Digestive enzyme

From Wikipedia, the free encyclopedia

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

Diagram of the digestive enzymes in the small intestine and pancreas

Digestive enzymes take part in the chemical process of digestion, which follows the mechanical process of digestion. Food consists of macromolecules of proteins, carbohydrates, and fats that need to be broken down chemically by digestive enzymes in the mouth, stomach, pancreas, and duodenum, before being able to be absorbed into the bloodstream. Initial breakdown is achieved by chewing (mastication) and the use of digestive enzymes of saliva. Once in the stomach further mechanical churning takes place mixing the food with secreted gastric acid. Digestive gastric enzymes take part in some of the chemical process needed for absorption. Most of the enzymatic activity, and hence absorption takes place in the duodenum.

Digestive enzymes are found in the digestive tracts of animals (including humans) and in the tracts of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival.

Digestive enzymes are classified based on their target substrates: lipases split fatty acids into fats and oils; proteases and peptidases split proteins into small peptides and amino acids; amylases split carbohydrates such as starch and sugars into simple sugars such as glucose, and nucleases split nucleic acids into nucleotides.

Types

Table of the different major digestive enzymes

Digestive enzymes are found throughout much of the gastrointestinal tract. In the human digestive system, the main sites of digestion are the mouth, stomach, and small intestine. Digestive enzymes are secreted by different exocrine glands including salivary glands, gastric glands, secretory cells in the pancreas, and secretory glands in the small intestine. In some carnivorous plants plant-specific digestive enzymes are used to break down their captured organisms.

Mouth

Complex food substances that are eaten must be broken down into simple, soluble, and diffusible substances before they can be absorbed. In the oral cavity, salivary glands secrete an array of enzymes and substances that aid in digestion and also disinfection. They include the following:

• Lingual lipase: Lipid digestion initiates in the mouth. Lingual lipase starts the digestion of the lipids/fats.
• Salivary amylase: Carbohydrate digestion also initiates in the mouth. Amylase, produced by the salivary glands, breaks complex carbohydrates, mainly cooked starch, to smaller chains, or even simple sugars. It is sometimes referred to as ptyalin.
• Lysozyme: Considering that food contains more than just essential nutrients, e.g. bacteria or viruses, the lysozyme offers a limited and non-specific, yet beneficial antiseptic function in digestion.

Of note is the diversity of the salivary glands. There are two types of salivary glands:

• Serous glands: These glands produce a secretion rich in water, electrolytes, and enzymes. A great example of a serous oral gland is the parotid gland.
• Mixed glands: These glands have both serous cells and mucous cells, and include sublingual and submandibular glands. Their secretion is mucinous and high in viscosity.

Stomach

The enzymes that are secreted in the stomach are gastric enzymes. The stomach plays a major role in digestion, both in a mechanical sense by mixing and crushing the food, and also in an enzymatic sense, by digesting it. The following are enzymes produced by the stomach and their respective function:

• Pepsin is the main gastric enzyme. It is produced in the stomach by gastric chief cells in its inactive form pepsinogen, which is a zymogen. Pepsinogen is then activated by the stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles, such as peptide fragments and amino acids. Protein digestion, therefore, primarily starts in the stomach, unlike carbohydrate and lipids, which start their digestion in the mouth (however, trace amounts of the enzyme kallikrein, which catabolises certain protein, is found in saliva in the mouth).
• Gastric lipase: Gastric lipase is an acidic lipase secreted by the gastric chief cells in the fundic mucosa of the stomach. It has a pH level of 3–6. Gastric lipase, together with lingual lipase, comprise the two acidic lipases. These lipases, unlike alkaline lipases (such as pancreatic lipase), do not require bile acid or colipase for optimal enzymatic activity. Acidic lipases make up 30% of lipid hydrolysis occurring during digestion in the human adult, with gastric lipase contributing the most of the two acidic lipases. In neonates, acidic lipases are much more important, providing up to 50% of total lipolytic activity.
• Cathepsin F: is a cysteine protease.

