the difference occurs in the second part of photosynthesis

8/8/2019 The Difference Occurs in the Second Part of Photosynthesis

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The difference occurs in the second part of photosynthesis, the Calvin -Benson cycle,which "fixes" CO2 into carbohydrates.

The Calvin-Benson cycle (in "normal", C3 plants) consists of three processes:1. The fixation of CO2 onto a 5-carbon "receptor" (ribulose 1,5-bisphosphate, better known as RuBP), which results in two 3-carbon molecules ( a sugar-phospate called

3-phosphoglycerate, or 3PG), a reaction catalyzed by the protein rubisco.2. The reduction of 3PG to form a carbohydrate, glyceraldehyde 3 -phosphate (G3P).3. Regeneration of the original receptor, RuBP.

Every "turn" of this cycle, one CO2 is fixed.

The problem comes in the first part of the cycle, where rubisco is used. Rubisco caneither grab onto CO2..._or_ O2. If it latches onto CO2 as i t should, then the first partof the cycle produces 2x 3PG, as it should. If it latches onto O2 instead, then the firstpart of the cycle produces one 3PG, and one glycolate. Now, C3 plants have evolvedways to reclaim at least some of the carbons channeled away as glycolate, byfeeding glycolate through a peroxisome and a mitochondrion, where it undergoes

several transformations and some of it is released back out as CO2 (this is thepathway called photorespiration). However, it reduces the net carbon fixa tion byabout 25%.

Rubisco has about 10x more affinity for CO2 than it does for O2, so under normalcircumstances this is not a problem. However, on very hot, dry days plants close thestomata in their leaves in order to minimize the loss of water -- and this interfereswith gas exchange as well. As CO2 is used up by the normal Calvin -Benson cycle,the balance of CO2:O2 inside the leaf alters in favor of O2, and rubsico starts to grabit instead. This both slows down photosynthesis and reduces its carbon f ixationoverall.

The C4 plants have introduced an extra bit into the Calvin -Benson cycle, an extraearly reaction that fixes CO2 into not *3*-carbon sugars, but *4*-carbon sugars calledoxaloacetate (hence the names, by the way, C3 for 3-carbon and C4 for 4-carbonsugars) -- by plunking CO2 onto a different receptor molecule (phosphoenolpyruvate,or PEP) by way of the enzyme PEP carboxylase.

PEP carboxylase has two advantages over rubisco: it has no affinity for O2 at all,and it finds and fixes CO2 even at very low CO2 levels. And oxaloacetate has anadvantage over 3PG, in low-CO2 circumstances -- some of it degrades to form CO2again in the mesophyll, the cells which carry CO2 to rubisco.

As a result, the C4 plants can close their stomata to retain moisture under hot, dryconditions, but still keep photosynthesis ticking over at good efficiency.

CAM plants (from "crassulacean acid metabolism", because this mechanism wasfirst described in members of plant family Crassulaceae) are a different kind of C4plant. In the C4 plants described above, the fixation of CO2 into 4 -carbon sugars andthe further fixation of CO2 into 3 -carbon sugars happens in different cells, separatedin space but at the same time. In CAM plants, the two different kinds of CO2 -fixationhappen in the same cells, but separated in time. In CAM plants the fixation of CO2



into oxaloacetate happens at night, when it is cooler and the stomata can open toensure a plentiful supply of CO2, and then the oxaloacetate is stored as malic acid.Then, during the day, the stomata close to minimize moisture loss, and the storedmalic acid is reclaimed and turned back into CO2 to power the normal Calvin -Benson cycle

Read more: What is the difference between C3, C4, and CAM photosynthesis? | Answerbag http://www.answerbag.com/q_view/32799#ixzz1Ae3MKeL3



C3 Photosynthesis : C3 plants.

y Called C3 because the CO2 is first incorporated into a 3-carbon compound.

y Stomata are open during the day.

y RUBISCO, the enzyme involved in photosynthesis, is also the enzyme involved in the

uptake of CO2.

y Photosynthesis takes place throughout the leaf.

y Adaptive Value: more efficient than C4 and CAM plants under cool and moist

conditions and under normal light because requires less machinery (fewer enzymes and

no specialized anatomy)..

y Most plants are C3.

