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Sunday, March 9, 2025

Bias

From Wikipedia, the free encyclopedia
Interpretations of the random patterns of craters on the Moon. A common example of a perceptual bias caused by pareidolia.

Bias is a disproportionate weight in favor of or against an idea or thing, usually in a way that is inaccurate, closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group, or a belief. In science and engineering, a bias is a systematic error. Statistical bias results from an unfair sampling of a population, or from an estimation process that does not give accurate results on average.

Etymology

The word appears to derive from Old Provençal into Old French biais, "sideways, askance, against the grain". Whence comes French biais, "a slant, a slope, an oblique".

It seems to have entered English via the game of bowls, where it referred to balls made with a greater weight on one side. Which expanded to the figurative use, "a one-sided tendency of the mind", and, at first especially in law, "undue propensity or prejudice". or ballast, used to lower the centre of gravity of a ship to increase stability and to keep the ship from one side.

Types

Cognitive biases

A cognitive bias is a repeating or basic misstep in thinking, assessing, recollecting, or other cognitive processes. That is, a pattern of deviation from standards in judgment, whereby inferences may be created unreasonably. People create their own "subjective social reality" from their own perceptions, their view of the world may dictate their behaviour. Thus, cognitive biases may sometimes lead to perceptual distortion, inaccurate judgment, illogical interpretation, or what is broadly called irrationality. However some cognitive biases are taken to be adaptive, and thus may lead to success in the appropriate situation. Furthermore, cognitive biases as an example through education may allow faster choice selection when speedier outcomes for a task are more valuable than precision. Other cognitive biases are a "by-product" of human processing limitations, coming about because of an absence of appropriate mental mechanisms, or just from human limitations in information processing.

Anchoring

Anchoring is a psychological heuristic that describes the propensity to rely on the first piece of information encountered when making decisions. According to this heuristic, individuals begin with an implicitly suggested reference point (the "anchor") and make adjustments to it to reach their estimate. For example, the initial price offered for a used car sets the standard for the rest of the negotiations, so that prices lower than the initial price seem more reasonable even if they are still higher than what the car is worth.

Apophenia

Apophenia, also known as patternicity, or agenticity, is the human tendency to perceive meaningful patterns within random data. Apophenia is well documented as a rationalization for gambling. Gamblers may imagine that they see patterns in the numbers which appear in lotteries, card games, or roulette wheels. One manifestation of this is known as the "gambler's fallacy".

Pareidolia is the visual or auditory form of apophenia. It has been suggested that pareidolia combined with hierophany may have helped ancient societies organize chaos and make the world intelligible.

Attribution bias

An attribution bias can happen when individuals assess or attempt to discover explanations behind their own and others' behaviors. People make attributions about the causes of their own and others' behaviors; but these attributions do not necessarily precisely reflect reality. Rather than operating as objective perceivers, individuals are inclined to perceptual slips that prompt biased understandings of their social world. When judging others we tend to assume their actions are the result of internal factors such as personality, whereas we tend to assume our own actions arise because of the necessity of external circumstances. There are a wide range of sorts of attribution biases, such as the ultimate attribution error, fundamental attribution error, actor-observer bias, and self-serving bias.

Examples of attribution bias:

Confirmation bias

A drawing of a man sitting on a stool at a writing desk
Confirmation bias has been described as an internal "yes man", echoing back a person's beliefs like Charles Dickens' character Uriah Heep.

Confirmation bias is the tendency to search for, interpret, favor, and recall information in a way that confirms one's beliefs or hypotheses while giving disproportionately less attention to information that contradicts it. The effect is stronger for emotionally charged issues and for deeply entrenched beliefs. People also tend to interpret ambiguous evidence as supporting their existing position. Biased search, interpretation and memory have been invoked to explain attitude polarization (when a disagreement becomes more extreme even though the different parties are exposed to the same evidence), belief perseverance (when beliefs persist after the evidence for them is shown to be false), the irrational primacy effect (a greater reliance on information encountered early in a series) and illusory correlation (when people falsely perceive an association between two events or situations). Confirmation biases contribute to overconfidence in personal beliefs and can maintain or strengthen beliefs in the face of contrary evidence. Poor decisions due to these biases have been found in political and organizational contexts.

Framing

Framing involves the social construction of social phenomena by mass media sources, political or social movements, political leaders, and so on. It is an influence over how people organize, perceive, and communicate about reality. It can be positive or negative, depending on the audience and what kind of information is being presented. For political purposes, framing often presents facts in such a way that implicates a problem that is in need of a solution. Members of political parties attempt to frame issues in a way that makes a solution favoring their own political leaning appear as the most appropriate course of action for the situation at hand. As understood in social theory, framing is a schema of interpretation, a collection of anecdotes and stereotypes, that individuals rely on to understand and respond to events. People use filters to make sense of the world, the choices they then make are influenced by their creation of a frame.

Cultural bias is the related phenomenon of interpreting and judging phenomena by standards inherent to one's own culture. Numerous such biases exist, concerning cultural norms for color, location of body parts, mate selection, concepts of justice, linguistic and logical validity, acceptability of evidence, and taboos. Ordinary people may tend to imagine other people as basically the same, not significantly more or less valuable, probably attached emotionally to different groups and different land.

Halo effect and horn effect

The halo effect and the horn effect are when an observer's overall impression of a person, organization, brand, or product influences their feelings about specifics of that entity's character or properties.

The name halo effect is based on the concept of the saint's halo, and is a specific type of confirmation bias, wherein positive sentiments in one area cause questionable or unknown characteristics to be seen positively. If the observer likes one aspect of something, they will have a positive predisposition toward everything about it. A person's appearance has been found to produce a halo effect. The halo effect is also present in the field of brand marketing, affecting perception of companies and non-governmental organizations (NGOs).

The opposite of the halo is the horn effect, when "individuals believe (that negative) traits are inter-connected." The term horn effect refers to Devil's horns. It works in a negative direction: if the observer dislikes one aspect of something, they will have a negative predisposition towards other aspects.