Pancreas

"Pancreatic enzyme" and "pancrease" redirect to this discussion of endogenous forms. For exogenous forms, see Pancreatic enzymes (medication).

The pancreas is both an endocrine, and an exocrine gland, in that it functions to produce endocrinic hormones released into the circulatory system (such as insulin, and glucagon), to control glucose metabolism, and also to secrete digestive / exocrinic pancreatic juice, which is secreted eventually via the pancreatic duct into the duodenum. Digestive or exocrine function of pancreas is as significant to the maintenance of health as its endocrine function.

Two of the population of cells in the pancreatic tissue make up its digestive enzymes:

• Ductal cells: Mainly responsible for production of bicarbonate (HCO₃), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to produce the hormone secretin and release to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice. Secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme.
• Acinar cells: Mainly responsible for production of the inactive pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin (CCK), which is a hormone/neurotransmitter produced by the intestinal cells (I cells) in the duodenum. CCK stimulates production of the pancreatic zymogens.

Pancreatic juice, composed of the secretions of both ductal and acinar cells, contains the following digestive enzymes:

• Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin.
• Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, turns into chymotrypsin and breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin.
• Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein
• Several elastases that degrade the protein elastin and some other proteins
• Pancreatic lipase that degrades triglycerides into two fatty acids and a monoglyceride
• Sterol esterase
• Phospholipase
• Several nucleases that degrade nucleic acids, like DNAase and RNAase
• Pancreatic amylase that breaks down starch and glycogen which are alpha-linked glucose polymers. Humans lack the cellulases to digest the carbohydrate cellulose which is a beta-linked glucose polymer.

Some of the preceding endogenous enzymes have pharmaceutical counterparts (pancreatic enzymes) that are administered to people with exocrine pancreatic insufficiency.

The pancreas's exocrine function owes part of its notable reliability to biofeedback mechanisms controlling secretion of the juice. The following significant pancreatic biofeedback mechanisms are essential to the maintenance of pancreatic juice balance/production:

• Secretin, a hormone produced by the duodenal "S cells" in response to the stomach chyme containing high hydrogen atom concentration (high acidity), is released into the blood stream; upon return to the digestive tract, secretion decreases gastric emptying, increases secretion of the pancreatic ductal cells, as well as stimulating pancreatic acinar cells to release their zymogenic juice.
• Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content. CCK also increases gallbladder contraction, resulting in bile squeezed into the cystic duct, common bile duct and eventually the duodenum. Bile of course helps absorption of the fat by emulsifying it, increasing its absorptive surface. Bile is made by the liver, but is stored in the gallbladder.
• Gastric inhibitory peptide (GIP) is produced by the mucosal duodenal cells in response to chyme containing high amounts of carbohydrate, proteins, and fatty acids. Main function of GIP is to decrease gastric emptying.
• Somatostatin is a hormone produced by the mucosal cells of the duodenum and also the "delta cells" of the pancreas. Somatostatin has a major inhibitory effect, including on pancreatic production.

Duodenum

The following enzymes/hormones are produced in the duodenum:

• secretin: This is an endocrine hormone produced by the duodenal "S cells" in response to the acidity of the gastric chyme.
• Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content. CCK also increases gallbladder contraction, causing release of pre-stored bile into the cystic duct, and eventually into the common bile duct and via the ampulla of Vater into the second anatomic position of the duodenum. CCK also decreases the tone of the sphincter of Oddi, which is the sphincter that regulates flow through the ampulla of Vater. CCK also decreases gastric activity and decreases gastric emptying, thereby giving more time to the pancreatic juices to neutralize the acidity of the gastric chyme.
• Gastric inhibitory peptide (GIP): This peptide decreases gastric motility and is produced by duodenal mucosal cells.
• motilin: This substance increases gastro-intestinal motility via specialized receptors called "motilin receptors".
• somatostatin: This hormone is produced by duodenal mucosa and also by the delta cells of the pancreas. Its main function is to inhibit a variety of secretory mechanisms.

