Lake Baikal (/baɪˈkɑːl, -ˈkæl/;[4] Russian:о́зеро Байка́л, tr.Ozero Baykal,IPA:[ˈozʲɪrə bɐjˈkaɫ]; Buryat: Байгал нуур, Baigal nuur; Mongolian: Байгал нуур, Baigal nuur, etymologically meaning, in Mongolian, "the Nature Lake")[5] is a rift lake in Russia, located in southern Siberia, between Irkutsk Oblast to the northwest and the Buryat Republic to the southeast.
Lake Baikal is the largest freshwater lake by volume in the world, containing 22–23% of the world's fresh surface water.[3][6][7] With 23,615.39km3 (5,670cumi) of fresh water,[1] it contains more water than the North American Great Lakes combined.[8] With a maximum depth of 1,642m (5,387ft),[1] Baikal is the world's deepest lake.[9] It is considered among the world's clearest[10] lakes and is considered the world's oldest lake[11] – at 25–30 million years.[12][13] It is the seventh-largest lake in the world by surface area.
Like Lake Tanganyika, Lake Baikal was formed as an ancient rift valley, having the typical long, crescent shape with a surface area of 31,722km2 (12,248sqmi). Baikal is home to thousands of species of plants and animals, many of which exist nowhere else in the world. The lake was declared a UNESCO World Heritage Site in 1996.[14] It is also home to Buryat tribes who reside on the eastern side of Lake Baikal,[15][16] raising goats, camels, cattle, sheep, and horses,[16] where the mean temperature varies from a winter minimum of −19°C (−2°F) to a summer maximum of 14°C (57°F).[17]
The region to the east of Lake Baikal is referred to as Transbaikalia, and the loosely defined region around the lake is sometimes known as simply Baikalia.
Geography and hydrography

Lake Baikal is in a rift valley, created by the Baikal Rift Zone, where the Earth's crust is slowly pulling apart.[18] At 636km (395mi) long and 79km (49mi) wide, Lake Baikal has the largest surface area of any freshwater lake in Asia, at 31,722km2 (12,248sqmi), and is the deepest lake in the world at 1,642m (5,387ft). The bottom of the lake is 1,186.5m (3,893ft) below sea level, but below this lies some 7km (4.3mi) of sediment, placing the rift floor some 8–11km (5.0–6.8mi) below the surface, the deepest continental rift on Earth.[18] In geological terms, the rift is young and active – it widens about 2cm (0.8in) per year. The fault zone is also seismically active; hot springs occur in the area and notable earthquakes happen every few years. The lake is divided into three basins: North, Central, and South, with depths about 900m (3,000ft), 1,600m (5,200ft), and 1,400m (4,600ft), respectively. Fault-controlled accommodation zones rising to depths about 300m (980ft) separate the basins. The North and Central basins are separated by Academician Ridge, while the area around the Selenga Delta and the Buguldeika Saddle separates the Central and South basins. The lake drains into the Angara tributary of the Yenisei. Notable landforms include Cape Ryty on Baikal's northwest coast.
Baikal's age is estimated at 25–30 million years, making it the most ancient lake in geological history.[12][13] It is unique among large, high-latitude lakes, as its sediments have not been scoured by overriding continental ice sheets. Russian, U.S., and Japanese cooperative studies of deep-drilling core sediments in the 1990s provide a detailed record of climatic variation over the past 6.7 million years.[19][20] Longer and deeper sediment cores are expected in the near future. Lake Baikal is the only confined freshwater lake in which direct and indirect evidence of gas hydrates exists.[21][22][23]
The lake is completely surrounded by mountains. The Baikal Mountains on the north shore, the Barguzin Range on the northeastern shore, and the taiga are technically protected as a national park. It contains 27 islands; the largest, Olkhon, is 72km (45mi) long and is the third-largest lake-bound island in the world. The lake is fed by as many as 330 inflowing rivers.[24] The main ones draining directly into Baikal are the Selenga River, the Barguzin River, the Upper Angara River, the Turka River, the Sarma River, and the Snezhnaya River. It is drained through a single outlet, the Angara River.

Cliffs on Olkhon Island
A sandy beach in the Kabansky District
Mountains on the Svyatoy Nos Peninsula, Zabaykalsky National Park
Turka River at its mouth before joining Lake Baikal

Water characteristics

Baikal is one of the clearest lakes in the world.[10] During the winter in open sections the water transparency can be as much as 30–40m (100–130ft), but during the summer it is typically 5–8m (15–25ft).[25] Baikal is rich in oxygen, even in deep sections,[25] which separates it from distinctly stratified bodies of water such as Lake Tanganyika and the Black Sea.[26][27]
In Lake Baikal, the water temperature varies significantly depending on location, depth, and time of the year. During the winter and spring, the surface freezes for 4–5 months; from early January to May–June (latest in the north), the entire lake surface is covered in ice.[28] On average, the ice reaches a thickness of 0.5to 1.4m (1.6–4.6ft),[29] but in some places with hummocks, it can be more than two metres (6.6ft).[28] During this period, the temperature slowly increases with depth in the lake, being coldest near the ice-covered surface at around freezing, and reaching about 3.5–3.8°C (38.3–38.8°F) at a depth of 200–250m (660–820ft).[30] After the surface ice breaks up, the surface water is slowly warmed up by the sun, and in May–June, the upper 300m (980ft) or so becomes homothermic (same temperature throughout) at around 4°C (39°F) because of water mixing.[25][30] The sun continues to heat up the surface layer, and at the peak in August can reach up to about 16°C (61°F) in the main sections[30] and 20–24°C (68–75°F) in shallow bays in the southern half of the lake.[25][31] During this time, the pattern is inverted compared to the winter and spring, as the water temperature falls with increasing depth. As the autumn begins, the surface temperature falls again and a second homothermic period at around 4°C (39°F) of the upper circa 300m (980ft) occurs in October–November.[25][30] In the deepest parts of the lake, from about 300m (980ft), the temperature is very stable at 3.1–3.4°C (37.6–38.1°F) with only minor annual variations.[30]
The average surface temperature has risen by almost 1.5 °C (2.7 °F) in the last 50 years, resulting in a shorter period where the lake is covered by ice.[13] At some locations, hydrothermal vents with water that is about 50°C (122°F) have been found. These are mostly in deep water, but locally have also been found in relatively shallow water. They have very little effect on the lake's temperature because of its huge volume.[30]
Stormy weather on the lake is common, especially during the summer and fall, and can result in waves as high as 4.5m (15ft).[25]

Lake Baikal as seen from the OrbView-2 satellite
Spring ice melt underway on Lake Baikal, on 4 May: Notice the ice-covered north, while much of the south already is ice-free.
Circle of thin ice, diameter of 4.4km (2.7mi) at the lake's southern tip, probably caused by convection
Delta of the Selenga River, Baikal's main tributary

Fauna and flora

Lake Baikal is rich in biodiversity. It hosts more than 1,000 species of plants and 2,500 species of animals based on current knowledge, but the actual figures for both groups are believed to be significantly higher.[25][32] More than 80% of the animals are endemic.[32]
Flora
The watershed of Lake Baikal has numerous floral species represented. The marsh thistle (Cirsium palustre) is found here at the eastern limit of its geographic range.[33]
Submerged macrophytic vascular plants are mostly absent, except in some shallow bays along the shores of Lake Baikal.[34] More than 85 species of submerged macrophytes have been recorded, including genera such as Ceratophyllum, Myriophyllum, Potamogeton, and Sparganium.[31] The invasive species Elodea canadensis was introduced to the lake in the 1950s.[34] Instead of vascular plants, aquatic flora is often dominated by several green algae species, notably Draparnaldioides, Tetraspora, and Ulothrix in water shallower than 20m (65ft); although Aegagrophila, Cladophora, and Draparnaldioides may occur deeper than 30m (100ft).[34] Except for Ulothrix, there are endemic Baikal species in all these green algae genera.[34] More than 400 diatom species, both benthic and planktonic, are found in the lake, and about half of these are endemic to Baikal; however, significant taxonomic uncertainties remain for this group.[34]
Mammals
The Baikal seal or nerpa (Pusa sibirica) is found throughout Lake Baikal.[35] It is one of only three entirely freshwater seal populations in the world, the other two being subspecies of ringed seals.
A wide range of land mammals can be found in the habitats around the lake, such as Eurasian brown bear, Eurasian wolf, red fox, sable, stoat, elk, Siberian red deer, reindeer, Siberian roe deer, Siberian musk deer, wild boar, red squirrel, Siberian chipmunk, marmot, lemming, and Alpine hare.[36] Until the Early Middle Ages, the wisent (European bison) was present near the lake, which was the easternmost part of its range.[37]
Birds

There are 236 species of birds that inhabit Lake Baikal, 29 of which are waterfowl.[38]
Fish

