Chapter 6- Photosynthesis Carbon Metabolism (1)

CHAPTER 6 PHOTOSYNTHESIS: CARBON METABOLISME

A summary of photosynthesis

Consist of two reactions: 

Photochemical reactions Occur in thylakoids and grana, produce O2, ATP, and reduced NADP+ (or NADPH).

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Biochemical reactions Known as Calvin cycle. Occur in stroma, uses the ATP and reduced NADP+ to reduce CO2 to carbohydrate. The byproducts ADP,

Pi, and NADP+ are returned from the Calvin cycle to the photochemical reactions

6.1 Photosynthetic Carbon Reduction (PCR) Cycle @ Calvin cycle 

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Pathway which all organisms (photosynthetic eukaryotic) incorporate CO2 into carbohydrate; known as carbon fixation @ Photosynthetic Carbon Reduction (PCR) cycle Referred as Calvin Cycle Consume ATP and NADPH produced by photosynthetic electron transport

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PCR cycle divided into 3 primary stage :

(i) Carboxylation fixes CO2 in the presence of 5-C acceptor molecule; ribulose bisphosphate (RuBP); and converts into 2 molecules of 3-C acid (ii) Reduction consume the ATP and NADPH produced by photosynthetic electron transport to convert the 3-C acid to tiose phosphate (iii) Regeneration

consume

(i) Carboxylation 

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Calvin’s study by using radiolabeled CO2; 14CO2 Radioactivity was found in a 3-Carbon acid (3-phosphoglycerate); 3-PGA 3-PGA first stable product of photosynthesis PCR cycle referred as C3 cycle Acceptor molecule ribulose-1,5biphosphate (RuBP)

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Carboxylation reaction; which CO2 added to RuBP forming sixcarbon intermediate which is transient & unstable six-carbon intermediate which is transient & unstable remain bound to enzyme and hydrolized to 2-molecules 3-PGA Carboxylation reaction Catalyzed by enzyme ribulose1,5-biphosphate carboxylaseoxygenase (Rubisco)

Carboxylation Reaction of the PCR cycle

O

CH20-P

C-COH

C=O HCOH

CH20-P

+ *CO2

HO

C=O HCOH

HCOH CH2OH-P

Ribulose-1,5biphosphate (RuBP)

CH2OH-P

Six-carbon intermediate

CH20-P HCOH CO2CO2HCOH CH20-P 3-phosphoglycerate (3-PGA)

* Ribulose-1,5-biphosphate carboxylase-oxygenase (Rubisco)

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Energy from light photosynthesis is points. i.e.:

reaction required

of at 2

i. Reduction of 3-PGA ii. Regeneration of RuBP acceptor molecule

(ii) Reduction of 3-PGA 

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For chloroplast continue take up CO2 Involved 2 conditions : i. Production of 3-PGA continually removed ii. Maintain an adequate supply of RuBP

Both require energy ATP and NADPH 3-PGA is removed by reduction

Reduction 3-PGA to G3P

CH20-P HCOH CO2-

ATP

NADPH

ADP CH20-P HCOH

NADP+ CH20-P HCOH CH

C O

O

+ Pi

O

P 3-PGA

1,3-biphosphoglycerate

Glycelaraldehyde-3phosphate(G3P)

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2-step reaction: 3-PGA phosphorylated to 1,3bisphosphoglycerate 1,3-bisphosphoglycerate reduced to glyceraldehyde -3-phosphate Both ATP and NADPH required in 2 step reaction Triose sugar-phosphate; G3P available for export to cytoplasm

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Regeneration of RuBP

Continuing supply of acceptor molecule; RUBP Accomplished by series of reaction involving 4-, 5-, 6-, and 7- carbon sugars Reactions include the condensation of 6-C fructose phosphate with triose-phosphate to forms 5-C and 4-C sugar Another triose join with 4-C

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7-C sugar combined with 3 rd triosephosphate 2 molecules 5-C sugar formed

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5-C sugars can be isomerized to form Ribulose-5-phosphate (Ru5P)

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Ru5P phosphorylated to regenerate required ribulose-1,5-biphosphate

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Net effect of reaction carbon from 5 out of 6 to regenerate 3 RuBP

