Corruption in South Africa

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Corruption_in_South_Africa 

 

Corruption in South Africa includes the improper use of public resources for private ends, including bribery and improper favouritism. The 2017 Transparency International Corruption Perceptions Index assigned South Africa a score of 43 out of 100, ranking South Africa 71 out of 180 countries; a high score and a low ranking signals that the country's public sector is perceived to be honest. There was a marginal improvement by 2021, when South Africa received a score of 44, ranking it 70 out of 180 countries. Nonetheless, this remains below its score of 45 in 2016. Countries with scores below 50 are believed to have serious corruption problems.

South Africa has a robust anti-corruption framework, but laws are inadequately enforced and accountability in public sectors such as healthcare remain below par. In addition, internal sanctions have been employed to discourage whistle-blowers from reporting corrupt activities in both the public and private sectors – according to a 2021 Afrobarometer survey, 76.2% of South Africans believe that ordinary people risk retaliation and other negative consequences if they report incidents of corruption.

A scandal involving the Gupta family and former South African President Jacob Zuma pushed Zuma out of office as a long list of corruption complaints against the former President resurfaced. Complaints against Zuma range from the former leader's lavish spending of state funds, to delegating contracts based on nepotism and businesses with familial connections or close ties benefiting through their association with him. The Zondo Commission was later created to investigate Zuma and his associates for corruption. On November 11, 2020, it was revealed that a historic anti-corruption blitz resulted in the arrest of more than 100 South African political, education, health, police and business officials on corruption charges.

Corruption has negatively impacted South Africa's ability to resolve the country's long running energy crisis.

Perception of corruption in South Africa

In 2013, Afrobarometer found a substantial increase in the public's perception of corruption since 2008. 66% of South Africans believed that the government could be doing more to curb corruption, compared to 56% elsewhere on the continent. South Africans' experience of first-hand corruption ranks among the least on a first-hand basis in Africa. At one stage only 15% of South Africans admit to having paid a bribe. The average percentage of all Africans having paid a bribe to government officials was around 30%.

Between 2011 and 2015, former President Jacob Zuma's public approval ratings almost halved, from 64% to 36%, possibly due to corruption scandals over that period. The majority of South Africans believe that various branches of government should oversee other branches of government's work. Around a quarter of South Africans feel they should be allowed to be responsible for holding elected representatives and leaders accountable.

In 2021, 9.1% of South Africans believed that corruption was the most important problem facing the country, meaning that corruption ranked second only to unemployment in the priorities of those surveyed. 60.5% believed that the government was doing "very badly" at fighting corruption, and another 15.4% believed that it was doing "fairly badly."

Apartheid and corruption

Apartheid, a system of discrimination and segregation on racial grounds, dominated the ideology and political system in South Africa from 1948 to 1994. This system, which intentionally excluded non-Afrikaners from civil service jobs, government positions, and politics entirely, came to an end in 1994 when the African National Congress headed by Nelson Mandela conducted negotiations with the South African government at the time.

Before the abolition of Apartheid, civil service was a pervasive medium for rent-seeking and the preferential treatment of Afrikaners. Policies favouring Afrikaner cultural and educational systems, the awarding of government contracts to Afrikaner businesses and the funding of parastatal Afrikaner organisations was a common phenomenon during the era of Apartheid. The building of rural homeland states in the 1980s created ideal projects for rent-seeking with many homeland leaders presiding over massive networks of patronage. A notable corruption scandal during apartheid was the Muldergate scandal that involved the improper use of public funds to conduct a pro-apartheid propaganda campaign; the scandal brought down the government of Prime Minister John Vorster.

During the anti-apartheid struggle the African National Congress (ANC) was receiving funds from foreign donors in order to build up stronger opposition parties to protest against South Africa's National Party and Apartheid. These leaders were given large sums of money without formal book-keeping. There is however no reports or evidence of corruption. The ANC's tradition of loyalty to the organization was shaped in the Apartheid era, with clear implications in modern-day ANC politics. In the middle years of Jacob Zuma's presidency, corruption became rampant in most government departments, intelligence agencies, the police and the military.

There is much ongoing debate regarding the origins of corruption and the definition of corruption in South Africa. The inherited bureaucracy and political culture which originated in the Apartheid era has rendered corruption issues hard to trace and tackle. The presidency of Jacob Zuma following 2009 created an environment where corruption has flourished under the new leadership. Both the new and old political order created their own types of corruption, benefiting those in their inner circles. Although forms of endemic corruption were passed on to the new order since 1994, new forms of corruption have emerged adding new layers of theft from the State's purse.

Types of corruption in South Africa

Although South Africa is subjected to various types of corruption, three notable forms are wasteful expenditure, state capture, and corruption related to or using Black Economic Empowerment (BEE) legislation. Recent state capture scandals involving South African politicians and the Gupta family have brought these types of corruption into the public spotlight. Petty Corruption is another relevant issue affecting public services and day-to-day life in South Africa. Local municipalities have also been noted as significantly impacted by corruption with Corruption Watch describing them as amoungst the most corrupt institutions in the country.

Most prevalent sectors with reports of corruption in 2021.

Sector	% of total reports	Most common type of corruption	Province with highest number of reports (percentage)
Police	10.0%	Abuse of authority	Gauteng (47%)
Schools	5.8%	Abuse of authority	Gauteng (37%)
COVID-19 related corruption	3.8%	Maladministration
Housing	3.1%	Maladministration	Gauteng (55%)
Health	2.7%	Procurement corruption	Gauteng (41%)
Traffic	2.7%	Bribery and extortion	Gauteng (83%)
Licensing	2.3%	Bribery and extortion
Mining	1.2%	Maladministration	Limpopo (31%)

State capture

Main article: State capture § South Africa

State capture, type of systemic political corruption in which private interests significantly influence a state's decision-making processes to their own advantage, became prevalent in South Africa during the presidency of Jacob Zuma (see section below). The most notable incident of state capture corruption is the Gupta family scandal (see section below). State capture in South Africa has been estimated by government to have cost the country up to R 250 billion (US$ 17 billion) between 2014 and 2017, and reduce the country's GDP growth rate by an estimated 4% a year. Former South African Treasury official Ismail Momoniat has stated that state capture during the Zuma administration had caused such severe damage to the South African economy that it had effectively undid all the efforts of the Mandela and Mbeki administrations to develop the country's economy.

Black Economic Empowerment

See also: Black Economic Empowerment § Corruption

Two notable types of corruption related to the government policies of BEE and Broad-Based Black Economic Empowerment (BBBEE) are BEE fronting and political corruption.

Political corruption

BEE and BBEEE requirements have been used to facilitate stated capture in South Africa with government contracts improperly awarded, at inflated prices, to politically connected "tenderpreneurs," sometimes to the detriment of quality and service delivery. A notable criticism of BBBEE is that the policy has been co-opted and repurposed by factions and powerful members of South Africa's political elite, mostly within the governing African National Congress (ANC), for the purposes of corrupt self-enrichment at the expense of South Africans who are not politically connected, thereby fueling the growth of corruption within South Africa, reducing economic growth, and increasing unemployment. Moeletsi Mbeki has argued that BEE is the biggest driver of corruption in South Africa.

BEE fronting

BEE fronting is an abuse of the rules governing BEE, where qualifying persons are given a seat on the Board of a company while having no decision-making power in the company, in order to qualify the company for government contracts in terms of BEE. In June 2017, Netcare, a company which operates the largest private hospital network in South Africa, was accused of BEE fronting. Since then, 17 other complaints have been filed against various South African companies regarding BEE fronting. Related to this is Cadre deployment and employment, which is an official ANC policy to colonize government with officials loyal to the ANC.