Self-serving bias

Self-serving bias is the tendency for cognitive or perceptual processes to be distorted by the individual's need to maintain and enhance self-esteem. It is the propensity to credit accomplishment to our own capacities and endeavors, yet attribute failure to outside factors, to dismiss the legitimacy of negative criticism, concentrate on positive qualities and accomplishments yet disregard flaws and failures. Studies have demonstrated that this bias can affect behavior in the workplace, in interpersonal relationships, playing sports, and in consumer decisions.

Status quo bias

Status quo bias is an emotional bias; a preference for the current state of affairs. The current baseline (or status quo) is taken as a reference point, and any change from that baseline is perceived as a loss. Status quo bias should be distinguished from a rational preference for the status quo ante, as when the current state of affairs is objectively superior to the available alternatives, or when imperfect information is a significant problem. A large body of evidence, however, shows that status quo bias frequently affects human decision-making.

Conflicts of interest

A conflict of interest is when a person or association has intersecting interests (financial, personal, etc.) which could potentially corrupt. The potential conflict is autonomous of actual improper actions, it can be found and intentionally defused before corruption, or the appearance of corruption, happens. "A conflict of interest is a set of circumstances that creates a risk that professional judgement or actions regarding a primary interest will be unduly influenced by a secondary interest." It exists if the circumstances are sensibly accepted to present a hazard that choices made may be unduly affected by auxiliary interests.

Corruption

A conflict of interest arises when a decision-maker participates in a corrupt act that seeks to influence the outcome in favor of a specific individual, organization, or entity in a decision-making process. For example, attempts to solicit a bribe or kickback in exchange for favoring a party creates a conflict of interest. A perceived conflict of interest may also arise in an individual who is offered such a payment, even if it is declined, particularly in situations where the attempt to bribe is not reported.

Laws restricting monetary transaction is appropriate can differ between jurisdictions based upon their criminal laws. For example, some nations criminalize the receipt of political campaign contributions in the form of cash, while other nations permit cash donations provided that donors otherwise adhere to election law.

Favoritism

Favoritism, sometimes known as in-group favoritism, or in-group bias, refers to a pattern of favoring members of one's in-group over out-group members. This can be expressed in evaluation of others, in allocation of resources, and in many other ways. This has been researched by psychologists, especially social psychologists, and linked to group conflict and prejudice. Cronyism is favoritism of long-standing friends, especially by appointing them to positions of authority, regardless of their qualifications. Nepotism is favoritism granted to relatives.

Lobbying

Box offered by tobacco lobbyists to Dutch Member of the European Parliament Kartika Liotard in September 2013

Lobbying is the attempt to influence choices made by administrators, frequently lawmakers or individuals from administrative agencies. Lobbyists may be among a legislator's constituencies, or not; they may engage in lobbying as a business, or not. Lobbying is often spoken of with contempt, the implication is that people with inordinate socioeconomic power are corrupting the law in order to serve their own interests. When people who have a duty to act on behalf of others, such as elected officials with a duty to serve their constituents' interests or more broadly the common good, stand to benefit by shaping the law to serve the interests of some private parties, there is a conflict of interest. This can lead to all sides in a debate looking to sway the issue by means of lobbyists.

Regulatory issues

Self-regulation is the process whereby an organization monitors its own adherence to legal, ethical, or safety standards, rather than have an outside, independent agency such as a third party entity monitor and enforce those standards. Self-regulation of any group can create a conflict of interest. If any organization, such as a corporation or government bureaucracy, is asked to eliminate unethical behavior within their own group, it may be in their interest in the short run to eliminate the appearance of unethical behavior, rather than the behavior itself.

Regulatory capture is a form of political corruption that can occur when a regulatory agency, created to act in the public interest, instead advances the commercial or political concerns of special interest groups that dominate the industry or sector it is charged with regulating. Regulatory capture occurs because groups or individuals with a high-stakes interest in the outcome of policy or regulatory decisions can be expected to focus their resources and energies in attempting to gain the policy outcomes they prefer, while members of the public, each with only a tiny individual stake in the outcome, will ignore it altogether. Regulatory capture is a risk to which a regulatory agency is exposed by its very nature.

Shilling

Shilling is deliberately giving spectators the feeling that one is an energetic autonomous client of a vendor for whom one is working. The effectiveness of shilling relies on crowd psychology to encourage other onlookers or audience members to purchase the goods or services (or accept the ideas being marketed). Shilling is illegal in some places, but legal in others. An example of shilling is paid reviews that give the impression of being autonomous opinions.

Statistical biases

Statistical bias is a systematic tendency in the process of data collection, which results in lopsided, misleading results. This can occur in any of a number of ways, in the way the sample is selected, or in the way data are collected. It is a property of a statistical technique or of its results whereby the expected value of the results differs from the true underlying quantitative parameter being estimated.

Forecast bias

A forecast bias is when there are consistent differences between results and the forecasts of those quantities; that is: forecasts may have an overall tendency to be too high or too low.

Observer-expectancy effect

The observer-expectancy effect is when a researcher's expectations cause them to subconsciously influence the people participating in an experiment. It is usually controlled using a double-blind system, and was an important reason for the development of double-blind experiments.

Reporting bias and social desirability bias

In epidemiology and empirical research, reporting bias is defined as "selective revealing or suppression of information" of undesirable behavior by subjects or researchers. It refers to a tendency to under-report unexpected or undesirable experimental results, while being more trusting of expected or desirable results. This can propagate, as each instance reinforces the status quo, and later experimenters justify their own reporting bias by observing that previous experimenters reported different results.

Social desirability bias is a bias within social science research where survey respondents can tend to answer questions in a manner that will be viewed positively by others. It can take the form of over-reporting laudable behavior, or under-reporting undesirable behavior. This bias interferes with the interpretation of average tendencies as well as individual differences. The inclination represents a major issue with self-report questionnaires; of special concern are self-reports of abilities, personalities, sexual behavior, and drug use.