Fewer than 65 native fish species occur in the lake basin, but more than half of these are endemic.[25][41] The families Abyssocottidae (deep-water sculpins), Comephoridae (golomyankas or Baikal oilfish), and Cottocomephoridae (Baikal sculpins) are entirely restricted to the lake basin.[25][42] All these are part of the Cottoidea and are typically less than 20cm (8in) long.[34] Of particular note are the two species of golomyanka (Comephorus baicalensis and C. dybowskii). These long-finned, translucent fish typically live in open water at depths of 100–500m (330–1,640ft), but occur both shallower and much deeper. Together with certain abyssocottid sculpins, they are the deepest living freshwater fish in the world, occurring to near the bottom of Lake Baikal.[43] The golomyankas are the primary prey of the Baikal seal and represent the largest fish biomass in the lake.[44] Beyond members of Cottoidea, there are few endemic fish species in the lake basin.[25][41]

The most important local species for fisheries is the omul (Coregonus migratorius), an endemic whitefish.[25] It is caught, smoked, and then sold widely in markets around the lake. Also, a second endemic whitefish inhabits the lake, C. baicalensis.[45] The Baikal black grayling (Thymallus baicalensis), Baikal white grayling (T. brevipinnis), and Baikal sturgeon (Acipenser baerii baicalensis) are other important species with commercial value. They are also endemic to the Lake Baikal basin.[39][40][46][47]
Invertebrates
The lake hosts a rich endemic fauna of invertebrates. The copepod Epischura baikalensis is endemic to Lake Baikal and the dominating zooplankton species there, making up 80 to 90% of total biomass.[48] It is estimated that the epischurans filter as much as a thousand cubic kilometers of water a year, or the lake’s entire volume every twenty-three years.[49]
Among the most diverse invertebrate groups are the amphipod and ostracod crustaceans, freshwater snails, annelid worms and turbellarian worms:
Amphipod and ostracod crustaceans

More than 350 species and subspecies of amphipods are endemic to the lake.[32] They are exceptionally diverse in ecology and appearance, ranging from the pelagic Macrohectopus to the relatively large deep-water Abyssogammarus and Garjajewia, the tiny herbivorous Micruropus, and the parasitic Pachyschesis (parasitic on other amphipods).[50] The "gigantism" of some Baikal amphipods, which has been compared to that seen in Antarctic amphipods, has been linked to the high level of dissolved oxygen in the lake.[51] Among the "giants" are several species of spiny Acanthogammarus and Brachyuropus (Acanthogammaridae) found at both shallow and deep depths.[52] These conspicuous and common amphipods are essentially carnivores (will also take detritus), and can reach a body length up to 7cm (2.8in).[50][52]
Similar to another ancient lake, Tanganyika, Baikal is a center for ostracod diversity. About 90% of the Lake Baikal ostracods are endemic,[53] meaning that there are c. 200 endemic species.[54] This makes it the second-most diverse group of crustacean in the lake, after the amphipods.[53] The vast majority of the Baikal ostracods belong in the families Candonidae (more than 100 described species) and Cytherideidae (about 50 described species),[53][55] but genetic studies indicate that the true diversity in at least the latter family has been heavily underestimated.[56] The morphology of the Baikal ostracods is highly diverse.[53]
Snails and bivalves
As of 2006, almost 150 freshwater snails are known from Lake Baikal, including 117 endemic species from the subfamilies Baicaliinae (part of the Amnicolidae) and Benedictiinae (part of the Lithoglyphidae), and the families Planorbidae and Valvatidae.[57] All endemics have been recorded between 20 and 30m (66 and 98ft), but the majority mainly live at shallower depths.[57] About 30 freshwater snail species can be seen deeper than 100m (330ft), which represents the approximate limit of the sunlight zone, but only 10 are truly deepwater species.[57] In general, Baikal snails are thin-shelled and small. Two of the most common species are Benedictia baicalensis and Megalovalvata baicalensis.[58] Bivalve diversity is lower with more than 30 species; about half of these, all in the families Euglesidae, Pisidiidae, and Sphaeriidae, are endemic (the only other family in the lake is the Unionidae with a single nonendemic species).[58][59] The endemic bivalves are mainly found in shallows, with few species from deep water.[60]
Aquatic worms
With almost 200 described species, including more than 160 endemics, the center of diversity for aquatic freshwater oligochaetes is Lake Baikal.[61] A smaller number of other freshwater annelids is known: 30 species of leeches (Hirudinea),[62] and 4 polychaetes.[61] Several hundred species of nematodes are known from the lake, but a large percentage of these are undescribed.[61]
More than 140 endemic flatworm (Plathelminthes) species are in Lake Baikal, where they occur on a wide range of bottom types.[63] Most of the flatworms are predatory, and some are relatively brightly marked. They are often very abundant in shallow waters, where they are typically less than 2cm (1in) long, but in deeper parts of the lake, the largest, Baikaloplana valida, can reach up to 30cm (1ft) when outstretched.[34][63]
Sponges

At least 18 species of sponges occur in the lake,[64] including 14 species from the endemic family Lubomirskiidae (the remaining are from the nonendemic family Spongillidae).[65] In the nearshore regions of Baikal, the largest benthic biomass is sponges.[64] Lubomirskia baicalensis, Baikalospongia bacillifera, and B. intermedia are unusually large for freshwater sponges and can reach 1m (3.3ft) or more.[64][66] These three are also the most common sponges in the lake.[64] While the Baikalospongia species typically have encrusting or carpet-like structures, L. baikalensis often has branching structures and in areas where common may form underwater "forests".[67] Most sponges in the lake are typically green when alive because of symbiotic chlorophytes (zoochlorella), but can also be brownish or yellowish.[68]
History
The Baikal area, sometimes known as Baikalia, has a long history of human habitation. An early known tribe in the area was the Kurykans.
Located in the former northern territory of the Xiongnu confederation, Lake Baikal is one site of the Han–Xiongnu War, where the armies of the Han dynasty pursued and defeated the Xiongnu forces from the second century BC to the first century AD. They recorded that the lake was a "huge sea" (hanhai) and designated it the North Sea (Běihǎi) of the semimythical Four Seas.[69] The Kurykans, a Siberian tribe who inhabited the area in the sixth century, gave it a name that translates to "much water". Later on, it was called "natural lake" (Baygal nuur) by the Buryats and "rich lake" (Bay göl) by the Yakuts.[70] Little was known to Europeans about the lake until Russia expanded into the area in the 17th century. The first Russian explorer to reach Lake Baikal was Kurbat Ivanov in 1643.[71]
Russian expansion into the Buryat area around Lake Baikal[72] in 1628–58 was part of the Russian conquest of Siberia. It was done first by following the Angara River upstream from Yeniseysk (founded 1619) and later by moving south from the Lena River. Russians first heard of the Buryats in 1609 at Tomsk. According to folktales related a century after the fact, in 1623, Demid Pyanda, who may have been the first Russian to reach the Lena, crossed from the upper Lena to the Angara and arrived at Yeniseysk.[73]
Vikhor Savin (1624) and Maksim Perfilyev (1626 and 1627–28) explored Tungus country on the lower Angara. To the west, Krasnoyarsk on the upper Yenisei was founded in 1627. A number of ill-documented expeditions explored eastward from Krasnoyarsk. In 1628, Pyotr Beketov first encountered a group of Buryats and collected yasak (tribute) from them at the future site of Bratsk. In 1629, Yakov Khripunov set off from Tomsk to find a rumored silver mine. His men soon began plundering both Russians and natives. They were joined by another band of rioters from Krasnoyarsk, but left the Buryat country when they ran short of food. This made it difficult for other Russians to enter the area. In 1631, Maksim Perfilyev built an ostrog at Bratsk. The pacification was moderately successful, but in 1634, Bratsk was destroyed and its garrison killed. In 1635, Bratsk was restored by a punitive expedition under Radukovskii. In 1638, it was besieged unsuccessfully.
In 1638, Perfilyev crossed from the Angara over the Ilim portage to the Lena River and went downstream as far as Olyokminsk. Returning, he sailed up the Vitim River into the area east of Lake Baikal (1640) where he heard reports of the Amur country. In 1641, Verkholensk was founded on the upper Lena. In 1643, Kurbat Ivanov went further up the Lena and became the first Russian to see Lake Baikal and Olkhon Island. Half his party under Skorokhodov remained on the lake, reached the Upper Angara at its northern tip, and wintered on the Barguzin River on the northeast side.
In 1644, Ivan Pokhabov went up the Angara to Baikal, becoming perhaps the first Russian to use this route, which is difficult because of the rapids. He crossed the lake and explored the lower Selenge River. About 1647, he repeated the trip, obtained guides, and visited a 'Tsetsen Khan' near Ulan Bator. In 1648, Ivan Galkin built an ostrog on the Barguzin River which became a center for eastward expansion. In 1652, Vasily Kolesnikov reported from Barguzin that one could reach the Amur country by following the Selenga, Uda, and Khilok Rivers to the future sites of Chita and Nerchinsk. In 1653, Pyotr Beketov took Kolesnikov's route to Lake Irgen west of Chita, and that winter his man Urasov founded Nerchinsk. Next spring, he tried to occupy Nerchensk, but was forced by his men to join Stephanov on the Amur. Nerchinsk was destroyed by the local Tungus, but restored in 1658.
The Trans-Siberian Railway was built between 1896 and 1902. Construction of the scenic railway around the southwestern end of Lake Baikal required 200 bridges and 33 tunnels. Until its completion, a train ferry transported railcars across the lake from Port Baikal to Mysovaya for a number of years. The lake became the site of the minor engagement between the Czechoslovak legion and the Red Army in 1918. At times during winter freezes, the lake could be crossed on foot, though at risk of frostbite and deadly hypothermia from the cold wind moving unobstructed across flat expanses of ice. In the winter of 1920, the Great Siberian Ice March occurred, when the retreating White Russian Army crossed frozen Lake Baikal. The wind on the exposed lake was so cold, many people died, freezing in place until spring thaw. Beginning in 1956, the impounding of the Irkutsk Dam on the Angara River raised the level of the lake by 1.4m (4.6ft).[74]
As the railway was built, a large hydrogeographical expedition headed by F.K. Drizhenko produced the first detailed contour map of the lake bed.[11]

