What are some examples of C3 plants
There are a number of plant species which, when exposed to high light intensity, are characterized by an increased and far more efficient net photosynthesis output than the others. Prime examples of this are several types of gramineae in warmer areas, such as maize and sugar cane.
At the beginning of the sixties H. KORTSCHAK (Hawaiian Sugar Planter's Association) established that the first product of photosynthesis in sugar cane was not C.3-Compound 3-phosphoglycerate, but a compound made up of four carbon atoms. The plant physiologists, the Australian M. D. HATCH and the Englishman C. R. SLACK confirmed the finding and identified the compound as oxalacetic acid (OAA), which is formed by the addition of a carbon dioxide molecule to phosphoenolpyruvate (PEP) (HATCH-SLACK cycle = C.4-Cycle). Plant species that follow this path are called C4Plants (or CAM plants), in contrast to the C3-Plants in which the absorbed carbon dioxide is introduced directly into the CALVIN cycle. In the majority of cases, the oxaloacetate is converted into malate, from which carbon dioxide is enzymatically split off again.
This carbon dioxide is now fed to the ribulose-1,5-diphosphate and via CALVIN cycle fixed.
Instead of malate, aspartate occurs as an intermediate in some species:
Oxaloacetate + L-glutamate> aspartate + alpha-Ketoglutarate.
The reversible binding of the carbon dioxide obviously serves its accumulation and storage. Since the process is energy consuming, one can speak of a carbon dioxide pump.
Photosynthesis in C4-Plants. In mesophyll cells, CO2 bound to phosphoenolpyruvate (PEP). The result is oxaloacetate and malate. This is released to the cells of the vascular bundle sheath ("wreath" cells), CO2 is split off and fed into the CALVIN cycle. The pyruvate migrates back into the mesophyll cells (active transport with consumption of energy) and is phosphorylated to PEP with additional consumption of ATP
The leaves of the C4Plants (with so-called "wreath" cells) differ fundamentally anatomically from those of the C.3-Plants. In C3-Plants, the chloroplasts are uniformly structured, in C4- There are two types of chloroplasts in plants. The mesophyll cells contain normally formed chloroplasts, while the cells of the vascular bundle sheath contain granalose and therefore only partially functional chloroplasts. However, the impairment relates only to the light reactions of photosynthesis. The Calvin cycle is not affected. The primary carbon dioxide binding (HATCH-SLACK reaction) takes place in the mesophyll cells, the incorporation into carbohydrates (CALVIN cycle) in the cells of the vascular bundle sheath. The two sections of photosynthesis are thus spatially separated from each other.
CAM is the abbreviation for Crassulacean Acid Metabolism. The name indicates that this metabolic pathway occurs primarily in the Crassulaceae (and other succulents). The chemical reaction of the carbon dioxide enrichment is similar to that in C.4-Plants, but here the two partial processes are not spatially but temporally separated from one another. CAM plants are mainly found in arid areas. Opening the stomata to produce carbon dioxide there is always associated with a particularly high loss of water. In order to contain or almost completely prevent it in strong sunlight (cuticular perspiration is retained), a control mechanism has been developed that enables carbon dioxide to be absorbed at night. The pre-fixed carbon dioxide is stored as malate (and isocitrate) in vacuoles and used for photosynthesis during the day.
The enzyme that is responsible for the primary carbon dioxide fixation of the C4- and CAM plants is catalyzed by phosphoenol pyruvate carboxylase (PEPC), whose affinity for carbon dioxide is far higher than the corresponding affinity of RuDP carboxylase (Rubisco), the first enzyme of the CALVIN cycle. C.4-Plants are thus enabled to use traces of carbon dioxide. PEPC also comes in small amounts (approx. 2 - 3%) in C.3Plants, and there, too, they play a key role in regulating the metabolism.
Carbon gain at C4 - and C3 -Plants of open grasslands in different geographical latitudes. In the temperate zone, the low light intensity is a disadvantage for C4 - Plants crucial, C3 -Plants have an advantage because the rate of photorespiration is low and there is no energy for the pre-fixation of CO2 needs to be used. (J. R. EHRLICHER, 1978).
Among other things, it provides NADH + H in growing roots (e.g. of maize seedlings)+, which is required for lipid synthesis. The following reactions take place:
- Phosphoenolpyruvate + HCO3-> oxaloacetate + Pi
- Oxaloacetate> malate. - During this step, NADH + H+ to NAD+ oxidized.