Throughout the lining of the small intestine there are numerous brush border enzymes whose function is to further break down the chyme released from the stomach into absorbable particles. These enzymes are absorbed whilst peristalsis occurs. Some of these enzymes include:

• Various exopeptidases and endopeptidases including dipeptidase and aminopeptidases that convert peptones and polypeptides into amino acids.
• Maltase: converts maltose into glucose.
• Lactase: This is a significant enzyme that converts lactose into glucose and galactose. A majority of Middle-Eastern and Asian populations lack this enzyme. This enzyme also decreases with age. As such lactose intolerance is often a common abdominal complaint in the Middle-Eastern, Asian, and older populations, manifesting with bloating, abdominal pain, and osmotic diarrhea.
• Sucrase: converts sucrose into glucose and fructose.
• Other disaccharidases

Plants

In carnivorous plants, digestive enzymes and acids break down insects and in some plants small animals. In some plants, the leaf collapses on the prey to increase contact, others have a small vessel of digestive liquid. Then digestion fluids are used to digest the prey to get at the needed nitrates and phosphorus. The absorption of the needed nutrients are usually more efficient than in other plants. Digestive enzymes independently came about in carnivorous plants and animals.

Some carnivorous plants like the Heliamphora do not use digestive enzymes, but use bacteria to break down the food. These plants do not have digestive juices, but use the rot of the prey.

Some carnivorous plants digestive enzymes:

• Hydrolytic process
• Esterase a hydrolase enzyme
• Proteases enzyme
• Nucleases enzyme
• Phosphatases enzyme
• Glucanases enzyme
• Peroxidases enzyme
• Ureas an organic compounds
• Chitinase enzyme

Clinical significance

Alpha-glucosidase inhibitors and alpha amylase inhibitors are found in several raw plants such as cinnamon. They are used as anti-diabetic drugs. Studies have shown that the use of raw cinnamon offers potential anti-diabetic therapeutic use.

Political system

From Wikipedia, the free encyclopedia

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

In political science, a political system means the form of political organization that can be observed, recognised or otherwise declared by a society or state.

It defines the process for making official government decisions. It usually comprizes the governmental legal and economic system, social and cultural system, and other state and government specific systems. However, this is a very simplified view of a much more complex system of categories involving the questions of who should have authority and what the government influence on its people and economy should be.

Along with a basic sociological and socio-anthropological classification, political systems can be classified on a social-cultural axis relative to the liberal values prevalent in the Western world, where the spectrum is represented as a continuum between political systems recognized as democracies, totalitarian regimes and, sitting between these two, authoritarian regimes, with a variety of hybrid regimes; and monarchies may be also included as a standalone entity or as a hybrid system of the main three.

Definition

According to David Easton, "A political system can be designated as the interactions through which values are authoritatively allocated for a society". Political system refers broadly to the process by which laws are made and public resources allocated in society, and to the relationships among those involved in making these decisions.

Basic classification

Social anthropologists generally recognize several kinds of political systems, often differentiating between ones that they consider uncentralized and ones they consider centralized.

• Uncentralized systems
  • Band society
    • Small family group, no larger than an extended family or clan; it has been defined as consisting of no more than 30 to 50 individuals.
    • A band can cease to exist if only a small group walks out.
  • Tribe
    • Generally larger, consisting of many families. Tribes have more social institutions, such as a chief or elders.
    • More permanent than bands. Many tribes are subdivided into bands.
• Centralized governments
  • Chiefdom
    • More complex than a tribe or a band society, and less complex than a state or a civilization
    • Characterized by pervasive inequality and centralization of authority.
    • A single lineage/family of the elite class becomes the ruling elite of the chiefdom
    • Complex chiefdoms have two or even three tiers of political hierarchy.
    • "An autonomous political unit comprising a number of villages or communities under the permanent control of a paramount chief"
  • Sovereign state
    • A sovereign state is a state with a permanent population, a defined territory, a government and the capacity to enter into relations with other sovereign states.
• Supranational political systems
  • Supranational political systems are created by independent nations to reach a common goal or gain strength from forming an alliance.
• Empires
  • Empires are widespread states consisting of people of different ethnicities under a single rule. Empires - such as the Romans, or British - often made considerable progress in ways of political structures, creating and building city infrastructures, and maintaining civility within the diverse communities. Because of the intricate organization of the empires, they were often able to hold a large majority of power on a universal level.
• Leagues
  • Leagues are international organizations composed of states coming together for a single common purpose. In this way, leagues are different from empires, as they only seek to fulfil a single goal. Often leagues are formed on the brink of a military or economic downfall. Meetings and hearings are conducted in a neutral location with representatives of all involved nations present.

