Search This Blog

Monday, October 10, 2022

Engineer

From Wikipedia, the free encyclopedia

Engineer
Kitty Joyner - Electrical Engineer - GPN-2000-001933.jpg
Electrical engineer Kitty Joyner in 1952

NamesEngineer
Occupation type
Profession
Activity sectors
Applied science
CompetenciesMathematics, science, design, analysis, critical thinking, engineering ethics, project management, engineering economics, creativity, problem solving, (See also: Glossary of engineering)
Education required
Engineering education
Fields of
employment
Research and development, industry, business
Related jobs
Scientist, architect, project manager, inventor, astronaut

Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the limitations imposed by practicality, regulation, safety and cost. The word engineer (Latin ingeniator) is derived from the Latin words ingeniare ("to contrive, devise") and ingenium ("cleverness"). The foundational qualifications of an engineer typically include a four-year bachelor's degree in an engineering discipline, or in some jurisdictions, a master's degree in an engineering discipline plus four to six years of peer-reviewed professional practice (culminating in a project report or thesis) and passage of engineering board examinations. A professional engineer is typically a person registered under an Engineering Council.

The work of engineers forms the link between scientific discoveries and their subsequent applications to human and business needs and quality of life.

Definition

In 1961, the Conference of Engineering Societies of Western Europe and the United States of America defined "professional engineer" as follows:

A professional engineer is competent by virtue of his/her fundamental education and training to apply the scientific method and outlook to the analysis and solution of engineering problems. He/she is able to assume personal responsibility for the development and application of engineering science and knowledge, notably in research, design, construction, manufacturing, superintending, managing and in the education of the engineer. His/her work is predominantly intellectual and varied and not of a routine mental or physical character. It requires the exercise of original thought and judgement and the ability to supervise the technical and administrative work of others. His/her education will have been such as to make him/her capable of closely and continuously following progress in his/her branch of engineering science by consulting newly published works on a worldwide basis, assimilating such information and applying it independently. He/she is thus placed in a position to make contributions to the development of engineering science or its applications. His/her education and training will have been such that he/she will have acquired a broad and general appreciation of the engineering sciences as well as thorough insight into the special features of his/her own branch. In due time he/she will be able to give authoritative technical advice and to assume responsibility for the direction of important tasks in his/her branch.

Roles and expertise

Design

Engineers develop new technological solutions. During the engineering design process, the responsibilities of the engineer may include defining problems, conducting and narrowing research, analyzing criteria, finding and analyzing solutions, and making decisions. Much of an engineer's time is spent on researching, locating, applying, and transferring information. Indeed, research suggests engineers spend 56% of their time engaged in various information behaviours, including 14% actively searching for information.

Engineers must weigh different design choices on their merits and choose the solution that best matches the requirements and needs. Their crucial and unique task is to identify, understand, and interpret the constraints on a design in order to produce a successful result.

Analysis

Engineers conferring on prototype design, 1954

Engineers apply techniques of engineering analysis in testing, production, or maintenance. Analytical engineers may supervise production in factories and elsewhere, determine the causes of a process failure, and test output to maintain quality. They also estimate the time and cost required to complete projects. Supervisory engineers are responsible for major components or entire projects. Engineering analysis involves the application of scientific analytic principles and processes to reveal the properties and state of the system, device or mechanism under study. Engineering analysis proceeds by separating the engineering design into the mechanisms of operation or failure, analyzing or estimating each component of the operation or failure mechanism in isolation, and recombining the components. They may analyze risk.

Many engineers use computers to produce and analyze designs, to simulate and test how a machine, structure, or system operates, to generate specifications for parts, to monitor the quality of products, and to control the efficiency of processes.

Specialization and management

NASA Launch Control Center Firing Room 2 as it appeared in the Apollo era

Most engineers specialize in one or more engineering disciplines. Numerous specialties are recognized by professional societies, and each of the major branches of engineering has numerous subdivisions. Civil engineering, for example, includes structural and transportation engineering and materials engineering include ceramic, metallurgical, and polymer engineering. Mechanical engineering cuts across most disciplines since its core essence is applied physics. Engineers also may specialize in one industry, such as motor vehicles, or in one type of technology, such as turbines or semiconductor materials.

Several recent studies have investigated how engineers spend their time; that is, the work tasks they perform and how their time is distributed among these. Research suggests that there are several key themes present in engineers' work: technical work (i.e., the application of science to product development), social work (i.e., interactive communication between people), computer-based work and information behaviors. Among other more detailed findings, a 2012 work sampling study found that engineers spend 62.92% of their time engaged in technical work, 40.37% in social work, and 49.66% in computer-based work. Furthermore, there was considerable overlap between these different types of work, with engineers spending 24.96% of their time engaged in technical and social work, 37.97% in technical and non-social, 15.42% in non-technical and social, and 21.66% in non-technical and non-social.