Selection bias

Sampling is supposed to collect of a representative sample of a population.

Selection bias is the conscious or unconscious bias introduced into a study by the way individuals, groups or data are selected for analysis, if such a way means that true randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed. This results in a sample that may be significantly different from the overall population.

Prejudices

Bias and prejudice are usually considered to be closely related. Prejudice is prejudgment, or forming an opinion before becoming aware of the relevant facts of a case. The word is often used to refer to preconceived, usually unfavorable, judgments toward people or a person because of gender, political opinion, social class, age, disability, religion, sexuality, race/ethnicity, language, nationality, or other personal characteristics. Prejudice can also refer to unfounded beliefs and may include "any unreasonable attitude that is unusually resistant to rational influence".

Ageism

Ageism is the stereotyping and/or discrimination against individuals or groups on the basis of their age. It can be used in reference to prejudicial attitudes towards older people, or towards younger people.

Classism

Classism is discrimination on the basis of social class. It includes attitudes that benefit the upper class at the expense of the lower class, or vice versa.

Lookism

Lookism is stereotypes, prejudice, and discrimination on the basis of physical attractiveness, or more generally to people whose appearance matches cultural preferences. Many people make automatic judgments of others based on their physical appearance that influence how they respond to those people.

Racism

Racism consists of ideologies based on a desire to dominate or a belief in the inferiority of another race. It may also hold that members of different races should be treated differently.

Sexism

A woman under arrrest walking between two policemen
Suffragette organizations campaigned for women's right to vote.
Sexism is prejudice or discrimination based on one's sex or gender. Sexism can affect anyone, but primarily affects women and girls. It has been linked to gender roles and stereotypes, and may include the belief that one sex or gender is intrinsically superior to another. Extreme sexism may foster sexual harassment, rape, and other forms of sexual violence. Discrimination in this context is defined as discrimination toward people based on their gender identity or their gender or sex differences. An example of this is workplace inequality. Sexism refers to violation of equal opportunities (formal equality) based on gender or refers to violation of equality of outcomes based on gender, also called substantive equality. Sexism may arise from social or cultural customs and norms.

Contextual biases

Biases in academia

Academic bias

Academic bias is the bias or perceived bias of scholars allowing their beliefs to shape their research and the scientific community. Claims of bias are often linked to claims by conservatives of pervasive bias against political conservatives and religious Christians. Some have argued that these claims are based upon anecdotal evidence which would not reliably indicate systematic bias, and have suggested that this divide is due to self-selection of conservatives choosing not to pursue academic careers. There is some evidence that perception of classroom bias may be rooted in issues of sexuality, race, class and sex as much or more than in religion.

Experimenter bias

In science research, experimenter bias occurs when experimenter expectancies regarding study results bias the research outcome. Examples of experimenter bias include conscious or unconscious influences on subject behavior including creation of demand characteristics that influence subjects, and altered or selective recording of experimental results themselves. It can also involve asking leading probes and not neutrally redirecting the subject back to the task when they ask for validation or questions.

Funding bias

Funding bias refers to the tendency of a scientific study to support the interests of the study's financial sponsor. This phenomenon is recognized sufficiently that researchers undertake studies to examine bias in past published studies. It can be caused by any or all of: a conscious or subconscious sense of obligation of researchers towards their employers, misconduct or malpractice, publication bias, or reporting bias.

Full text on net bias

Full text on net (or FUTON) bias is a tendency of scholars to cite academic journals with open access—that is, journals that make their full text available on the internet without charge—in their own writing as compared with toll access publications. Scholars can more easily discover and access articles that have their full text on the internet, which increases authors' likelihood of reading, quoting, and citing these articles, this may increase the impact factor of open access journals relative to journals without open access.

The related bias, no abstract available bias (NAA bias) is scholars' tendency to cite journal articles that have an abstract available online more readily than articles that do not.

Publication bias

Publication bias is a type of bias with regard to what academic research is likely to be published because of a tendency among researchers and journal editors to prefer some outcomes rather than others (e.g., results showing a significant finding), which leads to a problematic bias in the published literature. This can propagate further as literature reviews of claims about support for a hypothesis will themselves be biased if the original literature is contaminated by publication bias. Studies with significant results often do not appear to be superior to studies with a null result with respect to quality of design. However, statistically significant results have been shown to be three times more likely to be published compared to papers with null results.

Biases in law enforcement

Driving while black

Driving while black refers to the racial profiling of African American drivers. The phrase implies that a motorist might be pulled over by a police officer, questioned, and searched, because of a racial bias.

Racial profiling

Racial profiling, or ethnic profiling, is the act of suspecting or targeting a person of a certain race on the basis of racially observed characteristics or behavior, rather than on individual suspicion. Racial profiling is commonly referred to regarding its use by law enforcement, and its leading to discrimination against minorities.

Victim blaming

Victim blaming occurs when the victim of a wrongful act is held at fault for the harm that befell them. The study of victimology seeks to mitigate the perception of victims as responsible.

Biases in media

Media bias is the bias or perceived bias of journalists and news producers within the mass media in the selection of events, the stories that are reported, and how they are covered. The term generally implies a pervasive or widespread bias violating the standards of journalism, rather than the perspective of an individual journalist or article. The level of media bias in different nations is debated. There are also watchdog groups that report on media bias.

Practical limitations to media neutrality include the inability of journalists to report all available stories and facts, the requirement that selected facts be linked into a coherent narrative, government influence including overt and covert censorship, the influence of the owners of the news source, concentration of media ownership, the selection of staff, the preferences of an intended audience, and pressure from advertisers.

Bias has been a feature of the mass media since its birth with the invention of the printing press. The expense of early printing equipment restricted media production to a limited number of people. Historians have found that publishers often served the interests of powerful social groups.

Agenda setting

Agenda setting describes the capacity of the media to focus on particular stories, if a news item is covered frequently and prominently, the audience will regard the issue as more important. That is, its salience will increase.

