CHAPTER 6CHAPTER 6

PHOTOSYNTHESIS: PHOTOSYNTHESIS: CARBON METABOLISMECARBON METABOLISME

A summary of A summary of photosynthesis photosynthesis

Consist of two reactions:Consist of two reactions: Photochemical reactionsPhotochemical reactions

Occur in Occur in thylakoidsthylakoids and and granagrana, produce , produce OO22, , ATPATP, and , and reduced NADPreduced NADP+ + (or NADPH).(or NADPH).

Biochemical reactionsBiochemical reactions

Known as Known as Calvin cycleCalvin cycle. Occur in . Occur in stromastroma, , uses the ATPuses the ATP and reduced NADPand reduced NADP++ to reduce to reduce COCO22 to carbohydrate to carbohydrate. The . The byproducts ADP, byproducts ADP, Pi, and NADPPi, and NADP++ are returned from the Calvin are returned from the Calvin cycle to the photochemical reactionscycle to the photochemical reactions

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

cyclecycle

Pathway which all organisms Pathway which all organisms (photosynthetic eukaryotic) (photosynthetic eukaryotic) incorporate COincorporate CO22 into carbohydrate; into carbohydrate; known as known as carbon fixation @ carbon fixation @ Photosynthetic Carbon Reduction (PCR) Photosynthetic Carbon Reduction (PCR) cyclecycle

Referred as Calvin CycleReferred as Calvin Cycle

Consume Consume ATPATP and and NADPHNADPH produced by produced by photosynthetic electron transportphotosynthetic electron transport

PCR cycle divided into 3 primary stage :PCR cycle divided into 3 primary stage :

(i) Carboxylation (i) Carboxylation ⇨ ⇨ fixes COfixes CO22 in the in the presence of 5-C acceptor molecule; presence of 5-C acceptor molecule; ribulose bisphosphate (RuBP); and ribulose bisphosphate (RuBP); and converts into 2 molecules of 3-C acidconverts into 2 molecules of 3-C acid

(ii) Reduction (ii) Reduction ⇨ ⇨ consume the ATP and NADPH consume the ATP and NADPH produced by photosynthetic electron produced by photosynthetic electron transport to convert the 3-C acid to transport to convert the 3-C acid to tiose phosphatetiose phosphate

(iii) Regeneration (iii) Regeneration ⇨⇨ consume additional ATP consume additional ATP to convert some of the triose phosphate to convert some of the triose phosphate back into RuBP to ensure the capacity back into RuBP to ensure the capacity for the continuous fixation of COfor the continuous fixation of CO22

(i) Carboxylation(i) Carboxylation

Calvin’s study by using radiolabeled Calvin’s study by using radiolabeled COCO22; ; 1414COCO22

Radioactivity was found in a 3-Carbon Radioactivity was found in a 3-Carbon acid (3-phosphoglycerate); 3-PGAacid (3-phosphoglycerate); 3-PGA

3-PGA 3-PGA ⇨⇨ first stable product of first stable product of photosynthesisphotosynthesis

PCR cycle referred as C3 cyclePCR cycle referred as C3 cycle

Acceptor molecule Acceptor molecule ⇨⇨ ribulose-1,5- ribulose-1,5-biphosphate (RuBP)biphosphate (RuBP)

Carboxylation reaction; which COCarboxylation reaction; which CO22 added to RuBP forming six-added to RuBP forming six-carbon intermediate which is carbon intermediate which is transient & unstabletransient & unstable

six-carbon intermediate which is six-carbon intermediate which is transient & unstable remain transient & unstable remain bound to enzyme and hydrolized bound to enzyme and hydrolized to 2-molecules 3-PGAto 2-molecules 3-PGA

Carboxylation reaction Carboxylation reaction ⇨⇨ Catalyzed by enzyme ribulose-Catalyzed by enzyme ribulose-1,5-biphosphate carboxylase-1,5-biphosphate carboxylase-oxygenase (Rubisco)oxygenase (Rubisco)

CH20-P

C=O

HCOH

HCOH

CH2OH-P

CH20-P

HCOH

CO2-

O CH20-P

C-COH

HO C=O

HCOH

CH2OH-P

+ *CO2

CO2-

HCOH

CH20-P

3-phosphoglycerate 3-phosphoglycerate (3-PGA)(3-PGA)