Engineering is also an information-intensive field, with research finding that engineers spend 55.8% of their time engaged in various different information behaviors, including 14.2% actively information from other people (7.8%) and information repositories such as documents and databases (6.4%).

The time engineers spend engaged in such activities is also reflected in the competencies required in engineering roles. In addition to engineers’ core technical competence, research has also demonstrated the critical nature of their personal attributes, project management skills, and cognitive abilities to success in the role.

Types of engineers

There are many branches of engineering, each of which specializes in specific technologies and products. Typically, engineers will have deep knowledge in one area and basic knowledge in related areas. For example, mechanical engineering curricula typically include introductory courses in electrical engineering, computer science, materials science, metallurgy, mathematics, and software engineering.

An engineer may either be hired for a firm that requires engineers on a continuous basis, or may belong to an engineering firm that provides engineering consulting services to other firms.

When developing a product, engineers typically work in interdisciplinary teams. For example, when building robots an engineering team will typically have at least three types of engineers. A mechanical engineer would design the body and actuators. An electrical engineer would design the power systems, sensors, electronics, embedded software in electronics, and control circuitry. Finally, a software engineer would develop the software that makes the robot behave properly. Engineers that aspire to management engage in further study in business administration, project management and organizational or business psychology. Often engineers move up the management hierarchy from managing projects, functional departments, divisions and eventually CEOs of a multi-national corporation.

Branch Focus Related sciences Products
Automobile Engineering Focuses on the development of Automobiles and related technology Structural Engineering, Electronics, Electrical/electronics, Materials science, Safety, Fluid mechanics, Thermodynamics, engineering mathematics, energy, aesthetics, Ergonomics, Compliance, Road safety, Passenger Safety, Pedestrian Safety , Chemistry, hamolidation Automobiles
Aerospace engineering Focuses on the development of aircraft and spacecraft Aeronautics, astrodynamics, astronautics, avionics, control engineering, fluid mechanics, kinematics, materials science, thermodynamics Aircraft, robotics, spacecraft, trajectories
Agricultural engineering Focuses on the Engineering, Science and Technology for the production and processing of Food from Agriculture, such as the production of arable crops, soft fruit and livestock. A key goal of this discipline is to improve the efficacy and sustainability of agricultural practices for food production. Agricultural engineering often combines and converges many other engineering disciplines such as Mechanical engineering, Civil engineering, Electrical engineering, Chemical engineering, Biosystems engineering, Soil science, Environmental engineering, etc.. Livestock, Food, Horticulture, Forestry, Renewable Energy Crops.

Agricultural machinery such as Tractors, Combine Harvesters, Forage Harvesters, etc.

Agricultural technology incorporates such things as Robotics and Autonomous Vehicles.