Petty corruption

Petty corruption is a prevalent issue in South Africa's public service sphere. A survey conducted by the ISS National Victims of Crime tested the extent and nature of petty corruption in South Africa. One of the main issues highlighted by this survey is South Africans' lack of access to information regarding how to report corrupt acts. The fear of facing repercussions for whistle blowing and the pervasive belief that reporting corruption will not cause change are two other concerns revealed by the 2011 survey. Respondents of the survey were most likely to pay bribes to traffic officials, followed by police officers and officials in employment offices. These findings support the notion that the perception of corruption in local government departments such as traffic and municipal policing is high. The frequency of bribes involving police officers is concerning due to their role in tackling corruption and illicit behavior. Only 5.6% of South Africans reported experiencing forms of petty corruption involving either money, favours and gifts. Even though the percentage of experienced corruption is low, the high perception of corruption has led respondents to prioritize corruption as the second most prevalent crime in the country.

Corruption scandals

Jacob Zuma and corruption

Zuma's time in office has been marked by controversy and accusations of corruption. Zuma's resignation on February 14, 2018 came after months of pressure from the ANC. In April 2018 it was announced that Zuma would be prosecuted on 12 counts of fraud, one of racketeering, two of corruption and one of money laundering. Zuma faced corruption charges involving the US$2.5 billion South African Arms Deal. On 11 October 2019, a South African high court denied Zuma a motion to withdraw the recent criminal charges against him. Zuma was imprisoned for refusing to give evidence to the Zondo Commission resulting in the July 2021 unrest.

Nkandla

Main article: Nkandla homestead

The Nkandla homestead scandal involved the controversial, and possibly corrupt, funding of President Jacob Zuma's personal homestead at public expense. After a home invasion and the subsequent rape of one of Zuma's four wives, Zuma accessed the means to upgrade security measures of his homestead. A report by South Africa's public protector in 2014 found Zuma had inappropriately allocated state funds to finance additional home improvements such as the addition of a swimming pool, amphitheater, visitor centre and cattle enclosure to his property (among others). In 2018, South Africa's highest court found Mr. Zuma guilty of violating the constitution in regard to the lavish spending of R 246 million (US$19.14 million) of State funds towards Zuma's homestead in Nkandla.

Russian nuclear deal

Russian President Vladimir Putin alongside South African President Jacob Zuma in 2017.

In 2014 the Zuma administration and the Russian government put pressure on the South African government to sign a R1 trillion (US$66 billion) nuclear energy deal with the Russian state owned enterprise Rosatom to build and operate up to eight nuclear power plants in an effort to resolve the South African energy crisis; both the Russian government and the Zuma administration were criticized for forcing through the deal by attempting to circumvent South Africa's procurement laws. Then South African Finance Minister Nhlanhla Nene gave testimony to the Zondo Commission that he was fired for not approving a US$100 billion version of the deal in 2015. The deal was cancelled by court order in April 2017.

Gupta family scandal

Main article: Gupta family

 

A protest placard depicting Gupta family member Atul Gupta at a protest against corruption during the Zuma administration. The slogan "#Not My President" on the placard explicitly links Atul Gupta with corruption and state capture associated with the Presidency of Jacob Zuma.

Zuma's close relationship with the Gupta family has been highlighted in the former South African Public Protector's report on State Capture.

In 2016, the Public Protector, Thuli Madonsela, investigated the Gupta Family Scandal after receiving a formal complaint from a Catholic priest, called Father Stanslaus Muyebe. The State of Capture report released by the Public Protector in 2016 reports on the dangers of state capture in the case of South Africa. The 355-page document reports Mr. Zuma's preferential treatment of the Gupta family and the involvement of Jacob Zuma's son and the Gupta family. The allegations reported include the Gupta Family's involvement in the appointments and removals of members of the South African cabinet, the unlawful awarding of state contracts to Gupta linked companies or persons, the banks’ preferential treatment of Gupta owned companies and Zuma's conflict of interest concerning his position and business dealings. The Gupta brothers, who have been major players in South African business for over a decade began growing their relationship with former President Jacob Zuma in 2003 as he occupied the Office of Deputy President. Since 2003, both parties benefited from an understanding wherein the Gupta family financed the Zuma family while President Zuma appointed friendly officials and awarded lucrative state contracts to the Gupta empire. South African authorities are seeking to recover up to US$4.07 billion lost to these Gupta deals. Jacob Zuma's son, Duduzane and Gloria Ngeme Zuma, one of Mr. Zuma's wives, received large transfers and monthly salaries for positions held at one of the Gupta firms.

The Gupta family has engaged in a numerous suspicious transactions involving a series of shell companies and state-owned enterprises. The OCCRP has reported on Transnet's contract with South China Rail and subcontracts with Gupta run organizations. State controlled companies like Eskom and Transnet are in the centre of illicit deals with Gupta companies. An arrest warrant for Ajay Gupta was issued in February 2018. Some of the members of the Gupta family – the prime accused in the corruption case – were reported of residing in the emirate of Dubai as of 11 June 2021. The Bank of Baroda played an important role in facilitating irregular financial transactions for the Guptas.

The Arms Deal scandal

Main article: South African Arms Deal

The arms deal, formally known as the Strategic Defence Package, was a multi-billion dollar deal involving arms acquisitions from countries such as Germany and France. This arms deal set a precedent for cases of large-scale corruption and high levels of bribery and embezzlement in the African National Congress. This arms deal came at a time when the AIDS epidemic was rampant throughout Africa and poverty and inequality in the region still remained among the highest in the world. Taxpayers protested the arms deal through a public interest lawsuit claiming that the deal was unconstitutional and irrational. Various South African and international evidence teams have been investigating the arms deal since the early 2000s. Rumors of embezzlement, bribes and kick-backs by and between the external players in this procurement and that of the ANC, have prompted further investigations. In 2011, Former President Zuma appointed a Commission of Enquiry headed by Judge Seriti. to investigate allegations of impropriety, fraud and corruption around the 1998 Arms Deal.

As of March 2018, Jacob Zuma has been facing 16 charges of corruption, money laundering, racketeering and fraud involving the 1998 Arms Deal. In total Zuma has been accused of accepting 783 illegal payments, including receiving bribes from a French Arms firm via his financial advisor. In April 2018, two months after resigning from office, Zuma was charged with graft by a national jury. On 11 October 2019, a South African high court upheld all 16 charges against Zuma. A 15-month prison sentence which was issued to Zuma for contempt of court on this case would be reserved by the Constitutional Court of South Africa on 12 July 2021. On 20 July 2021, it was agreed that former South African President Jacob Zuma's criminal trial for this would case would begin on either August 10 or August 13, 2021. Prior to his imprisonment Zuma threatened that his arrest would result in nationwide riots; following his imprisonment in July 2021 triggered large scale riots and looting in KwaZulu-Natal and Gauteng provinces.

Other notable corruption scandals

1990s

"The A.N.C. has established a very clear pattern and the pattern is simple. You can be whatever you like as long as you are loyal. The minister of health may be responsible for 'Sarafina' but she is loyal, so she will be defended. The problem is you can't deal with corruption this way."

- Steve Friedman, Center for Policy Studies (1996)

• The Reverent Allan Boesak was found to have misappropriated donor funds in 1994 intended for his Foundation for Peace and Justice causing significant public controversy. Boesak was found guilty of stealing R332,000 in 1999 and convicted.
• In 1995 senior ANC member Winnie Mandela was accused of receiving a US$20,000 bribe from a building contracting company so they could secure government tenders to build public housing.
• Sarafina 2 was a government funded play based on the musical Sarafina! that was to increase awareness of the South African HIV/AIDS epidemic. In 1996 the ANC and South African government were criticized for not taking action against Health Minister Nkosazana Zuma and other ANC members accused of corruption and misconduct in organising the US$4 million play.
• ANC member and deputy minister of Environment and Tourism, Bantu Holomisa, was expelled from the ANC in 1996 after Holomisa accused former Transkei Prime Minister Stella Sigcau and a number of ANC members of being involved in a corrupt relationship with the South African casino magnate Sol Kerzner.
• Former President Jacob Zuma's financial advisor, Schabir Shaik, was sentenced to 15 years in 2005 for soliciting a bribe from a French arms company Thales in 1999. The bribe was solicited on behalf of Zuma when he was Deputy President of South Africa. Then presidential spokesperson Mac Maharaj was implicated in the bribery scandal when investigated by the Scorpions in 2003.