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Sum = 3 RuBP + 3 CO2 + G3P

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6 turns of cycle would regenerate 6 molecules RuBP and one additional hexose sugar as net product

recycle G3P molecules molecules 3 RuBP

6.1.2 

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Energy input in PCR cycle

3 turns of cycle ⇨ uptake of 3 molecules CO2 ⇨ total 6 molecules NADPH and 9 molecules ATP required Reduction of each molecule of CO2 requires 2 molecules NADPH and 3 molecules ATP ratio of ATP/NADPH of 3/2 @ 1.5 Each NADPH stores 2 electrons total of 4 electrons required to fix each molecule CO2

6.2 Activity and regulation of PCR Cycle 

Rate of carbon reduction is partly dependent on the availability of an adequate pool of acceptor molecules; CO2 and RuBP

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PCR cycle can utilize fixed carbon to increase the pool size through autocatalytic regeneration of RuBP

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Normally, extra carbon taken through PCR cycle accumulated as starch @ exported from chloroplast

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However, PCR cycle has potential to

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Amount of acceptor quickly built up within chloroplast to support rapid photosynthesis @ increase the rate of photosynthesis

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Time required to built up the levels of PCR cycle intermediates in transition from dark to night; called photosynthesis induction time

6.2.2 Regulation of Rubisco activity 

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Rapidly to zero when light is turn off Slowly when light is once again turned on Rubisco activity is regulated indirectly by light Involves complex interaction between Mg 2+ fluxes across the thylakoid, CO2 activation, chloroplast pH changes and

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Light –driven electron transport leads to proton net movement into lumen of thylakoid Proton across thylakoid membrane generates a proton gradient = pH 3.0 and increase pH of the stroma (pH 5.0 in the dark to pH 8.0 in the light) Light bring increase in the free Mg 2+ of the stroma Mg

2+

moves out of the lumen to

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CO2 reacts with amino group forming carbamate Carbamate requires the release of 2 proton ⇨ consequently, increasing pH Mg 2+ then coordinated to carbamate to form carbamate- Mg 2+ complex ⇨ active form of the enzyme

6.2.3 Regulation of other PCR enzymes 

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eg: ferredoxin/thioredoxin for light-driven enzyme activation in chloroplast

PSI drives the ferredoxin reduction which in turn reduces thioredoxin Thioredoxin reduces the appropriate disulphide bond (-S-S-) state to sulfhydryl (-SH-SH-) state

6.3

Photorespiration

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Photorespiration ⇨ process involves reoxidation of products assimilated in photosynthesis Involves activities of 3 different organelles: i. chloroplast ii. peroxisome iii. Mitochondria

Photorespiration consume oxygen, release carbon dioxide Because CO2 is evolved, results in net loss of carbon from cell

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Plants that incorporate carbon through PCR @ Calvin cycle known as C3 plants; cause the 1st product to incorporate CO2 in 3C acid PGA (3-PGA) C3 plants have built-in metabolic inefficiency in photosynthetic process Acquire through

CO2 from atmosphere stomata

when hot close to

dry weather stomata reduce water loss

3 CO2

3 RuBP

Rubisco

6 PGA 6 ATP

3 ADP

6 ADP

Calvin Cycle

3 ATP

6 1,3biphosphoglycerate 6 NADPH

5 G3P 6 G3P 1 G3P glucose

6 NADP+ 6Pi

H 2O

CO2

Light Chloroplast

NADP+ Photosynthe tic electron transport

ADP

Calvin Cycle

ATP NADPH

O2

CH2O (Glucose)

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6.3.1 Rubisco catalyzes fixation of both CO2 and O2

Glycolate (2-C) metabolism related to photorespiration Enzyme involved in process located in 3 organelles Bifunctional nature of Rubisco ⇨Rubisco catalyzes an in carboxylation reaction oxygenase reaction ⇨ Ribulose 1-5-biphosphate carboxylase-oxygenase RuBP + O2 3 PGA + phosphoglycolate (2C)

P-glycolate CO2 + recovery of remaining carbon of PCR 

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Known as C2 glycolate cycle @ PCO(Photosyntheitic carbon oxidation) cycle Begins with RuBP oxidation to 3-PGA and P-Glycolate

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Glycolate exported from chloroplast and diffuses to peroxisome Glycolate oxidized to glyoxylate and hidrogen peroxide Peroxide broken down by catalase Glyoxylate undergoes transamination reaction form amino acid glycine Glycine transferred to mitochondria ⇨ 2 mol. glycine (4-C) converted to 1 mol. serine (3-C) + 1 mol. CO2 (released) Serine leaves mitocondria ⇨returning to peroxisome; amino (-NH2) group given up in transmination The product (Hydroxy