Western socio-cultural paradigmatic-centric analysis

Further information: List of forms of government

The sociological interest in political systems is figuring out who holds power within the relationship between the government and its people and how the government’s power is used. According to Yale professor Juan José Linz, there are three main types of political systems today: democracies, totalitarian regimes and, sitting between these two, authoritarian regimes (with hybrid regimes). Another modern classification system includes monarchies as a standalone entity or as a hybrid system of the main three. Scholars generally refer to a dictatorship as either a form of authoritarianism or totalitarianism.

Democracy

Further information: Types of democracy

Democracy (from Ancient Greek: δημοκρατία, romanized: dēmokratía, dēmos 'people' and kratos 'rule') is a form of government in which political power is vested in the people or the population of a state. Under a minimalist definition of democracy, rulers are elected through competitive elections while more expansive or maximalist definitions link democracy to guarantees of civil liberties and human rights in addition to competitive elections.

Authoritarianism

Authoritarianism is a political system characterized by the rejection of political plurality, the use of strong central power to preserve the political status quo, and reductions in democracy, separation of powers, civil liberties, and the rule of law. Authoritarian regimes may be either autocratic or oligarchic and may be based upon the rule of a party or the military. States that have a blurred boundary between democracy and authoritarianism have some times been characterized as "hybrid democracies", "hybrid regimes" or "competitive authoritarian" states.

Totalitarian

Totalitarianism is a political system and a form of government that prohibits opposition from political parties, disregards and outlaws the political claims of individual and group opposition to the state, and completely controls the public sphere and the private sphere of society. In the field of political science, totalitarianism is the extreme form of authoritarianism, wherein all socio-political power is held by a dictator. This figure controls the national politics and peoples of the nation with continual propaganda campaigns that are broadcast by state-controlled and state-aligned private mass communications media.

Monarchy

A monarchy is a form of government in which a person, the monarch, reigns as head of state for rest of the life or until abdication. The extent of the authority of the monarch may vary from restricted and largely symbolic (constitutional monarchy), to fully autocratic (absolute monarchy), and may have representational, executive, legislative, and judicial functions.

The succession of monarchs has mostly been hereditary, often building dynasties; however, monarchies can also be elective and self-proclaimed. Aristocrats, though not inherent to monarchies, often function as the pool of persons from which the monarch is chosen, and to fill the constituting institutions (e.g. diet and court), giving many monarchies oligarchic elements. The political legitimacy of the inherited, elected or proclaimed monarchy has most often been based on claims of representation of people and land through some form of relation (e.g. kinship) and divine right or other achieved status.

Hybrid

Further information: Democratization and Democratic backsliding

A hybrid regime^[a] is a type of political system often created as a result of an incomplete democratic transition from an authoritarian regime to a democratic one (or vice versa). Hybrid regimes are categorized as having a combination of autocratic features with democratic ones and can simultaneously hold political repressions and regular elections. Hybrid regimes are commonly found in developing countries with abundant natural resources such as petro-states. Although these regimes experience civil unrest, they may be relatively stable and tenacious for decades at a time. There has been a rise in hybrid regimes since the end of the Cold War.