2000s

• New National Party politician Abe Williams was convicted in the year 2000 of corruption involving R240,112 and theft of R383,000 of donor funds intended for the upliftment of impoverished West Coast communities.
• The 2004 Oilgate scandal involved the transfer of R11 million from the state owned PetroSA to the ANC so as to help fund its re-election campaign in the run-up to the 2004 national elections. The Mail & Guardian newspaper was controversially gagged by the courts from publishing a report on the scandal following the election.
• Tony Yengeni, the ANC's chief parliamentary whip, was found guilty of fraud in 2005 after Yengeni received a large discount on a luxury car from a firm bidding for a government contract. In 2007 Goodwood police station commander, Siphiwu Hewana, was found guilty of attempting to defeat the ends of justice by tampering with the docket for convicted fraudster Tony Yengeni's arrest for driving under the influence.
• In 2005, the Travelgate scandal exposed Members of Parliament who were found to have illegally used parliamentary travel vouchers in a fraud exceeding R37,000,000 for personal use. As the CFO who identified the fraud reported: "I, Harry Charlton FCA, JP can confirm 6 travel agents and 435 then past and present MP's including 3 members of Mbeki's Cabinet were implicated - most of them ANC MPs. 50 plea bargained for sentences that would allow them to remain as MPs. There were 3 formats of fraud totalling in excess of R37 million (USD12million). The investigation went back 15 months but there was clear evidence that the fraud had been operating for a longer period. As I would not cooperate my services were terminated by Secretary Dingani in January 2016". Dingani was himself was terminated for fraud.
• Former South African President Thabo Mbeki and Sepp Blatter agreed to a $10m deal in 2007, which US prosecutors say was a "bribe" to secure the 2010 World Cup.

2010s

• Former National Police Commissioner and ex-President of Interpol, Jackie Selebi, was charged with graft in July 2010, for receiving (at least) R120,000 from alleged crime-syndicate boss, Glenn Agliotti.
• The Vrede Dairy Project was a 2012 government project established to empower black dairy farmers in the town of Vrede at a cost of R250 million. The Gupta family and Free State provincial premier Ace Magashule were implicated in the theft of almost all the project's funds.
• National secretary general of the ANC, Ace Magashule, was suspended from his position and arrested on 21 charges of corruption for his involvement in a 2014 corruption scandal that took place whilst he was Premier of the Free State. The R255 million contact to remove asbestos from public housing in the province was awarded to Blackhead Consulting that in turn made numerous payments to third parties at Magashule's instruction or with his knoweldge.
• In 2015 the German offices of the South African based private company Steinhoff International were raided. This publicly exposed one of the biggest cases of "corporate fraud in South Africa's history" implicating Steinhoff's South African CEO Markus Jooste.
• The collapse of the VBS Mutual Bank in 2018 due to fraud and corruption created a scandal that implicated municipalities, the ANC, and the Economic Freedom Fighters.

2020s

• The emergence of a “rogue unit” within the Western Cape Crime Intelligence Division of the South African Police Service (SAPS) resulting in the murder of investigating officer Charl Kinnear in 2020.
• Corruption within the Gauteng Provincial Government Department of Health resulting in the 2021 murder of Babita Deokaran.
• Digital Vibes scandal involving Minister of Health Zweli Mkhize awarding a large COVID-19 communications tender to family members.

Anti-corruption initiatives

Government initiatives against corruption are coordinated by the Department of Public Service and Administration. The Public Protector also plays a role in fighting corruption. A disbanded independent anti-corruption unit named the Scorpions was replaced by the Hawks. South Africa's Directorate for Priority Crime Investigation (DPCI), commonly known as the Hawks, was designed to target organized crime, economic crime and corruption. The group was established by the Zuma administration in 2008. South Africa has a well-developed framework and legislation outlining corruption initiatives. The Prevention and Combating of Corruption Act (PCCA) criminalizes corruption in public and private sectors and codifies specific offences making it easier for courts to use the legislation. This act especially condemns bribery, extortion, abuse of power and money laundering while obliging public officials to report corruption offences. Like many corruption regulations in South Africa, the PCCA is poorly implemented and the Act does not include any protection measures for whistle-blowers.

The National Anti-Corruption Forum provides an online guide to the Prevention and Combating of Corruption Act (PCCA). Other acts like the Promotion of Access to Information Act calls for an increased access to public information. The Public Finance Management Act examines government expenditures and the Code of Conduct for Assembly and Permanent Council members calls for the full disclosure of members to report gifts. Enforcement mechanisms remain weak for many of these preventative Acts, allow for corrupt actions to go unreported. South Africa has ratified the United Nations Convention against Corruption, the African Union Convention on Preventing and Combating Corruption and the OECD Anti-Bribery Convention.

In 2018 the Zondo Commission of Inquiry was set up to investigate allegations of State Capture and corruption during the administration of President Jacob Zuma. Testimony given during the inquiry implicated Bosasa, Bain & Company, the Gupta family and associates of former President Zuma. During the inquiry hearings a joint memorandum signed by the embassies of the United States, the United Kingdom, the Netherlands, Germany and Switzerland representing countries that make up 75% of foreign direct investment into South Africa warned that if unaddressed corruption would have a negative impact on future investment in South Africa. It called for President Ramaphosa at act against perpetrators of corruption. The South African government responded that it was disappointed by the memorandum not following "acceptable" diplomatic practices.

2020 anti-corruption blitz

On September 30, 2020, ousted Mangaung mayor Olly Mlamleli was arrested over a controversial asbestos contract which was issued during her time as a Member of the Free State Executive Council (MEC) for Cooperative Governance. Other co-defendats of Mlamleli would be arrested as well. All seven people charged in the asbestos corruption case were granted bail.

On 30 July 2020, Lieutenant-General Khomotso Phahlane, who was also former acting SAPS Commissioner, was dismissed from the SAPS, following 3 years on suspension, after he was found guilty of dishonest conduct. On 12 October 2020, Lieutenant-General Bonang Mgwenya, the country's second-most senior police official, was arrested on charges of corruption, fraud, theft and money laundering involving about R200-million and afterwards appeared in Ridge Magistrates’ court. At the time of Mgwenya's arrest, she and Phahlane were among 14 fellow South African officers who were charged with corruption. Mgwenya was suspended on 15 October 2020 and was dismissed from SAPS on 13 November 2020. On 23 December 2020, four Cape Town police officers attached to the national border control unit at Cape Town International Airport were arrested for extorting money from Chinese businesses.

On November 11, 2020, Business Insider reported that more than 100 people were arrested in recent weeks in an anti-corruption blitz. Those arrested included VBS Mutual Bank executives, federal Unemployment Insurance Fund (UIF) officials, numerous government officials in Gauteng and Limpopo, the mayor and municipal director of JB Marks, former Bosasa COO and Zuma whistleblower Angelo Agrizzi, ANC MP and former state security minister Bongani Bongo and numerous other government officials from hailing from Mpumalanga, ANC chair Ace Magashule, Christian minister Shepherd Bushiri, and former ANC MP Vincent Smith. In December 2020, Deputy Chief Justice of South Africa Raymond Zondo ordered Zuma to resume testifying before his Zondo Commission. Zondo also served two summonses which arranged for Zuma's required 2021 testimony to occur from January 18–22 and February 15–19.