Buryat shaman on Olkhon Island
Russian map circa 1700, Baikal (not to scale) is at top
Steam locomotive on the Circum-Baikal Railroad
Angara was launched in 1900 and is one of the oldest surviving icebreakers

Research

Several organizations are carrying out natural research projects on Lake Baikal. Most of them are governmental or associated with governmental organizations. The Baikalian Research Centre is an independent research organization carrying out environmental educational and research projects at Lake Baikal.[75]
In July 2008, Russia sent two small submersibles, Mir-1 and Mir-2, to descend 1,592m (5,223ft) to the bottom of Lake Baikal to conduct geological and biological tests on its unique ecosystem. Although originally reported as being successful, they did not set a world record for the deepest freshwater dive, reaching a depth of only 1,580m (5,180ft).[76] That record is currently held by Anatoly Sagalevich, at 1,637m (5,371ft) (also in Lake Baikal aboard a Pisces submersible in 1990).[76][77] Russian scientist and federal politician Artur Chilingarov, the leader of the mission, took part in the Mir dives[78] as did Russian leader Vladimir Putin.[79]
Since 1993, neutrino research has been conducted at the Baikal Deep Underwater Neutrino Telescope (BDUNT). The Baikal Neutrino Telescope NT-200 is being deployed in Lake Baikal, 3.6km (2.2mi) from shore at a depth of 1.1km (0.68mi). It consists of 192 optical modules.[80]
Economy

The lake, nicknamed "the Pearl of Siberia", drew investors from the tourist industry as energy revenues sparked an economic boom.[82] Viktor Grigorov's Grand Baikal in Irkutsk is one of the investors, who planned to build three hotels, creating 570 jobs. In 2007, the Russian government declared the Baikal region a special economic zone. A popular resort in Listvyanka is home to the seven-story Hotel Mayak. At the northern part of the lake, Baikalplan (a German NGO) built together with Russians in 2009 the Frolikha Adventure Coastline Track, a 100km (62mi)-long long-distance trail as example for a sustainable development of the region. Baikal was also declared a UNESCO World Heritage site in 1996. Rosatom plans to build a laboratory near Baikal, in conjunction with an international uranium plant and to invest $2.5 billion in the region and create 2,000 jobs in the city of Angarsk.[82]
Lake Baikal is a popular destination among tourists from all over the world. According to the Russian Federal State Statistics Service, in 2013, 79,179 foreign tourists visited Irkutsk and Lake Baikal; in 2014, 146,937 visitors. The most popular places to stay by the lake are Listvyanka village, Olkhon Island, Kotelnikovsky cape, Baykalskiy Priboi, resort Khakusy and Turka village. The popularity of Lake Baikal is growing from year to year, but there is no developed infrastructure in the area. For the quality of service and comfort from the visitor's point of view, Lake Baikal still has a long way to go.
The ice road to Olkhon Island is the only legal ice road on Lake Baikal. The route is prepared by specialists every year and it opens when the ice conditions allow it. In 2015, the ice road to Olkhon was open from February 17 to March 23. The thickness of the ice on the road is about 60cm (24in), maximum capacity allowed – 10t (9.8 long tons; 11 short tons); it is open to the public from 9 am to 6 pm. The road through the lake is 12km (7.5mi) long and it goes from the village Kurkut on the mainland, to Irkutskaya Guba on Olkhon Island.[83]
Environmental concerns
Baykalsk Pulp and Paper Mill

The Baykalsk Pulp and Paper Mill was constructed in 1966, directly on the shoreline of Lake Baikal. The plant bleached paper using chlorine and discharged waste directly into Lake Baikal. The decision to construct the plant on the Lake Baikal resulted in strong protests from Soviet scientists; according to them, the ultra-pure water of the lake was a significant resource and should have been used for innovative chemical production (for instance, the production of high-quality viscose for the aeronautics and space industries). The Soviet scientists felt that it was irrational to change Lake Baikal's water quality by beginning paper production on the shore. It was their position that it was also necessary to preserve endemic species of local biota, and to maintain the area around Lake Baikal as a recreation zone [84]. However, the objections of the Soviet scientists faced opposition from the industrial lobby and only after decades of protest, the plant was closed in November 2008 due to unprofitability.[85][86] In March 2009, the plant owner announced the paper mill would never reopen.[87] However, on January 4, 2010, production was resumed. Later that year on January 13, 2010, Russian Federation President Vladimir Putin introduced changes in the legislation legalising the operation of the plant; this action brought about a wave of protests from ecologists and local residents.[88] These changes were based on the determination President Putin made through a visual verification of Lake Baikal's condition from a miniature submarine, "I could see with my own eyes – and scientists can confirm – Baikal is in good condition and there is practically no pollution".[89] Despite this, in September 2013, the mill underwent a final bankruptcy, with the last 800 workers slated to lose their jobs by December 28, 2013.[90] On the day the plant was to close, December 28, 2013, the Russian government announced plans to build the Russian Nature Reserve's Expo Center in place of the closed paper mill.[91]
Planned East Siberia-Pacific Ocean oil pipeline

Russian oil pipelines state company Transneft[92] was planning to build a trunk pipeline that would have come within 800m (2,600ft) of the lake shore in a zone of substantial seismic activity. Environmental activists in Russia,[93] Greenpeace, Baikal pipeline opposition[94] and local citizens[95] were strongly opposed to these plans, due to the possibility of an accidental oil spill that might cause significant damage to the environment. According to the Transneft's president, numerous meetings with citizens near the lake were held in towns along the route, especially in Irkutsk.[96] Transneft agreed to alter its plans when Russian president Vladimir Putin ordered the company to consider an alternative route 40 kilometers (25mi) to the north to avoid such ecological risks.[97] Transneft has since decided to move the pipeline away from Lake Baikal, so that it will not pass through any federal or republic natural reserves.[98][99] Work began on the pipeline two days after President Putin agreed to changing the route away from Lake Baikal.[100]
Proposed uranium enrichment centre
In 2006, the Russian government announced plans to build the world's first international uranium enrichment centre at an existing nuclear facility in Angarsk, a city on the river Angara some 95km (59mi) downstream from the lake's shores. Critics and environmentalists argued it would be a disaster for the region and are urging the government to reconsider.[101]
After enrichment, only 10% of the uranium-derived radioactive material would be exported to international customers,[101] leaving 90% near the Lake Baikal region for storage. Uranium tailings contain radioactive and toxic materials, which if improperly stored, are potentially dangerous to humans and can contaminate rivers and lakes.[101]
Nonetheless, the enrichment centre was built in the end.[102]
Other pollution sources
According to The Moscow Times and Vice, an increasing number of an invasive species of algae thrives in the lake from hundreds of tons of liquid waste, including fuel and excrement, regularly disposed into the lake by tourist sites, and up to 25,000 tons of liquid waste are disposed of every year by local ships.[103][104]
Historical traditions

The first European to reach the lake is said to have been Kurbat Ivanov in 1643.[105]
In the past, the Baikal was referred to by many Russians as the "Baikal Sea" (Russian: Море Байкал, More Baikal), rather than merely "Lake Baikal" (Russian: Озеро Байкал, Ozero Baikal).[106]
This usage is attested already in the Life of Protopope Avvakum (1621–1682),[107] and on the late-17th-century maps by Semyon Remezov.[108] It is also attested in the famous song, now passed into the tradition, that opens with the words Славное море, священный Байкал (Glorious sea, [the] sacred Bajkal).
To this day, the strait between the western shore of the Lake and the Olkhon Island is called Maloye More (Малое Море), i.e. "the Little Sea".
Lake Baikal is nicknamed "Older sister of Sister Lakes (Lake Khövsgöl and Lake Baikal)".[109]

According to 19th-century traveler T. W. Atkinson, locals in the Lake Baikal Region had the tradition that Christ visited the area:
The people have a tradition in connection with this region which they implicitly believe. They say "that Christ visited this part of Asia and ascended this summit, whence he looked down on all the region around. After blessing the country to the northward, he turned towards the south, and looking across the Baikal, he waved his hand, exclaiming 'Beyond this there is nothing.'" Thus they account for the sterility of Daouria, where it is said "no corn will grow."[110]

Lake Baikal has been celebrated in several Russian folk songs. Two of these songs are well known in Russia and its neighboring countries, such as Japan.
"Glorious Sea, Sacred Baikal" (in Russian: Славное Mope, Священный Байкал) is about a katorga fugitive. The lyrics as documented and edited in the 19th century by Dmitriy P. Davydov (1811–1888).[111] See "Barguzin River" for sample lyrics.
"The Wanderer" (in Russian: Бродяга) is about a convict who had escaped from jail and was attempting to return home from Transbaikal.[112] The lyrics were collected and edited in the 20th century by Ivan Kondratyev.
The latter song was a secondary theme song for the Soviet Union's second color film, Ballad of Siberia (1947; in Russian: Сказание о земле Сибирской).

