Crassulacean acid metabolism Totally Explained

Everything about Crassulacean Acid Metabolism totally explained

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 acid malate. The is released during the day, where it's concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day; therefore it's 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's 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's converted to pyruvate. The pyruvate can be phosphorylated (for example 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's 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's 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 inundated 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))source|date=February 2008}}) 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're less efficient at absorption. This limits the amount of carbon they've available for growth.
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.

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's 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 isn't 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's subsequently transported into chloroplasts. There, depending on plant species, it's 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's 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 (for example orchids, bromeliads) or succulent xerophytes (for example cacti, cactoid Euphorbias), but it's also found in hemiepiphytes (for example Clusia), lithophytes (for example Sedum, Sempervivum), terrestrial bromeliads, hydrophytes (for example 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.

Division	Class/Angiosperm group	Order	Family	Plant Type	Clade involved
Lycopodiophyta	Isoetopsida	Isoetales	Isoetaceae	hydrophyte	Isoetes (the sole genus of class Isoetopsida) - I. howellii (seasonally submerged), I. macrospora, I. bolanderi, I. engelmanni, I. lacustris, I. sinensis, I. storkii, I. kirkii
Pteridophyta	Polypodiopsida	Polypodiales	Polypodiaceae	epiphyte, lithophyte	CAM is recorded from Microsorium, Platycerium and Polypodium, Pyrrosia and Drymoglossum and Microgramma
	Pteridopsida	Pteridales	Vittariaceae	epiphyte	VittariaAnetium citrifolium
Cycadophyta	Cycadopsida	Cycadales	Zamiaceae		Dioon edule
Pinophyta	Gnetopsida	Welwitschiales	Welwitschiaceae	xerophyte	Welwitschia mirabilis (the sole species of the order Welwitschiales)
Magnoliophyta	magnoliids	Magnoliales	Piperaceae	epiphyte	Peperomia
	eudicots	Caryophyllales	Plantaginaceae	hydrophyte	Littorella uniflora
			Cactaceae	xerophyte	all cacti have obligate Crassulacean Acid Metabolism in their stems; those few cacti with leaves have C3 Metabolism in those leaves; seedlings have C3 Metabolism.
			Portulacaceae	xerophyte	recorded in approximately half of the genera (note: Portulacaceae is paraphyletic with respect to Cactaceae and Didieraceae)
			Didiereaceae	xerophyte
		Saxifragales	Crassulaceae	hydrophyte, xerophyte, lithophyte	CAM is widespread in the family
	eudicots (rosids)	Vitales	Vitaceae, Cyphostemma
		Malpighiales	Clusiaceae	hemiepiphyte	Clusia	CAM is found is some species of Euphorbia including some formerly placed in the sunk genera Monadenium, and is also reported from Geranium pratense
		Cucurbitales	Cucurbitaceae		Xerosicyos danguyi, Dendrosicyos socotrana, Momordica
		Celastrales	Celastraceae
		Oxalidales	Oxalidaceae
		Brassicales	Moringaceae		Moringa
		Sapindales	Sapindaceae		Dodonaea viscosa
			Zygophyllaceae		Zygophyllum
	eudicots (asterids)	Ericales	Ebenaceae
		Solanales	Convolvulaceae		Ipomaea
		Gentianales	Rubiaceae	epiphyte	Hydnophytum and Myrmecodia
			Apocynaceae		CAM is found in subfamily Asclepidioideae (Hoya, Adenium, Huernia), and also in Carissa and Akocanthera
		Lamiales	Gesneriaceae	epiphyte	CAM was found Codonanthe crassifolia, but not in 3 other genera
			Lamiaceae		Plectranthus marrubioides, Coleus
		Apiales	Apiaceae	hydrophyte	Lilaeopsis lacustris
		Asterales	Asteraceae
Magnoliophyta	monocots	Alismatales	Hydrocharitaceae	hydrophyte	Hydrilla
		Poales	Bromeliaceae	epiphyte	Bromelioideae (91%), Puya (24%), Dyckia and related genera (all), Hechtia (all), Tillandsia (many)
			Cyperaceae	hydrophyte	Scirpus, Eleocharis
		Asparagales	Orchidaceae	epiphyte
			Agavaceae	xerophyte	Agave, Hesperaloe, Yucca
			Asphodelaceae	xerophyte	Aloe, Gasteria and Haworthia
			Ruscaceae		Sansevieria, Dracaena
		Commelinales	Commelinaceae		Callisia, Tradescantia, Tripogandra

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