On 4 June 2020, six senior Gauteng police officers where among 14 people arrested on corruption charges. Two other senior officers, now retired, were arrested as well. Among the Guateng-based SAPS officers charged with corruption included three brigadiers and a retired SAPS Lieutenant General. On December 17, 2020, former KwaZulu-Natal police commissioner Mmamonnye Ngobeni and her co-defendant, Durban businessman Thoshan Panday, returned to court on corruption charges.

Impact

Despite going anti-corruption efforts the South African Auditor-General reported that for the 2020/21 financial year "government spending that has ‘not been either recovered, condoned or written off stood at R488.14-billion’ " (roughly equivalent to US$34.28 billion) with a significant proportion of that amount being spent on COVID-19 related projects widely reported as being corrupt by the media. The Special Investigating Unit found that 66% of funding for COVID-19 PPE procurement was consumed by corruption or fraud.

Micelle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Micelle 

 

Micelle

IUPAC definition
Micelle	Particle of colloidal dimensions that exists in equilibrium with the molecules or ions in solution from which it is formed.
Micelle (polymers)	Organized auto-assembly formed in a liquid and composed of amphiphilic macromolecules, in general amphiphilic di- or tri-block copolymers made of solvophilic and solvophobic blocks.
Note 1	An amphiphilic behavior can be observed for water and an organic solvent or between two organic solvents.
Note 2	Polymeric micelles have a much lower critical micellar concentration (CMC) than soap (0.0001 to 0.001 mol/L) or surfactant micelles, but are nevertheless at equilibrium with isolated macromolecules called unimers. Therefore, micelle formation and stability are concentration-dependent.

Cross-section view of the structures that can be formed by phospholipids in aqueous solutions (unlike this illustration, micelles are usually formed by single-chain lipids, since it is difficult to fit two chains into this shape)

 

Scheme of a micelle formed by phospholipids in an aqueous solution

A micelle (/maɪˈsɛl/) or micella (/maɪˈsɛlə/) (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated colloidal system). A typical micelle in water forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the micelle centre.

This phase is caused by the packing behavior of single-tail lipids in a bilayer. The difficulty filling all the volume of the interior of a bilayer, while accommodating the area per head group forced on the molecule by the hydration of the lipid head group, leads to the formation of the micelle. This type of micelle is known as a normal-phase micelle (oil-in-water micelle). Inverse micelles have the head groups at the centre with the tails extending out (water-in-oil micelle).

Micelles are approximately spherical in shape. Other phases, including shapes such as ellipsoids, cylinders, and bilayers, are also possible. The shape and size of a micelle are a function of the molecular geometry of its surfactant molecules and solution conditions such as surfactant concentration, temperature, pH, and ionic strength. The process of forming micelles is known as micellisation and forms part of the phase behaviour of many lipids according to their polymorphism.

History

The ability of a soapy solution to act as a detergent has been recognized for centuries. However, it was only at the beginning of the twentieth century that the constitution of such solutions was scientifically studied. Pioneering work in this area was carried out by James William McBain at the University of Bristol. As early as 1913, he postulated the existence of "colloidal ions" to explain the good electrolytic conductivity of sodium palmitate solutions. These highly mobile, spontaneously formed clusters came to be called micelles, a term borrowed from biology and popularized by G.S. Hartley in his classic book Paraffin Chain Salts: A Study in Micelle Formation. The term micelle was coined in nineteenth century scientific literature as the ‑elle diminutive of the Latin word mica (particle), conveying a new word for "tiny particle".

Solvation

Individual surfactant molecules that are in the system but are not part of a micelle are called "monomers". Micelles represent a molecular assembly, in which the individual components are thermodynamically in equilibrium with monomers of the same species in the surrounding medium. In water, the hydrophilic "heads" of surfactant molecules are always in contact with the solvent, regardless of whether the surfactants exist as monomers or as part of a micelle. However, the lipophilic "tails" of surfactant molecules have less contact with water when they are part of a micelle—this being the basis for the energetic drive for micelle formation. In a micelle, the hydrophobic tails of several surfactant molecules assemble into an oil-like core, the most stable form of which having no contact with water. By contrast, surfactant monomers are surrounded by water molecules that create a "cage" or solvation shell connected by hydrogen bonds. This water cage is similar to a clathrate and has an ice-like crystal structure and can be characterized according to the hydrophobic effect. The extent of lipid solubility is determined by the unfavorable entropy contribution due to the ordering of the water structure according to the hydrophobic effect.

Micelles composed of ionic surfactants have an electrostatic attraction to the ions that surround them in solution, the latter known as counterions. Although the closest counterions partially mask a charged micelle (by up to 92%), the effects of micelle charge affect the structure of the surrounding solvent at appreciable distances from the micelle. Ionic micelles influence many properties of the mixture, including its electrical conductivity. Adding salts to a colloid containing micelles can decrease the strength of electrostatic interactions and lead to the formation of larger ionic micelles. This is more accurately seen from the point of view of an effective charge in hydration of the system.

Energy of formation

See also: Thermodynamics of micellization

Micelles form only when the concentration of surfactant is greater than the critical micelle concentration (CMC), and the temperature of the system is greater than the critical micelle temperature, or Krafft temperature. The formation of micelles can be understood using thermodynamics: Micelles can form spontaneously because of a balance between entropy and enthalpy. In water, the hydrophobic effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules is unfavorable in terms of both enthalpy and entropy of the system. At very low concentrations of the surfactant, only monomers are present in solution. As the concentration of the surfactant is increased, a point is reached at which the unfavorable entropy contribution, from clustering the hydrophobic tails of the molecules, is overcome by a gain in entropy due to release of the solvation shells around the surfactant tails. At this point, the lipid tails of a part of the surfactants must be segregated from the water. Hence, they start to form micelles. In broad terms, above the CMC, the loss of entropy due to assembly of the surfactant molecules is less than the gain in entropy by setting free the water molecules that were "trapped" in the solvation shells of the surfactant monomers. Also important are enthalpic considerations, such as the electrostatic interactions that occur between the charged parts of surfactants.

Micelle packing parameter

The micelle packing parameter equation is utilized to help "predict molecular self-assembly in surfactant solutions":

${\displaystyle {\frac {v_{o}}{a_{e}\ell _{o}}}}$

where ${\displaystyle v_{o}}$ is the surfactant tail volume, ${\displaystyle \ell _{o}}$ is the tail length, and ${\displaystyle a_{e}}$ is the equilibrium area per molecule at the aggregate surface.

Block copolymer micelles

The concept of micelles was introduced to describe the core-corona aggregates of small surfactant molecules, however it has also extended to describe aggregates of amphiphilic block copolymers in selective solvents. It is important to know the difference between these two systems. The major difference between these two types of aggregates is in the size of their building blocks. Surfactant molecules have a molecular weight which is generally of a few hundreds of grams per mole while block copolymers are generally one or two orders of magnitude larger. Moreover, thanks to the larger hydrophilic and hydrophobic parts, block copolymers can have a much more pronounced amphiphilic nature when compared to surfactant molecules.

Because of these differences in the building blocks, some block copolymer micelles behave like surfactant ones, while others don't. It is necessary therefore to make a distinction between the two situations. The former ones will belong to the dynamic micelles while the latter will be called kinetically frozen micelles.

Dynamic micelles

Certain amphiphilic block copolymer micelles display a similar behavior as surfactant micelles. These are generally called dynamic micelles and are characterized by the same relaxation processes assigned to surfactant exchange and micelle scission/recombination. Although the relaxation processes are the same between the two types of micelles, the kinetics of unimer exchange are very different. While in surfactant systems the unimers leave and join the micelles through a diffusion-controlled process, for copolymers the entry rate constant is slower than a diffusion controlled process. The rate of this process was found to be a decreasing power-law of the degree of polymerization of the hydrophobic block to the power 2/3. This difference is due to the coiling of the hydrophobic block of a copolymer exiting the core of a micelle.