Gatekeeping

Gatekeeping is the way in which information and news are filtered to the public, by each person or corporation along the way. It is the "process of culling and crafting countless bits of information into the limited number of messages that reach people every day, and it is the center of the media's role in modern public life. [...] This process determines not only which information is selected, but also what the content and nature of the messages, such as news, will be."

Sensationalism

Sensationalism is when events and topics in news stories and pieces are overhyped to present skewed impressions of events, which may cause a misrepresentation of the truth of a story. Sensationalism may involve reporting about insignificant matters and events, or the presentation of newsworthy topics in a trivial or tabloid manner contrary to the standards of professional journalism.

Other contexts

Educational bias

Bias in education refers to real or perceived bias in the educational system. The content of school textbooks is often the issue of debate, as their target audience is young people, and the term "whitewashing" is used to refer to selective removal of critical or damaging evidence or comment. Religious bias in textbooks is observed in countries where religion plays a dominant role. There can be many forms of educational bias. Some overlooked aspects, occurring especially with the pedagogical circles of public and private schools—sources that are unrelated to fiduciary or mercantile impoverishment which may be unduly magnified—include teacher bias as well as a general bias against women who are going into STEM research.

Inductive bias

Inductive bias occurs within the field of machine learning. In machine learning one seeks to develop algorithms that are able to learn to anticipate a particular output. To accomplish this, the learning algorithm is given training cases that show the expected connection. Then the learner is tested with new examples. Without further assumptions, this problem cannot be solved exactly as unknown situations may not be predictable. The inductive bias of the learning algorithm is the set of assumptions that the learner uses to predict outputs given inputs that it has not encountered. It may bias the learner towards the correct solution, the incorrect, or be correct some of the time. A classical example of an inductive bias is Occam's Razor, which assumes that the simplest consistent hypothesis is the best.

Insider trading

Insider trading is the trading of a public company's stock or other securities (such as bonds or stock options) by individuals with access to non-public information about the company. In various countries, trading based on insider information is illegal because it is seen as unfair to other investors who do not have access to the information as the investor with insider information could potentially make far larger profits that a typical investor could make.

Match fixing

In organized sports, match fixing occurs when a match is played to a completely or partially pre-determined result, violating the rules of the game and often the law. There is a variety of reasons for this, but the most common is in exchange for a payoff from gamblers. Players might also intentionally perform poorly to get an advantage in the future (such as a better draft pick, or an easier opponent in a playoff), or to rig a handicap system. Match-fixing generally refers to fixing the final result of the game. Another form of match-fixing, known as spot-fixing, involves fixing small events within a match which can be gambled upon, but which are unlikely to prove decisive in determining the final result of the game.

Implicit bias

An implicit bias, or implicit stereotype, is the unconscious attribution of particular qualities to a member of a certain social group.

Implicit stereotypes are shaped by experience and based on learned associations between particular qualities and social categories, including race and/or gender. Individuals' perceptions and behaviors can be influenced by the implicit stereotypes they hold, even if they are unaware/unintentionally hold such stereotypes. Implicit bias is an aspect of implicit social cognition: the phenomenon that perceptions, attitudes, and stereotypes operate without conscious intention. For example, researchers may have implicit bias when designing survey questions and as a result, the questions do not produce accurate results or fail to encourage survey participation. The existence of implicit bias is supported by a variety of scientific articles in psychological literature. Implicit stereotype was first defined by psychologists Mahzarin Banaji and Anthony Greenwald in 1995.

Cell biology

From Wikipedia, the free encyclopedia

Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and functioning of organisms. Cell biology is the study of the structural and functional units of cells. Cell biology encompasses both prokaryotic and eukaryotic cells and has many subtopics which may include the study of cell metabolism, cell communication, cell cycle, biochemistry, and cell composition. The study of cells is performed using several microscopy techniques, cell culture, and cell fractionation. These have allowed for and are currently being used for discoveries and research pertaining to how cells function, ultimately giving insight into understanding larger organisms. Knowing the components of cells and how cells work is fundamental to all biological sciences while also being essential for research in biomedical fields such as cancer, and other diseases. Research in cell biology is interconnected to other fields such as genetics, molecular genetics, molecular biology, medical microbiology, immunology, and cytochemistry.

History

Cells were first seen in 17th-century Europe with the invention of the compound microscope. In 1665, Robert Hooke referred to the building blocks of all living organisms as "cells" (published in Micrographia) after looking at a piece of cork and observing a structure reminiscent of a monastic cell; however, the cells were dead. They gave no indication to the actual overall components of a cell. A few years later, in 1674, Anton Van Leeuwenhoek was the first to analyze live cells in his examination of algae. Many years later, in 1831, Robert Brown discovered the nucleus. All of this preceded the cell theory which states that all living things are made up of cells and that cells are organisms' functional and structural units. This was ultimately concluded by plant scientist Matthias Schleiden and animal scientist Theodor Schwann in 1838, who viewed live cells in plant and animal tissue, respectively. 19 years later, Rudolf Virchow further contributed to the cell theory, adding that all cells come from the division of pre-existing cells. Viruses are not considered in cell biology – they lack the characteristics of a living cell and instead are studied in the microbiology subclass of virology.