See also
Russian Far East
Sarma (wind)

Literature
Detlev Henschel: Kayak Adventure in Siberia: The first solo circumnavigation of Lake Baikal. Amazon ISBN978-3-7375-6102-0.
Colin Thubron In Siberia,
Leonid Borodin: Year Of Miracle And Grief, Quartet Books, 1988.

 at the Encyclopædia Britannica

What is the deepest point in Lake Baikal?


Park Chung-hee, president of South Korea, was assassinated on Friday, October 26, 1979 at 7:41 p.m. during a dinner at a Korean Central Intelligence Agency (KCIA) safehouse inside the Blue House presidential compound, in Gungjeong-dong, Seoul. The assassination was carried out by Kim Jae-gyu, the then director of KCIA and the president's security chief. Park was shot in the chest and head, and died almost immediately. Four bodyguards and a presidential chauffeur were also killed. The incident is often referred to as "10.26" or the "10.26 incident" in South Korea.[1]
There is still a great deal of controversy surrounding Kim's motives, as it remains uncertain whether the act was part of a planned coup d'état or was merely impulsive.  The chief investigator Yi Hak-bong famously concluded that the assassination was "too careless for a  deliberate act and yet too elaborate for an impulsive act." [2]
Background
President Park's dictatorship

By the time of his assassination, President Park had exercised dictatorial power over South Korea for nearly 18 years.[3]
The Korean Central Intelligence Agency was created in 1961 to coordinate both international and domestic intelligence activities, including those of the military. [4] Almost immediately following its creation, the KCIA was used to suppress any domestic opposition to Park's regime using its broad powers to wiretap, arrest, and torture anyone without a court order.  KCIA was heavily involved in many behind-the-scene political manoeuvrings aimed at weakening the opposition parties through bribing, blackmailing, threatening, or arresting opposition lawmakers.  President Park nevertheless nearly lost the presidential election to Kim Dae-jung in 1971 despite spending ten percent of the national budget on his election campaign.  Park therefore established the Yushin Constitution in 1972 to ensure his perpetual dictatorship.[5] This abolished the direct vote in presidential elections and replaced it with an indirect voting system involving delegates, allotted one third of the National Assembly seats to the president, gave the president the authority to issue emergency decrees and suspend the Constitution, gave the president the authority to appoint all judges and dismiss the National Assembly, and repealed a term limit to presidency. When opposition to the Yushin Constitution arose, Park issued a number of emergency decrees, the first of which made any act of opposition or denial of the Yushin Constitution punishable by imprisonment for up to 15 years through a military tribunal.
The last year of his rule was particularly turbulent with increasing opposition from the New Democratic Party (NDP), which was emboldened after winning the 1978 election by 1.1% despite Park's complete control of the media, money, and all the institutions of government.  Because of the Yushin Constitution, which allowed President Park to appoint one third of National Assembly seats, Park's Democratic Republican Party (DRP) remained in power; nevertheless, it was an embarrassing situation for Park. In May 1979, Kim Young Sam was elected as the chairman of New Democratic Party (NDP) despite intense behind-the-scene manoeuvrings by KCIA to back a more pliable candidate, Yi Chul-seung.  Kim took the hardline policy of never compromising or cooperating with Park until the repeal of the Yushin Constitution.  In August 1979, 2,000 policemen stormed the NDP headquarters, which was used by female workers at a wig company for their sit-in demonstration.  In the process, one female worker died and many lawmakers trying to protect them were severely beaten, some requiring hospitalization.  After this incident, which garnered widespread criticism of the government, Park was determined to remove Kim from the political scene in the same manner the imprisoned Kim Dae-joong was dispatched. The KCIA was duly instructed to engineer such a move.
In September 1979, the courts obliged by ordering the nullification of Kim's chairmanship of the NDP, and Park's Democratic Republican Party (DRP) expelled him from the National Assembly in a secret session on October 5, which led all 66 NDP lawmakers to submit their resignation to the National Assembly in protest. The Carter administration in the U.S. recalled its ambassador to Seoul in protest as well.  When it became known that the government was planning to accept the resignations selectively, uprisings broke out in Kim's hometown of Busan (the second largest city in South Korea) on October 16, 1979, resulting in arson attacks on 30 police stations over several days. It was the largest demonstration since the days of President Syngman Rhee and spread to nearby Masan on October 19, 1979 and other cities, with students and citizens calling for repeal of the Yushin Constitution. The KCIA Director, Kim, went to Busan to investigate the situation and found that the demonstrations were not riots by some college students, but more like a "popular uprising joined by regular citizens" to resist the regime. He warned President Park that the uprisings would spread to five other large cities, including Seoul.  Park said that he himself would give direct orders to the security forces to fire upon demonstrators if the situation got worse.  Less than a week later, he was assassinated by his own security chief.
Rivalry between Kim and Cha
While President Park faced an increasing opposition to his dictatorship outside Blue House, another kind of conflict was intensifying inside Blue House between Kim Jae-kyu, who was appointed to directorship of KCIA in December 1976 and Chief Bodyguard Cha Ji-chul, who was appointed to his position in 1974 after Park's wife Yuk Young-soo was killed in an assassination by Moon Se-gwang, an ethnic Korean from Japan.[6]
The rivalry stemmed largely from Cha's increasing encroachment into KCIA turf and arrogant behavior that belittled Kim in public.  Almost universally disliked yet feared, Cha served Park in close proximity and became his favorite and most trusted advisor in the process.  Cha appropriated tanks, helicopters, and troops from the Army so that the presidential security apparatus had a division-level firepower under Cha's direct command.
The rivalry between Cha and Kim, whose KCIA was until then the most feared government apparatus, was heightened further with a series of political crises in late 1979 as they clashed over the approach in dealing with growing opposition to the regime.  In the NDP's election for its chairman in 1979, KCIA backed Yi Chul-seung to prevent the election of hardliner Kim Young Sam, but Cha Ji-chul interfered in KCIA's political sabotage with its own behind-scene manoeuvrings.  When Kim Young Sam was elected as the NDP chairman, Cha laid the blame on KCIA, which infuriated Director Kim.