Carboxylation Reaction of the PCR cycle

Ribulose-Ribulose-1,5-1,5-biphosphate biphosphate (RuBP)(RuBP)

Six-carbon intermediate

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

Energy from light reaction of Energy from light reaction of photosynthesis is required at 2 photosynthesis is required at 2 points. i.e.:points. i.e.:

i. Reduction of 3-PGAi. Reduction of 3-PGA

ii. Regeneration of RuBP ii. Regeneration of RuBP acceptor moleculeacceptor molecule

(ii) Reduction of 3-PGA(ii) Reduction of 3-PGA

For chloroplast continue take up COFor chloroplast continue take up CO22

Involved 2 conditions :Involved 2 conditions :

i. Production of 3-PGA continually i. Production of 3-PGA continually removedremoved

ii. Maintain an adequate supply of ii. Maintain an adequate supply of RuBPRuBP

Both require energy Both require energy ⇨⇨ ATP and NADPH ATP and NADPH 3-PGA is removed by reduction to 3-PGA is removed by reduction to glyceraldehyde-3-phosphateglyceraldehyde-3-phosphate

CH20-P

HCOH

CO2-

CH20-P

HCOH

CH

O

CH20-P

HCOH

C

O O

P

ATP ADP NADP+NADPH

+ Pi

3-PGA Glycelaraldehyde-3-phosphate(G3P)1,3-biphosphoglycerate

Reduction 3-PGA to G3P

2-step reaction: 2-step reaction:

⇨⇨ 3-PGA phosphorylated to 3-PGA phosphorylated to 1,3bisphosphoglycerate 1,3bisphosphoglycerate

⇨⇨ 1,3-bisphosphoglycerate reduced 1,3-bisphosphoglycerate reduced to to glyceraldehyde -3-phosphateglyceraldehyde -3-phosphate

Both ATP and NADPH required in 2 Both ATP and NADPH required in 2 step reactionstep reaction

Triose sugar-phosphate; G3P Triose sugar-phosphate; G3P available for export to cytoplasmavailable for export to cytoplasm

(iii) Regeneration of RuBP(iii) Regeneration of RuBP

Continuing supply of acceptor molecule; Continuing supply of acceptor molecule; RUBP RUBP

Accomplished by series of reaction Accomplished by series of reaction involving 4-, 5-, 6-, and 7- carbon involving 4-, 5-, 6-, and 7- carbon sugarssugars

Reactions include the condensation of 6-Reactions include the condensation of 6-C fructose phosphate with triose-C fructose phosphate with triose-phosphate to forms 5-C and 4-C sugarphosphate to forms 5-C and 4-C sugar

Another triose join with 4-C sugar to Another triose join with 4-C sugar to produce produce 7- C sugar7- C sugar

7-C sugar combined with 3 rd triose-7-C sugar combined with 3 rd triose-phosphate phosphate ⇨ 2 molecules 5-C sugar ⇨ 2 molecules 5-C sugar formedformed

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

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

Net effect of reaction Net effect of reaction ⇨⇨ recycle recycle carbon from 5 out of 6 G3P molecules carbon from 5 out of 6 G3P molecules ⇨ to r⇨ to regenerate egenerate 3 RuBP molecules3 RuBP molecules

Sum = 3 RuBP + 3 COSum = 3 RuBP + 3 CO22 3 RuBP 3 RuBP + G3P+ G3P

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

6.1.2 Energy input in PCR 6.1.2 Energy input in PCR cyclecycle 3 turns of cycle 3 turns of cycle ⇨⇨ uptake of 3 uptake of 3

molecules COmolecules CO2 2 ⇨ total 6 molecules NADPH ⇨ total 6 molecules NADPH and 9 molecules ATP requiredand 9 molecules ATP required

Reduction of each molecule of COReduction of each molecule of CO22 requires 2 molecules NADPH and 3 requires 2 molecules NADPH and 3 molecules ATP molecules ATP ⇨ ⇨ ratio of ATP/NADPH of ratio of ATP/NADPH of 3/2 @ 1.53/2 @ 1.5

Each NADPH stores 2 electrons Each NADPH stores 2 electrons ⇨ ⇨ total total of 4 electrons required to fix each of 4 electrons required to fix each molecule COmolecule CO22