Techniques

Cell biology research looks at different ways to culture and manipulate cells outside of a living body to further research in human anatomy and physiology, and to derive medications. The techniques by which cells are studied have evolved. Due to advancements in microscopy, techniques and technology have allowed scientists to hold a better understanding of the structure and function of cells. Many techniques commonly used to study cell biology are listed below:

  • Cell culture: Utilizes rapidly growing cells on media which allows for a large amount of a specific cell type and an efficient way to study cells. Cell culture is one of the major tools used in cellular and molecular biology, providing excellent model systems for studying the normal physiology and biochemistry of cells (e.g., metabolic studies, aging), the effects of drugs and toxic compounds on the cells, and mutagenesis and carcinogenesis. It is also used in drug screening and development, and large scale manufacturing of biological compounds (e.g., vaccines, therapeutic proteins).
  • Fluorescence microscopy: Fluorescent markers such as GFP, are used to label a specific component of the cell. Afterwards, a certain light wavelength is used to excite the fluorescent marker which can then be visualized.
  • Phase-contrast microscopy: Uses the optical aspect of light to represent the solid, liquid, and gas-phase changes as brightness differences.
  • Confocal microscopy: Combines fluorescence microscopy with imaging by focusing light and snap shooting instances to form a 3-D image.
  • Transmission electron microscopy: Involves metal staining and the passing of electrons through the cells, which will be deflected upon interaction with metal. This ultimately forms an image of the components being studied.
  • Cytometry: The cells are placed in the machine which uses a beam to scatter the cells based on different aspects and can therefore separate them based on size and content. Cells may also be tagged with GFP-fluorescence and can be separated that way as well.
  • Cell fractionation: This process requires breaking up the cell using high temperature or sonification followed by centrifugation to separate the parts of the cell allowing for them to be studied separately.

Cell types

A drawing of a prokaryotic cell

There are two fundamental classifications of cells: prokaryotic and eukaryotic. Prokaryotic cells are distinguished from eukaryotic cells by the absence of a cell nucleus or other membrane-bound organelle. Prokaryotic cells are much smaller than eukaryotic cells, making them the smallest form of life. Prokaryotic cells include Bacteria and Archaea, and lack an enclosed cell nucleus.  Eukaryotic cells are found in plants, animals, fungi, and protists. They range from 10 to 100 μm in diameter, and their DNA is contained within a membrane-bound nucleus. Eukaryotes are organisms containing eukaryotic cells. The four eukaryotic kingdoms are Animalia, Plantae, Fungi, and Protista.

They both reproduce through binary fission. Bacteria, the most prominent type, have several different shapes, although most are spherical or rod-shaped. Bacteria can be classed as either gram-positive or gram-negative depending on the cell wall composition. Gram-positive bacteria have a thicker peptidoglycan layer than gram-negative bacteria. Bacterial structural features include a flagellum that helps the cell to move, ribosomes for the translation of RNA to protein, and a nucleoid that holds all the genetic material in a circular structure. There are many processes that occur in prokaryotic cells that allow them to survive. In prokaryotes, mRNA synthesis is initiated at a promoter sequence on the DNA template comprising two consensus sequences that recruit RNA polymerase. The prokaryotic polymerase consists of a core enzyme of four protein subunits and a σ protein that assists only with initiation. For instance, in a process termed conjugation, the fertility factor allows the bacteria to possess a pilus which allows it to transmit DNA to another bacteria which lacks the F factor, permitting the transmittance of resistance allowing it to survive in certain environments.

Structure and function

Structure of eukaryotic cells

A diagram of an animal cell

Eukaryotic cells are composed of the following organelles:

  • Nucleus: The nucleus of the cell functions as the genome and genetic information storage for the cell, containing all the DNA organized in the form of chromosomes. It is surrounded by a nuclear envelope, which includes nuclear pores allowing for the transportation of proteins between the inside and outside of the nucleus. This is also the site for replication of DNA as well as transcription of DNA to RNA. Afterwards, the RNA is modified and transported out to the cytosol to be translated to protein.
  • Nucleolus: This structure is within the nucleus, usually dense and spherical. It is the site of ribosomal RNA (rRNA) synthesis, which is needed for ribosomal assembly.
  • Endoplasmic reticulum (ER): This functions to synthesize, store, and secrete proteins to the Golgi apparatus. Structurally, the endoplasmic reticulum is a network of membranes found throughout the cell and connected to the nucleus. The membranes are slightly different from cell to cell and a cell's function determines the size and structure of the ER.
  • Mitochondria: Commonly known as the powerhouse of the cell is a double membrane bound cell organelle. This functions for the production of energy or ATP within the cell. Specifically, this is the place where the Krebs cycle or TCA cycle for the production of NADH and FADH occurs. Afterwards, these products are used within the electron transport chain (ETC) and oxidative phosphorylation for the final production of ATP.
  • Golgi apparatus: This functions to further process, package, and secrete the proteins to their destination. The proteins contain a signal sequence that allows the Golgi apparatus to recognize and direct it to the correct place. Golgi apparatus also produce glycoproteins and glycolipids.
  • Lysosome: The lysosome functions to degrade material brought in from the outside of the cell or old organelles. This contains many acid hydrolases, proteases, nucleases, and lipases, which break down the various molecules. Autophagy is the process of degradation through lysosomes which occurs when a vesicle buds off from the ER and engulfs the material, then, attaches and fuses with the lysosome to allow the material to be degraded.
  • Ribosomes: Functions to translate RNA to protein. it serves as a site of protein synthesis.
  • Cytoskeleton: Cytoskeleton is a structure that helps to maintain the shape and general organization of the cytoplasm. It anchors organelles within the cells and makes up the structure and stability of the cell. The cytoskeleton is composed of three principal types of protein filaments: actin filaments, intermediate filaments, and microtubules, which are held together and linked to subcellular organelles and the plasma membrane by a variety of accessory proteins.
  • Cell membrane: The cell membrane can be described as a phospholipid bilayer and is also consisted of lipids and proteins. Because the inside of the bilayer is hydrophobic and in order for molecules to participate in reactions within the cell, they need to be able to cross this membrane layer to get into the cell via osmotic pressure, diffusion, concentration gradients, and membrane channels.
  • Centrioles: Function to produce spindle fibers which are used to separate chromosomes during cell division.

Eukaryotic cells may also be composed of the following molecular components:

  • Chromatin: This makes up chromosomes and is a mixture of DNA with various proteins.
  • Cilia: They help to propel substances and can also be used for sensory purposes.