C4 Photosynthesis : C4 plants.

y Called C4 because the CO2 is first incorporated into a 4-carboncompound.

y Stomata are open during the day.

y Uses PEPCarboxylase for the enzyme involved in the uptake of CO2.

This enzyme allows CO2 to be taken into the plant very quickly, and then

it "delivers" the CO2 directly to RUBISCO for photsynthesis.

y Photosynthesis takes place in inner cells (requires special anatomy called Kranz

Anatomy)

y Adaptive Value:

o Photosynthesizes faster than C3 plants under high light intensity and hightemperatures because the CO2 is delivered directly to RUBISCO, not allowing it

to grab oxygen and undergo photorespiration.

o Has better Water Use Efficiency because PEPCarboxylase brings in CO2 faster

and so does not need to keep stomata open as much (less water lost by

transpiration) for the same amount of CO2 gain for photosynthesis.

y C4 plants include several thousand species in at least 19 plant families. Example:

fourwing saltbush pictured here, corn, and many of our summer annual plants.

CAM Photosynthesis : CAM plants. CAM stands for Crassulacean Acid Metabolism

y Called CAM after the plant family in which it was first found

(Crassulaceae) and because the CO2 is stored in the form of an

acid before use in photosynthesis.

y Stomata open at night (when evaporation rates are usually lower)



and are usually closed during the day. The CO2 is converted to an acid and

stored during the night. During the day, the acid is broken down and the CO2 is

released to RUBISCO for photosynthesis

y Adaptive Value:

o B

etter Water U

seE

fficiency than C3 plants under arid conditions due to openingstomata at night when transpiration rates are lower (no sunlight, lower

temperatures, lower wind speeds, etc.).

o May CAM-idle. When conditions are extremely arid, CAM plants can just leave

their stomata closed night and day. Oxygen given off in photosynthesis is used for

respiration and CO2 given off in respiration is used for photosynthesis. This is a

little like a perpetual energy machine, but there are costs associated with running

the machinery for respiration and photosynthesis so the plant cannot CAM-idle

forever. But CAM-idling does allow the plant to survive dry spells, and it allows

the plant to recover very quickly when water is available again (unlike plants that

drop their leaves and twigs and go dormant during dry spells).

y CAM plants include many succulents such as cactus and agaves and also some

orchids.



ComparingAlternative

Mechanismsof

CarbonFixation to

C3 Fixation



Though there are alternativemechanisms besides C 3 carbon

fixation, which include C 4 and CAM fixation, they all share similaritiesand differences with each other.Carbon dioxide (CO2 ) fixation is a

photosynthetic reaction in whichcarbon dioxide is attached to anorganic compound. Not only are thealternative mechanisms of carbonfixation different, the areas in which

certain plants grow determinewhat kind mechanism of fixationthe plant will undergo. This paper will illustrate the differences and

similarities between carbon fixationin C 4 and C 3 plants. It will alsoillustrate the differences and similarities between CAM and C 3



plants. Before outlining each, it should be noted that there are

considerably moredifferences than similaritiesbetween each.

C 3 carbon fixation is a

metabolic pathway for carbon fixation in photosynthesis. This process

converts carbon dioxide and ribulose bisphosphate (RuBP,

a 5-carbon sugar) into 3- phosphoglycerate (3PG)through the followingreaction:

6 CO2 + 6R

uBP

12 3-phosphoglycerateIn C 3 plants, carbon fixation isalwaysthe first step in the Calvin Cycle.