Block copolymers which form dynamic micelles are some of the tri-block Poloxamers under the right conditions.

Kinetically frozen micelles

When block copolymer micelles don't display the characteristic relaxation processes of surfactant micelles, these are called kinetically frozen micelles. These can be achieved in two ways: when the unimers forming the micelles are not soluble in the solvent of the micelle solution, or if the core forming blocks are glassy at the temperature in which the micelles are found. Kinetically frozen micelles are formed when either of these conditions is met. A special example in which both of these conditions are valid is that of polystyrene-b-poly(ethylene oxide). This block copolymer is characterized by the high hydrophobicity of the core forming block, PS, which causes the unimers to be insoluble in water. Moreover, PS has a high glass transition temperature which is, depending on the molecular weight, higher than room temperature. Thanks to these two characteristics, a water solution of PS-PEO micelles of sufficiently high molecular weight can be considered kinetically frozen. This means that none of the relaxation processes, which would drive the micelle solution towards thermodynamic equilibrium, are possible. Pioneering work on these micelles was done by Adi Eisenberg. It was also shown how the lack of relaxation processes allowed great freedom in the possible morphologies formed. Moreover, the stability against dilution and vast range of morphologies of kinetically frozen micelles make them particularly interesting, for example, for the development of long circulating drug delivery nanoparticles.

Inverse/reverse micelles

In a non-polar solvent, it is the exposure of the hydrophilic head groups to the surrounding solvent that is energetically unfavourable, giving rise to a water-in-oil system. In this case, the hydrophilic groups are sequestered in the micelle core and the hydrophobic groups extend away from the center. These inverse micelles are proportionally less likely to form on increasing headgroup charge, since hydrophilic sequestration would create highly unfavorable electrostatic interactions.

It is well established that for many surfactant/solvent systems a small fraction of the inverse micelles spontaneously acquire a net charge of +q_e or -q_e. This charging takes place through a disproportionation/comproportionation mechanism rather than a dissociation/association mechanism and the equilibrium constant for this reaction is on the order of 10⁻⁴ to 10⁻¹¹, which means about every 1 in 100 to 1 in 100 000 micelles will be charged.

Supermicelles

Electron micrograph of the windmill-like supermicelle, scale bar 500 nm.

Supermicelle is a hierarchical micelle structure (supramolecular assembly) where individual components are also micelles. Supermicelles are formed via bottom-up chemical approaches, such as self-assembly of long cylindrical micelles into radial cross-, star- or dandelion-like patterns in a specially selected solvent; solid nanoparticles may be added to the solution to act as nucleation centers and form the central core of the supermicelle. The stems of the primary cylindrical micelles are composed of various block copolymers connected by strong covalent bonds; within the supermicelle structure they are loosely held together by hydrogen bonds, electrostatic or solvophobic interactions.

Uses

When surfactants are present above the critical micelle concentration (CMC), they can act as emulsifiers that will allow a compound that is normally insoluble (in the solvent being used) to dissolve. This occurs because the insoluble species can be incorporated into the micelle core, which is itself solubilized in the bulk solvent by virtue of the head groups' favorable interactions with solvent species. The most common example of this phenomenon is detergents, which clean poorly soluble lipophilic material (such as oils and waxes) that cannot be removed by water alone. Detergents clean also by lowering the surface tension of water, making it easier to remove material from a surface. The emulsifying property of surfactants is also the basis for emulsion polymerization.

Micelles may also have important roles in chemical reactions. Micellar chemistry uses the interior of micelles to harbor chemical reactions, which in some cases can make multi-step chemical synthesis more feasible. Doing so can increase reaction yield, create conditions more favorable to specific reaction products (e.g. hydrophobic molecules), and reduce required solvents, side products, and required conditions (e.g. extreme pH). Because of these benefits, Micellular chemistry is thus considered a form of Green chemistry. However, micelle formation may also inhibit chemical reactions, such as when reacting molecules form micelles that shield a molecular component vulnerable to oxidation.

Micelle formation is essential for the absorption of fat-soluble vitamins and complicated lipids within the human body. Bile salts formed in the liver and secreted by the gall bladder allow micelles of fatty acids to form. This allows the absorption of complicated lipids (e.g., lecithin) and lipid-soluble vitamins (A, D, E, and K) within the micelle by the small intestine.

During the process of milk-clotting, proteases act on the soluble portion of caseins, κ-casein, thus originating an unstable micellar state that results in clot formation.

Micelles can also be used for targeted drug delivery as gold nanoparticles.

Vesicle (biology and chemistry)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Vesicle_(biology_and_chemistry)

Scheme of a liposome formed by phospholipids in an aqueous solution.

In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form naturally during the processes of secretion (exocytosis), uptake (endocytosis) and transport of materials within the plasma membrane. Alternatively, they may be prepared artificially, in which case they are called liposomes (not to be confused with lysosomes). If there is only one phospholipid bilayer, they are called unilamellar liposome vesicles; otherwise they are called multilamellar. The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane, and intracellular vesicles can fuse with the plasma membrane to release their contents outside the cell. Vesicles can also fuse with other organelles within the cell. A vesicle released from the cell is known as an extracellular vesicle.

Vesicles perform a variety of functions. Because it is separated from the cytosol, the inside of the vesicle can be made to be different from the cytosolic environment. For this reason, vesicles are a basic tool used by the cell for organizing cellular substances. Vesicles are involved in metabolism, transport, buoyancy control, and temporary storage of food and enzymes. They can also act as chemical reaction chambers.

Sarfus image of lipid vesicles. IUPAC definition

The 2013 Nobel Prize in Physiology or Medicine was shared by James Rothman, Randy Schekman and Thomas Südhof for their roles in elucidating (building upon earlier research, some of it by their mentors) the makeup and function of cell vesicles, especially in yeasts and in humans, including information on each vesicle's parts and how they are assembled. Vesicle dysfunction is thought to contribute to Alzheimer's disease, diabetes, some hard-to-treat cases of epilepsy, some cancers and immunological disorders and certain neurovascular conditions.

Types of vesicular structures

Electron micrograph of a cell containing a food vacuole (fv) and transport vacuole (tv) in a malaria parasite.

Vacuoles

Vacuoles are cellular organelles that contain mostly water.

• Plant cells have a large central vacuole in the center of the cell that is used for osmotic control and nutrient storage.
• Contractile vacuoles are found in certain protists, especially those in Phylum Ciliophora. These vacuoles take water from the cytoplasm and excrete it from the cell to avoid bursting due to osmotic pressure.

Lysosomes

• Lysosomes are involved in cellular digestion. Food can be taken from outside the cell into food vacuoles by a process called endocytosis. These food vacuoles fuse with lysosomes which break down the components so that they can be used in the cell. This form of cellular eating is called phagocytosis.
• Lysosomes are also used to destroy defective or damaged organelles in a process called autophagy. They fuse with the membrane of the damaged organelle, digesting it.

Transport vesicles

• Transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus.
• Membrane-bound and secreted proteins are made on ribosomes found in the rough endoplasmic reticulum. Most of these proteins mature in the Golgi apparatus before going to their final destination which may be to lysosomes, peroxisomes, or outside of the cell. These proteins travel within the cell inside of transport vesicles.

Secretory vesicles

Secretory vesicles contain materials that are to be excreted from the cell. Cells have many reasons to excrete materials. One reason is to dispose of wastes. Another reason is tied to the function of the cell. Within a larger organism, some cells are specialized to produce certain chemicals. These chemicals are stored in secretory vesicles and released when needed.