Architectural engineering and building engineering Focuses on building and construction Architecture, architectural technology Buildings and bridges
Biomedical engineering Focuses on closing the gap between engineering and medicine to advance various health care treatments. Biology, physics, chemistry, medicine Prostheses, medical devices, regenerative tissue growth, various safety mechanisms, genetic engineering
Chemical engineering Focuses on the manufacturing of chemicals and or extraction of chemical species from natural resources Chemistry, thermodynamics, chemical thermodynamics, process engineering, Transport phenomena, nanotechnology, biology, Chemical kinetics, genetic engineering medicine, Fluid mechanics, Textile Chemicals, Hydrocarbons, Fuels, medicines, raw materials, food and drink, Waste treatment, Pure gases, Plastics, Coatings, Water treatment, Textiles
Civil engineering Focuses on the construction of large systems, structures, and environmental systems Statics, fluid mechanics, soil mechanics, structural engineering, transportation engineering, geotechnical engineering, environmental engineering, hydraulic engineering Roads, bridges, dams, buildings, structural system, foundation, earthworks, waste management, water treatment
Computer engineering Focuses on the design and development of computer hardware & software systems Computer science, mathematics, electrical engineering Microprocessors, microcontrollers, operating systems, embedded systems, computer networks
Electrical engineering Focuses on application of electricity, electronics, and electromagnetism Mathematics, probability and statistics, engineering ethics, engineering economics, instrumentation, materials science, physics, network analysis, electromagnetism, linear system, electronics, electric power, logic, computer science, data transmission, systems engineering, control engineering, signal processing Electricity generation and equipment, remote sensing, robotics, control system, computers, home appliances, Internet of things, consumer electronics, avionics, hybrid vehicles, spacecraft, unmanned aerial vehicles, optoelectronics, embedded systems
Fire protection engineering Focuses on application of science and engineering principles to protect people, property, and their environments from the harmful and destructive effects of fire and smoke. Fire, smoke, fluid dynamics, thermodynamics, heat transfer, combustion, physics, materials science, chemistry, statics, dynamics, probabilistic risk assessment or risk management, environmental psychology, engineering ethics, engineering economics, systems engineering, reliability, fire suppression, fire alarm, building fire safety, wildfire, building codes, measurement and simulation of fire phenomena, mathematics, probability and statistics. Fire suppression systems, fire alarm systems, passive fire protection, smoke control systems, sprinkler systems, Code consulting, fire and smoke modeling, emergency management, water supply systems, fire pumps, structural fire protection, foam extinguishing systems, gaseous fire suppression systems, oxygen reduction systems, flame detection, aerosol fire suppression.
Industrial engineering Focuses on the design, optimization, and operation of production, logistics, and service systems and processes Operations research, engineering statistics, applied probability and stochastic processes, automation engineering, methods engineering, production engineering, manufacturing engineering, systems engineering, logistics engineering, ergonomics quality control systems, manufacturing systems, warehousing systems, supply chains, logistics networks, queueing systems, business process management
Mechatronics engineering Focuses on the technology and controlling all the industrial field Process control, automation Robotics, controllers, CNC
Mechanical engineering Focuses on the development and operation of energy systems, transport systems, manufacturing systems, machines and control systems Dynamics, kinematics, statics, fluid mechanics, materials science, metallurgy, strength of materials, thermodynamics, heat transfer, mechanics, mechatronics, manufacturing engineering, control engineering Cars, airplanes, machines, power generation, spacecraft, buildings, consumer goods, manufacturing, HVAC
Metallurgical engineering/materials engineering Focuses on extraction of metals from its ores and development of new materials Material science, thermodynamics, extraction of metals, physical metallurgy, mechanical metallurgy, nuclear materials, steel technology Iron, steel, polymers, ceramics, metals
Mining engineering Focuses on the use of applied science and technology to extract various minerals from the earth, not to be confused with metallurgical engineering, which deals with mineral processing of various ores after they have already been mined Rock mechanics, geostatistics, soil mechanics, control engineering, geophysics, fluid mechanics, drilling and blasting Gold, silver, coal, iron ore, potash, limestone, diamond, rare-earth element, bauxite, copper
Software engineering Focuses on the design and development of software systems Computer science, information theory, systems engineering, formal language Application software, Mobile apps, Websites, Operating systems, Embedded systems, Real-time computing, Video games, Virtual reality, AI software, Edge computing, Distributed systems, Computer vision, Music sequencer, Digital audio workstation, Software synthesizer, Robotics, CGI, Medical software, Computer-assisted surgery, Internet of things, Avionics software, Computer simulation, Quantum programming, Satellite navigation software, Antivirus software, Electronic design automation, Computer-aided design, Self-driving car, Educational software

Ethics

The Challenger disaster is held as a case study of engineering ethics.

Engineers have obligations to the public, their clients, employers, and the profession. Many engineering societies have established codes of practice and codes of ethics to guide members and inform the public at large. Each engineering discipline and professional society maintains a code of ethics, which the members pledge to uphold. Depending on their specializations, engineers may also be governed by specific statute, whistleblowing, product liability laws, and often the principles of business ethics.

An engineer receiving his Order of the Engineer ring

Some graduates of engineering programs in North America may be recognized by the iron ring or Engineer's Ring, a ring made of iron or stainless steel that is worn on the little finger of the dominant hand. This tradition began in 1925 in Canada with The Ritual of the Calling of an Engineer, where the ring serves as a symbol and reminder of the engineer's obligations to the engineering profession. In 1972, the practice was adopted by several colleges in the United States including members of the Order of the Engineer.

Education

Most engineering programs involve a concentration of study in an engineering specialty, along with courses in both mathematics and the physical and life sciences. Many programs also include courses in general engineering and applied accounting. A design course, often accompanied by a computer or laboratory class or both, is part of the curriculum of most programs. Often, general courses not directly related to engineering, such as those in the social sciences or humanities, also are required.

Accreditation is the process by which engineering programs are evaluated by an external body to determine if applicable standards are met. The Washington Accord serves as an international accreditation agreement for academic engineering degrees, recognizing the substantial equivalency in the standards set by many major national engineering bodies. In the United States, post-secondary degree programs in engineering are accredited by the Accreditation Board for Engineering and Technology.

