The key differences between C3, C4, and CAM photosynthesis are seen in the way that carbon dioxide is extracted from sunlight. Plants, algae, and many species of bacteria utilize one of these photosynthetic processes in a chemical reaction that creates energy. Whether an organic compound uses C3, C4, or CAM photosynthesis depends largely on the conditions of the organic compound's habitat.Other People Are Reading
In photosynthesis, plants and other organic compounds use the energy from sunlight to extract nutrients from air and water. Photosynthetic organisms feature a green compound known as chlorophyll that contains the enzymes ATP and NADPH. With the energy absorbed from sunlight, photosynthetic compounds convert these enzymes to ADP and NADP+. The energy from the converted enzymes is used to extract carbon dioxide from air and water, which is then used to produce sugar molecules such as glucose. Through photosynthesis, plants excrete waste molecules including oxygen, which makes the air breathable for animal organisms.C3
Photosynthetic organisms that undergo C3 photosynthesis begin the process of energy conversion, known as the Calvin cycle, by producing a three-carbon compound called 3-phosphoglyceric acid. This is the reason for the title "C3." C3 photosynthesis is a one-stage process that takes place inside of the chloroplast organelles, which act as storage centers for sunlight energy. The energy is then used to combine ATP and NADPH into ordered sugar molecules. Roughly 85 percent of the plants on earth utilize C3 photosynthesis.C4
C4 photosynthesis is a two-stage process in which a four-carbon intermediate compound is produced. The photosynthetic process occurs in the chloroplast of a thin-walled mesophyll cell. Once created, the intermediate compound is pumped into a thick-walled bundle sheath cell, where the compound is split into carbon dioxide and a three-carbon compound. The carbon dioxide then undergoes the Calvin cycle, as in C3 photosynthesis. The benefit of C4 photosynthesis is that it produces a higher concentration of carbon, making C4 organisms more adept at surviving in habitats with low light and water.CAM
CAM is an abbreviation of crassulacean acid metabolism. In this type of photosynthesis, organisms absorb sunlight energy during the day, then use the energy to fix carbon dioxide molecules during the night. During the day, the organism's stomata close up to resist dehydration, while the carbon dioxide from the night prior undergoes the Calvin cycle. CAM photosynthesis allows plants to survive in arid climates, and therefore is the type of photosynthesis used by cacti and other desert plants. However, CAM photosynthesis is also observed in non-desert plants including pineapples and epiphyte plants such as orchids.
"Beauty is in the eye of the beer-holder." - Affluxlove?C4 and Cam Photosynthesis
Date Submitted: 04/26/2011 06:59 PM Flesch-Kincaid Score: 45.9 Words: 692 Essay Grade: no grades Flag
C4 and CAM photosynthesis are considered advantageous to plants that exhibit them because of the special “add-on” features they display. C4 photosynthesis reduces photorespiration and water loss. The reduction of photorespiration is due to the fact that carbon dioxide is moved to a specialized cell known as a bundle sheath cell which surrounds the leaf veins and they themselves are surrounded by mesophyll cells. (Photosynthesizing cells) Bundle sheath cells rarely ever come in contact with an intercellular space thus minimal oxygen reaches them. This lack of oxygen allows rubisco to fix with carbon dioxide without having to compete with oxygen. Therefore, little photorespiration takes place and photosynthesis is more efficient.
The higher rates of photosynthesis in C4 plants allow them to reduce the amount of time their stomata are open which reduces water loss. (Stomata open to allow carbon dioxide to enter) This advantage allows C4 plants to live in dry, arid climates such as deserts. Examples of C4 plants include sugarcane and crab grass.
CAM plants follow a similar pathway to C4 plants with some minor differences. Instead of OOA (oxaloacetate) being converted into malate, it is converted into malic acid. The malic acid is then transported to the vacuole of a cell instead of bundle sheath cells like in a C4 plant. The main advantage of CAM plants is that their stomata are open at night which greatly reduces water loss. During this time, the malic acid is transported out of the vacuole and converted back to OOA, releasing carbon dioxide. The carbon dioxide is fixed with rubisco and photosynthesis can proceed during the day.
Three major differences between C3 photosynthesis and C4 and CAM photosynthesis are the fixing enzymes they use, their production rates, and their ability to adjust to arid climates. C3 photosynthesis uses the fixing enzyme rubisco to fix carbon dioxide into PGA. However, both C4 and CAM.Comments
Express your owns thoughts and ideas on this essay by writing a grade and/or critique.
C3, C4 Photosynthesis, and CAM
Allen, E. B. 1982. Germination and competition of Salsola kali with native C3 and C4 species under three temperature regimes. Bulletin of the Torrey Botanical Club 109:39-46.
Basiouny, F. M. T. K. Van, and R. H. Biggs. 1978. Some morphological and biochemical characteristics of C3 and C4 plants irradiated with UV-B. Physiologia Plantarum 42:29-32.
Bauwe, H. 1984. Photosynthetic enzyme activities in C3 and C3-C4 intermediate species of Moricandia and in Panicum milioides . Photosynthetica 18:201-209.