The term hybrid regime arises from a polymorphic view of political regimes that oppose the dichotomy of autocracy or democracy. Modern scholarly analysis of hybrid regimes focuses attention on the decorative nature of democratic institutions (elections do not lead to a change of power, different media broadcast the government point of view and the opposition in parliament votes the same way as the ruling party, among others), from which it is concluded that democratic backsliding, a transition to authoritarianism is the most prevalent basis of hybrid regimes. Some scholars also contend that hybrid regimes may imitate a full dictatorship.

Marxist/Dialectical materialistic analysis

See also: Dialectical materialism

19th-century German-born philosopher Karl Marx analysed that the political systems of "all" state societies are the dictatorship of one social class, vying for its interests against that of another one; with which class oppressing which other class being, in essence, determined by the developmental level of that society, and its repercussions implicated thereof, as the society progresses through the passage of time. In capitalist societies, this characterises as the dictatorship of the bourgeoisie or capitalist class, in which the economic and political system is designed to work in their interests collectively as a class, over those of the proletariat or working class.

Marx devised this theory by adapting his forerunner-contemporary Georg Wilhelm Friedrich Hegel's notion of dialectics into the framework of materialism.

Galvanic cell

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

Galvanic cell with no cation flow

A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous oxidation–reduction reactions. An example of a galvanic cell consists of two different metals, each immersed in separate beakers containing their respective metal ions in solution that are connected by a salt bridge or separated by a porous membrane.

Volta was the inventor of the voltaic pile, the first electrical battery. Common usage of the word battery has evolved to include a single Galvanic cell, but the first batteries had many Galvanic cells.

History

In 1780, Luigi Galvani discovered that when two different metals (e.g., copper and zinc) are in contact and then both are touched at the same time to two different parts of a muscle of a frog leg, to close the circuit, the frog's leg contracts. He called this "animal electricity". The frog's leg, as well as being a detector of electrical current, was also the electrolyte (to use the language of modern chemistry).

A year after Galvani published his work (1790), Alessandro Volta showed that the frog was not necessary, using instead a force-based detector and brine-soaked paper (as electrolyte). (Earlier Volta had established the law of capacitance C = ⁠Q/V⁠ with force-based detectors). In 1799 Volta invented the voltaic pile, which is a stack of galvanic cells each consisting of a metal disk, an electrolyte layer, and a disk of a different metal. He built it entirely out of non-biological material to challenge Galvani's (and the later experimenter Leopoldo Nobili)'s animal electricity theory in favor of his own metal-metal contact electricity theory. Carlo Matteucci in his turn constructed a battery entirely out of biological material in answer to Volta. Volta's contact electricity view characterized each electrode with a number that we would now call the work function of the electrode. This view ignored the chemical reactions at the electrode-electrolyte interfaces, which include H₂ formation on the more noble metal in Volta's pile.

Although Volta did not understand the operation of the battery or the galvanic cell, these discoveries paved the way for electrical batteries; Volta's cell was named an IEEE Milestone in 1999.

Some forty years later, Faraday (see Faraday's laws of electrolysis) showed that the galvanic cell—now often called a voltaic cell—was chemical in nature. Faraday introduced new terminology to the language of chemistry: electrode (cathode and anode), electrolyte, and ion (cation and anion). Thus Galvani incorrectly thought the source of electricity (or source of electromotive force (emf), or seat of emf) was in the animal, Volta incorrectly thought it was in the physical properties of the isolated electrodes, but Faraday correctly identified the source of emf as the chemical reactions at the two electrode-electrolyte interfaces. The authoritative work on the intellectual history of the voltaic cell remains that by Ostwald.

It was suggested by Wilhelm König in 1940 that the object known as the Baghdad battery might represent galvanic cell technology from ancient Parthia. Replicas filled with citric acid or grape juice have been shown to produce a voltage. However, it is far from certain that this was its purpose—other scholars have pointed out that it is very similar to vessels known to have been used for storing parchment scrolls.

