lecture 9 photosynthesis -ii chapter 8

8/9/2019 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

lecture 9 photosynthesis -ii chapter 8

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