Regulation

In many countries, engineering tasks such as the design of bridges, electric power plants, industrial equipment, machine design and chemical plants, must be approved by a licensed professional engineer. Most commonly titled professional engineer is a license to practice and is indicated with the use of post-nominal letters; PE or P.Eng. These are common in North America, as is European engineer (EUR ING) in Europe. The practice of engineering in the UK is not a regulated profession but the control of the titles of chartered engineer (CEng) and incorporated engineer (IEng) is regulated. These titles are protected by law and are subject to strict requirements defined by the Engineering Council UK. The title CEng is in use in much of the Commonwealth.

Many skilled and semi-skilled trades and engineering technicians in the UK call themselves engineers. A growing movement in the UK is to legally protect the title 'Engineer' so that only professional engineers can use it; a petition was started to further this cause.

In the United States, engineering is a regulated profession whose practice and practitioners are licensed and governed by law. Licensure is generally attainable through combination of education, pre-examination (Fundamentals of Engineering exam), examination (professional engineering exam), and engineering experience (typically in the area of 5+ years). Each state tests and licenses professional engineers. Currently, most states do not license by specific engineering discipline, but rather provide generalized licensure, and trust engineers to use professional judgment regarding their individual competencies; this is the favoured approach of the professional societies. Despite this, at least one of the examinations required by most states is actually focused on a particular discipline; candidates for licensure typically choose the category of examination which comes closest to their respective expertise. In the United States, an "industrial exemption" allows businesses to employ employees and call them an "engineer", as long as such individuals are under the direct supervision and control of the business entity and function internally related to manufacturing (manufactured parts) related to the business entity, or work internally within an exempt organization. Such person does not have the final authority to approve, or the ultimate responsibility for, engineering designs, plans, or specifications that are to be incorporated into fixed works, systems, or facilities on the property of others or made available to the public. These individuals are prohibited from offering engineering services directly to the public or other businesses, or engage in practice of engineering unless the business entity is registered with the state's board of engineering, and the practice is carried on or supervised directly only by engineers licensed to engage in the practice of engineering. In some instances, some positions, such as a "sanitation engineer", does not have any basis in engineering sciences. Although some states require a BS degree in engineering accredited by the Engineering Accreditation Commission (EAC) of Accreditation Board for Engineering and Technology (ABET) with no exceptions, about two thirds of the states accept BS degrees in engineering technology accredited by the Engineering Technology Accreditation Commission (ETAC) of ABET to become licensed as professional engineers. Each state has different requirements on years of experience to take the Fundamentals of Engineering (FE) and Professional Engineering (PE) exams. A few states require a graduate MS in engineering to sit for the exams as further learning. After seven years of working after graduation, two years of responsibility for significant engineering work, continuous professional development, some highly qualified PEs are able to become International Professional Engineers Int(PE). These engineers must meet the highest level of professional competencies and this is a peer reviewed process. Once the IntPE title is awarded, the engineer can gain easier admission to national registers of a number of members jurisdictions for international practice.

In Canada, engineering is a self-regulated profession. The profession in each province is governed by its own engineering association. For instance, in the Province of British Columbia an engineering graduate with four or more years of post graduate experience in an engineering-related field and passing exams in ethics and law will need to be registered by the Association for Professional Engineers and Geoscientists (APEGBC) in order to become a Professional Engineer and be granted the professional designation of P.Eng allowing one to practice engineering.

In Continental Europe, Latin America, Turkey, and elsewhere the title is limited by law to people with an engineering degree and the use of the title by others is illegal. In Italy, the title is limited to people who hold an engineering degree, have passed a professional qualification examination (Esame di Stato) and are enrolled in the register of the local branch of National Associations of Engineers (a public body). In Portugal, professional engineer titles and accredited engineering degrees are regulated and certified by the Ordem dos Engenheiros. In the Czech Republic, the title "engineer" (Ing.) is given to people with a (masters) degree in chemistry, technology or economics for historical and traditional reasons. In Greece, the academic title of "Diploma Engineer" is awarded after completion of the five-year engineering study course and the title of "Certified Engineer" is awarded after completion of the four-year course of engineering studies at a Technological Educational Institute (TEI).

Perception

The perception and definition of the term 'engineer' varies across countries and continents.