Bazzaz, F. A. K. Garbutt, E. G. Reekie, and W. E. Williams. 1989. Using growth analysis to interpret competition between a C3 and a C4 annual under ambient and elevated CO2. Oecologia 79:223-235.
Biran, I. B. Bravdo, I. Bushkin-Harav, and E. Rawitz. 1981. Water consumption and growth rate of 11 turfgrasses as affected by mowing height, irrigation frequency, and soil moisture. Agronomy Journal 73:85-90.
Boryslawski, Z. and B. L. Bentley. 1985. The effect of nitrogen and clipping on interference between C3 and C4 grasses. Journal of Ecology 73:113-121.
Bouton, J. H. R. H. Brown, P. T. Evans, and J. A. Jernstedt. 1986. Photosynthesis, leaf anatomy, and morphology of progeny from hybrids between C3 and C3/C4 Panicum species. Plant Physiology 80:487-492.
Boutton, T. W. G. N. Cameron, and B. N. Smith. 1978. Insect herbivory on C3 and C4 grasses. Oecologia 36:21-32.
Boutton, T. W. A. T. Harrison, and B. N. Smith. 1980. Distribution of biomass of species differing in photosynthetic pathway along an altitudinal transect in southwestern Wyoming grassland. Oecologia 45:287-298.
Brown, R. H. 1978. A difference in N efficiency in C3 and C4 plants and its implications in adaptation and evolution. Crop Science 18:93-98.
Brown, R. H. and P. W. Hattersley. 1989. Leaf anatomy of C3-C4 species as related to evolution of C4 photosynthesis. Plant Physiology 91:1543-1550.
Brown, R. H. and R. E. Simmons. 1979. Photosynthesis of grass species differing in CO2 fixation pathways. I. Water-use efficiency. Crop Science 19:375-379.
Brown, R. H. C. L. Bassett, R. G. Cameron, P. T. Evans, Bouton, J.H. C. C. Black, L. O'Reilly Sternberg, and M. J. DeNiro. 1986. Photosynthesis of F1 hybrids between C4 and C3-C4 species of Flaveria. Plant Physiology 82:211-217.
Bunce, J. A. 1985. Effects of day and night temperature and temperature variation on photosynthetic characteristics. Photosynthesis Research 6:175-182.
Burzynski, W. and Z. Lechowski. 1983. The effect of temperature and light intensity on the photosynthesis of Panicum species of the C3, C3-C4, and C4 type. Acta Physiologiae Plantarum 5:93-104.
Byrd, G. T. and R. H. Brown. 1989. Environmental effects on photorespiration of C3-C4 species. I. Influence of CO2 and O2 during growth on photorespiratory characteristics and leaf anatomy. Plant Physiology 90:1022-1028.
Caldwell, M. M. 1972. Adaptability and productivity of species possessing C3 and C4 photosynthesis in a cool desert environment. Pages 27-30 in USSR Academy of Sciences, Eco-physiological foundation of ecosystems productivity in arid zone. International Symposium, USSR. Nauka, Leningrad, USSR.
Caldwell, M. M. R. S. White, R. T. Moore, and L. B. Camp. 1977. Carbon balance, productivity, and water use of cold-winter desert shrub communities dominated by C3 and C4 species. Oecologia 29:275-300.
Carter, D. R. and K. M. Peterson. 1983. Effects of a CO2-enriched atmosphere on the growth and competitive interaction of a C3 and a C4 grass. Oecologia 58:188-192.
Cavagnaro, J. B. 1988. Distribution of C3 and C4 grasses at different altitudes in a temperate arid region of Argentina. Oecologia 76:273-277.
Cheng, S. H. B. D. Moore, J. Wu, G. E. Edwards, and M. S. B. Ku. 1989. Photosynthetic plasticity in Flaveria brownii - growth irradiance and the expression of C4 photosynthesis. Plant Physiology 89:1129-1135.
Cock, J. H. N. M. Riano, El-Sharkawy, Y. F. Lopez, and G. Bastidas. 1987. C3-C4 intermediate photosynthetic characteristics of cassava (Manihot esculenta Crantz). II. Initial products of 14CO2 fixation. Photosynthesis Research 12:237-241.
Crespo, H. M. C. F. Cresswell, and J. Tew. 1979. The occurrence of both C3 and C4 photosynthetic characteristics on a single Zea mays plant. Planta 147:257-263.
Day, T. A. and J. K. Detling. 1990. Changes in grass leaf water relations following bison urine deposition. American Midland Naturalist 123:171-178.
De Jong, T. M. 1978. Comparative gas exchange and growth responses of C3 and C4 beach species grown at different salinities. Oecologia 36:59-68.
Dunn, R. S. M. Thomas, A. J. Keyes, and S. P. Long. 1987. A comparison of the growth of the C4 grass Spartina anglica with the C3 grass Lolium perenne at different temperatures. Journal of Experimental Botany 38:433-441.