Photosynthetic reduction process Photosynthetic reduction process represents an energy storage represents an energy storage efficiencyefficiency

6.2 Activity and regulation of 6.2 Activity and regulation of PCR CyclePCR Cycle

Rate of carbon reduction is partly dependent Rate of carbon reduction is partly dependent on the availability of an adequate pool of on the availability of an adequate pool of acceptor molecules; COacceptor molecules; CO22 and RuBP and RuBP

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

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

However, PCR cycle has potential to supplies However, PCR cycle has potential to supplies of acceptor by retaining that extra carbon of acceptor by retaining that extra carbon and diverting it toward generating and diverting it toward generating increasing amounts of RuBP increasing amounts of RuBP

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

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

6.2.2 Regulation of Rubisco 6.2.2 Regulation of Rubisco activityactivity Rapidly to zero when light is turn Rapidly to zero when light is turn offoff

Slowly when light is once again Slowly when light is once again turned onturned on

Rubisco activity is regulated Rubisco activity is regulated indirectly by lightindirectly by light

Involves complex interaction between Involves complex interaction between Mg Mg 2+2+ fluxes across the thylakoid, fluxes across the thylakoid, COCO22 activation, chloroplast pH activation, chloroplast pH changes and activating proteinchanges and activating protein

Light –driven electron transport Light –driven electron transport leads to proton net movement into leads to proton net movement into lumen of thylakoidlumen of thylakoid

Proton across thylakoid membrane Proton across thylakoid membrane generates a proton gradient = pH 3.0 generates a proton gradient = pH 3.0 and increase pH of the stroma (pH and increase pH of the stroma (pH 5.0 in the dark to pH 8.0 in the 5.0 in the dark to pH 8.0 in the light)light)

Light bring increase in the free Mg Light bring increase in the free Mg 2+2+ of the stroma of the stroma

Mg Mg 2+ 2+ moves out of the lumen to moves out of the lumen to compensate for the proton flux in compensate for the proton flux in opposite directionopposite direction

COCO22 reacts with amino group reacts with amino group forming carbamateforming carbamate

Carbamate requires the release of Carbamate requires the release of 2 proton 2 proton ⇨ consequently, ⇨ consequently, increasing pHincreasing pH

Mg Mg 2+ 2+ then coordinated to carbamate then coordinated to carbamate to form carbamate- Mg to form carbamate- Mg 2+2+ complex complex ⇨ ⇨ active form of the enzymeactive form of the enzyme

6.2.3 Regulation of other PCR 6.2.3 Regulation of other PCR enzymesenzymes

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

PSI drives the PSI drives the ferredoxin ferredoxin reduction reduction which in turn reduces which in turn reduces thioredoxinthioredoxin

Thioredoxin reduces the appropriate Thioredoxin reduces the appropriate disulphide bond (-S-S-) state to disulphide bond (-S-S-) state to sulfhydryl (-SH-SH-) statesulfhydryl (-SH-SH-) state

Reduction mediated by the enzyme; Reduction mediated by the enzyme; ferredoxin-thioredoxin reductaseferredoxin-thioredoxin reductase

6.3 Photorespiration6.3 Photorespiration

Photorespiration Photorespiration ⇨ process involves ⇨ process involves reoxidation of products assimilated in reoxidation of products assimilated in photosynthesisphotosynthesis

Involves activities of 3 different Involves activities of 3 different organelles: organelles:

i. chloroplast i. chloroplast

ii. peroxisome ii. peroxisome

iii. Mitochondriaiii. Mitochondria Photorespiration consume oxygen, Photorespiration consume oxygen, release carbon dioxiderelease carbon dioxide

Because COBecause CO22 is evolved, results in net is evolved, results in net loss of carbon from cellloss of carbon from cell

Plants that incorporate carbon through Plants that incorporate carbon through PCR @ Calvin cycle known as PCR @ Calvin cycle known as C3 plantsC3 plants; ; cause the 1cause the 1stst product to incorporate CO product to incorporate CO22 in 3C acid PGA in 3C acid PGA (3-PGA)(3-PGA)

C3 plants have built-in metabolic C3 plants have built-in metabolic inefficiency in photosynthetic processinefficiency in photosynthetic process