Types

• Synaptic vesicles are located at presynaptic terminals in neurons and store neurotransmitters. When a signal comes down an axon, the synaptic vesicles fuse with the cell membrane releasing the neurotransmitter so that it can be detected by receptor molecules on the next nerve cell.
• In animals endocrine tissues release hormones into the bloodstream. These hormones are stored within secretory vesicles. A good example is an endocrine tissue found in the islets of Langerhans in the pancreas. This tissue contains many cell types that are defined by which hormones they produce.
• Secretory vesicles hold the enzymes that are used to make the cell walls of plants, protists, fungi, bacteria and Archaea cells as well as the extracellular matrix of animal cells.
• Bacteria, Archaea, fungi and parasites release membrane vesicles (MVs) containing varied but specialized toxic compounds and biochemical signal molecules, which are transported to target cells to initiate processes in favour of the microbe, which include invasion of host cells and killing of competing microbes in the same niche.

Extracellular vesicles

Extracellular vesicles (EVs) are lipid bilayer-delimited particles produced by all domains of life including complex eukaryotes, both Gram-negative and Gram-positive bacteria, mycobacteria, and fungi.

Types

• Ectosomes/microvesicles are shed directly from the plasma membrane and can range in size from around 30 nm to larger than a micron in diameter. These may include large particles such as apoptotic blebs released by dying cells, large oncosomes released by some cancer cells, or "exophers," released by nematode neurons and mouse cardiomyocytes.
• Exosomes: membranous vesicles of endocytic origin (30-100 nm diameter).

Different types of EVs may be separated based on density (by gradient differential centrifugation), size, or surface markers. However, EV subtypes have an overlapping size and density ranges, and subtype-unique markers must be established on a cell-by-cell basis. Therefore, it is difficult to pinpoint the biogenesis pathway that gave rise to a particular EV after it has left the cell.

In humans, endogenous extracellular vesicles likely play a role in coagulation, intercellular signaling and waste management. They are also implicated in the pathophysiological processes involved in multiple diseases, including cancer. Extracellular vesicles have raised interest as a potential source of biomarker discovery because of their role in intercellular communication, release into easily accessible body fluids and the resemblance of their molecular content to that of the releasing cells. The extracellular vesicles of (mesenchymal) stem cells, also known as the secretome of stem cells, are being researched and applied for therapeutic purposes, predominantly degenerative, auto-immune and/or inflammatory diseases.

In Gram-negative bacteria, EVs are produced by the pinching off of the outer membrane; however, how EVs escape the thick cell walls of Gram-positive bacteria, mycobacteria and fungi is still unknown. These EVs contain varied cargo, including nucleic acids, toxins, lipoproteins and enzymes and have important roles in microbial physiology and pathogenesis. In host-pathogen interactions, gram negative bacteria produce vesicles which play roles in establishing a colonization niche, carrying and transmitting virulence factors into host cells and modulating host defense and response.

Ocean cyanobacteria have been found to continuously release vesicles containing proteins, DNA and RNA into the open ocean. Vesicles carrying DNA from diverse bacteria are abundant in coastal and open-ocean seawater samples.

Other types

Main article: Gas vesicle

Gas vesicles are used by Archaea, bacteria and planktonic microorganisms, possibly to control vertical migration by regulating the gas content and thereby buoyancy, or possibly to position the cell for maximum solar light harvesting. These vesicles are typically lemon-shaped or cylindrical tubes made out of protein; their diameter determines the strength of the vesicle with larger ones being weaker. The diameter of the vesicle also affects its volume and how efficiently it can provide buoyancy. In cyanobacteria natural selection has worked to create vesicles that are at the maximum diameter possible while still being structurally stable. The protein skin is permeable to gasses but not water, keeping the vesicles from flooding.

Matrix vesicles are located within the extracellular space, or matrix. Using electron microscopy they were discovered independently in 1967 by H. Clarke Anderson and Ermanno Bonucci. These cell-derived vesicles are specialized to initiate biomineralisation of the matrix in a variety of tissues, including bone, cartilage and dentin. During normal calcification, a major influx of calcium and phosphate ions into the cells accompanies cellular apoptosis (genetically determined self-destruction) and matrix vesicle formation. Calcium-loading also leads to formation of phosphatidylserine:calcium:phosphate complexes in the plasma membrane mediated in part by a protein called annexins. Matrix vesicles bud from the plasma membrane at sites of interaction with the extracellular matrix. Thus, matrix vesicles convey to the extracellular matrix calcium, phosphate, lipids and the annexins which act to nucleate mineral formation. These processes are precisely coordinated to bring about, at the proper place and time, mineralization of the tissue's matrix unless the Golgi are non-existent.

Multivesicular body, or MVB, is a membrane-bound vesicle containing a number of smaller vesicles.

Formation and transport

Main article: Membrane vesicle trafficking

 

Cell biology
Animal cell diagram
Components of a typical animal cell: Nucleolus Nucleus Ribosome (dots as part of 5) Vesicle Rough endoplasmic reticulum Golgi apparatus (or, Golgi body) Cytoskeleton Smooth endoplasmic reticulum Mitochondrion Vacuole Cytosol (fluid that contains organelles; with which, comprises cytoplasm) Lysosome Centrosome Cell membrane

Some vesicles are made when part of the membrane pinches off the endoplasmic reticulum or the Golgi complex. Others are made when an object outside of the cell is surrounded by the cell membrane.

Vesicle coat and cargo molecules

The vesicle "coat" is a collection of proteins that serve to shape the curvature of a donor membrane, forming the rounded vesicle shape. Coat proteins can also function to bind to various transmembrane receptor proteins, called cargo receptors. These receptors help select what material is endocytosed in receptor-mediated endocytosis or intracellular transport.

There are three types of vesicle coats: clathrin, COPI and COPII. The various types of coat proteins help with sorting of vesicles to their final destination. Clathrin coats are found on vesicles trafficking between the Golgi and plasma membrane, the Golgi and endosomes and the plasma membrane and endosomes. COPI coated vesicles are responsible for retrograde transport from the Golgi to the ER, while COPII coated vesicles are responsible for anterograde transport from the ER to the Golgi.

The clathrin coat is thought to assemble in response to regulatory G protein. A protein coat assembles and disassembles due to an ADP ribosylation factor (ARF) protein.

Vesicle docking

Surface proteins called SNAREs identify the vesicle's cargo and complementary SNAREs on the target membrane act to cause fusion of the vesicle and target membrane. Such v-SNARES are hypothesised to exist on the vesicle membrane, while the complementary ones on the target membrane are known as t-SNAREs.

Often SNAREs associated with vesicles or target membranes are instead classified as Qa, Qb, Qc, or R SNAREs owing to further variation than simply v- or t-SNAREs. An array of different SNARE complexes can be seen in different tissues and subcellular compartments, with 36 isoforms currently identified in humans.

Regulatory Rab proteins are thought to inspect the joining of the SNAREs. Rab protein is a regulatory GTP-binding protein and controls the binding of these complementary SNAREs for a long enough time for the Rab protein to hydrolyse its bound GTP and lock the vesicle onto the membrane.

SNAREs proteins in plants are understudied compared to fungi and animals. The cell botanist Natasha Raikhel has done some of the basic research in this area, including Zheng et al 1999 in which she and her team found AtVTI1a to be essential to Golgi⇄vacuole transport.

Vesicle fusion

Further information: Vesicle fusion

Vesicle fusion can occur in one of two ways: full fusion or kiss-and-run fusion. Fusion requires the two membranes to be brought within 1.5 nm of each other. For this to occur water must be displaced from the surface of the vesicle membrane. This is energetically unfavorable and evidence suggests that the process requires ATP, GTP and acetyl-coA. Fusion is also linked to budding, which is why the term budding and fusing arises.