CO2 adds to an already existing 5-

carbonmolecule, ribulose 1, 5-biphosphate, RuBP,to form an unstable 6-carbon

intermediate. Theintermediate instantly splits intotwo 3-carbonmolecules called 3- phosphoglycerate (3PG). If this

reaction occurs three times, threemolecules of CO2 (three carbons)react with three molecules of RuBP (3 x

15 carbons = 15 carbons) to produce six molecules of 3PG (6 x 13 carbons =18 carbons). Since the first



compound to be produced contains three

carbon atoms, the Calvin cycle isalso knownas C 3 photosynthesis. Most plantsare called C 3 plants because they

undergo this form of carbonfixation. The reactions are catalyzed by the enzyme ribulosebisphosphatecarboxylase/oxygenase

(RuBisCO).A process that largely affects the

rate of photosynthesis is called

photorespiration. It is

essentially the oxidation of R uBP by rubisco and oxygen in the light

to form glycolate, which



subsequently releases carbondioxide. Photorespiration

decreases the production of carbohydrates by photosynthesis,since it removes 3PG moleculesfrom the Calvin Cycle. When

photorespiration occurs, one 3PG molecule and one 2-carbonglycolate molecule are formed fromone 5-carbon RuBP, instead of thetwo 3PG molecules that are formed

when carbon dioxide reacts.Glycolate is partially converted toCO2 but three of the four carbons intwo glycolate molecules are

subsequently returned to the Calvincycle as G3P (glyceraldehyde-3- phosphate). C 3 plants are moreefficient in cool, moist



environments with moderate light intensity.

C 4 plants are slightly different when it comes to fixing carbondioxide. In the C 4 pathway, theenzyme phosphenolpyruvate

carboxylase (PEP carboxylase)fixes CO2 by catalyzing theattachment of CO2 to three-carbon PEP, forming afour carbon molecule called

oxaloacetate (OAA). This reactionoccurs in thecytoplasm,notthe

chloroplast.OAA isconverted tomalate.



Malate, afour-carbon

acid, diffusesfrom themesophyll cells into the bundle-sheath cells through cell-cell

connections called plasmodesmata.Here a carbon dioxide portion is removed from the malatemolecule(decarboxylation). This reaction

converts 4-carbon malate intoPEP. This reaction is endergonic and uses one ATP. The Co2 that was removed from malate in

the bundle sheath cellsenters the C 3 Calvin cycle in asecond fixation reaction, this



time catalyzed by rubisco in thebundle sheath cell.

That is the major differencebetween the C 3 carbonfixation and C 4 carbon fixationbecause it required a previous

step before it continues with theCalvin cycle. Also, this method of carbon fixation reduces the amount of photorespiration that takes placeby continually pumping CO2

molecules from the mesophyll cellsto the bundle-sheath cells (viamalate), where rubisco brings theminto the Calvin cycle. The

concentration of CO2 in bundle-sheath cells is kept high so that CO2 outcompetes O2 in binding torubisco. Photorespiration is



minimized and sugar production ismaximized.

However, it costs the plant two ATP molecules to transport a CO2 molecule into a bundle-sheath cell.Since six CO2 molecules are

processed by the Calvin cycle to produce the equivalent of oneglucosemolecule, the cell must expend theenergy of 12 additional ATP

molecules to fix six CO2 molecules.In C 3 photosynthesis, 18 molecules of ATP are used to produce one

glucose molecule, whereas, in C 4 plants,30 ATP are used ± almost twice asmany. Nevertheless, the process is



advantageous in hot, tropical climates where photorespiration

would otherwise convert more thanhalf of the glucose produced back to CO2 .C 4 plants typically live in warmer,

drier climates than normal C 3 plantscan withstand. When theoutside air is hot and dry, C 3 plantsmust close their stomata or they risk losing too much water via

transpiration, but closing thestomata also cuts off the supply of CO2 . As the influx of sunlight drives photosynthesis, CO2 levels fall and

O2 levels rise. Due to the fact that rubisco can fix oxygen as well ascarbon dioxide, some of themolecules needed for regular



photosynthesis become oxidized and useless O2 levels get too high.