Acquire COAcquire CO22 from atmosphere through from atmosphere through stomatastomata

when hot dry weather stomata close to when hot dry weather stomata close to reduce water lossreduce water loss

COCO22 concentration in interior leaves concentration in interior leaves dropdrop

Rubisco combinesRubisco combines OO22 rather than CO rather than CO22

3 CO2

6 PGA

glucose

6 1,3-biphosphoglycerate

1 G3P

5 G3P

6 ATP

6 G3P

3 RuBP

6 ADP

6 NADP+ 6Pi

6 NADPH

3 ADP

3 ATPCalvin Cycle

Rubisco

ADP

ATP

NADP+

NADPH

H2O

O2

CO2

CH2O (Glucose)

Light

Calvin CyclePhotosynthetic electron transport

Chloroplast

6.3.1 Rubisco catalyzes 6.3.1 Rubisco catalyzes fixation of both fixation of both

COCO22 and O and O22 Glycolate (2-C) metabolism related to Glycolate (2-C) metabolism related to photorespirationphotorespiration

Enzyme involved in process located in 3 Enzyme involved in process located in 3 organellesorganelles

Bifunctional nature of RubiscoBifunctional nature of Rubisco ⇨⇨Rubisco catalyzes an in carboxylation Rubisco catalyzes an in carboxylation reaction oxygenase reaction reaction oxygenase reaction ⇨ ⇨ Ribulose Ribulose 1-5-biphosphate carboxylase-oxygenase1-5-biphosphate carboxylase-oxygenase

RuBP + ORuBP + O22 3 PGA + 3 PGA + phosphoglycolate (2C)phosphoglycolate (2C)

P-glycolate COP-glycolate CO22 + recovery + recovery of remaining of remaining carbon of PCRcarbon of PCR

Known as C2 glycolate cycle @ Known as C2 glycolate cycle @ PCO(Photosyntheitic carbon PCO(Photosyntheitic carbon oxidation) cycleoxidation) cycle

Begins with RuBP oxidation to 3-Begins with RuBP oxidation to 3-PGA and P-GlycolatePGA and P-Glycolate

Glycolate exported from chloroplast and Glycolate exported from chloroplast and diffuses to peroxisomediffuses to peroxisome

Glycolate oxidized to glyoxylate and Glycolate oxidized to glyoxylate and hidrogen peroxidehidrogen peroxide

Peroxide broken down by catalase Peroxide broken down by catalase Glyoxylate undergoes transamination reaction Glyoxylate undergoes transamination reaction form amino acid glycine form amino acid glycine

Glycine transferred to mitochondria Glycine transferred to mitochondria ⇨ 2 mol. ⇨ 2 mol. glycine (4-C) converted to 1 mol. serine (3-glycine (4-C) converted to 1 mol. serine (3-C) + 1 mol. COC) + 1 mol. CO2 2 (released)(released)

Serine leaves mitocondria Serine leaves mitocondria ⇨returning to ⇨returning to peroxisome; amino (-NHperoxisome; amino (-NH22) group given up in ) group given up in transmination transmination

The product (Hydroxy pyruvate)reduced to The product (Hydroxy pyruvate)reduced to glycerateglycerate

Glycerate returned to chloroplast Glycerate returned to chloroplast ⇨ ⇨ phosphorylated to 3-PGAphosphorylated to 3-PGA

6.4 C4 Syndrome6.4 C4 Syndrome

C-4 plants:C-4 plants:

⇨⇨ First product is 4-carbon acid oxaloacetate(OAA)First product is 4-carbon acid oxaloacetate(OAA)

⇨⇨ exhibit specific anatomical, physiological and exhibit specific anatomical, physiological and

biochemical characteristic constitute C4 biochemical characteristic constitute C4 syndromesyndrome

C-4 Leaves anatomy :C-4 Leaves anatomy :

⇨⇨ presence of 2 distinct photosynthesis tissues; presence of 2 distinct photosynthesis tissues;

between vascular bundles mesophyll cells calledbetween vascular bundles mesophyll cells called

Kranz anatomyKranz anatomy

⇨⇨ Vascular bundles are close togetherVascular bundles are close together

⇨⇨ Each bundle is surrounded by fitted layer of Each bundle is surrounded by fitted layer of cells; called bundle sheathcells; called bundle sheath