Later when NDP chief Kim Young Sam called on the U.S. to stop supporting Park's regime in an interview with New York Times reporter Henry Stokes, Cha pushed for Kim's expulsion from the National Assembly,[7] which Director Kim feared to be a disastrous development [8] (as it turned out to be true when it led to uprisings in Busan and Masan).  Cha easily bested his opponent as his hardline approach was favored by Park, and he blamed worsening development on Director Kim's weak leadership of KCIA at every opportunity.  As Cha came to control the scheduling of President Park's meetings and briefings and thus access to the president, KCIA briefings, which were usually the first business in the morning, were pushed down to afternoons.  By October, there were wide rumors that Kim would be soon replaced as KCIA director.
Event
On the day of the assassination, Park and his entourage visited ribbon-cutting ceremonies for a dam in Sap-gyeo-cheon and a KBS TV transmitting station in Dang-jin. KCIA Director Kim was expected to accompany him since the TV station was under KCIA jurisdiction, but after Chief Bodyguard Cha blocked him from riding in the same helicopter as President Park, Director Kim angrily excused himself from the trip.
After the trip, President Park instructed KCIA to prepare for one of his numerous banquets – on the average of ten per month according to KCIA Chief Agent Park Seon-ho, one of the conspirators – at a KCIA safehouse inside the Blue House presidential compound, Seoul, South Korea.
It was to be attended by President Park, KCIA Director Kim, Chief Bodyguard Cha, Chief Secretary Kim Gye-won, and two young women – rising singer Shim Soo-bong and a college student named Shin Jae-soon. After Director Kim was notified of the banquet, he called South Korean Army Chief of Staff Jeong Seung-hwa 15 minutes later to invite him to the KCIA safehouse and arranged to have him dine with KCIA Deputy Director Kim Jeong-seop in a nearby KCIA building in the same compound.[9]
Just before the dinner, Director Kim told Chief Secretary Kim Gye-won that he would get rid of Chief Bodyguard Cha.  It is not clear whether Kim Gye-won misheard or misunderstood Director Kim or he ignored Kim's words.
During the dinner, volatile political issues including demonstrations in Busan and the opposition leader Kim Young Sam were discussed with President Park and Chief Bodyguard Cha taking a hardline and Director Kim calling for moderate measures while Chief Secretary Kim was trying to steer the topic of discussion to small talk.
President Park rebuked Director Kim for not being repressive enough in dealing with protesters and Kim Young Sam, whom Park wanted to be arrested. Each time discussion drifted to other subjects, Cha continued to bring up the inability of KCIA to end the crisis and suggested that demonstrators and opposition lawmakers should be "mowed down with tanks."
The rebukes from Park and especially Cha riled up Kim, who left the dining room to convene with his closest subordinate, former Marine colonel and KCIA Chief Agent Park Seon-ho and Army colonel and Director Kim's secretary, Park Heung-ju (no relations), and said to them: "Chief of Staff and Deputy Director are here as well.  Today is the day."  Asked if President Park is included as a target, Kim said yes.[10]
Kim reentered the meeting room with a semi-automatic Walther PPK pistol and opened fire, shooting Chief Bodyguard Cha in the arm and President Park in the left chest before the PPK jammed as he attempted to kill Cha, who fled to a bathroom adjacent to the dining room. Kim came back with his subordinate Park Sun-ho's revolver Smith & Wesson Model 36 Chief Special and killed Cha with a shot to the abdomen before speaking to Park and shooting him in the head execution-style.
Upon hearing the initial shots, Park Seon-ho held two bodyguards in the waiting room at gunpoint and ordered them to put their hands up in hope of preventing further bloodshed, especially since he was a friend of one of the bodyguards. When the other bodyguard attempted to reach for a gun, Park shot them dead as Colonel Park Heung-ju and two other KCIA agents stormed the kitchen area and killed the remaining bodyguard.
In all, six people were killed: President Park, Chief Bodyguard Cha and three presidential bodyguards in the safe house, and a presidential chauffeur outside.[11]
Aftermath
After killing President Park, KCIA Director Kim asked Chief Secretary Kim to secure the safehouse and ran to the nearby KCIA building where Army Chief of Staff Jeong Seung-hwa was waiting; Jeong heard the shootings and was discussing them with KCIA Deputy Director Kim Jeong-seop when Director Kim came in breathless to tell them that an emergency situation occurred.
In the car, Kim notified Jeong that President Park had died, but without explaining how.  Kim hoped that Jeong and Chief Secretary Kim would support him in the coup as both were appointed to their position on his recommendation, and Chief Secretary Kim was especially close with him. The car initially headed to KCIA Headquarters in Namsan district, but eventually went to Army Headquarters in Yongsan district,  since the Army would have to be involved in declaring emergency martial law.
Many historians believe that Kim made a critical mistake in not going to KCIA HQ where he would be in control; however, his failure to gain Jeong's support sealed the fate of the conspirators.
Meanwhile, Chief Secretary Kim took President Park's body to the Army hospital and ordered doctors to save him at all costs (without revealing Park's identity), and went to Prime Minister Choi Kyu-ha to reveal what happened that night.
When Chief of Staff Jeong learned of what happened from Chief Secretary Kim, he ordered Major General Chun Doo-hwan, commander of Security Command who later became the president of South Korea through a military coup, to take Director Kim into custody and investigate the incident.
Director Kim was arrested after he was lured to a secluded area outside Army HQ on the pretext of meeting with Army Chief of Staff.  Eventually, everyone involved in the assassination was arrested, tortured, and later executed.  In the process, Chun Doo-hwan emerged as a new political force by investigating and subjugating KCIA, the most feared government agency until then, under his Security Command and later by arresting the chief martial law administrator Jeong Seung-hwa (and Chief Secretary Kim) on suspicion of conspiring with Director Kim.
Both were eventually released after Chun Doo-hwan seized power in a military coup in May 1980, after being on death row at one time.[12]
Theories on the motive