C 4 plants solve this problem by not having carbon-fixation occur in the palisademesophyll cells, where oxygen

concentrations are high due to thesplitting of water molecules(photolysis). Photolysis isnecessary to replenish theelectrons lost in reducing carbon

compounds tocarbohydrates. Instead, whencarbon dioxide enters the leaf of aC 4 plant, it is bonded to a three-

carboncompound called phosphenolpyruvate (PEP) by anenzyme called PEP carboxylase.



PEP is much morespecific for carbon dioxide than

rubisco is, so there is less risk of photorespiration.

The most important differencebetween CAM (crassulacean acid

metabolism) plants and C 3 plants is the temporal separation of the fixation of carbon. Water-storing plants (known assucculents),

such as cacti and pineapples, and members of the Crassulacea family (including the jade plants), opentheir stomata at night and close

them during the day ± the reverse of other plants. CAM is a mechanismwhereby CO2 is concentrated around RuBisCO by day, while the



enzyme is operating at peak capacity.

This concentration of CO2 increasesRuBisCO's efficiency, as it is proneto operate in the "reverse" direction via photorespiration -

utilizing oxygen to break down thereaction products the plant would rather it was producing. It differsfrom C 4 metabolism, which spatially concentrates CO2 around

RuBisCO. CAM plants are moreefficient in hot, dry environmentswith high light intensity. They areunlike C 3 plants, which are more

efficient in cool, moist environments with moderate light intensity.



During the night, CAM plantsopen their stomata during the

cooler and more humid night-timehours, permitting the uptake of carbon dioxide with minimum water loss. The carbon dioxide is

converted to soluble molecules,which can be readily stored by the plant at a sensible concentration.The precise chemical pathway involves a three-carbon compound

phosphenolpyruvate (PEP), towhich aCO2 molecule is added - forming anew molecule, oxaloacetate. This

forms malate. Oxaloacetate and malate are built around a skeletonof four carbons - hence the term C 4.Malate can be readily stored by



les within individual cells. The next day,malate can be broken down on

demand,releasing a molecule of CO2 as it isconverted to pyruvate. The pyruvate can be

phosphorylated toregenerate the PEP with which westarted, ready tobe spurred into action the next night, but it is the

release of CO2 that makes the cycleworth the plant'swhile. It is directed to the stroma of chloroplasts: the

sites at which photosynthesis ismost active. There, it is provided to RuBisCO in greater concentration,



increasing the efficiency of themolecule, and

therefore producing more sugars per unit photosynthesis.

The similarities between C 3 and

C 4 / CAM plants are so miniscule, they only constitute one paragraph. Though C 4 plants fix carbon in two

compartments, it occurs at thesame time of the day.C 3 plants fix carbon in onecompartment, but like C 4

plants, the fixation occurs at thesame time of theday. CAM plants don¶t go undergo

carbon fixation at



the same time of the day as C3

plants; they do this

during the night and then releasethe CO2 for

photosynthesisduring the day. However, this all

occurs inone compartment, just like C 3 plants. The prevalent similarity between all the methods of carbonfixation is that they all undergo the

Calvin cycle. Though C 4 and CAM plants have an alternatemechanism before the Calvin cycle,it is nonetheless still the same.

The C 4 and CAM pathwaysrepresent evolutionary solutions tothe problem of maintain



photosynthesis when stomata closeon sunny, hot, dry days. Both

methods initially produce organic acids that eventually transfer to CO2 to the C 3 Calvin cycle. In C 4 plants,these two processes occur in two

different types of cells that areconnected by plasmodesmata(spatial separation). In CAM plants,thetwo processes occur in the same

compartment, but at different times(temporal separation): carbonfixation into organic acids duringthe night and the Calvin cycle

during the day. The C 4 pathway usesalmost twice as much ATP as the C 3 pathway to produce glucose, but



plant. Even through all thealternative mechanisms of carbon

fixation, they all eventually returntheCalvin cycle.Figure 1 - Overview of the Calvin cycle



Figure 2 - Overview of C4 processes.

Figure 3 - Picture outlining some similarities and

differences

the difference occurs in the second part of photosynthesis

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