19th-century engineer Isambard Kingdom Brunel by the launching chains of the SS Great Eastern

UK

British school children in the 1950s were brought up with stirring tales of "the Victorian Engineers", chief among whom were Brunel, Stephenson, Telford, and their contemporaries. In the UK, "engineering" has more recently been erroneously styled as an industrial sector consisting of employers and employees loosely termed "engineers" who include tradespeople. However, knowledgeable practitioners reserve the term "engineer" to describe a university-educated professional of ingenuity represented by the Chartered (or Incorporated) Engineer qualifications. A large proportion of the UK public incorrectly thinks of "engineers" as skilled tradespeople or even semi-skilled tradespeople with a high school education. Also, many UK skilled and semi-skilled tradespeople falsely style themselves as "engineers". This has created confusion in the eyes of some members of the public in understanding what professional engineers actually do, from fixing car engines, television sets and refrigerators (technicians, handymen) to designing and managing the development of aircraft, spacecraft, power stations, infrastructure and other complex technological systems (engineers).

France

In France, the term ingénieur (engineer) is not a protected title and can be used by anyone who practices this profession.

However, the title ingénieur diplomé (graduate engineer) is an official academic title that is protected by the government and is associated with the Diplôme d'Ingénieur, which is a renowned academic degree in France. Anyone misusing this title in France can be fined a large sum and jailed, as it is usually reserved for graduates of French engineering grandes écoles. Engineering schools which were created during the French revolution have a special reputation among the French people, as they helped to make the transition from a mostly agricultural country of late 18th century to the industrially developed France of the 19th century. A great part of 19th-century France's economic wealth and industrial prowess was created by engineers that have graduated from Ecole Centrale Paris, Ecole des Mines de Paris, Ecole Polytechnique or Télécom Paris. This was also the case after WWII when France had to be rebuilt. Before the "réforme René Haby" in the 1970s, it was very difficult to be admitted to such schools, and the French ingénieurs were commonly perceived as the nation's elite. However, after the Haby reform and a string of further reforms (Modernization plans of French universities), several engineering schools were created which can be accessed with relatively lower competition.

In France, engineering positions are now shared between the ingénieurs diplomés graduating from engineering grandes écoles; and the holders of a Master's degree in Science from public universities. Engineers are less highlighted in current French economy as industry provides less than a quarter of the GDP.

Italy

In Italy, only people who hold a formal engineering qualification of at least a bachelor's degree are permitted to describe themselves as an engineer. So much so that people holding such qualifications are entitled to use the pre-nominal title of "Ingegnere" (or "Ingegnera" if female - in both cases often abbreviated to "Ing.") in lieu of "Signore", "Signorina" or "Signora" (Mr, Miss and Mrs respectively) in the same manner as someone holding a doctorate would use the pre-nominal title "Doctor".

Spanish-speaking countries

Certain Spanish-speaking countries follow the Italian convention of engineers using the pre-nominal title, in this case "ingeniero" (or "ingeniera" if female). Like in Italy, it is usually abbreviated to "Ing." In Spain this practice is not followed.

The engineering profession enjoys high prestige in Spain, ranking close to medical doctors, scientists and professors, and above judges, journalists or entrepreneurs.

Europe

As of 2022, thirty two countries in Europe (including nearly all 27 countries of the EU) now recognise the title of 'European Engineer' which permits the use of the pre-nominal title of "EUR ING" (always fully capitalised). Each country sets its own precise qualification requirement for the use of the title (though they are all broadly equivalent). Holding the requisite qualification does not afford automatic entitlement. The title has to be applied for (and the appropriate fee paid). The holder is entitled to use the title in their passport. EUR INGs are allowed to describe themselves as professionally qualified engineers and practise as such in any of the 32 participating countries including those where the title of engineer is regulated by law.

United States

In the United States, the practice of professional engineering is highly regulated and the title "professional engineer" is legally protected, meaning that it is unlawful to use it to offer engineering services to the public unless permission, certification or other official endorsement is specifically granted by that state through a professional engineering license.

Canada

In Canada, engineering is a regulated profession whose practice and practitioners are licensed and governed by law. Licensed professional engineers are referred to as P.Eng. A 2002 study by the Ontario Society of Professional Engineers revealed that engineers are the third most respected professionals behind doctors and pharmacists. Many Canadian engineers wear an Iron Ring.

In all Canadian provinces, the title "Engineer" is protected by law and any non-licensed individual or company using the title is committing a legal offence and is subject to fines and restraining orders. Companies usually prefer not to use the title except for license holders because of liability reasons; for example, if the company filed a lawsuit and the judge, investigators, or lawyers found that the company is using the word engineer for non-licensed employees this could be used by opponents to hinder the company's efforts.

Asia and Africa

In the Indian subcontinent, Russia, Middle East, Africa, and China, engineering is one of the most sought after undergraduate courses, inviting thousands of applicants to show their ability in highly competitive entrance examinations.