Under condition of high fluence rates and Under condition of high fluence rates and high temperature (30-40ºC), high temperature (30-40ºC), photosynthesis rates of C4 species may 2 photosynthesis rates of C4 species may 2 -3 times greater than C3 species-3 times greater than C3 species

Maintain active photosynthesis under of Maintain active photosynthesis under of water stress water stress ⇨ ⇨ lead stomatal closurelead stomatal closure

COCO22 concentrated in bundle sheath cellsconcentrated in bundle sheath cells Hatch & Slack proposed a cyclic mechanism Hatch & Slack proposed a cyclic mechanism ⇨⇨ carbon incorporated into C4 acid carbon incorporated into C4 acid ⇨ ⇨ ββ ––carboxyl carbon transferred as COcarboxyl carbon transferred as CO22 to PCR to PCR cyclecycle

C4 photosynthetic carbon assimilation cycle

6.4.1 C4 photosynthetic carbon assimilation cycle

Carboxylation phosphoenol pyruvate (PEP) Carboxylation phosphoenol pyruvate (PEP) using HCOusing HCO33

-- by PEPcase by PEPcase Oxaloacetate (OAA) formed is unstableOxaloacetate (OAA) formed is unstable OAA reduced to malate or transaminated to OAA reduced to malate or transaminated to aspartateaspartate

Transported from mesophyll cell into Transported from mesophyll cell into bundle sheath cellbundle sheath cell

C4-Acid undergoes decarboxylation, COC4-Acid undergoes decarboxylation, CO22 formed reduce to triose sugars via PCR formed reduce to triose sugars via PCR cyclecycle

C3-acid (pyruvate or alanine) transported C3-acid (pyruvate or alanine) transported back into mesophyll cell back into mesophyll cell ⇨ ⇨ tthen converted hen converted to pyruvateto pyruvate

Pyruvate phosphorylated to regenerate PEP Pyruvate phosphorylated to regenerate PEP

C4 Syndrome Ecological C4 Syndrome Ecological SignificanceSignificance

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

Photosynthesis of C4 is not inhibited by Photosynthesis of C4 is not inhibited by oxygenoxygen

Photorespiraton is absent of C4 plants or Photorespiraton is absent of C4 plants or the process suppressed the process suppressed

C4 plants have a low COC4 plants have a low CO22 compensation compensation point(range of 0 to 5 ul/l COpoint(range of 0 to 5 ul/l CO22 ) ; While ) ; While for C3 plants, range of 20 to 100 ul/l COfor C3 plants, range of 20 to 100 ul/l CO22

COCO22 compensation point : compensation point :⇨ ⇨ ambient COambient CO22 concentration at which the concentration at which the rate of COrate of CO22 uptake (photosynthesis) is uptake (photosynthesis) is balanced by the rate of CO2 evolution balanced by the rate of CO2 evolution (respiration)(respiration)

High level of COHigh level of CO22 in bundle sheath cell in bundle sheath cell outcompeting Ooutcompeting O22 for binding with Rubisco for binding with Rubisco

Adaptations of C4 leaves ensure COAdaptations of C4 leaves ensure CO22 that that might escape bundle-sheath cell is trapped might escape bundle-sheath cell is trapped and reassimilated by PEPcase in mesophyll and reassimilated by PEPcase in mesophyll cells before escape from leaf cells before escape from leaf ⇨ ⇨ thus,thus, C4 C4 leaves efficient in COleaves efficient in CO22 absorbers, trap and absorbers, trap and recirculate COrecirculate CO2 2 produced in leafproduced in leaf

C4 plants have higher temperature optimum C4 plants have higher temperature optimum (30-45(30-45ooC) than C3 plants (20-25C) than C3 plants (20-25ooC)C)

High temperature (between 40 High temperature (between 40 ooC and 50 C and 50 ooC) C) ⇨⇨ rate of photosynthesis decrease to greater rate of photosynthesis decrease to greater extent than rate of photorepirationextent than rate of photorepiration

As a results, quantum yield of As a results, quantum yield of photosynthesis in C4 plants remains photosynthesis in C4 plants remains constant; while C3 plants decline with constant; while C3 plants decline with temperature increasetemperature increase