Ehleringer, J. R. 1979. Photosynthesis and photorespiration: biochemistry, physiology, and ecological implications. HortScience 14:217-222.
Ehleringer, J. R. 1978. Implications of quantum yield differences on the distributions of C3 and C4 grasses. Oecologia 31:255-267.
Ehleringer, J. and R.W. Pearcy. 1983. Variation in quantum yield for CO2 uptake among C3 and C4 plants. Plant Physiology 73:555-559.
El-Sharkawy, M.A. and J.H. Cock. 1987. C3-C4 intermediate photosynthetic characteristics of cassava (Manihot esculenta Crantz). I. Gas exchange. Photosynthesis Research 12:219-235.
Feldhake, C. M. and D. G. Boyer. 1985. Resistance to water loss from warm and cool-season forage canopies in a growth chamber. Agricultural and Forest Meteorology 34:269-275.
Feldhake, C. M. and D. G. Boyer. 1986. Effect of soil temperature on evaporation by C3 and C4 grasses. Agricultural and Forest Meteorology 37:309-318.
Fladung, M. and J. Hesselbach. 1989. Effect of varying environments on photosynthetic parameters of C3, C3-C4 and C4 species of Panicum . Oecologia 79:168-173.
Frean, M. L. D. Ariovich, and C. F. Cresswell. 1983. C3 and C4 photosynthetic and anatomical forms of Alloteropsis simialata (R. Br.) Hitchcock 2. A comparative investigation of leaf ultrastructure and distribution of chlorenchyma in the two forms. Annals of Botany 51:811-822.
Furbank, R. T. and R. C. Leegood. 1984. Carbon metabolism and gas exchange in leaves of Zea mays L. Interaction between the C3 and C4 pathways during photosynthetic induction. Planta 162:457-462.
Gebauer, G. B. Schubert, M. I. Schuhmacher, H. Rehder, and H. Ziegler. 1987. Biomass production and nitrogen content of C3- and C4-grasses in pure and mixed culture with different nitrogen supply. Oecologia 71:613-617.
Gurevitch, J. 1986. Restriction of a C3 grass to dry ridges in a semiarid grassland. Canadian Journal of Botany 64:1006-1011.
Gurevitch, J. 1986. Competition and the local distribution of the grass Stipa neomexicana. Ecology 67:46-57.
Hattersley, P. W. S.-C. Wong, S. Perry, and Z. Roksandic. 1986. Comparative ultrastructure and gas exchange characteristics of the C3-C4intermediate Neurachne minor S.T. Blake (Poaceae). Plant, Cell and Environment 9:217-233.
Hicks, R. A. D. D. Briske, C. A. Call, and R. J. Ansley. 1990. Co-existence of a perennial C3 bunchgrass in a C4 dominated grassland: An evaluation of gas exchange characteristics. Photosynthetica 24:63-74.
Holaday, A. S. and R. Chollet. 1983. Photosynthetic/photorespiratory carbon metabolism in the C3-C4 intermediate species, Moricandia arvensis and Panicum milioides. Plant Physiology 73:740-745.
Huber, W. E. R. H. Brown, J. H. Bouton, and L. O. Sternberg. 1989. CO2 exchange, cytogenetics, and leaf anatomy of hybrids between photosyntehtically distinct Flaveria species. Plant Physiology 89:839-844.
Huber, W. and N. Sankhla. 1976. C4 pathway and regulation of the balance between C4 and C3 metabolism. Pages 335-363 in O.L. Lange, L. Kappen, and E.-D. Schulze, editors. Ecological studies, volume 19. Water and plant life; problems and modern approaches. Springer-Verlag, New York, New York, USA.
Imai, K. and Y. Murata. 1979. Effect of carbon dioxide concentration on growth and dry matter production of crop plants. VI. Effect of oxygen concentration on the carbon dioxide-dry matter production relationship in some C3- and C4-crop species. Japanese Journal of Crop Science 48:58-65.
Imai, K. and Y. Murata. 1979. Effect of carbon dioxide concentration on growth and dry matter production of crop plants. VII. Influence of light intensity and temperature on the effect of carbon dioxide-enrichment in some C3- and C4-species. Japanese Journal of Crop Science 48:409-417.
Jones, M. B. 1988. Photosynthetic responses of C3 and C4 wetland species in a tropical swamp. Journal of Ecology 76:253-262.
Ku, M. S. B. M. R. Schmitt, and G. E. Edwards. 1979. Quantitative determination of RuBP carboxylase-oxygenase protein in leaves of several C3 and C4 plants. Journal of Experimental Botany 30:89-98.
Lanning, F. C. and L. N. Eleuterius. 1989. Silica deposition in some C3 and C4 species of grasses, sedges and composites in the USA. Annals of Botany 64:395-410.
Leaney, F. W. C. B. Osmond, G. B. Allison, and H. Ziegler. 1985. Hydrogen-isotope composition of leaf water in C3 and C4 plants: its relationship to the hydrogen-isotope composition. Planta 164:215-220.