In receptor downregulation

Membrane proteins serving as receptors are sometimes tagged for downregulation by the attachment of ubiquitin. After arriving an endosome via the pathway described above, vesicles begin to form inside the endosome, taking with them the membrane proteins meant for degradation; When the endosome either matures to become a lysosome or is united with one, the vesicles are completely degraded. Without this mechanism, only the extracellular part of the membrane proteins would reach the lumen of the lysosome and only this part would be degraded.

It is because of these vesicles that the endosome is sometimes known as a multivesicular body. The pathway to their formation is not completely understood; unlike the other vesicles described above, the outer surface of the vesicles is not in contact with the cytosol.

Preparation

Isolated vesicles

Producing membrane vesicles is one of the methods to investigate various membranes of the cell. After the living tissue is crushed into suspension, various membranes form tiny closed bubbles. Big fragments of the crushed cells can be discarded by low-speed centrifugation and later the fraction of the known origin (plasmalemma, tonoplast, etc.) can be isolated by precise high-speed centrifugation in the density gradient. Using osmotic shock, it is possible temporarily open vesicles (filling them with the required solution) and then centrifugate down again and resuspend in a different solution. Applying ionophores like valinomycin can create electrochemical gradients comparable to the gradients inside living cells.

Vesicles are mainly used in two types of research:

• To find and later isolate membrane receptors that specifically bind hormones and various other important substances.
• To investigate transport of various ions or other substances across the membrane of the given type. While transport can be more easily investigated with patch clamp techniques, vesicles can also be isolated from objects for which a patch clamp is not applicable.

Artificial vesicles

See also: Unilamellar liposome

Artificial vesicles are classified into three groups based on their size: small unilamellar liposomes/vesicles (SUVs) with a size range of 20–100 nm, large unilamellar liposomes/vesicles (LUVs) with a size range of 100–1000 nm and giant unilamellar liposomes/vesicles (GUVs) with a size range of 1–200 µm. Smaller vesicles in the same size range as trafficking vesicles found in living cells are frequently used in biochemistry and related fields. For such studies, a homogeneous phospholipid vesicle suspension can be prepared by extrusion or sonication, or by rapid injection of a phospholipid solution into an aqueous buffer solution. In this way, aqueous vesicle solutions can be prepared of different phospholipid composition, as well as different sizes of vesicles. Larger synthetically made vesicles such as GUVs are used for in vitro studies in cell biology in order to mimic cell membranes. These vesicles are large enough to be studied using traditional fluorescence light microscopy. A variety of methods exist to encapsulate biological reactants like protein solutions within such vesicles, making GUVs an ideal system for the in vitro recreation (and investigation) of cell functions in cell-like model membrane environments. These methods include microfluidic methods, which allow for a high-yield production of vesicles with consistent sizes.

Tissue (biology)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Tissue_(biology)

Microscopic view of a histologic specimen of human lung, consisting of various tissues: blood, connective tissue, vascular endothelium and respiratory epithelium, stained with hematoxylin and eosin.

In biology, tissue is a biological organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues.

The English word "tissue" derives from the French word "tissu", the past participle of the verb tisser, "to weave".

The study of tissues is known as histology or, in connection with disease, as histopathology. Xavier Bichat is considered as the "Father of Histology". Plant histology is studied in both plant anatomy and physiology. The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope. Developments in electron microscopy, immunofluorescence, and the use of frozen tissue-sections have enhanced the detail that can be observed in tissues. With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis.

Plant tissue

Cross-section of a flax plant stem with several layers of different tissue types:

1. Pith
2. Protoxylem
3. Xylem
4. Phloem
5. Sclerenchyma (bast fibre)
6. Cortex
7. Epidermis

In plant anatomy, tissues are categorized broadly into three tissue systems: the epidermis, the ground tissue, and the vascular tissue.

• Epidermis – Cells forming the outer surface of the leaves and of the young plant body.
• Vascular tissue – The primary components of vascular tissue are the xylem and phloem. These transport fluids and nutrients internally.
• Ground tissue – Ground tissue is less differentiated than other tissues. Ground tissue manufactures nutrients by photosynthesis and stores reserve nutrients.

Plant tissues can also be divided differently into two types:

1. Meristematic tissues
2. Permanent tissues.

Meristematic tissue

See also: Plant stem cell

Meristematic tissue consists of actively dividing cells and leads to increase in length and thickness of the plant. The primary growth of a plant occurs only in certain specific regions, such as in the tips of stems or roots. It is in these regions that meristematic tissue is present. Cells of this type of tissue are roughly spherical or polyhedral to rectangular in shape, with thin cell walls. New cells produced by meristem are initially those of meristem itself, but as the new cells grow and mature, their characteristics slowly change and they become differentiated as components of meristematic tissue, being classified as:

• Apical meristem : Present at the growing tips of stems and roots, they increase the length of the stem and root. They form growing parts at the apices of roots and stems and are responsible for the increase in length, also called primary growth. This meristem is responsible for the linear growth of an organ.
• Lateral meristem: Cells which mainly divide in one plane and cause the organ to increase in diameter and girth. Lateral meristem usually occurs beneath the bark of the tree as cork cambium and in vascular bundles of dicotyledons as vascular cambium. The activity of this cambium forms secondary growth.
• Intercalary meristem: Located between permanent tissues, it is usually present at the base of the node, internode, and on leaf base. They are responsible for growth in length of the plant and increasing the size of the internode. They result in branch formation and growth.

The cells of meristematic tissue are similar in structure and have a thin and elastic primary cell wall made of cellulose. They are compactly arranged without inter-cellular spaces between them. Each cell contains a dense cytoplasm and a prominent cell nucleus. The dense protoplasm of meristematic cells contains very few vacuoles. Normally the meristematic cells are oval, polygonal, or rectangular in shape.

Meristematic tissue cells have a large nucleus with small or no vacuoles because they have no need to store anything, as opposed to their function of multiplying and increasing the girth and length of the plant, with no intercellular spaces.

Permanent tissues

Permanent tissues may be defined as a group of living or dead cells formed by meristematic tissue and have lost their ability to divide and have permanently placed at fixed positions in the plant body. Meristematic tissues that take up a specific role lose the ability to divide. This process of taking up a permanent shape, size and a function is called cellular differentiation. Cells of meristematic tissue differentiate to form different types of permanent tissues. There are 2 types of permanent tissues:

1. simple permanent tissues
2. complex permanent tissues

Simple permanent tissue

Simple permanent tissue is a group of cells which are similar in origin, structure, and function . They are of three types:

1. Parenchyma
2. Collenchyma
3. Sclerenchyma

Parenchyma

Parenchyma (Greek, para – 'beside'; enchyma– infusion – 'tissue') is the bulk of a substance. In plants, it consists of relatively unspecialized living cells with thin cell walls that are usually loosely packed so that intercellular spaces are found between cells of this tissue. These are generally isodiametric, in shape. They contain small number of vacuoles or sometimes they even may not contain any vacuole. Even if they do so the vacuole is of much smaller size than of normal animal cells. This tissue provides support to plants and also stores food. Chlorenchyma is a special type of parenchyma that contains chlorophyll and performs photosynthesis. In aquatic plants, aerenchyma tissues, or large air cavities, give support to float on water by making them buoyant. Parenchyma cells called idioblasts have metabolic waste. Spindle shape fiber also contained into this cell to support them and known as prosenchyma, succulent parenchyma also noted. In xerophytes, parenchyma tissues store water.

Collenchyma

Cross section of collenchyma cells

Collenchyma (Greek, ‘Colla’ means gum and ‘enchyma’ means infusion) is a living tissue of primary body like Parenchyma. Cells are thin-walled but possess thickening of cellulose, water and pectin substances (pectocellulose) at the corners where a number of cells join. This tissue gives tensile strength to the plant and the cells are compactly arranged and have very little inter-cellular spaces. It occurs chiefly in hypodermis of stems and leaves. It is absent in monocots and in roots.