In Egypt, the educational system makes engineering the second-most-respected profession in the country (after medicine); engineering colleges at Egyptian universities requires extremely high marks on the General Certificate of Secondary Education (Arabic: الثانوية العامة al-Thānawiyyah al-`Āmmah)—on the order of 97 or 98%—and are thus considered (along with the colleges of medicine, natural science, and pharmacy) to be among the "pinnacle colleges" (كليات القمة kullīyāt al-qimmah).

In the Philippines and Filipino communities overseas, engineers who are either Filipino or not, especially those who also profess other jobs at the same time, are addressed and introduced as Engineer, rather than Sir/Madam in speech or Mr./Mrs./Ms. (G./Gng./Bb. in Filipino) before surnames. That word is used either in itself or before the given name or surname.

Corporate culture

In companies and other organizations, there is sometimes a tendency to undervalue people with advanced technological and scientific skills compared to celebrities, fashion practitioners, entertainers, and managers. In his book, The Mythical Man-Month, Fred Brooks Jr says that managers think of senior people as "too valuable" for technical tasks and that management jobs carry higher prestige. He tells how some laboratories, such as Bell Labs, abolish all job titles to overcome this problem: a professional employee is a "member of the technical staff." IBM maintains a dual ladder of advancement; the corresponding managerial and engineering or scientific rungs are equivalent. Brooks recommends that structures need to be changed; the boss must give a great deal of attention to keeping managers and technical people as interchangeable as their talents allow.

Free-radical theory of aging

From Wikipedia, the free encyclopedia

The free radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time. A free radical is any atom or molecule that has a single unpaired electron in an outer shell. While a few free radicals such as melanin are not chemically reactive, most biologically relevant free radicals are highly reactive. For most biological structures, free radical damage is closely associated with oxidative damage. Antioxidants are reducing agents, and limit oxidative damage to biological structures by passivating them from free radicals.

Strictly speaking, the free radical theory is only concerned with free radicals such as superoxide ( O2 ), but it has since been expanded to encompass oxidative damage from other reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), or peroxynitrite (OONO).

Denham Harman first proposed the free radical theory of aging in the 1950s, and in the 1970s extended the idea to implicate mitochondrial production of ROS.

In some model organisms, such as yeast and Drosophila, there is evidence that reducing oxidative damage can extend lifespan. However, in mice, only 1 of the 18 genetic alterations (SOD-1 deletion) that block antioxidant defences, shortened lifespan. Similarly, in roundworms (Caenorhabditis elegans), blocking the production of the naturally occurring antioxidant superoxide dismutase has recently been shown to increase lifespan. Whether reducing oxidative damage below normal levels is sufficient to extend lifespan remains an open and controversial question.

Background

The free radical theory of aging was conceived by Denham Harman in the 1950s, when prevailing scientific opinion held that free radicals were too unstable to exist in biological systems. This was also before anyone invoked free radicals as a cause of degenerative diseases. Two sources inspired Harman: 1) the rate of living theory, which holds that lifespan is an inverse function of metabolic rate which in turn is proportional to oxygen consumption, and 2) Rebbeca Gershman's observation that hyperbaric oxygen toxicity and radiation toxicity could be explained by the same underlying phenomenon: oxygen free radicals. Noting that radiation causes "mutation, cancer and aging", Harman argued that oxygen free radicals produced during normal respiration would cause cumulative damage which would eventually lead to organismal loss of functionality, and ultimately death.

In later years, the free radical theory was expanded to include not only aging per se, but also age-related diseases. Free radical damage within cells has been linked to a range of disorders including cancer, arthritis, atherosclerosis, Alzheimer's disease, and diabetes. There has been some evidence to suggest that free radicals and some reactive nitrogen species trigger and increase cell death mechanisms within the body such as apoptosis and in extreme cases necrosis.

In 1972, Harman modified his original theory. In its current form, this theory proposes that reactive oxygen species (ROS) that are produced in the mitochondria, causes damage to certain macromolecules including lipids, proteins and most importantly mitochondrial DNA. This damage then causes mutations which lead to an increase of ROS production and greatly enhance the accumulation of free radicals within cells. This mitochondrial theory has been more widely accepted that it could play a major role in contributing to the aging process.

Since Harman first proposed the free radical theory of aging, there have been continual modifications and extensions to his original theory.

Processes

In chemistry, a free radical is any atom, molecule or ion with an unpaired valence electron.

Free radicals are atoms or molecules containing unpaired electrons. Electrons normally exist in pairs in specific orbitals in atoms or molecules. Free radicals, which contain only a single electron in any orbital, are usually unstable toward losing or picking up an extra electron, so that all electrons in the atom or molecule will be paired.