Longstreth, D. J. T. L. Hartsock, and P. S. Nobel. 1980. Mesophyll and cell properties for some C3 and C4 species with photosynthetic rates. Physiologia Plantarum 48:494-498.
Lüttge, U. and K. Fischer. 1980. Light-dependent net CO-evolution by C3 and C4 plants. Planta 149:59-63.
Marks, S. and B. R. Strain. 1989. Effects of drought and CO2 enrichment on competition between two old-field perennials. New Phytologist 111:181-186.
Martin, C. E. F. S. Harris, and F. J. Norman. 1992. Ecophsyiological responses of C3 forbes and C4 grasses to drought and rain on a tallgrass prairie in northeastern Kansas. Botanical Gazette 152:257-.
Martin, C. E. F. S. Harris, and F. J. Norman. 1992. Ecophsyiological responses of C3 forbes and C4 grasses to drought and rain on a tallgrass prairie in northeastern Kansas. Botanical Gazette 152:257-.
Monson, R. K. and B. d. Moore. 1989. On the significance of C3-C4 intermediate photosynthesis to the evolution of C4 photosynthesis: opinion. Plant, Cell and Environment 12:689-699.
Monson, R. K. G. E. Edwards, and M. S. B. Ku. 1984. C3-C4 intermediate photosynthesis in plants. BioScience 34:563-574.
Monson, R. K. B. D. Moore, M. S. B. Ku, and G. E. Edwards. 1986. Co-function of C3, C4, and C4-C3 intermediate Flaveria species. Planta 168:493-502.
Monson, R. K. M. R. Sackschewsky, and G. J. Williams. 1986. Field measurements of photosynthesis, water-use efficiency, and growth in Agropyron smithii (C3) and Bouteloua gracilis (C4) in the Colorado shortgrass steppe. Oecologia 68:400-409.
Morden, C. J. D. Brotherson, and B. N. Smith. 1986. Ecological differences of C3 and C4 plant species from central Utah habitats and mineral composition. Great Basin Naturalist 46:140-147.
Peisker, M. 1986. Models of carbon metabolism in C3-C4 intermediate plants as applied ot the evolution of C4 photosynthesis. Plant, Cell and Environment 9:627-635.
Peisker, M. 1985. Modelling carbon metabolism in C3-C4 intermediate species. II. Carbon isotope discrimination. Photosynthetica 19:300-311.
Peisker, M. and H. Bauwe. 1984. Modeling carbon metabolism in C3-C4 intermediate species. I. CO2 compensation concentration and its O2 dependence. Photosynthetica 18:9-19.
Rawthorne, S. and C. M. Hyton. 1991. The relationship between the post-illumination CO2 burst and glycine metabolism in leaves of C3 and C4 intermediate species of. Planta 186:122-.
Robichaux, R. H. and R. W. Pearcy. 1980. Photosynthetic responses of C3 and C4 species from cool shaded habitats in Hawaii. Oecologia 47:106-109.
Rumpho, M. E. M. S. B. Ku, S.-H. Cheng, and G. E. Edwards. 1984. Photosynthetic characteristics of C3-C4 intermediate Flaveria species. Plant Physiology 75:993-996.
Rundel, P. W. 1980. The ecological distribution of C4 and C3 grasses in the Hawaiian Islands. Oecologia 45:354-359.
Schmidt, H.-L. and F. J. Winkler. 1979. Sole reasons for the range of variation of 13C-values found in C3- and C4-plants. Berichte der Deutschen Botanischen Gesellschaft 92:185-191.
Sternberg, L. and M. J. De Niro. 1983. Isotopic composition of cellulose from C3, C4, and CAM plants growing near one another. Science 220:947-948.
Takeda, T. W. Agata, S. Hakoyama, and H. Tanaka. 1977. Studies on weed vegetation in non-cultivated paddy fields. II. The relation between the ecological distribution of Gramineous C3- and C4-weeds and the soil moisture condition in non-cultivated paddy fields. Japanese Journal of Crop Science 46:558-568.
Tenhunen, J. D. 1982. The diurnal course of leaf gas exchange of the C4 species Amaranthus retroflexus under field conditions in a `cool' climate: comparison with the C3 species Glycine max and Chenopodium album. Oecologia 53:310-316.
Van, T. K. and L. A. Garrard. 1975. Effect of UV-B radiation on net photosynthesis of some C3 and C3 crop plants. Soil and Crop Science Society of Florida Proceedings 35:1-3.
Vogel, J. C. A. Fuls, and A. Danin. 1986. Geographical and environmental distribution of C3 and C4 grasses in the Sinai, Negev, and Judean deserts. Oecologia 70:258-265.
Vu, C. V. L. H. Allen, Jr. and L. A. Gerrard. 1982. Effects of supplemental UV-B radiation on primary photosynthetic carboxylating enzymes and soluble proteins in leaves of C3 and C4 crop plants. Physiologia Plantarum 55:11-16.
Welkie, G. W. and M. M. Caldwell. 1970. Leaf anatomy of species in some dicotyledon families as related to the C3 and C4 pathways of carbon fixation. Canadian Journal of Botany 48:2135-2146.