Kim Jae-kyu's motive in killing his long-time benefactor President Park has been controversial and the subject of much discussion. There are many theories on Kim's true motive of killing Park.  The following are just some of these theories.
Motive
It was an unplanned, impulsive act on the spur of the moment.
Kim did not hide the gun in the second floor study of the same building but had to go to another building to get a gun.
Kim had virtually no plan for the aftermath of Park's assassination.
Kim went to Army HQ instead of KCIA HQ.
Kim notified his closest subordinates of the assassination plan just before its execution.  (Park Seun-ho's only regret toward Kim was that Kim did not give him enough instructions or explanations, which prevented Park from handling the aftermath more effectively at KCIA.)
Kim was severely rebuked by Cha and President Park for incompetence during the dinner.  For several months, he had been under extreme pressure with a series of political crises, Cha's aggressive encroachment into KCIA turf, President Park turning deaf ears to his urgent warnings and always siding with Cha, and failing health.
Kim deliberately planned the assassination of President Park.
Kim invited Army Chief of Staff to a dinner at 4:15 pm after learning that he would have a dinner with President Park at 4:00pm.  Kim appears to have decided on assassination at 4:15 pm at the latest.
It was not customary for Kim to carry a gun to dinner with President Park.
Kim claimed that he had been planning to end Park's dictatorship for seven years (The Yusin Constitution of 1972 was ratified seven years before that). He claimed that he attempted to assassinate Park three times: in 1974 and twice in 1979.  There are some pieces of evidence that partially corroborate these statements.
In April 1979, Kim invited the three Chiefs of Staff of the Army, Navy, and Air Force to a dinner in the manner similar to the day of assassination.  Kim, however, called off the assassination plan.
Shortly before the dinner banquet, Kim told Chief Secretary Kim Gye-won that he would get rid of Chief Bodyguard Cha Ji-chul.
Kim was not entirely misguided in trusting Chief Secretary Kim, Army Chief of Staff Jeong Seung-hwa, and the mood of the military, which was under pressure to crack down on demonstrations in Busan and Masan.  Jeong largely followed Kim's lead until Chief Secretary Kim Gye-won revealed the truth of the situation to Jeong.  Furthermore, Jeong said that Park's assassination was not a tragedy for the entire country and people and that Park's regime acted wrongfully in some cases while Kim was on trial.  He even quoted Kim's statements word for word to make his point and appeared to defend Kim's actions.  Chief Secretary Kim was a close friend of Director Kim since the former saved the latter in a car accident.  Both were recommended to their position by Director Kim.  After Park's assassination, 50 out of 52 generals in the military voted to repeal the Yushin Constitution, which was a significant rebuke of Park's regime.  Although the military dictatorship continued under Chun Doo-hwan, the Yushin Constitution was repealed a year later on October 27, 1980.
The main proponents of the theory that the assassination was unplanned were Kim Gye-won and Jeong Seung-hwa, who had a vested interest in portraying the event as an impulsive act as they were both suspected of being co-conspirators.
Murderer
Kim assassinated Park out of jealousy of Chief Bodyguard Cha when he was losing his status and power as No.2 man in the regime.
Kim had been a loyal henchman to President Park throughout his career, who placed him in the innermost sanctum of power.  As KCIA Director, he was virtually No.2 man in the regime and was responsible for all the crimes that KCIA perpetrated as the regime's weapon of repression including tortures, unlawful imprisonments, and murders such as the brutal arrest of female factory workers in September 1979 and the infamous assassination of former KCIA director Kim Hyong-uk in Paris in October 1979.  KCIA's very function was to defend the Yushin Constitution and suppress internal enemies of Park's regime, which included the opposition parties, democracy activists, leftist students, and intellectuals.  His role as the KCIA director, whose chief job was to maintain Park's dictatorship, makes it hard to believe that he was indeed a believer in democracy.  (However, it is also possible that Kim sought to become a mitigating influence on Park and KCIA, and he pursued moderate measures and preferred compromise over brute force.)
When Kim shot Park, his rallying cry was not about democracy but rather reflected his resentment of Chief Bodyguard Cha.
Kim worked tirelessly to sabotage the opposition party's election and prevent Kim Young Sam's chairmanship of the party.
As Kim testified in his trial, his relationship with Park was that of real brothers.  They came from the same hometown and were classmates at the South Korean Army Academy.
Kim assassinated Park for democracy.
Kim said in court: "I shot the heart of Yushin in the heart of beast. I did that for democracy of this country. There were no ambition and greed."
He gave five motives for assassinating Park in his last statement at the trial: "firstly, to restore free democracy; secondly, to prevent further bloodshed of Korean people; thirdly, to prevent North Korean aggression; fourthly, to completely restore the close relationship with our strong ally the United States, which fell to the worst point since the founding of South Korea and advance our national interest through closer cooperation in defense, diplomacy, and economy; and fifthly, to restore [South] Korea's honor in the international community by cleansing the bad image of [South] Korea as a dictatorship country."
According to people close to Kim, President Park promised to voters in the 1971 presidential election that it would be his last term on Kim's suggestion.  Kim was very disappointed when Park broke his promise and ratified the Yushin Constitution, which guaranteed Park's dictatorship for life.
According to Kim's subordinates when he was the commander of Third Army Group in 1972, Kim was very disturbed by the Yushin Constitution.  Kim claimed that he planned to arrest Park and force his resignation if he were to visit his base during his tour of military bases.  The wire fence of a small house in the base that were to detain Park was indeed modified to prevent exit from within rather than entry from without.
Declassified U.S. diplomatic cables revealed that Kim was thought of as an unusual KCIA director who often spoke of democracy and a more approachable figures who often carried Washington's messages on human rights to President Park.
He also made contacts with opposition leaders, which was revealed long after his death. According to the eldest son of the widely respected opposition leader Jang Jun-ha, Jang told him that Kim was a patriotic soldier who would one day work together with them for democracy.[13]  They pretended to run into each other accidentally when they met, according to Jang's son.  Kim claimed to his lawyer that his first attempt to assassinate Park was in September 14, 1974 when he was appointed to be Minister of Construction.  A newsreel of this event does show something protruding out of Kim's pocket when he shook hands with Park.  According to Rev. Yi Hae-hak, a cellmate with Jang Jun-ha when Jang was sentenced to fifteen years for a petition campaign against the Yushin Constitution, Jang knew of Kim's plan to assassinate Park and was very disappointed that it did not take place.  (In 1975, Jang died in a suspicious circumstance while climbing a mountain.  According to Jang's son, Kim helped Jang's family financially in secret.  When Kim later became KCIA director, he met Jang's son to tell him with deep regret that Jang's death was not accidental but that the regime was involved.[14]
According to Cardinal Kim Sou-hwan, another leading figure in the democracy movement, Director Kim (then KCIA deputy director) came to see him whenever there was a political crisis and once asked him to talk to President Park to come up with a "third way," that is, to somehow amend the Yushin Constitution in a way that is acceptable to Park. He was surprised when Kim compared President Park to "a sick patient."  Deputy director Kim believed Cardinal Kim, as a Catholic cardinal, was the only person who could speak frankly to Park without repercussion and was disappointed when the talk was fruitless.[15]  Kim's association with two key figures of the democracy movement – Jang Jun-ha and Cardinal Kim Sou-hwan – led some to reconsider Kim's motive in assassinating Park.
In 1979, he often wrote calligraphy about freedom and democracy, which were found in his house after his arrest. Kim asked his relative and consul serving in Japan to draft a "third way" compromise which would allow Park to maintain military power but yield political power to a civilian government.[16]
Central Intelligence Agency and other theories
One theory is that the American Central Intelligence Agency (CIA) was behind the assassination of President Park to prevent development of a nuclear weapon by South Korea, which Park was pursuing. Later the United States recognized Chun Doo-hwan's legitimacy on the condition of abandoning the nuclear weapons program.
Kim claimed that the United States was behind him. The United States Ambassador denied any American involvement in his diplomatic cable to the State Department. On the other hand, diplomatic cables show that United States Ambassador William Gleysteen worried about the possibility of Kim claiming that he and his predecessor incited him to assassinate Park. In any case, it is possible that Kim believed that his coup would have the support of the United States if successful. In 1999, Gleysteen said that the U.S. became unwittingly involved in Park's assassination without explaining further.[17]
Kim had frequent meetings with Robert G. Brewster, CIA chief in Seoul, and other American diplomats. He met with United States Ambassador William Gleysteen on the day of the assassination, just five hours before the shooting.[18]
Kim cited the worsening of American-South Korean diplomatic relations as one of his reasons for assassinating President Park.
Another theory is that Kim was protected by the CIA and was even seen alive after his "alleged" execution. However, this claim is not widely believed.
Kim planned and then assassinated President Park to seize power for himself (the official announcement of Chun Doo-hwan's investigation.)
Kim had momentary insanity from hepatic encephalopathy related to his liver disease.  However, his physician Kim Jeong-Ryong claimed that his liver disease was well controlled and not serious enough to affect daily activities.
There was a combination of various factors that led to the assassination – i.e. Kim had planned to assassinate Park, but the actual assassination was a more or less impulsive act in connection with Chief Bodyguard Cha.
Only Kim can know the true circumstances of and motive for his assassination of Park.  (This appears to be the most widely accepted view as there are many contradictions about him.)
Korean Central Intelligence Agency conspirators
Kim Jae-gyu: Hanged on May 24, 1980.
Park Heung-ju, Kim Jae-gyu's secretary and former aide-de-camp of Kim: Shot dead by a firing squad on March 6, 1980; he was executed first because he was on active military service at the time of the assassination.
Park Seon-ho, senior KCIA agent and pupil of Kim Jae-kyu when the latter was a middle school teacher: Hanged on May 24, 1980.
Yoo Seong-ok, driver in the KCIA safehouse: Hanged on May 24, 1980.
Lee Ki-ju, head of safehouse security service: Hanged on May 24, 1980.
Kim Tae-won, safehouse security agent: Hanged on May 24, 1980. He did not actually kill anyone, but was heavily involved in the planning and fired an automatic rifle on the victim's bodies on Park Seon-ho's orders to disguise the shooting as an attack by North Korean commandos.
Seo Young-jun, safehouse security agent: Released after imprisonment.
Except for Kim Jae-kyu, Park Heung-ju and Park Seon-ho, the co-conspirators followed their superior's order without knowing who and why they were shooting.
Witnesses
Kim Gye-won: chief secretariat
Sim Soo-bong: famous female singer
Shin Jae-soon: a female student of Hanyang University
See also
Bu-Ma Democratic Protests
The President's Last Bang: a 2005 black comedy film satirizing the event
Blue House Raid
Jeong Seung-hwa

