Crassulacean acid metabolism in the context of "Vacuoles"

A cactus (pl.: cacti, cactuses, or less commonly, cactus) is a member of the plant family Cactaceae (/kækˈteɪsi.iː, -ˌaɪ/), a family of the order Caryophyllales comprising about 127 genera with some 1,750 known species. The word cactus derives, through Latin, from the Ancient Greek word κάκτος (káktos), a name originally used by Theophrastus for a spiny plant whose identity is now not certain. Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north, with the exception of Rhipsalis baccifera, which is also found in Africa and Sri Lanka. Cacti are adapted to live in very dry environments, including the Atacama Desert, one of the driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. In the absence of true leaves, cacti's enlarged stems carry out photosynthesis.

Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled. Many cacti have short growing seasons and long dormancies and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system that quickly absorbs any water reaching the ground surface. Cactus stems are often ribbed or fluted with a number of ribs which corresponds to a number in the Fibonacci numbers (2, 3, 5, 8, 13, 21, 34 etc.). This allows them to expand and contract easily for quick water absorption after rain, followed by retention over long drought periods. Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis. Transpiration, during which carbon dioxide enters the plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night. The plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in photosynthesis. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced.

The Calvin cycle, light-independent reactions, bio synthetic phase, dark reactions, or photosynthetic carbon reduction (PCR) cycle of photosynthesis is a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. The Calvin cycle is present in all photosynthetic eukaryotes and also many photosynthetic bacteria. In plants, these reactions occur in the stroma, the fluid-filled region of a chloroplast outside the thylakoid membranes. These reactions take the products (ATP and NADPH) of light-dependent reactions and perform further chemical processes on them. The Calvin cycle uses the chemical energy of ATP and the reducing power of NADPH from the light-dependent reactions to produce sugars for the plant to use. These substrates are used in a series of reduction-oxidation (redox) reactions to produce sugars in a step-wise process; there is no direct reaction that converts several molecules of CO₂ to a sugar. There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration.

Though it is also called the "dark reaction", the Calvin cycle does not occur in the dark or during nighttime. This is because the process requires NADPH, which is short-lived and comes from light-dependent reactions. In the dark, plants instead release sucrose into the phloem from their starch reserves to provide energy for the plant. The Calvin cycle thus happens when light is available independent of the kind of photosynthesis (C3 carbon fixation, C4 carbon fixation, and crassulacean acid metabolism (CAM)); CAM plants store malic acid in their vacuoles every night and release it by day to make this process work.

C₃ carbon fixation is the most common of three metabolic pathways for carbon fixation in photosynthesis, the other two being C₄ and CAM. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 5-carbon sugar) into two molecules of 3-phosphoglycerate through the following reaction:

This reaction was first discovered by Melvin Calvin, Andrew Benson and James Bassham in 1950. C₃ carbon fixation occurs in all plants as the first step of the Calvin–Benson cycle. (In C₄ and CAM plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.)

The Crassulaceae (/ˈkræsjuːleɪsiːˌiː, -siˌaɪ/, from Latin crassus, thick), also known as the crassulas, the stonecrops or the orpine family, are a diverse family of flowering plants primarily characterized by succulent leaves and a form of photosynthesis known as crassulacean acid metabolism (CAM), in which plants photosynthesize in the daytime and exchange gases during the cooler temperatures of the night. The blossoms of crassulas generally have five floral parts. Crassulaceae are usually herbaceous, though there are some subshrubs, and relatively few trees or aquatic plants.

The Crassulaceae is a medium-sized family in the core eudicots clade, along with the order Saxifragales, whose diversity has made infrafamilial classification very difficult. The family includes approximately 1,400 species and 34–35 genera—depending on the circumscription of the genus Sedum—distributed over three subfamilies. Members of the Crassulaceae are found worldwide, though are mostly concentrated in the Northern Hemisphere and Southern Africa (where the most species are found). They are adapted to thrive in typically dry, arid (hot or cold) areas where water may be scarce, and richer, organic substrates may be at a minimum or nonexistent. The Sempervivum (houseleeks) of Central and Southern Europe, or the Orostachys (dunce-caps) of Asia, for example, grow in rocky crevices at higher elevations, where soil is at a minimum but precipitation, sun exposure, and winds may be intense; these plants have thus adapted to absorb water by having succulent leaves, despite living often on slopes or near-vertical cliff faces, from which water typically drains quickly. Their roots are adhesive and grip to any rock, dirt, wood or other surface they come into contact with, while also gleaning minerals from said material. Seedlings that may sprout near more permanent sources of water, such as pooling rainwater or seeps, may experience rot or discoloration over time, and not survive.