Wong, S. C. 1980. Elevated atmospheric partial pressure of CO2 and plant growth. I. Interactions of nitrogen nutrition and photosynthetic capacity in C3 and C4 plants. Oecologia 44:68-74.
2 Introduction C 3, C 4 and CAM photosynthesis are the three types of photosynthesis in green plants. C 3 photosynthesis is the photosynthesis we have learned about in class. C 4 and CAM photosynthesis are adaptations to arid conditions.
3 C 3 Photosynthesis C 3 is so called because the first compound made when carbon dioxide is fixed from the atmosphere has three carbons (PGAL). Stomata are open during the day. RuBisco is the enzyme involved in carbon fixation.
4 C 3 Photosynthesis Photosynthesis takes place throughout the leaf. Most plants are C3 because it requires fewer enzymes and less specialized machinery than C4 and CAM photosynthesis. It is the most efficient form of photosynthesis under normal light intensity, lower temperature and normal moisture.
5 C 4 Photosynthesis Called C 4 because carbon dioxide is fixed into a four carbon compound. Stomata open during the day. Dark Reactions take place in inner cells called Kranz Anatomy.
6 C 4 Photosynthesis Uses the enzyme PEP carboxylase to fix carbon dioxide into a four carbon compound (usually malic acid) and then delivers it to RuBisCO to provide carbon dioxide and continue photosynthesis similar to the C 3 pathway.
7 C 4 Photosynthesis This works faster than C 3 photosynthesis under high light intensity and high temperature because it delivers CO 2 directly to RuBisCO thereby maximizing carbohydrate formation and preventing product loss due to photorespiration.
8 Photorespiration Definition Photorespiration is a process in plant metabolism by which RuBP (a sugar) has oxygen added to it by RuBisCO, instead of carbon dioxide during normal photosynthesis. This process reduces efficiency of photosynthesis in C3 plants. Oxygen acts as a competitive inhibitor.
9 C 4 Photosynthesis It is also more efficient in terms of water use because PEP carboxylase brings in CO 2 faster and therefore does not need to keep the stomata open for as long, thereby minimizing water loss. C 4 photosynthesis is common in plants that grow mainly during the intense heat of summer in North America (i.e. Corn).
10 CAM Photosynthesis Crassulacean acid metabolism CAM) photosynthesis is another adaptation for plants that are in arid conditions. Stomata are closed during the day and open at night to reduce water loss through transpiration.
11 CAM Photosynthesis These plants fix carbon dioxide into vacuoles during the night and incorporate it into malic acid similar to the C4 pathway. During the day, the malic acid transfers carbon dioxide to RuBisCO and carbohydrate is made.
12 CAM Photosynthesis CAM plants often have thick, reduced leaves with a low SA to V ratio; thick cuticle; and stomata sunken into pits. Some store water in vacuoles (succulent plants). CAM plants can also be recognized as plants whose leaves have an increasing sour taste during the night yet become sweeter-tasting during the day. This is due to malic acid stored in the vacuoles of the plants' cells during the night and then used up during the day. That is why we let pineapple ripen before eating!
13 CAM Photosynthesis CAM plants can “CAM-idle” which allows them to keep their stomata closed at all times during extremely arid conditions and therefore any oxygen they give off in photosynthesis is immediately used in cellular respiration and vice versa. This allows the plant to survive dry periods and recover very quickly when water returns.
14 CAM vs. C 4 Photosynthesis Similarities Differences Both are a response to arid conditions. Both use water more efficiently than C3 plants. Both minimize the amount of photorespiration by proving carbon dioxide directly to RuBisCO. CAM plants provide CO 2 temporally (stockpile it at night and provide it during the day) whereas C4 plants provide CO 2 spatially (take it from the outer cells and provide it to the inner Kranz Anatomy). C4 plants require special Krantz Anatomy. C4 plants have stomata open during the day while CAM plants have stomata open at night.
C4 Photosynthesis. This mechanism of photosynthesis occurs in two adjoining types of cells, the mesophyll and bundle sheath cells in plant species called C4 plants. Both C3 and C4 cycles operate in the non-light-requiring or Dark Reactions of photosynthesis but spatially. that is, in different cells: C4 in the mesophyll cells immediately followed by C3 cycle in the bundle sheath cells.
CO2 first enters the leaf and into the mesophyll cell. It is then hydrated to produce bicarbonate ion (HCO3-) in the cytoplasm with carbonic anhydrase (CA ) as catalyst. This is the first step in C4 photosynthesis, followed by carboxylation reaction utilizing HCO3- instead of CO2 as the inorganic carbon substrate, Hatch and Burnell (1990) emphasized.
HCO3- reacts with the three-carbon acid phosphoenolpyruvate (PEP or PEPA, C3H5O6P) to form oxaloacetate (OAA. oxaloacetic acid= C4H4O5). The reaction is catalyzed by the carboxylating enzyme phosphoenolpyruvate carboxylase (PEPcase. PEPC or PEPCO). OAA is a four-carbon product, hence the term C4 photosynthesis.