Principles

Schematic of Zn–Cu galvanic cell

Galvanic cells are extensions of spontaneous redox reactions, but have been merely designed to harness the energy produced from said reaction. For example, when one immerses a strip of zinc metal (Zn) in an aqueous solution of copper sulfate (CuSO₄), dark-colored solid deposits will collect on the surface of the zinc metal and the blue color characteristic of the Cu⁺⁺ ion disappears from the solution. The depositions on the surface of the zinc metal consist of copper metal, and the solution now contains zinc ions. This reaction is represented by

Zn_(s) + Cu⁺⁺
_(aq) ⟶ Zn⁺⁺
_(aq) + Cu_(s)

In this redox reaction, Zn is oxidized to Zn⁺⁺ and Cu⁺⁺ is reduced to Cu. When electrons are transferred directly from Zn to Cu⁺⁺, the enthalpy of reaction is lost to the surroundings as heat. However, the same reaction can be carried out in a galvanic cell, allowing some of the chemical energy released to be converted into electrical energy. In its simplest form, a half-cell consists of a solid metal (called an electrode) that is submerged in a solution; the solution contains cations (+) of the electrode metal and anions (−) to balance the charge of the cations. The full cell consists of two half-cells, usually connected by a semi-permeable membrane or by a salt bridge that prevents the ions of the more noble metal from plating out at the other electrode.

A specific example is the Daniell cell (see figure), with a zinc (Zn) half-cell containing a solution of ZnSO₄ (zinc sulfate) and a copper (Cu) half-cell containing a solution of CuSO₄ (copper sulfate). A salt bridge is used here to complete the electric circuit.

If an external electrical conductor connects the copper and zinc electrodes, zinc from the zinc electrode dissolves into the solution as Zn⁺⁺ ions (oxidation), releasing electrons that enter the external conductor. To compensate for the increased zinc ion concentration, via the salt bridge zinc ions (cations) leave and sulfate ions (anions) enter the zinc half-cell. In the copper half-cell, the copper ions plate onto the copper electrode (reduction), taking up electrons that leave the external conductor. Since the Cu⁺⁺ ions (cations) plate onto the copper electrode, the latter is called the cathode. Correspondingly the zinc electrode is the anode. The electrochemical reaction is

${\displaystyle {\text{Zn}}_{\mathsf{(}\mathsf{\text{s}}\mathsf{)}}^{}+{\text{Cu}}_{\mathsf{(}\mathsf{\text{aq}}\mathsf{)}}^{++}⟶{\text{Zn}}_{\mathsf{(}\mathsf{\text{aq}}\mathsf{)}}^{++}+{\text{Cu}}_{\mathsf{(}\mathsf{\text{s}}\mathsf{)}}^{}}$

This is the same reaction as given in the previous example. In addition, electrons flow through the external conductor, which is the primary application of the galvanic cell.

As discussed under cell voltage, the electromotive force of the cell is the difference of the half-cell potentials, a measure of the relative ease of dissolution of the two electrodes into the electrolyte. The emf depends on both the electrodes and on the electrolyte, an indication that the emf is chemical in nature.

Half reactions and conventions

A half-cell contains a metal in two oxidation states. Inside an isolated half-cell, there is an oxidation-reduction (redox) reaction that is in chemical equilibrium, a condition written symbolically as follows (here, "M" represents a metal cation, an atom that has a charge imbalance due to the loss of n electrons):

Mⁿ⁺
_{(oxidized species)} + n e⁻ ⇌ M_{(reduced species)}

A galvanic cell consists of two half-cells, such that the electrode of one half-cell is composed of metal A, and the electrode of the other half-cell is composed of metal B; the redox reactions for the two separate half-cells are thus:

A ⁿ⁺ + n e⁻ ⇌ A

B ^m+ + m e⁻ ⇌ B

The overall balanced reaction is:

m A + n B ^m+ ⇌ n B + m A ⁿ⁺

In other words, the metal atoms of one half-cell are oxidized while the metal cations of the other half-cell are reduced. By separating the metals in two half-cells, their reaction can be controlled in a way that forces transfer of electrons through the external circuit where they can do useful work.