6.4

C4 Syndrome

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C-4 plants:

⇨ First product is 4-carbon acid oxaloacetate(OAA) ⇨ exhibit specific anatomical, physiological and biochemical characteristic constitute C4 syndrome C-4 Leaves anatomy : ⇨ presence of 2 distinct photosynthesis tissues; between vascular bundles mesophyll cells called

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Under condition of high fluence rates and high temperature (30-40ºC), photosynthesis rates of C4 species may 2 -3 times greater than C3 species Maintain active photosynthesis under of water stress ⇨ lead stomatal closure CO2 concentrated in bundle sheath cells Hatch & Slack proposed a cyclic mechanism ⇨ carbon incorporated into C4 acid ⇨ β –carboxyl carbon transferred as CO2 to PCR cycle

C4 photosynthetic carbon assimilation cycle

6.4.1     

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C4 photosynthetic carbon assimilation cycle

Carboxylation phosphoenol pyruvate (PEP) using HCO3- by PEPcase Oxaloacetate (OAA) formed is unstable OAA reduced to malate or transaminated to aspartate Transported from mesophyll cell into bundle sheath cell C4-Acid undergoes decarboxylation, CO2 formed reduce to triose sugars via PCR cycle C3-acid (pyruvate or alanine) transported back into mesophyll cell

C4 Syndrome Ecological Significance

Comparison of C4 and C3 plants (Refer to Table 5.4 page 112; Textbook)   

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Photosynthesis of C4 is not inhibited by oxygen Photorespiraton is absent of C4 plants or the process suppressed C4 plants have a low CO2 compensation point(range of 0 to 5 ul/l CO2 ) ; While for C3 plants, range of 20 to 100 ul/l CO2 CO2 compensation point : ambient CO2 concentration at which

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High cell with

level of CO2 in bundle sheath outcompeting O2 for binding Rubisco

Adaptations of C4 leaves ensure CO2 that might escape bundle-sheath cell is trapped and reassimilated by PEPcase in mesophyll cells before escape from leaf ⇨ thus, C4 leaves efficient in CO2 absorbers, trap and recirculate CO2 produced in leaf C4 plants have higher temperature optimum (30-45oC) than C3 plants (20-25oC) High temperature (between 40 oC and 50 oC) ⇨ rate of photosynthesis decrease to greater extent than rate

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C4 plants can maintain photosynthesis when stomata partially closed to conserve water; while C3 plants , moderate water stress will close stomata Transpiration low (range of plants higher 1000)

ratio of C4 plants is 200 to 350), while C3 ( range of 500 to

Does not saturate, even in full sunlight; while light saturation of C3 occur about 25 % of ful,l

Crassulacean Acid Metabolism (CAM)

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Studied most extensively in Family Crassulaceae Now found in 23 different flowering plants (Cactaceae, Euphobiaceae) fern (Polypodiaceae) primitive plant (Welwitschia) Survive in extremely dry or xerophytic habitats Succulent plants ⇨ characteristics by thick, fleshy leaves, cells contain large water-filled vacuoles)

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Involve Carboxylation phosphoenol pyruvate (PEP) @ enzyme PEP carboxylase Immediate product OAA reduce to malate which is store in vacuole During daylight hours

⇨ malate retrieved from vacuole ⇨ decarboxylated ⇨ CO2 diffuses into chloroplast and converted to triose

Comparison between CAM and C4 syndrome  CAM similar to C4

⇨ use PEPcase to form C4 acids from PEP and bicarbonate ⇨ Acids decarboxylated to provide CO2 for PCR cycle

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Significant differences ⇨ C4 requires a special anatomy by which C4 carboxylation spatially separated from C3 PCR cycle. In CAM both occur in same cell but separate time ⇨ CAM no close cycle of carbon intermediate as in C4 plants. PEP

Significant of CAM  

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Represents a adaptation to dry habitat Growing in shallow & sandy soil with little available water CO2 uptake at night (during minimum evaporative water loss) Daylight hours ⇨ stomata closed to reduce water loss ⇨ photosynthesis can proceed using the reservoir of stored CO2 Daily carbon assimilation by CAM plants about ½ of C3 plants, 1/3 of C4 plants CO2 uptake by CAM plants continue under water stress conditions which in