(1) Hydration of CO2 (catalyzing enzyme is carbonic anhydrase):
(2) Carboxylation of HCO3- (catalyzing enzyme is PEPcase):
HCO3- + PEP ---------->OAA
The summary reaction is commonly written as shown below in which the hydration reactions leading to the formation of HCO3- and its carboxylation are skipped :
OAA is then reduced to malate (malic acid= C4H6O5) or transaminated to aspartate (aspartic acid= C4H7NO4) and transported to the adjacent bundle-sheath cells. As to malate, it is utilized in two ways: for the regeneration of PEP, and for the supply of CO2 for the succeeding C3 cycle. First, malate is decarboxylated in which CO2 is removed and pyruvate (pyruvic acid= C3H4O3) is formed. Pyruvate goes back to the mesophyll cell where it is phosphorylated to PEP, the CO2 acceptor in the C4 cycle. The freed CO2 enters the C3 cycle within the bundle sheath cell.
As in C3 photosynthesis, the product of the biochemical reactions in the bundle sheath cells is the three-carbon sugar glyceraldehyde-3-phosphate (G3P. C3H7O6P), also called triose phosphate and phosphoglyceraldehyde (PGAL). Similarly, some molecules of G3P undergo reactions to regenerate RuBP, the CO2 acceptor in the C3 cycle. Other molecules of G3P leave the cycle and proceed with the formation of glucose and other organic compounds that plants need.
Contrasted to C3 photosynthesis, the C4 photosynthetic pathway is more efficient based on resistance to photorespiration which is a wasteful process. Unlike in C3 photosynthesis, the initial CO2-fixing enzyme PEPcase in C4 cycle does not act as oxygenase and therefore it does not fix O2 even when it is in high concentration within the cell. This enzyme initially fixes atmospheric CO2 in the mesophyll cells which is then delivered to the bundle sheath cells in the form of organic acids.
The C4 cycle in C4 photosynthesis therefore serves as a CO2-concentrating mechanism for the bundle sheath cells. The high concentration of CO2 favors the fixing of CO2, instead of O2, by rubisco. Photorespiration is thus suppressed.
However, the C4 pathway of CO2 reduction expends more energy (5 ATP and 2 NADPH) than C3 pathway (3 ATP and 2 NADPH) (Hopkins 1999). Nevertheless, the former is efficient under conditions of high light intensity, high temperature, and limited water.
(Ben G. Bareja Aug. 2013)
BI 203 - Study Guide for Final Exam
In addition to the material below, you should also be familiar with the material from the previous two study guides because the final is cumulative.
C3, C4, and CAM photosynthesis
What is photorespiration? How does it differ from dark respiration? What is the cost of photorespiration to the plant? Why does it happen? What conditions lead to higher rates of photorespiration?
How does the C4 pathway solve problems of photorespiration? What is the role of the Calvin cycle in C4 photosynthesis? What is the primary enzyme, the precursors, the products and the cost of the C4 pathway? Is this a cycle? What is Kranz anatomy? Why don�t all plants have C4 photosynthesis? In what regions is C4 particularly important? Why?
How is CAM photosynthesis similar to and different from both C3 and C4? What are the advantages and disadvantages of each?
Water in plants
Reading: pp. 76-81, Chap.31
What are the components of the full water potential equation? How does each affect the free energy and movement of water - between cells, and between plant and soil?
What is transpiration and why is it a "necessary evil"? How important is transpirational water loss in the whole water budget of a plant? Why can�t capillarity and air pressure account for transpirational movement of water through plants? How does water potential and the tension-cohesion mechanism account for this movement? What is the water potential gradient through a plant?
What factors, both biotic and abiotic control rates of transpiration?
What components of water potential account for night-time root pressure in some plants? What is guttation? What is hydraulic lift and how does it work? How does it relate to water potential and water potential gradients? What are some ecological consequences of hydraulic lift?
What moves through phloem? What is meant by "source-sink" relationships in phloem flow? How does phloem flow differ from xylem flow? What is the pressure flow hypothesis and the three main steps involved? How does each step work?
Reading: Chap. 30, pp. 726-731, 736-742
What is an "essential element"? What is the difference between macronutrients and micronutrients? Who are the macronutrients, where do they come from, and what is at least one major function of each?
What two factors contribute to the deficiency symptoms for the different nutrients? Which macronutrients have high, intermediate and low mobility?
Why is nitrogen important to plants? In what forms is N available to plants in soils? What steps are important in plant assimilation of nitrogen and how does this differ among the different available forms?
What are the main steps of the nitrogen cycle? what are the main pools and fluxes? What organisms mediate the different steps? Why is N-fixation important to the nitrogen cycle?
What happens in the process of N-fixation? Who does it? What is the mutualistic tradeoff in symbiotic N-fixation? How specific is this process? What are the steps in nodule formation? What factors potentially constrain N-fixation and how does the plant solve the oxygen problem?