Collenchymatous tissue acts as a supporting tissue in stems of young plants. It provides mechanical support, elasticity, and tensile strength to the plant body. It helps in manufacturing sugar and storing it as starch. It is present in the margin of leaves and resists tearing effect of the wind.

Sclerenchyma

Sclerenchyma (Greek, Sclerous means hard and enchyma means infusion) consists of thick-walled, dead cells and protoplasm is negligible. These cells have hard and extremely thick secondary walls due to uniform distribution and high secretion of lignin and have a function of providing mechanical support. They do not have inter-molecular space between them. Lignin deposition is so thick that the cell walls become strong, rigid and impermeable to water which is also known as a stone cell or sclereids. These tissues are mainly of two types: sclerenchyma fiber and sclereids. Sclerenchyma fibre cells have a narrow lumen and are long, narrow and unicellular. Fibers are elongated cells that are strong and flexible, often used in ropes. Sclereids have extremely thick cell walls and are brittle, and are found in nutshells and legumes.

Epidermis

The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. The entire surface of the plant has this outer layer of the epidermis. Hence it is also called surface tissue. Most of the epidermal cells are relatively flat. The outer and lateral walls of the cell are often thicker than the inner walls. The cells form a continuous sheet without intercellular spaces. It protects all parts of the plant. The outer epidermis is coated with a waxy thick layer called cutin which prevents loss of water. The epidermis also consists of stomata (singular:stoma) which helps in transpiration.

Complex permanent tissue

The complex permanent tissue consists of more than one type of cells having a common origin which work together as a unit. Complex tissues are mainly concerned with the transportation of mineral nutrients, organic solutes (food materials), and water. That's why it is also known as conducting and vascular tissue. The common types of complex permanent tissue are:

• Xylem (or wood)
• Phloem (or bast).

Xylem and phloem together form vascular bundles.

Xylem

Xylem (Greek, xylos = wood) serves as a chief conducting tissue of vascular plants. It is responsible for the conduction of water and inorganic solutes. Xylem consists of four kinds of cells:

• Tracheids
• Vessels (or tracheae)
• Xylem fibres or Xylem sclerenchyma
• Xylem parenchyma

Cross section of 2-year-old Tilia americana, highlighting xylem ray shape and orientation

Xylem tissue is organised in a tube-like fashion along the main axes of stems and roots. It consists of a combination of parenchyma cells, fibers, vessels, tracheids, and ray cells. Longer tubes made up of individual cellssels tracheids, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The end overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell.

Though most conduction in xylem tissue is vertical, lateral conduction along the diameter of a stem is facilitated via rays. Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium.

Phloem

Phloem consists of:

• Sieve tube
• Companion cell
• Phloem fibre
• Phloem parenchyma.

Phloem is an equally important plant tissue as it also is part of the 'plumbing system' of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls. The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers, parenchyma and ray cells. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Sieve-tube members that are alive contain a polymer called callose, a carbohydrate polymer, forming the callus pad/callus, the colourless substance that covers the sieve plate. Callose stays in solution as long as the cell contents are under pressure. Phloem transports food and materials in plants upwards and downwards as required.

Animal tissue

Animal tissues are grouped into four basic types: connective, muscle, nervous, and epithelial. Collections of tissues joined in units to serve a common function compose organs. While most animals can generally be considered to contain the four tissue types, the manifestation of these tissues can differ depending on the type of organism. For example, the origin of the cells comprising a particular tissue type may differ developmentally for different classifications of animals. Tissue appeared for the first time in the diploblasts, but modern forms only appeared in triploblasts.

The epithelium in all animals is derived from the ectoderm and endoderm (or their precursor in sponges), with a small contribution from the mesoderm, forming the endothelium, a specialized type of epithelium that composes the vasculature. By contrast, a true epithelial tissue is present only in a single layer of cells held together via occluding junctions called tight junctions, to create a selectively permeable barrier. This tissue covers all organismal surfaces that come in contact with the external environment such as the skin, the airways, and the digestive tract. It serves functions of protection, secretion, and absorption, and is separated from other tissues below by a basal lamina.

The connective tissue and the muscular are derived from the mesoderm. The neural tissue is derived from the ectoderm.

Epithelial tissue

Main article: Epithelium

The epithelial tissues are formed by cells that cover the organ surfaces, such as the surface of skin, the airways, surfaces of soft organs, the reproductive tract, and the inner lining of the digestive tract. The cells comprising an epithelial layer are linked via semi-permeable, tight junctions; hence, this tissue provides a barrier between the external environment and the organ it covers. In addition to this protective function, epithelial tissue may also be specialized to function in secretion, excretion and absorption. Epithelial tissue helps to protect organs from microorganisms, injury, and fluid loss.

Functions of epithelial tissue:

• The principle function of epithelial tissues are covering and lining of free surface
• The cells of the body's surface form the outer layer of skin.
• Inside the body, epithelial cells form the lining of the mouth and alimentary canal and protect these organs.
• Epithelial tissues help in the elimination of waste.
• Epithelial tissues secrete enzymes and/or hormones in the form of glands.
• Some epithelial tissue perform secretory functions. They secrete a variety of substances including sweat, saliva, mucus, enzymes.

There are many kinds of epithelium, and nomenclature is somewhat variable. Most classification schemes combine a description of the cell-shape in the upper layer of the epithelium with a word denoting the number of layers: either simple (one layer of cells) or stratified (multiple layers of cells). However, other cellular features such as cilia may also be described in the classification system. Some common kinds of epithelium are listed below:

• Simple squamous (pavement) epithelium
• Simple cuboidal epithelium
• Simple Columnar epithelium
• Simple ciliated (pseudostratified) columnar epithelium
• Simple glandular columnar epithelium
• Stratified non-keratinized squamous epithelium
• Stratified keratinized epithelium
• Stratified transitional epithelium

Connective tissue

Main article: Connective tissue

Connective tissues are fibrous tissues made up of cells separated by non-living material, which is called an extracellular matrix. This matrix can be liquid or rigid. For example, blood contains plasma as its matrix and bone's matrix is rigid. Connective tissue gives shape to organs and holds them in place. Blood, bone, tendon, ligament, adipose, and areolar tissues are examples of connective tissues. One method of classifying connective tissues is to divide them into three types: fibrous connective tissue, skeletal connective tissue, and fluid connective tissue.

Muscular tissue

Main article: Muscle tissue

Muscle cells form the active contractile tissue of the body known as muscle tissue or muscular tissue. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle tissue is separated into three distinct categories: visceral or smooth muscle, found in the inner linings of organs; skeletal muscle, typically attached to bones, which generate gross movement; and cardiac muscle, found in the heart, where it contracts to pump blood throughout an organism.

Nervous tissue

Main article: Nervous tissue

Cells comprising the central nervous system and peripheral nervous system are classified as nervous (or neural) tissue. In the central nervous system, neural tissues form the brain and spinal cord. In the peripheral nervous system, neural tissues form the cranial nerves and spinal nerves, inclusive of the motor neurons.

Mineralized tissues

Main article: Mineralized tissues

Mineralized tissues are biological tissues that incorporate minerals into soft matrices. Such tissues may be found in both plants and animals,

History

Xavier Bichat (1771–1802)

Xavier Bichat introduced word tissue into the study of anatomy by 1801. He was "the first to propose that tissue is a central element in human anatomy, and he considered organs as collections of often disparate tissues, rather than as entities in themselves". Although he worked without a microscope, Bichat distinguished 21 types of elementary tissues from which the organs of the human body are composed, a number later reduced by other authors.