: a recollection of Park's assassination.


The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside environment (the extracellular space) which protects the cell from its environment[1][2] consisting of a lipid bilayer with embedded proteins. The cell membrane controls the movement of substances in and out of cells and organelles. In this way, it is selectively permeable to ions and organic molecules.[3] In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall, the carbohydrate layer called the glycocalyx, and the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.[4][5][6]
History
While Robert Hooke’s discovery of cells in 1665 led to the proposal of the Cell Theory, Hooke misled the cell membrane theory that all cells contained a hard cell wall since only plant cells could be observed at the time.[7] Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made. In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated. This theory extended to include animal cells to suggest a universal mechanism for cell protection and development. By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls. However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes weren’t the equivalent of a cell wall to plant cell. It was also inferred that cell membranes weren’t vital components to all cells. Many refuted the existence of a cell membrane still towards the end of the 19th century. In 1890, an update to the Cell Theory stated that cell membranes existed, but were merely secondary structures. It wasn’t until later studies with osmosis and permeability that cell membranes gained more recognition.[7] In 1895, Ernest Overton proposed that cell membranes were made of lipids.[8]
The lipid bilayer hypothesis, proposed in 1925 by Gorter and Grendel,[9] created speculation to the description of the cell membrane bilayer structure based on crystallographic studies and soap bubble observations. In an attempt to accept or reject the hypothesis, researchers measured membrane thickness.[7] In 1925 it was determined by Fricke that the thickness of erythrocyte and yeast cell membranes ranged between 3.3 and 4nm, a thickness compatible with a lipid monolayer. The choice of the dielectric constant used in these studies was called into question but future tests could not disprove the results of the initial experiment. Independently, the leptoscope was invented in order to measure very thin membranes by comparing  the intensity of light reflected from a sample to the intensity of a membrane standard of known thickness. The instrument could resolve thicknesses that depended on pH measurements and the presence of membrane proteins that ranged from 8.6 to 23.2nm, with the lower measurements supporting the lipid bilayer hypothesis. Later in the 1930s, the membrane structure model developed in general agreement to be the paucimolecular model of Davson and Danielli (1935). This model was based on studies of surface tension between oils and echinoderm eggs. Since the surface tension values appeared to be much lower than would be expected for an oil–water interface, it was assumed that some substance was responsible for lowering the interfacial tensions in the surface of cells. It was suggested that a lipid bilayer was in between two thin protein layers. The paucimolecular model immediately became popular and it dominated cell membrane studies for the following 30 years, until it became rivaled by the fluid mosaic model of Singer and Nicolson (1972).[10][7]
Despite the numerous models of the cell membrane proposed prior to the fluid mosaic model, it remains the primary archetype for the cell membrane long after its inception in the 1970s.[7] Although the fluid mosaic model has been modernized to detail contemporary discoveries, the basics have remained constant: the membrane is a lipid bilayer composed of hydrophilic exterior heads and a hydrophobic interior where proteins can interact with hydrophilic heads through polar interactions, but proteins that span the bilayer fully or partially have hydrophobic amino acids that interact with the non-polar lipid interior. The fluid mosaic model not only provided an accurate representation of membrane mechanics, it enhanced the study of hydrophobic forces, which would later develop into an essential descriptive limitation to describe biological macromolecules.[7]
For many centuries, the scientists cited disagreed with the significance of the structure they were seeing as the cell membrane. For almost two centuries, the membranes were seen but mostly disregarded this as an important structure with cellular function. It was not until the 20th century that the significance of the cell membrane as it was acknowledged. Finally, two scientists Gorter and Grendel (1925) made the discovery that the membrane is “lipid-based”. From this, they furthered the idea that this structure would have to be in a formation that mimicked layers. Once studied further, it was found by comparing the sum of the cell surfaces and the surfaces of the lipids, a 2:1 ratio was estimated; thus, providing the first basis of the bilayer structure known today. This discovery initiated many new studies that arose globally within various fields of scientific studies, confirming that the structure and functions of the cell membrane are widely accepted.[7]
The structure has been variously referred to by different writers as the ectoplast (de Vries, 1885),[11] Plasmahaut (plasma skin, Pfeffer, 1877, 1891),[12] Hautschicht (skin layer, Pfeffer, 1886; used with a different meaning by Hofmeister, 1867), plasmatic membrane (Pfeffer, 1900),[13] plasma membrane, cytoplasmic membrane, cell envelope and cell membrane.[14][15] Some authors who did not believe that there was a functional permeable boundary at the surface of the cell preferred to use the term plasmalemma (coined by Mast, 1924) for the external region of the cell.[16][17][18]
Composition
Cell membranes contain a variety of biological molecules, notably lipids and proteins. Composition is not set, but constantly changing for fluidity and changes in the environment, even fluctuating during different stages of cell development. Specifically, the amount of cholesterol in human primary neuron cell membrane changes, and this change in composition affects fluidity throughout development stages.[19]
Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:
Fusion of intracellular vesicles with the membrane (exocytosis) not only excretes the contents of the vesicle but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs around extracellular material that pinch off to become vesicles (endocytosis).
If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), there is an exchange of molecules between the lipid and aqueous phases.
Lipids
The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and sterols. The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant, often contributing for over 50% of all lipids in plasma membranes.[20][21] Glycolipids only account for a minute amount of about 2% and sterols make up the rest. In RBC studies, 30% of the plasma membrane is lipid. However, for the majority of eukaryotic cells, the composition of plasma membranes is about half lipids and half proteins by weight.
The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always "cis". The length and the degree of unsaturation of fatty acid chains have a profound effect on membrane fluidity as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting temperature (increasing the fluidity) of the membrane.[20][21] The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation.
The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Under physiological conditions phospholipid molecules in the cell membrane are in the liquid crystalline state. It means the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present.[20] However, the exchange of phospholipid molecules between intracellular and extracellular leaflets of the bilayer is a very slow process. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.[21] Also, a fraction of the lipid in direct contact with integral membrane proteins,  which is tightly bound to the protein surface is called annular lipid shell; it behaves as a part of protein complex.
In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane.[3] Additionally, the amount of cholesterol in biological membranes varies between organisms, cell types, and even in individual cells. Cholesterol, a major component of animal plasma membranes, regulates the fluidity of the overall membrane, meaning that cholesterol controls the amount of movement of the various cell membrane components based on its  concentrations.[3] In high temperatures, cholesterol inhibits the movement of phospholipid fatty acid chains,causing a reduced permeability to small molecules and reduced membrane fluidity. The opposite is true for the role of cholesterol in cooler temperatures. Cholesterol production, and thus concentration, is up-regulated (increased) in response to cold temperature. At cold temperatures, cholesterol interferes with fatty acid chain interactions. Acting as antifreeze, cholesterol maintains the fluidity of the membrane. Cholesterol is more abundant in cold-weather animals than warm-weather animals. In plants, which lack cholesterol, related compounds called sterols perform the same function as cholesterol.[3]
Phospholipids forming lipid vesicles
Lipid vesicles or liposomes are approximately spherical pockets that are enclosed by a lipid bilayer.[22] These structures are used in laboratories to study the effects of chemicals in cells by delivering these chemicals directly to the cell, as well as getting more insight into cell membrane permeability. Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution then agitating the mixture through sonication, resulting in a vesicle. By measuring the rate of efflux from that of the inside of the vesicle to the ambient solution, allows researcher to better understand membrane permeability. Vesicles can be formed with molecules and ions inside the vesicle by forming the vesicle with the desired molecule or ion present in the solution. Proteins can also be embedded into the membrane through solubilizing the desired proteins in the presence of detergents and attaching them to the phospholipids in which the liposome is formed. These provide researchers with a tool to examine various membrane protein functions.
Carbohydrates
Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides). Carbohydrates are important in the role of cell-cell recognition in eukaryotes; they are located on the surface of the cell where they recognize host cells and share information, viruses that bind to cells using these receptors cause an infection [23] For the most part, no glycosylation occurs on membranes within the cell; rather generally glycosylation occurs on the extracellular surface of the plasma membrane. The glycocalyx is an important feature in all cells, especially epithelia with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, lymphocyte homing,[23] and many others. The penultimate sugar is galactose and the terminal sugar is sialic acid, as the sugar backbone is modified in the Golgi apparatus. Sialic acid carries a negative charge, providing an external barrier to charged particles.
Proteins

The cell membrane has large content of proteins, typically around 50% of membrane volume[24] These proteins are important for cell because they are responsible for various biological activities. Approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms.[22] Membrane proteins consist of three main types: Integral proteins, peripheral proteins, and lipid-anchored proteins.[3]
As shown in the adjacent table, integral proteins are amphipathic transmembrane proteins. Examples of integral proteins include ion channels, proton pumps, and g-protein coupled receptors. Ion channels allow inorganic ions such as sodium, potassium, calcium, or chlorine to diffuse down their electrochemical gradient across the lipid bilayer through hydrophilic pores across the membrane. The electrical behavior of cells (i.e. nerve cells) are controlled by ion channels.[3] Proton pumps are protein pumps that are embedded in the lipid bilayer that allow protons to travel through the membrane by transferring from one amino acid side chain to another. Processes such as electron transport and generating ATP use proton pumps.[3] A G-protein coupled receptor is a single polypeptide chain that crosses the lipid bilayer seven times responding to signal molecules (i.e. hormones and neurotransmitters). G-protein coupled receptors are used in processes such as cell to cell signaling, the regulation of the production of cAMP, and the regulation of ion channels.[3]
The cell membrane, being exposed to the outside environment, is an important site of cell–cell communication. As such, a large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane. Functions of membrane proteins can also include cell–cell contact, surface recognition, cytoskeleton contact, signaling, enzymatic activity, or transporting substances across the membrane.

Most membrane proteins must be inserted in some way into the membrane.[25] For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to the endoplasmic reticulum, which inserts the proteins into a lipid bilayer. Once inserted, the proteins are then transported to their final destination in vesicles, where the vesicle fuses with the target membrane.
Function