Reading: Chap. 28 to p.693
What is the definition of a hormone? What factors control the expression of a hormone-induced response? What is signal transduction pathway and what is at least one example from the hormones that we studied?
What is the primary natural auxin? Where is it made and how is it transported? What are the effects of auxin and how do they work? What is an experiment that demonstrates the role of auxin in apical dominance? What auxin mimics are important and why?
What are cytokinins, where are they made, and what are their effects? How do they interact with IAA? How were cytokinins discovered and how are they used today? What is totipotency?
What are giberellins, where are they made and what are their functions? How do they interact with the aleurone layer in monocot seeds?
What is unique about ethylene? What is its structure and what are its effects? What is a climacteric fruit and some examples of that type of fruit? How is ethylene used commercially now? How has this use contributed (indirectly) to the downfall of tomato flavor?
How does abscisic acid (ABA) affect stomatal closure?
Plant response to the environment
Reading: chap. 29, pp. 702-714.
What is a positive tropism and what is a negative tropism? How does gravitopism work in roots and shoots? How is the mechanism and response similar and how different in these different tissues? What is phototropism and what is its mechanism of action?
What are the different photoperiod plant types? What role does phytochrome play in photoperiod sensation? What is phytochrome's sensitivity to different wavelengths of light? How does a plant measure the length of a dark cycle? How do we know these things from studies of seed germination and flowering?
What other roles does phytochrome play in etiolation and growth responses in shade? How might these be ecologically important to a plant?
Alternative forms of photosynthesis are used by specific types of plants, called C4 and CAM plants, to alleviate problems of photorespiration and excess water loss.
Photosynthesis is the physiological process whereby plants use the sun’s radiant energy to produce organic molecules. The backbone of all such organic compounds is a skeleton composed of carbon atoms. Plants use carbon dioxide from the atmosphere as their carbon source.
The overwhelming majority of plants use a single chemical reaction to attach carbon dioxide from the atmosphere onto an organic compound, a process referred to as carbon fixation. This process takes place inside specialized structures within the cells of green plants known as chloroplasts.
The enzyme that catalyzes this fixation is ribulose bisphosphate carboxylase (Rubisco), and the first stable organic product is a three-carbon molecule. This three-carbon compound is involved in the biochemical pathway known as the Calvin cycle. Plants using carbon fixation are referred to as C3 plants because the first product made with carbon dioxide is a three-carbon molecule.
For many years scientists thought that the only way photosynthesis occurred was through C3 photosynthesis. In the early 1960’s, however, researchers studying the sugarcane plant discovered a biochemical pathway that involved incorporation of carbon dioxide into organic products at two different stages.
First, carbon dioxide from the atmosphere enters the sugarcane leaf, and fixation is accomplished by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase). This step takes place within the cytoplasm, not inside the chloroplasts. The first stable product is a four-carbon organic compound that is an acid, usually malate. Sugarcane and other plants with this photosynthetic pathway are known as C plants.
In C4 plants, this photosynthetic pathway is tied to a unique leaf anatomy known as Kranz anatomy. This term refers to the fact that in C4 plants the cells that surround the water- and carbohydrate conducting system (known as the vascular system) are packed very tightly together and are called bundle sheath cells.
Surrounding the bundle sheath is a densely packed layer of mesophyll cells. The densely packed mesophyll cells are in contact with air spaces in the leaf. and because of their dense packing they keep the bundle sheath cells from contact with air. This Kranz anatomy plays a major role in C4 photosynthesis.
In C4 plants the initial fixation of carbon dioxide from the atmosphere takes place in the densely packed mesophyll cells. After the carbon dioxide is fixed into a four-carbon organic acid, the malate is transferred through tiny tubes from these cells to the specialized bundle sheath cells.
Inside the bundle sheath cells, the malate is chemically broken down into a smaller organic molecule, and carbon dioxide is released. This carbon dioxide then enters the chloroplast of the bundle sheath cell and is fixed a second timewith the enzyme Rubisco and continues through the C3 pathway.
Advantages of Double-Carbon Fixation
The double-carbon fixation pathway confers a greater photosynthetic efficiency on C4 plants over C3 plants, because the C3 enzyme Rubisco is highly inefficient in the presence of elevated levels of oxygen. In order for the enzyme to operate, carbon dioxide must first attach to the enzyme at a particular location known as the active site.
However, oxygen is also able to attach to this active site and prevent carbon dioxide from attaching, a process known as photorespiration. As a consequence, there is an ongoing competition between these two gases for attachment at the active site of the Rubisco enzyme. Not only does the oxygen outcompete carbon dioxide; when oxygen binds to Rubisco, it also destroys some of the molecules in the Calvin
At any given time, the winner of this competition is largely dictated by the relative concentrations of these two gases. When a plant opens its stomata (the pores in its leaves), the air that diffuses in will be at equilibrium with the atmosphere, which is 21 percent oxygen and 0.04 percent carbon dioxide.