The unpaired electron does not imply charge; free radicals can be positively charged, negatively charged, or neutral.

Damage occurs when the free radical encounters another molecule and seeks to find another electron to pair its unpaired electron. The free radical often pulls an electron off a neighboring molecule, causing the affected molecule to become a free radical itself. The new free radical can then pull an electron off the next molecule, and a chemical chain reaction of radical production occurs. The free radicals produced in such reactions often terminate by removing an electron from a molecule which becomes changed or cannot function without it, especially in biology. Such an event causes damage to the molecule, and thus to the cell that contains it (since the molecule often becomes dysfunctional).

The chain reaction caused by free radicals can lead to cross-linking of atomic structures. In cases where the free radical-induced chain reaction involves base pair molecules in a strand of DNA, the DNA can become cross-linked.

DNA cross-linking can in turn lead to various effects of aging, especially cancer. Other cross-linking can occur between fat and protein molecules, which leads to wrinkles. Free radicals can oxidize LDL, and this is a key event in the formation of plaque in arteries, leading to heart disease and stroke. These are examples of how the free-radical theory of aging has been used to neatly "explain" the origin of many chronic diseases.

Free radicals that are thought to be involved in the process of aging include superoxide and nitric oxide. Specifically, an increase in superoxide affects aging whereas a decrease in nitric oxide formation, or its bioavailability, does the same.

Antioxidants are helpful in reducing and preventing damage from free radical reactions because of their ability to donate electrons which neutralize the radical without forming another. Ascorbic acid, for example, can lose an electron to a free radical and remain stable itself by passing its unstable electron around the antioxidant molecule.

This has led to the hypothesis that large amounts of antioxidants, with their ability to decrease the numbers of free radicals, might lessen the radical damage causing chronic diseases, and even radical damage responsible for aging.

Evidence

Numerous studies have demonstrated a role for free radicals in the aging process and thus tentatively support the free radical theory of aging. Studies have shown a significant increase in superoxide radical (SOR) formation and lipid peroxidation in aging rats. Chung et al. suggest ROS production increases with age and indicated the conversion of XDH to XOD may be an important contributing factor. This was supported by a study that showed superoxide production by xanthine oxidase and NO synthase in mesenteric arteries was higher in older rats than young ones.

Hamilton et al. examined the similarities in impaired endothelial function in hypertension and aging in humans and found a significant overproduction of superoxide in both. This finding is supported by a 2007 study which found that endothelial oxidative stress develops with aging in healthy men and is related to reductions in endothelium-dependent dilation. Furthermore, a study using cultured smooth muscle cells displayed increased ROS in cells derived from older mice. These findings were supported by a second study using Leydig cells isolated from the testes of young and old rats.

The Choksi et al. experiment with Ames dwarf (DW) mice suggests the lower levels of endogenous ROS production in DW mice may be a factor in their resistance to oxidative stress and long life. Lener et al. suggest Nox4 activity increases oxidative damage in human umbilical vein endothelial cells via superoxide overproduction. Furthermore, Rodriguez-Manas et al. found endothelial dysfunction in human vessels is due to the collective effect of vascular inflammation and oxidative stress.

Sasaki et al. reported superoxide-dependent chemiluminescence was inversely proportionate to maximum lifespan in mice, Wistar rats, and pigeons. They suggest ROS signalling may be a determinant in the aging process. In humans, Mendoza-Nunez et al. propose an age of 60 years or older may be linked with increased oxidative stress. Miyazawa found mitochondrial superoxide anion production can lead to organ atrophy and dysfunction via mitochondrial- mediated apoptosis. In addition, they suggest mitochondrial superoxide anion plays an essential part in aging. Lund et al. demonstrated the role of endogenous extracellular superoxide dismutase in protecting against endothelial dysfunction during the aging process using mice.

Modifications of the theory

One of the main criticisms of the free radical theory of aging is directed at the suggestion that free radicals are responsible for the damage of biomolecules, thus being a major reason for cellular senescence and organismal aging. Several modifications have been proposed to integrate current research into the overall theory.

Mitochondrial theory of aging

Major sources of reactive oxygen species in living systems

The mitochondrial theory of aging was first proposed in 1978, and two years later, the mitochondrial free-radical theory of aging was introduced. The theory implicates the mitochondria as the chief target of radical damage, since there is a known chemical mechanism by which mitochondria can produce ROS, mitochondrial components such as mtDNA are not as well protected as nuclear DNA, and by studies comparing damage to nuclear and mtDNA that demonstrate higher levels of radical damage on the mitochondrial molecules. Electrons may escape from metabolic processes in the mitochondria like the Electron transport chain, and these electrons may in turn react with water to form ROS such as the superoxide radical, or via an indirect route the hydroxyl radical. These radicals then damage the mitochondria's DNA and proteins, and these damage components in turn are more liable to produce ROS byproducts. Thus a positive feedback loop of oxidative stress is established that, over time, can lead to the deterioration of cells and later organs and the entire body.