The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to hold them together to form tissues. Fungi, bacteria, most archaea, and plants also have a cell wall, which provides a mechanical support to the cell and precludes the passage of larger molecules.
The cell membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or "active", requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs a number of transport mechanisms that involve biological membranes:
1. Passive osmosis and diffusion: Some substances (small molecules, ions) such as carbon dioxide (CO2) and oxygen (O2), can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Diffusion occurs when small molecules and ions move freely from high concentration to low concentration in order to equilibrate the membrane. It is considered a passive transport process because it does not require energy and is propelled by the concentration gradient created by each side of the membrane.[26] Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water. Osmosis, in biological systems involves a solvent, moving through a semipermeable membrane similarly to passive diffusion as the solvent still moves with the concentration gradient and requires no energy. While water is the most common solvent in cell, it can also be other liquids as well as supercritical liquids and gases.[27]
2. Transmembrane protein channels and transporters: Transmembrane proteins extend through the lipid bilayer of the membranes; they function on both sides of the membrane to transport molecules across it.[28] Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules can diffuse passively through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters. Protein channel proteins, also called permeases, are usually quite specific, and they only recognize and transport a limited variety of  chemical substances, often limited to a single substance. Another example of a transmembrane protein is a cell-surface receptor, which allow cell signaling molecules to communicate between cells.[28]
3. Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured.This invagination is caused by proteins on the outside on the cell membrane, acting as receptors and clustering into depressions that eventually promote accumulation of more proteins and lipids on the cytosolic side of the membrane.[29] The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles ("cell eating" or phagocytosis), small molecules and ions ("cell drinking" or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.
4. Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell
Prokaryotes
Prokaryotes are divided into two different groups, Archaea and Bacteria, with bacteria dividing further into gram-positive and gram-negative. Gram-negative bacteria have both a plasma membrane and an outer membrane separated by periplasm, however, other prokaryotes have only a plasma membrane. These two membranes differ in many aspects. The outer membrane of the gram-negative bacteria differ from other prokaryotes due to phospholipids forming the exterior of the bilayer, and lipoproteins and phospholipids forming the interior.[30] The outer membrane typically has a porous quality due to its presence of membrane proteins, such as gram-negative porins, which are pore-forming proteins. The inner, plasma membrane is also generally symmetric whereas the outer membrane is asymmetric because of proteins such as the aforementioned.  Also, for the prokaryotic membranes, there are multiple things that can affect the fluidity.  One of the major factors that can affect the fluidity is fatty acid composition.  For example, when the bacteria Staphylococcus aureus was grown in 37◦C for 24h, the membrane exhibited a more fluid state instead of a gel-like state. This supports the concept that in higher temperatures, the membrane is more fluid than in colder temperatures. When the membrane is becoming more fluid and needs to become more stabilized, it will make longer fatty acid chains or saturated fatty acid chains in order to help stabilize the membrane.[31] Bacteria are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cell walls, but none that are made of peptidoglycan. The outer membrane of gram negative bacteria is rich in lipopolysaccharides, which are combined poly- or oligosaccharide and carbohydrate lipid regions that stimulate the cell’s natural immunity.[32] The outer membrane can bleb out into periplasmic protrusions under stress conditions or upon virulence requirements while encountering a host target cell, and thus such blebs may work as virulence organelles.[33] Bacterial cells provide numerous examples of the diverse ways in which prokaryotic cell membranes are adapted with structures that suit the organism’s niche. For example, proteins on the surface of certain bacterial cells aid in their gliding motion.[34] Many gram-negative bacteria have cell membranes which contain ATP-driven protein exporting systems.[34]
Structures
Fluid mosaic model
According to the fluid mosaic model of S. J. Singer and G. L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.[35] Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton, and potentially lipid rafts.
Lipid bilayer

Lipid bilayers form through the process of self-assembly. The cell membrane consists primarily of a thin layer of amphipathic phospholipids that spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding water while the hydrophilic "head" regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. Hydrophobic interactions (also known as the hydrophobic effect) are the major driving forces in the formation of lipid bilayers.  An increase in interactions between  hydrophobic molecules (causing clustering of hydrophobic regions) allows water molecules to bond more freely with each other, increasing the entropy of the system.  This complex interaction can include noncovalent interactions such as van der Waals, electrostatic and hydrogen bonds.
Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates.
Flippases and scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moieties moving through the membrane.
Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.
Membrane polarity

The apical membrane of a polarized cell is the surface of the plasma membrane that faces inward to the lumen. This is particularly evident in epithelial and endothelial cells, but also describes other polarized cells, such as neurons. The basolateral membrane of a polarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces. It faces outwards, towards the interstitium, and away from the lumen. Basolateral membrane is a compound phrase referring to the terms "basal (base) membrane" and "lateral (side) membrane", which, especially in epithelial cells, are identical in composition and activity. Proteins (such as ion channels and pumps) are free to move from the basal to the lateral surface of the cell or vice versa in accordance with the fluid mosaic model. Tight junctions join epithelial cells near their apical surface to prevent the migration of proteins from the basolateral membrane to the apical membrane. The basal and lateral surfaces thus remain roughly equivalent to one another, yet distinct from the apical surface.
Membrane structures

Cell membrane can form different types of "supramembrane" structures such as caveola, postsynaptic density, podosome, invadopodium, focal adhesion, and different types of cell junctions. These structures are usually responsible for cell adhesion, communication, endocytosis and exocytosis. They can be visualized by electron microscopy or fluorescence microscopy. They are composed of specific proteins, such as integrins and cadherins.
Cytoskeleton
The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Indeed, cytoskeletal elements interact extensively and intimately with the cell membrane.[36] Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrate gut — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia, which are microtubule-based extensions covered by the cell membrane, and filopodia, which are actin-based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a bleb.
Intracellular membranes
The content of the cell, inside the cell membrane, is composed of numerous membrane-bound organelles, which contribute to the overall function of the cell. The origin, structure, and function of each organelle leads to a large variation in the cell composition due to the individual uniqueness associated with each organelle.
Mitochondria and chloroplasts are considered to have evolved from bacteria, known as the endosymbiotic theory. This theory arose from the idea that Paracoccus and Rhodopseaudomonas, types of bacteria, share similar functions to mitochondria and blue-green algae, or cyanobacteria, share similar functions to chloroplasts. The endosymbiotic theory proposes that through the course of evolution, a eukaryotic cell engulfed these 2 types of bacteria, leading to the formation of mitochondria and chloroplasts inside eukaryotic cells. This engulfment lead to the 2 membranes systems of these organelles in which the outer membrane originated from the host's plasma membrane and the inner membrane was the endosymbiont's plasma membrane. Considering that mitochondria and chloroplasts both contain their own DNA is further support that both of these organelles evolved from engulfed bacteria that thrived inside a eukaryotic cell.[37]
In eukaryotic cells, the nuclear membrane separates the contents of the nucleus from the cytoplasm of the cell.[38] The nuclear membrane is formed by an inner and outer membrane, providing the strict regulation of materials in to and out of the nucleus. Materials move between the cytosol and the nucleus through nuclear pores in the nuclear membrane. If a cell’s nucleus is more active in transcription, its membrane will have more pores. The protein composition of the nucleus can vary greatly from the cytosol as many proteins are unable to cross through pores via diffusion. Within the nuclear membrane, the inner and outer membranes vary in protein composition, and only the outer membrane is continuous with the endoplasmic reticulum (ER) membrane.  Like the ER, the outer membrane also possesses ribosomes responsible for producing and transporting proteins into the space between the two membranes. The nuclear membrane disassembles during the early stages of mitosis and reassembles in later stages of mitosis.[39]
The ER, which is part of the endomembrane system, which makes up a very large portion of the cell's total membrane content. The ER is an enclosed network of tubules and sacs, and its main functions include protein synthesis, and lipid metabolism. There are 2 types of ER, smooth and rough. The rough ER has ribosomes attached to it used for protein synthesis, while the smooth ER is used more for the processing of toxins and calcium regulation in the cell.[40]
The Golgi apparatus has two interconnected round Golgi cisternae. Compartments of the apparatus forms multiple tubular-reticular networks responsible for organization, stack connection and cargo transport that display a continuous grape-like stringed vesicles ranging from 50-60nm. The apparatus consists of three main compartments, a flat disc-shaped cisterna with tubular-reticular networks and vesicles.[41]
Variations
The cell membrane has different lipid and protein compositions in distinct types of cells and may have therefore specific names for certain cell types.
Sarcolemma in myocytes: “Sarcolemma” is the name given to the cell membrane of myocytes (also known as muscle cells).[42]  Although the sarcolemma is similar to other cell membranes, it has other functions that set it apart.  For instance, the sarcolemma transmits synaptic signals, helps generate action potentials, and is very involved in muscle contractions.[43] Unlike other cell membranes, the sarcolemma makes up small channels called “t-tubules” that pass through the entirety of muscle cells.  It has also been found that the average sarcolemma is 10 m thick as opposed to the 4 m thickness of a general cell membrane.[44][42]
Oolemma is the cell membrane in oocytes: The oolemma of oocytes, (immature egg cells) are not consistent with a lipid bilayer as they lack a bilayer and do not consist of lipids.[45] Rather, the structure has an inner layer, the fertilization envelope, and the exterior is made up of the vitelline layer, which is made up of glycoproteins; however, channels and proteins are still present for their functions in the membrane.
Axolemma: The specialized plasma membrane on the axons of nerve cells that is responsible for the generation of the action potential. It consists of a granular, densely packed lipid bilayer that works closely with the cytoskeleton components spectrin and actin. These cytoskeleton components are able to bind to and interact with transmembrane proteins in the axolemma.[46][47]
Permeability

The permeability of a membrane is the rate of passive diffusion of molecules through the membrane. These molecules are known as permeant molecules. Permeability depends mainly on the electric charge and polarity of the molecule and to a lesser extent the molar mass of the molecule. Due to the cell membrane's hydrophobic nature, small electrically neutral molecules pass through the membrane more easily than charged, large ones. The inability of charged molecules to pass through the cell membrane results in pH partition of substances throughout the fluid compartments of the body.
See also

Annular lipid shell
Artificial cell
Bacterial cell structure
Bangstad syndrome
Cell cortex
Cell damage, including damage to cell membrane
Cell theory
Cytoneme
Elasticity of cell membranes
Gram-positive bacteria
Membrane models
Membrane nanotubule
History of cell membrane theory
Lipid raft
Trogocytosis

Notes and references