During hot, dry weather, excess water vapor diffuses out, and under these conditions plants face certain desiccation if the stomata are left open continuously.When these pores are closed, the concentration of gases will change. As photosynthesis proceeds, carbon dioxide will be consumed and oxygen generated.
When the concentration of carbon dioxide drops below 0.01 percent, oxygen will outcompete carbon dioxide at the active site, and no net photosynthesis occurs. C4 plants, however, are able to prevent photorespiration, because the PEP carboxylase enzyme is not inhibited by oxygen.
Thus, when the stomata are closed, this enzyme continues to fix carbon inside the leaf until it is consumed. Because the bundle sheath is isolated from the leaf’s air spaces, it is not affected by the rising oxygen levels, and the C3 cycle functions without interference. C4 photosynthesis is found in at least nineteen families of flowering plants.
No family is exclusively composed of C4 plants. Because C4 photosynthesis is an adaptation to hot, dry environments, especially climates found in tropical regions, C4 plants are often able to out compete C3 plants in those areas. In more temperate regions, they have less of an advantage and are therefore less common.
A second alternative photosynthetic pathway, known as crassulacean acid metabolism (CAM), exists in succulents such as cacti and other desert plants. These plants have the same two carbon-fixing steps as are present in C4 plants, but rather than being spatially separated between the mesophyll and bundle sheath cells, CAM plants have both carbon dioxide-fixing enzymes within the same cell.
These enzymes are active at different times, PEP carboxylase during the day and Rubisco at night. Just as Kranz anatomy is unique to C4 plants, CAM plants are unique in that the stomata are open at night and largely closed during the day.
The biochemical pathway of photosynthesis in CAM plants begins at night. With the stomata open, carbon dioxide diffuses into the leaf and into mesophyll cells, where it is fixed by the C4 enzyme PEP carboxylase. The product is malate, as in C4 photosynthesis, but it is transformed into malic acid (a nonionic form of malate) and is stored in the cell’s vacuoles (cavities within the cytoplasm) until the next day.
Although the malic acid will be used as a carbon dioxide source for the C3 cycle, just as in C4 photosynthesis, it is stored until daylight because the C3 cycle requires light as an energy source. The vacuoles will accumulate malic acid through most of the night.
A few hours before daylight, the vacuole will fill up, and malic acid will begin to accumulate in the cytoplasm outside the vacuole. As it does, the pH of the cytoplasm will become acidic, causing the enzyme to stop functioning for the rest of the night.
When the sun rises the stomata will close, and photosynthesis by the C3 cycle will quickly deplete the atmosphere within the leaf of all carbon dioxide. At this time, the malic acid will be transported out of the vacuole to the cytoplasmof the cell. There it will be broken down, and the carbon dioxide will enter the chloroplast and be used by the C3 cycle; thus, photosynthesis is able to continue with closed stomata.
Crassulacean acid metabolism derives its name from the fact that it involves a daily fluctuation in the level of acid within the plant and that it was first discovered to be common in species within the stonecrop family, Crassulaceae.
The discovery of this photosynthetic pathway dates back to the 1960’s. The observation that succulent plants become very acidic at night, however, dates back to at least the seventeenth century, when it was noted that cactus tastes sour in the morning and bitter in the afternoon.
CAM Plant Ecosystems
There are two distinctly different ecological environments where CAM plants may be found. Most are terrestrial plants typical of deserts or other harsh, dry sites.
In these environments, the pattern of stomatal opening and closing provides an important advantage for surviving arid conditions: When the stomata are open, water is lost; however, the rate of loss decreases as the air temperature decreases. By restricting the time period of stomatal opening to the nighttime, CAM plants are extremely good at conserving water.
The other ecological setting where CAM plants are found is in certain aquatic habitats. When this environment was first discovered, it seemed quite odd, because in these environments conserving water would be of little value to a plant. It was found, however, that there are aspects of the aquatic environment which make CAM photosynthesis advantageous.
In shallow bodies of water, the photosynthetic consumption of carbon dioxide may proceed at a rate in excess of the rate of diffusion of carbon dioxide from the atmosphere into the water, largely because gases diffuse several times more slowly in water than in air.
Consequently, pools of water may be completely without carbon dioxide for large parts of the day. Overnight, carbon dioxide is replenished, and aquatic CAM plants take advantage of this condition to fix the plentiful supply of carbon dioxide available at night and store it as malic acid.
Hence, during the day, when the ambient carbon dioxide concentration is zero, these plants have their own internal supply of carbon dioxide for photosynthesis. Thus, two very different ecological conditions have selected for the identical biochemical pathway.
These two modified photosynthetic pathways adequately describe what happens in most terrestrial plants, although there is much variation. For example, there are species that appear in many respects to have photosynthetic characteristics intermediate to C3 and C4 plants.
Other plants are capable of switching from exclusively C3 photosynthesis to CAM photosynthesis at different times of the year. Photosynthesis by aquatic plants appears to present even more variation. C3-C4 intermediate plants seem to be relatively common compared to the terrestrial flora. and several species have C4 photosynthesis but lack Kranz anatomy.