "rasses
%nitially &x carbon usin" P'P carboxylase inmesophyll cells
Produces oxaloacetate$ converted to malate$transported to bundle!sheath cells
(ithin the bundle!sheath cells$ malate isdecarboxylated to produce pyruvate and CO2
Carbon &xation then by rubisco and the Calvincycle
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C# pathay$ althou"h it overcomes the
problems of photorespiration$ does have acost
)o produce a sin"le "lucose re*uires +2additional A)P compared ith the Calvincycle alone
C# photosynthesis is advanta"eous in hot dryclimates here photorespiration ouldremove more than half of the carbon &xed bythe usual C, pathay alone
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CA- plants-any succulent .ater!storin"/ plants$ such as
cacti$ pineapples$ and some members ofabout to dozen other plant "roups
0tomata open durin" the ni"ht and closedurin" the day
Reverse of that in most plants
Fix CO2 usin" P'P carboxylase durin" theni"ht and store in vacuole
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(hen stomata closed durin" the day$ or"anicacids are decarboxylated to yield hi"h levelsof CO2
1i"h levels of CO2 drive the Calvin cycle and
minimize photorespiration
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)ypes of photosynthesisC,Plants that &x carbon usin" only C,
photosynthesis .the Calvin cycle/C# and CA-
Add CO2 to P'P to form # carbon molecule
se P'P carboxylase
3reater a4nity for CO2$ no oxidase activity
C# 5 spatial solution
CA- 5 temporal solution
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Compare C# and CA-Both use both C, and C# pathays
C# 5 to pathays occur in di6erent cells
CA- 5 C# pathay at ni"ht and the C, pathay durin" the day
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Overview of
"hotosynthesis
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Plant :ner#y Cycle