Crassulacean acid metabolism

Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some plants. These plants fix carbon dioxide during the night, storing it as the four carbon sugar malate. The is released during the day, where it is concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day; therefore it is especially common in plants adapted to arid conditions.

Historical background
CAM was first discovered in the late 1940s. It was observed by the botanists Ransom and Thomas, in the Crassulaceae family of succulents (which includes jade plants and sedums). Its name refers to acid metabolism in Crassulaceae, not the metabolism of Crassulacean acid.

Overview of CAM: a two-part cycle
CAM is a mechanism whereby is concentrated around RuBisCO by day, while the enzyme is operating at peak capacity. This concentration of increases RuBisCO's efficiency, as it is prone to operate in the "reverse" direction via photorespiration - utilising oxygen to break down the reaction products the plant would rather it was producing. It differs from metabolism, which spatially concentrates  around RuBisCO.

During the night
CAM plants open their stomata during the cooler and more humid night-time hours, permitting the uptake of carbon dioxide with the 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 phosphoenolpyruvate (PEP), to which a molecule is added via carboxylation - forming a new molecule, oxaloacetate. This is then reduced, forming malate. Oxaloacetate and malate are built around a skeleton of four carbons - hence the term. Malate can be readily stored by the plant in vacuoles within individual cells.

The next day...
Malate can be broken down on demand, releasing a molecule of as it is converted to pyruvate. The pyruvate can be phosphorylated (i.e. have a phosphate group added by the "energy carrier" ATP) to regenerate the PEP with which we started, ready to be spurred into action the next night. But it is the release of that makes the cycle worth the plant's while. It is directed to the stroma of chloroplasts: the sites at which photosynthesis is most active. There, it is provided to RuBisCO in great concentrations, increasing the efficiency of the molecule, and therefore producing more sugars per unit photosynthesis.

The benefits of CAM
A great deal of energy is expended during CAM by the production and subsequent destruction of malate. This is in part countered by the increased efficiency of RuBisCO, but the more important benefit to the plant is the ability to leave 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. plants, for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by CAM plants.

Comparison with metabolism
The pathway bears resemblance to CAM; both act to concentrate  around RuBisCO, thereby increasing its efficiency. CAM concentrates it in time, providing during the day, and not at night, when respiration is the dominant reaction. plants, on the contrary, concentrate spatially, with a RuBisCO reaction centre in a "bundle sheath cell" being innundated with.

How to spot a CAM plant
CAM can be considered an adaptation to arid conditions. CAM plants often display other xerophytic characters, such as thick, reduced leaves with a low surface-area-to-volume ratio; thick cuticle; and stomata sunken into pits. Some shed their leaves during the dry season; others (the succulents) store water in vacuoles.

CAM plants are not only good at retaining water, but use nitrogen very efficiently. However, due to their stomata being closed by day, they are less efficient at absorption. This limits the amount of carbon they have available for growth.

Biochemistry of Crassulacean Acid Metabolism
Plants with Crassulacean Acid Metabolism (CAM plants) must control storage of carbon dioxide and its reduction to branched carbohydrates in space and time.

At low temperatures (frequently at night), when CAM plants open their guard cells, carbon dioxide molecules diffuse into the spongy mesophyll's intracellular spaces and finally get into the cytoplasm. Here, they can meet phosphoenolpyruvate (PEP), which is a phosphorylated triosephosphate. During this time, CAM plants are synthesizing a protein called PEP carboxylase kinase (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). Phosphorylation dramatically enhanced the enzyme‘s capability to catalyze the formation of oxalacetate that can be subsequently transformed into malate by NAD malate dehydrogenase. Malate is then transported via malate shuttles into the vacuole, where it is converted into the storage form maleic acid. 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 dephosphorylation.

At daylight, CAM plants close their guard cells and discharged malate that is subsequently transported into chloroplasts. 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 Calvin cycle, a coupled and self-recovering enzyme system, which is used to build branched carbohydrates. The by-product pyruvate can be further degraded in the mitochondrial citric acid cycle 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 ATP and an additional phosphate. In the following cold night, PEP is finally exported into the cytoplasm, where it is involved in fixing carbon dioxide via malate.

Ecological and Taxonomic Distribution of CAM Plants
The majority of plants possessing Crassulacean Acid Metabolism are either epiphytes (e.g. orchids, bromeliads) or succulent xerophytes (e.g. cacti, cactoid Euphorbias), but it is also found in hemiepiphytes (e.g. Clusia), lithophytes (e.g. Sedum, Sempervivum), terrestrial bromeliads, hydrophytes (e.g. Isoetes, Crassula (Tillaea), and from a halophyte (Mesembryanthemum crystallinum), a non-succulent terrestrial plant (Dodonaea viscosa) and a mangrove associate (Sesuvium portulacastrum). Portulacaria afra is the only plant known to display both CAM and C4 pathways.

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 quillworts (relatives of club mosses), in ferns, and in gymnosperms, but the great majority of CAM plants are angiosperms (flowering plants).

The following list summarises the taxonomic distribution of CAM plants.