- Malate> pyruvate + CO2.
Here the reduction of NAD to NADH + H takes place+.
Nitrogen fixation takes place in the leguminous root nodules. In order to incorporate the ammonia produced by bacterial activity, carbon skeletons must be provided in sufficient quantities. The CO2-Fixing via The PEPC reaction therefore also serves as an important supplement to replenish the carbon skeleton supply
Furthermore, the PEPC is used to produce intermediate products of the citric acid cycle (oxaloacetate and / or malate) in order to overcome bottlenecks if necessary. PEPC activities are controlled by external factors, the length of the day being one of the decisive factors. In some investigated cases, different isoenzymes have been detected in different tissues, the production of which is controlled by different triggers.
In C3-Plants can lose up to 20 percent of the carbon fixed in the CALVIN cycle through photorespiration at high radiation intensity. With high light intensity, photorespiration is about 1.5 - 3.5 times as high as normal dark breathing. In C4-Plant the photorespiration is drastically reduced, maybe it is no longer detectable at all. In other words:
The net photosynthesis rate (and thus the net production of biomass) of the C4-Plants is far higher than that of the C at high light intensities3-Plants. The temperature optimum of photosynthesis is below that of dark breathing. As a result, respiratory losses play a greater role at higher temperatures than at low temperatures. Where lack of light is a limiting factor and temperatures are low (in temperate climates), C is3- Plants at an advantage, C4-Plants rarely occur (one of the exceptions is Spartina townsendii). C.4-Plants (they are almost always herbs or shrubs) are more successful in open terrain in warmer climates.
It should be noted that in the HATCH-SLACK cycle, two ATP molecules are consumed per fixed carbon dioxide molecule.
C.4Plants belong to numerous, phylogenetically unrelated families of the mono- and dicotyledons. In addition, one found C4-Activities including the blue-green algae Anacystis nidulans as well as some dinoflagellates.
Since with the higher plants with the alternative C3 or C4 If considerable anatomical changes in the leaves are connected, one must assume that the genetic potential for realizing both paths is widespread in the plant kingdom and that, depending on ecological requirements, one path is chosen for one species and the other for a related one.
The genus is a well-studied example Atriplex in which both ways are realized. Accordingly, C3- Kinds of kin, C4-Types of a different one. In individual cases, hybrids between C3- and C4-Create species.
Influence of various parameters on the efficiency of carbon dioxide uptake (ordinate) at a C3 - plant (Atriplex patula - yellow lines) and a C4 - plant (Atriplex rosea - green lines). Measured parameters from left to right: light intensity, leaf temperature and carbon dioxide concentrations in the intercellular areas (after O. BJÖRKMAN and J. BERRY, 1973)
With several plant species from the genera Zea, Mollugo, Moricandia, Flaveria Among other things, both paths are followed in a plant. Photosynthesis usually takes place on the C in young leaves3-, in older ones on the C4-Path. The amount of the C4-Proportion is regulated by location factors.
CAM has been detected in over 1000 angiosperms from 17 different families. Usually it is associated with plant succulence, but not all Crassulaceae e.g. show CAM, and succulence is not a prerequisite for CAM. The Bromeliacee Tillandsia usneoides is not succulent, but is characterized by CAM. Mesembryanthemum crystallinum (a leaf succulent) can the C3-Take the path, but switch to CAM when growing on salty soils. Switching can be achieved experimentally by increasing the NaCl content of the nutrient medium (K. WINTER and D. J. von WILLERT, 1972). While the advantage of the C4 Plants come into play with high light intensities, in CAM plants it is primarily temperature, humidity and salinity that regulate the degree of CAM influence. Strong and weak CAM plants are known. In the case of the weak, CAM only appears when there are certain differences between day and night temperatures. CAM plants that have stored a lot of malate and, because of the osmotic value caused by it, also a lot of water, are usually less frost-resistant than C.3-Plants. However, because of their high acid content, they are also less heat-resistant. For species in arid (dry) areas it is therefore necessary to dismantle the malate pool during the day (R. LÖSCH and H. KAPPEN, University of Kiel, 1985). In general, the C4-Way and CAM from each other. As an exception, the succulent C4-Dicots Portulaca oleracea called, which can take the optimal path in each case in natural locations.
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