Cell metabolism

Cell metabolism is necessary for the production of energy for the cell and therefore its survival and includes many pathways and also sustaining the main cell organelles such as the nucleus, the mitochondria, the cell membrane etc. For cellular respiration, once glucose is available, glycolysis occurs within the cytosol of the cell to produce pyruvate. Pyruvate undergoes decarboxylation using the multi-enzyme complex to form acetyl coA which can readily be used in the TCA cycle to produce NADH and FADH2. These products are involved in the electron transport chain to ultimately form a proton gradient across the inner mitochondrial membrane. This gradient can then drive the production of ATP and H2O during oxidative phosphorylation. Metabolism in plant cells includes photosynthesis which is simply the exact opposite of respiration as it ultimately produces molecules of glucose.

Cell signaling

Cell signaling or cell communication is important for cell regulation and for cells to process information from the environment and respond accordingly. Signaling can occur through direct cell contact or endocrine, paracrine, and autocrine signaling. Direct cell-cell contact is when a receptor on a cell binds a molecule that is attached to the membrane of another cell. Endocrine signaling occurs through molecules secreted into the bloodstream. Paracrine signaling uses molecules diffusing between two cells to communicate. Autocrine is a cell sending a signal to itself by secreting a molecule that binds to a receptor on its surface. Forms of communication can be through:

  • Ion channels: Can be of different types such as voltage or ligand gated ion channels. They allow for the outflow and inflow of molecules and ions.
  • G-protein coupled receptor (GPCR): Is widely recognized to contain seven transmembrane domains. The ligand binds on the extracellular domain and once the ligand binds, this signals a guanine exchange factor to convert GDP to GTP and activate the G-α subunit. G-α can target other proteins such as adenyl cyclase or phospholipase C, which ultimately produce secondary messengers such as cAMP, Ip3, DAG, and calcium. These secondary messengers function to amplify signals and can target ion channels or other enzymes. One example for amplification of a signal is cAMP binding to and activating PKA by removing the regulatory subunits and releasing the catalytic subunit. The catalytic subunit has a nuclear localization sequence which prompts it to go into the nucleus and phosphorylate other proteins to either repress or activate gene activity.
  • Receptor tyrosine kinases: Bind growth factors, further promoting the tyrosine on the intracellular portion of the protein to cross phosphorylate. The phosphorylated tyrosine becomes a landing pad for proteins containing an SH2 domain allowing for the activation of Ras and the involvement of the MAP kinase pathway.

Growth and development

Eukaryotic cell cycle

The process of cell division in the animal cell cycle

Cells are the foundation of all organisms and are the fundamental units of life. The growth and development of cells are essential for the maintenance of the host and survival of the organism. For this process, the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, and cell death.

The cell cycle is divided into four distinct phases: G1, S, G2, and M. The G phase – which is the cell growth phase – makes up approximately 95% of the cycle. The proliferation of cells is instigated by progenitors. All cells start out in an identical form and can essentially become any type of cells. Cell signaling such as induction can influence nearby cells to determinate the type of cell it will become. Moreover, this allows cells of the same type to aggregate and form tissues, then organs, and ultimately systems. The G1, G2, and S phase (DNA replication, damage and repair) are considered to be the interphase portion of the cycle, while the M phase (mitosis) is the cell division portion of the cycle. Mitosis is composed of many stages which include, prophase, metaphase, anaphase, telophase, and cytokinesis, respectively. The ultimate result of mitosis is the formation of two identical daughter cells.

The cell cycle is regulated in cell cycle checkpoints, by a series of signaling factors and complexes such as cyclins, cyclin-dependent kinase, and p53. When the cell has completed its growth process and if it is found to be damaged or altered, it undergoes cell death, either by apoptosis or necrosis, to eliminate the threat it can cause to the organism's survival.

Cell mortality, cell lineage immortality

The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. The immortality of a cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. In sexual organisms, continuity of the germline depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes, as well as in prokaryotes, provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination.

Cell cycle phases

The cell cycle is a four-stage process that a cell goes through as it develops and divides. It includes Gap 1 (G1), synthesis (S), Gap 2 (G2), and mitosis (M). The cell either restarts the cycle from G1 or leaves the cycle through G0 after completing the cycle. The cell can progress from G0 through terminal differentiation. Finally, the interphase refers to the phases of the cell cycle that occur between one mitosis and the next, and includes G1, S, and G2. Thus, the phases are:

  • G1 phase: the cell grows in size and its contents are replicated.
  • S phase: the cell replicates each of the 46 chromosomes.
  • G2 phase: in preparation for cell division, new organelles and proteins form.
  • M phase: cytokinesis occurs, resulting in two identical daughter cells.
  • G0 phase: the two cells enter a resting stage where they do their job without actively preparing to divide.

Pathology

The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer but also in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a screening test used to detect cervical cancer, and precancerous cervical lesions that may lead to cervical cancer.

Cell cycle checkpoints and DNA damage repair system

The cell cycle is composed of a number of well-ordered, consecutive stages that result in cellular division. The fact that cells do not begin the next stage until the last one is finished, is a significant element of cell cycle regulation. Cell cycle checkpoints are characteristics that constitute an excellent monitoring strategy for accurate cell cycle and divisions. Cdks, associated cyclin counterparts, protein kinases, and phosphatases regulate cell growth and division from one stage to another. The cell cycle is controlled by the temporal activation of Cdks, which is governed by cyclin partner interaction, phosphorylation by particular protein kinases, and de-phosphorylation by Cdc25 family phosphatases. In response to DNA damage, a cell's DNA repair reaction is a cascade of signaling pathways that leads to checkpoint engagement, regulates, the repairing mechanism in DNA, cell cycle alterations, and apoptosis. Numerous biochemical structures, as well as processes that detect damage in DNA, are ATM and ATR, which induce the DNA repair checkpoints.