• The electrodes are connected with a metal wire in order to conduct the electrons that participate in the reaction.

In one half-cell, dissolved metal B cations combine with the free electrons that are available at the interface between the solution and the metal B electrode; these cations are thereby neutralized, causing them to precipitate from solution as deposits on the metal B electrode, a process known as plating.

This reduction reaction causes the free electrons throughout the metal B electrode, the wire, and the metal A electrode to be pulled into the metal B electrode. Consequently, electrons are wrestled away from some of the atoms of the metal A electrode, as though the metal B cations were reacting directly with them; those metal A atoms become cations that dissolve into the surrounding solution.

As this reaction continues, the half-cell with the metal A electrode develops a positively charged solution (because the metal A cations dissolve into it), while the other half-cell develops a negatively charged solution (because the metal B cations precipitate out of it, leaving behind the anions); unabated, this imbalance in charge would stop the reaction. The solutions of the half-cells are connected by a salt bridge or a porous plate that allows ions to pass from one solution to the other, which balances the charges of the solutions and allows the reaction to continue.

By definition:

• The anode is the electrode where oxidation (loss of electrons) takes place (metal A electrode); in a galvanic cell, it is the negative electrode, because when oxidation occurs, electrons are left behind on the electrode. These electrons then flow through the external circuit to the cathode (positive electrode) (while in electrolysis, an electric current drives electron flow in the opposite direction and the anode is the positive electrode).

• The cathode is the electrode where reduction (gain of electrons) takes place (metal B electrode); in a galvanic cell, it is the positive electrode, as ions get reduced by taking up electrons from the electrode and plate out (while in electrolysis, the cathode is the negative terminal and attracts positive ions from the solution). In both cases, the statement 'the cathode attracts cations' is true.

By their nature, galvanic cells produce direct current.

The Weston cell has an anode composed of cadmium mercury amalgam, and a cathode composed of pure mercury. The electrolyte is a (saturated) solution of cadmium sulfate. The depolarizer is a paste of mercurous sulfate. When the electrolyte solution is saturated, the voltage of the cell is very reproducible; hence, in 1911, it was adopted as an international standard for voltage.

• In the strictest sense, a battery is a set of two or more galvanic cells that are connected in series to form a single source of voltage.

For instance, a typical 12 V lead–acid battery has six galvanic cells connected in series, with the anodes composed of lead and cathodes composed of lead dioxide, both immersed in sulfuric acid.

Large central office battery rooms – in a telephone exchange to provide power for subscribers' land-line telephones, for instance – may have many cells, connected both in series and parallel: Individual cells are connected in series as a battery of cells with some standard voltage (c. 40 V), and banks of such serial batteries, themselves connected in parallel, to provide adequate amperage to supply a typical peak demand for telephone connections.

Cell voltage

The voltage (electromotive force E^o) produced by a galvanic cell can be estimated from the standard Gibbs free energy change in the electrochemical reaction according to:

$${\displaystyle E_{\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}^{\mathsf{o}}=-\frac{{\mathrm{\Delta }}_r{G}^{\mathsf{o}}}{{\nu }_{\mathsf{e}}F}}$$

where ν_e is the number of electrons transferred in the balanced half reactions, and F is Faraday's constant. However, it can be determined more conveniently by the use of a standard potential table for the two half cells involved. The first step is to identify the two metals and their ions reacting in the cell. Then one looks up the standard electrode potential, E^o, in volts, for each of the two half reactions. The standard potential of the cell is equal to the more positive E^o value minus the more negative E^o value.

