During the night, the CAM plant's stomata are open, allowing CO2 to enter and be fixated as organic acids that are stored in vacuoles. During the day the stomata are closed (thus preventing water loss) and the carbon is released to the Calvin Cycle so that photosynthesis may take place.
The carbon dioxide is fixed in the mesophyll cell's cytoplasm by a PEP reaction similar to that of . But, unlike C4 plants, the resulting organic acids are stored in vacuoles for later use; that is, they are not immediately passed on to the . Of course, the latter cannot operate during night because the light reactions which provide it with and cannot take place without light.
At low temperatures (frequently at night), when CAM plants open their , carbon dioxide molecules diffuse into the spongy 's intracellular spaces and finally get into the . Here, they can meet (PEP), which is a phosphorylated triose. During this time, CAM plants are synthesizing a protein called PEP carboxylase (PEP-C kinase), which expression can be inhibited by high temperatures (frequently at daylight) and the presence of malate. PEP-C kinase phosphorylates its target enzyme PEP carboxylase (PEP-C). dramatically enhanced the enzyme‘s capability to catalyze the formation of that can be subsequently transformed into by NAD malate dehydrogenase. Malate is then transported via malate shuttles into the vacuole, where it is converted into the storage form . In contrast to PEP-C kinase, PEP-C is synthesized all the time but almost inhibited at daylight either by dephosphorylation via PEP-C phosphatase or directly by binding malate. The latter is not possible at low temperatures, since malate is efficiently transported into the vacuole whereas PEP-C kinase readily inverts .
At daylight, CAM plants close their guard cells and discharged malate that is subsequently transported into . There, depending on plant species, it is cleaved into pyruvate and carbon dioxide either by malic enzyme or PEP carboxykinase. Carbon dioxide is then introduced into the , a coupled and self-recovering enzyme system, which is used to build branched carbohydrates. The by-product can be further degraded in the mitochondrial and therefore, provides additional carbon dioxide molecules for the calvin cycle. Alternatively, pyruvate can be also used to recover PEP via pyruvate phosphate dikinase, a high energy step, which requires and an additional . In the following cold night, PEP is finally exported into the cytoplasm, where it is involved in fixing carbon dioxide via malate.
The majority of plants possessing Crassulacean Acid Metabolism are either (e.g. orchids, bromeliads) or succulent (e.g. cacti, cactoid Euphorbias), but it is also found in (e.g. Clusia), (e.g. Sedum, Sempervivum), terrestrial bromeliads, (e.g. Isoetes, Crassula (Tillaea), and from a (Mesembryanthemum crystallinum), a non-succulent terrestrial plant (Dodonaea viscosa) and a mangrove associate (Sesuvium portulacastrum). afra is the only plant known to display both CAM and C4 pathways.
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?
Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate pathway in some . These plants fix carbon dioxide (CO2) during the night, storing it as the four carbon acid . The CO2 is released during the day, where it is concentrated around the , increasing the . The CAM pathway allows to remain shut during the day; therefore it is especially common in plants adapted to arid conditions.
The most important benefit to the plant is the ability to leave most leaf stomata closed during the day. CAM plants are most common in environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing CAM plants to grow in environments that would otherwise be far too dry. , for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by CAM plants.
The bears resemblance to CAM; both act to concentrate CO2 around RuBisCO, thereby increasing usefulness. CAM concentrates it in time, providing CO2 during the day, and not at night, when respiration is the dominant reaction. C4 plants, on the contrary, concentrate CO2 spatially, with a RuBisCO reaction centre in a " cell" being inundated with CO2.
CAM can be considered an adaptation to arid conditions. CAM plants often display other characters, such as thick, reduced leaves with a low -to-volume ratio; thick ; and sunken into pits. Some shed their leaves during the dry season; others (the succulents) store water in .
These pathways of carbon fixation, know as the C4 and the CAM pathways, take place in the cytoplasm of the cell.
CAM plants can also be recognised as plants which have sour tasting leaves increasing during nights but sweet tasting leaves increasing during days. This is due to the malic acid being stored in the vacuoles of the plant cells during the night, and its being used up during the day.
Crassulacean Acid Metabolism has evolved convergently many times. It occurs in 16,000 species (about 7% of plants), belonging to over 300 genera and around 40 families. It is found in (relatives of ), in ferns, and in , but the great majority of CAM plants are angiosperms (flowering plants).