Essay 1: Answer following: Chemical form and Essay 1: 1) Compare and Contrast the C3, C4 and CAM pathway. Include the following: function, eolutionary form and function, and ecological form and function. !) In addition, a ddition, thoroughly e"plain each pathway and how the functions of stomatal opening#closure affects 3) plant growth, geographic location and photorespiration. 1) $he C4 and CAM pathways are designed to aoid the pro%lem of photorespiration &when '! out( competes C'! %inding to u*+, which happens when '! leels rise and C'! leels fall, such as when stomata are closed on a hot, arid day). $his results in less glucose %eing formed or no glucose formed at all. ach of the three %iochemical pathways &C3, C4 and CAM) is designed &eoled) to produce the ma"imum amount of glucose &C-1!'-) for / plants ha%itat as efficiently efficiently as possi%le. C3 plants &standard Calin Cycle) hae not particular shortage of C'! and therefore hae not deeloped any specific way to deal with the pro%lem of photorespiration. C4 plants aoid photorespiration %y using a spatial solution &the %iochemical processes occur in two different cells). CAM plants aoid photorespiration %y using a temporal solution &the %iochemical processes occur at different times of the day). *oth processes store store C'! as an organic acid and release the C'! directly to the C3 pathway, aoiding photorespiration. photorespiration. $he %ig difference difference is that the C4 plants can open their stomata during the day until it gets too dry and hot, while the CAM plants lie in regions &desert) that do not allow the stoma to %e open during the day at all &too hot and arid, too much !' loss) and therefore can only open their stomata at night when the conditions are not as harsh. $his all falls into the %asic form and function concept: if the enironment is not too arid then the plant can open it stomata most of the time, if it is too arid part of the time C 4 deeloped %ecause the stomata can open during the day part of the time, if it is always to arid during the da y, then CAM deeloped where the stoma only open at night and store the C'! as an organic acid. !) $he C'! acceptor for the C3 pathway &and indirectly the other pathways as %oth the C4 and CAM feed C'! to the C3) is u*+ 0a car%on compound2 which forms u%isco 0a - car%on compound2. ice, wheat and soy%eans are e"amples of C3 plants. $he C'! acceptor for the C4 pathway is phosphoenolpyruate &++) with the help of ++ car%o"ylase. ++ car%o"ylase has a much greater affinity affinity for C'! than u*+ and ' affinity for '!, thus %ypassing photorespiration. &in addition this highlights once more the importance of enymes5) $his occurs in the mesophyll cells and then the product &malate) is sent to the %undle(sheath cells thus the C4 pathway is separated spatially. $he organic molecule that forms from the ++ and C'! is o"aloacetate a 4(car%on compound and thus the name C4 pathway. Another 4(car%on compound called malate is conerted from the o"aloacetate and releases their %ound C'! to the C3 cycle in the %undle( sheath cells. Corn and sugarcane are e"amples of C4 plants. CAM &crassulacean acid meta%olism) plants which hae temporally separated %iochemical pathways. pathways. $he acid is made at night with the C'! %eing %rought in through the open stomata when the enironment is not as hot or arid as in the day. $he organic acid is stored in acuoles in the mesophyll cells until the stomta close in the daytime and the light reaction starts. 6hen the stomata close during the day, the acid releases its C'! to the C3 pathway as the light reaction is occurring. Catcti, and succulents &pineapple, aloe) a re good e"amples of CAM plants. C3 plants lie in temperate or hot and humid climates where photorespiration is not a significant significant factor. C4 plants lie in enironments that are often hot and arid, %ut not always hot and arid. CAM plants &mostly succulents) lie in regions that are so hot and arid that the stomata are not eer opened o pened during the day, only o nly at night. 3) +lants growth is the %est in regions where the stomata can %e open at any time and do not hae to incorporate e"tra %iochemical pathways &C4 and CAM) to aoid photorespiration#wilting, such as in a tropical rainforest. When going over a new concept, such as the C4 and CAM pathways, always think about for and function, how the concept evolved, how the concept affects hoeostasis in the organis in !uestion, and how this applies to the real world the organis lives in"
C3, C4, and CAM plants all carry out the same photosynthetic functions. $hey all hae light(dependent reactions and the Calin(*enson cycle. $he ma7or difference in C4 and CAM plants is when and where the car%on fi"ation initially occurs. In C3 plants, the light reactions occur in the palisade mesophyll cells. Car%on fi"ation also occ urs in these cells during the Calin(*enson cycle. An enyme called ri%ulose %isphosphate car%o"ylase o"ygenase &ru%isco for short) fi"es incoming car%on dio"ide molecules into three car%on molecules called glyceraldehyde(3(phosphate &83+). $he 83+ goes thorough a series of reductions to form car%ohydrate molecules. *ecause these %iochemical pathways are irtually identical in all three types of plants, I won9t ela%orate on them. C4 plants typically lie in warmer, drier climates than normal C3 plants can withstand. 6hen the outside air is hot and dry, C3 plants must close their stomata or they ris losing too much water ia transpiration. *ut closing the stomata also cuts off the supply o f C'!. As the influ" of sunlight dries photosynthesis, C'! leels fall and '! leels rise. *ecause ru%isco can fi" o"ygen as well as car%on dio"ide, some of the molecules needed for regular photosynthesis %ecome o"idied and useless if '! leels get too high. C4 plants sole this pro%lem %y not haing car%on(fi"ation occur in the palisade mesophyll cells, where o"ygen concentrations are high due to the splitting of water molecules &photolysis). +hotolysis is necessary to replenish the electrons ;lost; in reducing car%on compounds to car%ohydrates. Instead, when car%on dio"ide enters the leaf of a C4 plant, it is %onded to a three(car%on compound called phosphoenolpyruate &++) %y an enyme called ++ car%o"ylase. ++ is much more specific for car%on dio"ide than ru%isco is, so there is less ris of photorespiration. $he resulting four(car%on molecule is called o"aloacetic acid &'AA). It is conerted into another four(car%on molecule &malic acid), and actiely transported to the %undle sheath cells. It9s from this four(car%on ;ta"i; molecule that C4 photosynthesis gets its name. *ecause light reactions are not occurring in the %undle sheath cells, the '! leels aren9t high there. $herefore, car%on can %e safely fi"ed %y ru%isco in the o"ygen(poor %undle sheath cells. 'nce malic acid reaches the %undle sheath cells, it is relieed of its fourth car%on, which conerts it %ac into a three(car%on molecule. It returns to the mesophyll cells to gather more car%on dio"ide. Meanwhile, the left( %ehind car%on molecule is fi"ed into 83+ %y ru%isco and the rest is clocwor.
C4 plants carry out their light(dependent and Calin(*enson reactions at the same time, %ut in different places: the light reactions tae place in the mesophyll cells while car%on fi"ation occurs in the %undle sheath cells. CAM, lie C3 plants, perform their light(dependent and Calin(*enson reactions in the same place and at the same time, %ut they get their car%on from a store they %uild up oernight.