For example, in the figure above the solutions are CuSO₄ and ZnSO₄. Each solution has a corresponding metal strip in it, and a salt bridge or porous disk connecting the two solutions and allowing SO²⁻
₄ ions to flow freely between the copper and zinc solutions. To calculate the standard potential one looks up copper and zinc's half reactions and finds:

Cu⁺⁺ + 2
e⁻
⇌ Cu   :   E^o = +0.34 V

Zn⁺⁺ + 2
e⁻
⇌ Zn   :   E^o = −0.76 V

Thus the overall reaction is:

Cu⁺⁺ + Zn ⇌ Cu + Zn⁺⁺

The standard potential for the reaction is then +0.34 V − (−0.76 V) = +1.10 V . The polarity of the cell is determined as follows. Zinc metal is more strongly reducing than copper metal because the standard (reduction) potential for zinc is more negative than that of copper. Thus, zinc metal will lose electrons to copper ions and develop a positive electrical charge. The equilibrium constant, K, for the cell is given by:

$${\displaystyle {\log }_{e}K=\frac{{\nu }_{\mathsf{e}}F{E}_{\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}^{\mathsf{o}}}{RT}}$$

where

F is the Faraday constant,

R is the gas constant, and

T is the absolute temperature in Kelvins.

For the Daniell cell K ≈ 1.5×10³⁷ . Thus, at equilibrium, a few electrons are transferred, enough to cause the electrodes to be charged.

Actual half-cell potentials must be calculated by using the Nernst equation as the solutes are unlikely to be in their standard states:

$${\displaystyle E_{\mathsf{h}\mathsf{a}\mathsf{l}\mathsf{f}\mathsf{-}\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}=E^{\mathsf{o}}-\frac{RT}{{\nu }_{\mathsf{e}}F}{\log }_{e}Q}$$

where Q is the reaction quotient. When the charges of the ions in the reaction are equal, this simplifies to:

$${\displaystyle E_{\mathsf{h}\mathsf{a}\mathsf{l}\mathsf{f}\mathsf{-}\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}=E^{\mathsf{o}}-2.303\frac{RT}{{\nu }_{\mathsf{e}}F}{\log }_{10}\left\{{\mathsf{M}}^{n+}\right\}}$$

where M ⁿ⁺ is the activity of the metal ion in solution. In practice concentration in ⁠ mol / L ⁠ is used in place of activity. The metal electrode is in its standard state so by definition has unit activity. The potential of the whole cell is obtained as the difference between the potentials for the two half-cells, so it depends on the concentrations of both dissolved metal ions. If the concentrations are the same the Nernst equation is not needed, and ${\displaystyle E_{\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}=E_{\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}^{\mathsf{o}}}$ under the conditions assumed here.

The value of 2.303 ⁠ R / F ⁠ is 1.9845×10⁻⁴ ⁠ V / K ⁠ , so at   T = 25 °C (298.15 K) the half-cell potential will change by only ⁠0.05918 V/ ν_e ⁠ if the concentration of a metal ion is increased or decreased by a factor of 10 .

$${\displaystyle E_{\mathsf{h}\mathsf{a}\mathsf{l}\mathsf{f}\mathsf{-}\mathsf{c}\mathsf{e}\mathsf{l}\mathsf{l}}=E^{\mathsf{o}}-\frac{0.05918\mathsf{V}}{{\nu }_{\mathsf{e}}}{\log }_{10}\left\{{\mathsf{M}}^{n+}\right\}}$$

These calculations are based on the assumption that all chemical reactions are in equilibrium. When a current flows in the circuit, equilibrium conditions are not achieved and the cell voltage will usually be reduced by various mechanisms, such as the development of overpotentials. Also, since chemical reactions occur when the cell is producing power, the electrolyte concentrations change and the cell voltage is reduced. A consequence of the temperature dependency of standard potentials is that the voltage produced by a galvanic cell is also temperature dependent.

Galvanic corrosion

Main article: Galvanic corrosion

Galvanic corrosion is the electrochemical erosion of metals. Corrosion occurs when two dissimilar metals are in contact with each other in the presence of an electrolyte, such as salt water. This forms a galvanic cell, with hydrogen gas forming on the more noble (less active) metal. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material. A concentration cell can be formed if the same metal is exposed to two different concentrations of electrolyte.