C4 plants can maintain photosynthesis C4 plants can maintain photosynthesis when stomata partially closed to when stomata partially closed to conserve water; while C3 plants , conserve water; while C3 plants , moderate water stress will close stomatamoderate water stress will close stomata

Transpiration ratio of C4 plants is low Transpiration ratio of C4 plants is low (range of 200 to 350), while C3 plants (range of 200 to 350), while C3 plants higher ( range of 500 to 1000)higher ( range of 500 to 1000)

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

CO2 supply limit photosynthesis in C3CO2 supply limit photosynthesis in C3

Crassulacean Acid Crassulacean Acid Metabolism Metabolism

(CAM)(CAM)

Studied most extensively in Family Studied most extensively in Family CrassulaceaeCrassulaceae

Now found in 23 different flowering plants Now found in 23 different flowering plants (Cactaceae, Euphobiaceae) fern (Cactaceae, Euphobiaceae) fern (Polypodiaceae) primitive plant (Polypodiaceae) primitive plant (Welwitschia)(Welwitschia)

Survive in extremely dry or xerophytic Survive in extremely dry or xerophytic habitatshabitats

Succulent plants Succulent plants ⇨⇨ characteristics by characteristics by thick, fleshy leaves, cells contain large thick, fleshy leaves, cells contain large water-filled vacuoles)water-filled vacuoles)

Inverted stomatal cycle Inverted stomatal cycle ⇨⇨ stomata open stomata open during nighttime hours and closed during the during nighttime hours and closed during the day; COday; CO22 uptake at night uptake at night

Involve Carboxylation phosphoenol Involve Carboxylation phosphoenol pyruvate (PEP) @ enzyme PEP pyruvate (PEP) @ enzyme PEP carboxylasecarboxylase

Immediate product OAA reduce to Immediate product OAA reduce to malate which is store in vacuolemalate which is store in vacuole

During daylight hours During daylight hours

⇨⇨ malate retrieved from vacuole malate retrieved from vacuole

⇨⇨ decarboxylated decarboxylated

⇨ ⇨ COCO22 diffuses into chloroplast diffuses into chloroplast and converted to triose phosphate and converted to triose phosphate by C3 PCR cycleby C3 PCR cycle

Comparison between CAM and C4 Comparison between CAM and C4 syndromesyndrome CAM similar to C4CAM similar to C4

⇨⇨ use PEPcase to form C4 acids from PEP and use PEPcase to form C4 acids from PEP and bicarbonate bicarbonate ⇨⇨ Acids decarboxylated to provide COAcids decarboxylated to provide CO22 for for PCR cyclePCR cycle

Significant differencesSignificant differences ⇨⇨ C4 requires a special anatomy by which C4 C4 requires a special anatomy by which C4 carboxylation spatially separated from C3 PCR carboxylation spatially separated from C3 PCR cycle. cycle.

In CAM both occur in same cell but separate In CAM both occur in same cell but separate timetime ⇨⇨ CAM no close cycle of carbon intermediate CAM no close cycle of carbon intermediate as in C4 plants. PEP required as substrate as in C4 plants. PEP required as substrate for carboxylation reaction derived for carboxylation reaction derived from from stored carbohydratestored carbohydrate

Significant of CAMSignificant of CAM Represents a adaptation to dry habitat Represents a adaptation to dry habitat Growing in shallow & sandy soil with little Growing in shallow & sandy soil with little available water available water

COCO22 uptake at night (during minimum uptake at night (during minimum evaporative water loss)evaporative water loss)

Daylight hours Daylight hours ⇨ ⇨ stomata closed to reduce water loss stomata closed to reduce water loss ⇨⇨ photosynthesis can proceed using the photosynthesis can proceed using the reservoir of stored COreservoir of stored CO22

Daily carbon assimilation by CAM plants Daily carbon assimilation by CAM plants about ½ of C3 plants, 1/3 of C4 plantsabout ½ of C3 plants, 1/3 of C4 plants

CO2 uptake by CAM plants continue under CO2 uptake by CAM plants continue under water stress conditions which in C3 plants water stress conditions which in C3 plants photosynthesis will stop and severely photosynthesis will stop and severely restrict carbon uptake by C4 plants restrict carbon uptake by C4 plants