The cell cycle is a sequence of activities in which cell organelles are duplicated and subsequently separated into daughter cells with precision. There are major events that happen during a cell cycle. The processes that happen in the cell cycle include cell development, replication and segregation of chromosomes.  The cell cycle checkpoints are surveillance systems that keep track of the cell cycle's integrity, accuracy, and chronology. Each checkpoint serves as an alternative cell cycle endpoint, wherein the cell's parameters are examined and only when desirable characteristics are fulfilled does the cell cycle advance through the distinct steps. The cell cycle's goal is to precisely copy each organism's DNA and afterwards equally split the cell and its components between the two new cells. Four main stages occur in the eukaryotes. In G1, the cell is usually active and continues to grow rapidly, while in G2, the cell growth continues while protein molecules become ready for separation. These are not dormant times; they are when cells gain mass, integrate growth factor receptors, establish a replicated genome, and prepare for chromosome segregation. DNA replication is restricted to a separate Synthesis in eukaryotes, which is also known as the S-phase. During mitosis, which is also known as the M-phase, the segregation of the chromosomes occur. DNA, like every other molecule, is capable of undergoing a wide range of chemical reactions. Modifications in DNA's sequence, on the other hand, have a considerably bigger impact than modifications in other cellular constituents like RNAs or proteins because DNA acts as a permanent copy of the cell genome. When erroneous nucleotides are incorporated during DNA replication, mutations can occur. The majority of DNA damage is fixed by removing the defective bases and then re-synthesizing the excised area. On the other hand, some DNA lesions can be mended by reversing the damage, which may be a more effective method of coping with common types of DNA damage. Only a few forms of DNA damage are mended in this fashion, including pyrimidine dimers caused by ultraviolet (UV) light changed by the insertion of methyl or ethyl groups at the purine ring's O6 position.

Mitochondrial membrane dynamics

Mitochondria are commonly referred to as the cell's "powerhouses" because of their capacity to effectively produce ATP which is essential to maintain cellular homeostasis and metabolism. Moreover, researchers have gained a better knowledge of mitochondria's significance in cell biology because of the discovery of cell signaling pathways by mitochondria which are crucial platforms for cell function regulation such as apoptosis. Its physiological adaptability is strongly linked to the cell mitochondrial channel's ongoing reconfiguration through a range of mechanisms known as mitochondrial membrane dynamics, including endomembrane fusion and fragmentation (separation) and ultrastructural membrane remodeling. As a result, mitochondrial dynamics regulate and frequently choreograph not only metabolic but also complicated cell signaling processes such as cell pluripotent stem cells, proliferation, maturation, aging, and mortality. Mutually, post-translational alterations of mitochondrial apparatus and the development of transmembrane contact sites among mitochondria and other structures, which both have the potential to link signals from diverse routes that affect mitochondrial membrane dynamics substantially, Mitochondria are wrapped by two membranes: an inner mitochondrial membrane (IMM) and an outer mitochondrial membrane (OMM), each with a distinctive function and structure, which parallels their dual role as cellular powerhouses and signaling organelles. The inner mitochondrial membrane divides the mitochondrial lumen into two parts: the inner border membrane, which runs parallel to the OMM, and the cristae, which are deeply twisted, multinucleated invaginations that give room for surface area enlargement and house the mitochondrial respiration apparatus. The outer mitochondrial membrane, on the other hand, is soft and permeable. It, therefore, acts as a foundation for cell signaling pathways to congregate, be deciphered, and be transported into mitochondria. Furthermore, the OMM connects to other cellular organelles, such as the endoplasmic reticulum (ER), lysosomes, endosomes, and the plasma membrane. Mitochondria play a wide range of roles in cell biology, which is reflected in their morphological diversity. Ever since the beginning of the mitochondrial study, it has been well documented that mitochondria can have a variety of forms, with both their general and ultra-structural morphology varying greatly among cells, during the cell cycle, and in response to metabolic or cellular cues. Mitochondria can exist as independent organelles or as part of larger systems; they can also be unequally distributed in the cytosol through regulated mitochondrial transport and placement to meet the cell's localized energy requirements. Mitochondrial dynamics refers to the adaptive and variable aspect of mitochondria, including their shape and subcellular distribution.

Autophagy

Autophagy is a self-degradative mechanism that regulates energy sources during growth and reaction to dietary stress. Autophagy also cleans up after itself, clearing aggregated proteins, cleaning damaged structures including mitochondria and endoplasmic reticulum and eradicating intracellular infections. Additionally, autophagy has antiviral and antibacterial roles within the cell, and it is involved at the beginning of distinctive and adaptive immune responses to viral and bacterial contamination. Some viruses include virulence proteins that prevent autophagy, while others utilize autophagy elements for intracellular development or cellular splitting. Macro autophagy, micro autophagy, and chaperon-mediated autophagy are the three basic types of autophagy. When macro autophagy is triggered, an exclusion membrane incorporates a section of the cytoplasm, generating the autophagosome, a distinctive double-membraned organelle. The autophagosome then joins the lysosome to create an autolysosome, with lysosomal enzymes degrading the components. In micro autophagy, the lysosome or vacuole engulfs a piece of the cytoplasm by invaginating or protruding the lysosomal membrane to enclose the cytosol or organelles. The chaperone-mediated autophagy (CMA) protein quality assurance by digesting oxidized and altered proteins under stressful circumstances and supplying amino acids through protein denaturation. Autophagy is the primary intrinsic degradative system for peptides, fats, carbohydrates, and other cellular structures. In both physiologic and stressful situations, this cellular progression is vital for upholding the correct cellular balance. Autophagy instability leads to a variety of illness symptoms, including inflammation, biochemical disturbances, aging, and neurodegenerative, due to its involvement in controlling cell integrity. The modification of the autophagy-lysosomal networks is a typical hallmark of many neurological and muscular illnesses. As a result, autophagy has been identified as a potential strategy for the prevention and treatment of various disorders. Many of these disorders are prevented or improved by consuming polyphenol in the meal. As a result, natural compounds with the ability to modify the autophagy mechanism are seen as a potential therapeutic option. The creation of the double membrane (phagophore), which would be known as nucleation, is the first step in macro-autophagy. The phagophore approach indicates dysregulated polypeptides or defective organelles that come from the cell membrane, Golgi apparatus, endoplasmic reticulum, and mitochondria. With the conclusion of the autophagocyte, the phagophore's enlargement comes to an end. The auto-phagosome combines with the lysosomal vesicles to formulate an auto-lysosome that degrades the encapsulated substances, referred to as phagocytosis.

