8/8/2019 The Difference Occurs in the Second Part of Photosynthesis http://slidepdf.com/reader/full/the-difference-occurs-in-the-second-part-of-photosynthesis 1/27 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 can either grab onto CO2..._or_ O2. If it latches onto CO2 as i t should, then the first part of the cycle produces 2x 3PG, as it should. If it latches onto O2 instead, then the first part of the cycle produces one 3PG, and one glycolate. Now, C3 plants have evolved ways to reclaim at least some of the carbons channele d away as glycolate, by feeding glycolate through a peroxisome and a mitochondrion, where it undergoes several transformations and some of it is released back out as CO2 (this is the pathway called photorespiration). However, it reduces the net carbon fixa tion by about 25%. Rubisco has about 10x more affinity for CO2 than it does for O2, so under normal circumstances this is not a problem. However, on very hot, dry days plants close the stomata in their leaves in order to minimize the loss of water -- and this interferes with 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 grab it instead. This both slows down photosynthesis and reduces its carbon f ixation overall. The C4 plants have introduced an extra bit into the Calvin -Benson cycle, an extra early reaction that fixes CO2 into not *3* -carbon sugars, but *4*-carbon sugars called oxaloacetate (hence the names, by the way, C3 for 3-carbon and C4 for 4-carbon sugars) -- 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 an advantage over 3PG, in low-CO2 circumstances -- some of it degrades to form CO2 again 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, dry conditions, but still keep photosynthesis ticking over at good efficiency. CAM plants (from "crassulacean acid metabolism", because this mechanism was first described in members of plant family Crassulaceae) are a different kind of C4 plant. In the C4 plants described above, the fixation of CO2 into 4 -carbon sugars and the further fixation of CO2 into 3 -carbon sugars happens in different cells, separated in space but at the same time. In CAM plants, the two different kinds of CO2 -fixation happen in the same cells, but separated in time. In CAM plants the fixation of CO2
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8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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.
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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.
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis
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
8/8/2019 The Difference Occurs in the Second Part of Photosynthesis