This theory has been widely debated and it is still unclear how ROS induced mtDNA mutations develop. Conte et al. suggest iron-substituted zinc fingers may generate free radicals due to the zinc finger proximity to DNA and thus lead to DNA damage.

Afanas'ev suggests the superoxide dismutation activity of CuZnSOD demonstrates an important link between life span and free radicals. The link between CuZnSOD and life span was demonstrated by Perez et al. who indicated mice life span was affected by the deletion of the Sod1 gene which encodes CuZnSOD.

Contrary to the usually observed association between mitochondrial ROS (mtROS) and a decline in longevity, Yee et al. recently observed increased longevity mediated by mtROS signaling in an apoptosis pathway. This serves to support the possibility that observed correlations between ROS damage and aging are not necessarily indicative of the causal involvement of ROS in the aging process but are more likely due to their modulating signal transduction pathways that are part of cellular responses to the aging process.

Epigenetic oxidative redox shift (EORS) theory of aging

Brewer proposed a theory which integrates the free radical theory of aging with the insulin signalling effects in aging. Brewer's theory suggests "sedentary behaviour associated with age triggers an oxidized redox shift and impaired mitochondrial function". This mitochondrial impairment leads to more sedentary behaviour and accelerated aging.

Metabolic stability theory of aging

The metabolic stability theory of aging suggests it is the cells ability to maintain stable concentration of ROS which is the primary determinant of lifespan. This theory criticizes the free radical theory because it ignores that ROS are specific signalling molecules which are necessary for maintaining normal cell functions.

Mitohormesis

Oxidative stress may promote life expectancy of Caenorhabditis elegans by inducing a secondary response to initially increased levels of ROS. In mammals, the question of the net effect of reactive oxygen species on aging is even less clear. Recent epidemiological findings support the process of mitohormesis in humans, and even suggest that the intake of exogenous antioxidants may increase disease prevalence in humans (according to the theory, because they prevent the stimulation of the organism's natural response to the oxidant compounds which not only neutralizes them but provides other benefits as well).

Effects of calorie restriction

Studies have demonstrated that calorie restriction displays positive effects on the lifespan of organisms even though it is accompanied by increases in oxidative stress. Many studies suggest this may be due to anti-oxidative action, oxidative stress suppression, or oxidative stress resistance which occurs in calorie restriction. Fontana et al. suggest calorie restriction influenced numerous signal pathways through the reduction of insulin-like growth factor I (IGF-1). Additionally they suggest antioxidant SOD and catalase are involved in the inhibition of this nutrient signalling pathway.

The increase in life expectancy observed during some calorie restriction studies which can occur with lack of decreases or even increases in O2 consumption is often inferred as opposing the mitochondrial free radical theory of aging. According to a study by G. Barja, significant decreases in mitochondrial oxygen radical production (per unit of O2 consumed) occur during dietary restriction, aerobic exercise, chronic exercise, and hyperthyroidism. Additionally, mitochondrial oxygen radical generation is lower in long-lived birds than in short-lived mammals of comparable body size and metabolic rate. Thus, mitochondrial ROS production must be regulated independently of O2 consumption in a variety of species, tissues and physiologic states.

Challenges to the theory

Naked mole-rat

The naked mole-rat is a long-lived (32 years) rodent. As reviewed by Lewis et al., (2013), levels of ROS production in the naked mole rat are similar to that of another rodent, the relatively short-lived mouse (4 years). They concluded that it is not oxidative stress that modulates health-span and longevity in these rodents, but rather other cytoprotective mechanisms that allow animals to deal with high levels of oxidative damage and stress. In the naked mole-rat, a likely important cytoprotective mechanism that could provide longevity assurance is elevated expression of DNA repair genes involved in several key DNA repair pathways. (See DNA damage theory of aging.) Compared with the mouse, the naked mole rat had significantly higher expression levels of genes essential for the DNA repair pathways of DNA mismatch repair, non-homologous end joining and base excision repair.

Birds

Among birds, parrots live about five times longer than quail. ROS production in heart, skeletal muscle, liver and intact erythrocytes was found to be similar in parrots and quail and showed no correspondence with longevity difference. These findings were concluded to cast doubt on the robustness of the oxidative stress theory of aging.

Samaritans

From Wikipedia, the free encyclopedia https://en.wikipedia.org/w...