Carbon-based life

From Wikipedia, the free encyclopedia
The Lewis structure of a carbon atom, showing its four valence electrons

Carbon is a primary component of all known life on Earth, and represents approximately 45–50% of all dry biomass. Carbon compounds occur naturally in great abundance on Earth. Complex biological molecules consist of carbon atoms bonded with other elements, especially oxygen and hydrogen and frequently also nitrogen, phosphorus, and sulfur (collectively known as CHNOPS).

Because it is lightweight and relatively small in size, carbon molecules are easy for enzymes to manipulate. Carbonic anhydrase is part of this process. Carbon has an atomic number of 6 on the periodic table. The carbon cycle is a biogeochemical cycle that is important in maintaining life on Earth over a long time span. The cycle includes carbon sequestration and carbon sinks. Plate tectonics are needed for life over a long time span, and carbon-based life is important in the plate tectonics process. Iron- and sulfur-based Anoxygenic photosynthesis life forms that lived from 3.80 to 3.85 billion years ago on Earth produced an abundance of black shale deposits. These shale deposits increase heat flow and crust buoyancy, especially on the sea floor, helping to increase plate tectonics. Talc is another organic mineral that helps drive plate tectonics. Inorganic processes also help drive plate tectonics. Carbon-based photosynthesis life caused a rise in oxygen on Earth. This increase of oxygen helped plate tectonics form the first continents. It is frequently assumed in astrobiology that if life exists elsewhere in the Universe, it will also be carbon-based. Critics, like Carl Sagan in 1973, refer to this assumption as carbon chauvinism.

Characteristics

Carbon is capable of forming a vast number of compounds, more than any other element, with almost ten million compounds described to date, and yet that is but a fraction of the number of compounds that are theoretically possible under standard conditions. The enormous diversity of carbon compounds, known as organic compounds, has led to a distinction between them and the inorganic compounds that do not contain carbon. The branch of chemistry that studies organic compounds is known as organic chemistry.

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen.

The most important characteristics of carbon as a basis for the chemistry of cellular life are that each carbon atom is capable of forming up to four valence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive. Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long macromolecules and polymers in a process known as catenation. "What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", per Stephen Hawking in a 2008 lecture, "carbon [...] has the richest chemistry."

Norman Horowitz was the head of the Jet Propulsion Laboratory's bioscience section for the first U.S. mission, Viking Lander of 1976, to successfully land an unmanned probe on the surface of Mars. He considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. However, the results of this mission indicated that Mars was presently extremely hostile to carbon-based life. He also considered that, in general, there was only a remote possibility that non-carbon life forms would be able to evolve with genetic information systems capable of self-replication and adaptation.

Key molecules

The most notable classes of biological macromolecules used in the fundamental processes of living organisms include:

Schematic of photosynthesis in plants. The carbohydrates produced are stored in or used by the plant. Photosynthesis is foundation of food on Earth

Liquid water is essential for carbon-based life. Chemical bonding of carbon molecules requires liquid water. Water has the chemical property to make compound-solvent pairing. Water provides the reversible hydration of carbon dioxide. Hydration of carbon dioxide is needed in carbon-based life. All life on Earth uses the same biochemistry of carbon. Water is important in life's carbonic anhydrase the interaction of between carbon dioxide and water. Carbonic anhydrase needs a family of carbon base enzymes for the hydration of carbon dioxide and acid–base homeostasis, that regulates PH levels in life.  In plant life, liquid water is needed for photosynthesis, the biological process plants use to convert light energy and carbon dioxide into chemical energy. Water makes up 55% to 60% of the human body by weight.

Other candidates

A few other elements have been proposed as candidates for supporting biological systems and processes as fundamentally as carbon does, for example, processes such as metabolism. The most frequently suggested alternative is silicon. Silicon, atomic number of 14, more than twice the size of carbon, shares a group in the periodic table with carbon, can also form four valence bonds, and also bonds to itself readily, though generally in the form of crystal lattices rather than long chains. Despite these similarities, silicon is considerably more electropositive than carbon, and silicon compounds do not readily recombine into different permutations in a manner that would plausibly support lifelike processes. Silicon is abundant on Earth, but as it is more electropositive and in a water based environment it forms Si–O bonds rather than Si–Si bonds. Boron does not react with acids and does not form chains naturally. Thus boron is not a candidate for life. Arsenic is toxic to life, and its possible candidacy has been rejected. In the past (1960s-1970s) other candidates for life were plausible, but with time and more research, only carbon has the complexity and stability to make large molecules and polymers essential for life.

Fiction

Speculations about the chemical structure and properties of hypothetical non-carbon-based life have been a recurring theme in science fiction. Silicon is often used as a substitute for carbon in fictional lifeforms because of its chemical similarities. In cinematic and literary science fiction, when man-made machines cross from non-living to living, this new form is often presented as an example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, such machines are often classed as "silicon-based life". Other examples of fictional "silicon-based life" can be seen in the 1967 episode "The Devil in the Dark" from Star Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon. 1994 The X-Files episode "Firewalker", in which a silicon-based organism is discovered in a volcano.

In the 1984 film adaptation of Arthur C. Clarke's 1982 novel 2010: Odyssey Two, a character argues, "Whether we are based on carbon or on silicon makes no fundamental difference; we should each be treated with appropriate respect."

In JoJolion, the eighth part of the larger JoJo's Bizarre Adventure series, a mysterious race of silicon-based lifeforms "Rock Humans" serve as the primary antagonists.

Globular cluster

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Globular_cluster     Globular cluster...