Internal respiration . Types and process


 Internal respiration also known as  cellular respiration is the set of biochemical reactions why particular organic compounds are completely degraded by oxidation , to become inorganic substances, a process that provides useful energy for the cell (mainly in the form of ATP ).

Types of internal respiration 

  • Aerobic respiration . The final electron acceptor is molecular oxygen , which is reduced to water . It is carried out by the vast majority of organisms, including humans. The organisms that carry out this type of breathing are called aerobic organisms .
  • Anaerobic breathing . The final electron acceptor is an inorganic molecule other than oxygen, more rarely an organic molecule. It is a type of metabolism very common in many microorganisms , especially prokaryotes . It should not be confused with fermentation , anaerobic process, but in which nothing like an electron transport chain intervenes.

Aerobic respiration

The aerobic respiration is a type of energy metabolism in the living extract energy from organic molecules , such as glucose , a complex process in which carbon is oxidized and in which the oxygen from the air is the oxidant used. In other variants of respiration, very rare, the oxidant is different from oxygen ( anaerobic respiration ).

Aerobic respiration is the responsible process that most living beings, so-called aerobic, require oxygen. Aerobic respiration is characteristic of eukaryotic organisms in general and of some types of bacteria .

The oxygen that, like any gas, crosses without obstructing the biological membranes , crosses first the plasma membrane and then the mitochondrial membranes , being in the matrix of the mitochondria where it is united to electrons and protons (that add up constitute atoms of hydrogen ) forming water . In that final oxidation, which is complex, and in previous processes the energy required for the phosphorylation of ATP is obtained .

In the presence of oxygen, the pyruvic acid , obtained during the first anaerobic phase or glycolysis , is oxidized to provide energy, carbon dioxide and water. This series of reactions is known as aerobic respiration.

The overall chemical reaction of respiration is as follows:6 H 12 O 6 + 6O 2 ? 6CO 2 + 6H 2 O + energy ( ATP )


During glycolysis , a glucose molecule is oxidized and divided into two molecules of pyruvic acid (pyruvate). In this metabolic path two net molecules of ATP are obtained and two molecules of NAD + are reduced ; the number of carbons remains constant (6 in the initial glucose molecule, 3 in each of the pyruvic acid molecules). The entire process is performed in the cell’s cytosol .

The glycerine (glycerol) formed in the lipolysis of triglycerides incorporated into glycolysis at the level of glyceraldehyde 3 – phosphate .

The oxidative deamination of certain amino acids also yields pyruvate; which have the same metabolic fate as that obtained by glycolysis.

Oxidative decarboxylation of pyruvic acid edit ]

Pyruvic acid enters the mitochondrial matrix where it is processed by the enzymatic complex pyruvate dehydrogenase , which carries out the oxidative decarboxylation of pyruvate; decarboxylation because starts one of the three carbons of pyruvic acid (which appears in the form of CO 2 ) Oxidative because, simultaneously tear off two atoms of hydrogen (oxidation by dehydrogenation ), which are captured by the NAD + , which is reduced to NADH . So; pyruvate is transformed into a radicalacetyl (-CO-CH 3 , acetic acid without the hydroxyl group ) which is taken up by coenzyme A (which passes to acetyl-CoA ), which is responsible for transporting it to the Krebs cycle.

Krebs Cycle

The Krebs cycle is a cyclic metabolic pathway that is carried out in the mitochondrial matrix and in which the two acetyls transported by acetyl coenzyme A from pyruvate are oxidized to produce two CO 2 molecules , releasing energy in usable form, ie reducing power ( NADH , FAD H 2 ) and GTP .

Two complete turns of the Krebs cycle occur for each glucose, since two molecules of acetyl coenzyme A had been produced in the previous step; thus gaining 2 GTPs and releasing 4 molecules of CO 2 . These four molecules, together with the oxidative decarboxylation of pyruvate, make a total of six, which is the number of CO 2 molecules produced in aerobic respiration (see general equation).

Respiratory chain and oxidative phosphorylation 

They are the last stages of aerobic or anaerobic respiration and have two basic purposes:

  1. Reoxidize the coenzymes that have been reduced in the previous steps ( NADH and FAD H 2 ) in order that they are again free to accept electrons and protons from new oxidizable substrates.
  2. Produce usable energy in the form of ATP .

These two phenomena are closely related and mutually coupled. They occur in a series of enzymatic complexes located (in eukaryotes ) in the internal membrane of the mitochondria ; four complexes perform the oxidation of the aforementioned coenzymes by transporting the electrons and harnessing their energy to pump protons from the mitochondrial matrix to the intermembrane space. These protons can only return to the matrix through the ATP synthase , the enzyme that fail the electrochemical gradient created for phosphorylating the ADP to ATP , a process known as oxidative phosphorylation .

The electrons and protons involved in these processes are definitely assigned to O 2 which is reduced to water . Note that the atmospheric oxygen obtained by pulmonary ventilation hashas the sole purpose of acting as final acceptor of electrons and protons in aerobic respiration.

Anaerobic respiration

The anaerobic respiration (or anaerobic ) is a biological process oxidorreducción of monosaccharides and other compounds in which the terminal electron acceptor is an inorganic molecule other than the oxygen , and rarely an organic molecule through an electron transport chain analogous to that of the mitochondria in aerobic respiration .  It should not be confused with fermentation , which is also an anaerobic process, but in which nothing similar to a transport chain of electrons and the final electron acceptor is always an organic molecule such as pyruvate . Anaerobic respiration is a type of metabolic process unique to certain prokaryotic microorganisms .



In this process no oxygen is used , but another different oxidizing substance such as sulfate or nitrate . In bacteria with anaerobic respiration there is also an electron transport chain in which the reduced coenzymes are reoxidized during the oxidation of the nutrient substrates ; is the analog of aerobic respiration , since it is composed of the same elements ( cytochromes , quinones , proteins ferrous sulphides, etc.). The only difference, therefore, lies in the fact that the ultimate electron acceptor is not oxygen.

All possible acceptors in anaerobic respiration have a lower reduction potential than O 2 , so that, starting from the same substrates ( glucose , amino acids , triglycerides ), less energy is generated in this metabolism than in conventional aerobic respiration .

Anaerobic respiration should not be confused with fermentation , in which there is no electron transport chain at all , and the final electron acceptor is an organic molecule ; these two types of metabolism have only in common not being dependent on oxygen.

The following table shows different electron acceptors of their products and some examples of microorganisms that perform such processes:


Acceptor Final product Microorganism
Nitrate Nitrites , nitrogen oxides and N 2 Pseudomonas , Bacillus
Sulfate Sulphides Desulfovibrio , Clostridium
Sulfur Sulphides Thermoplasma
Thiosulfate Sulfate and sulfide Thermotogae , Thermoanaerobacteriales
CO 2 Methane Methanococcus , Methanosarcine , Methanopyrus
Fe 3+ Fe 2+ Shewanella , Geobacter , Geospirillum , Geovibrio
Mn 4+ Mn 2+ Shewanella putrefaciens
Seleniato Selenito B. selenatarsenatis
Arsenic Arsenite Desulfotomaculum , Chrysiogenetes
Fumarate Succinate Wolinella succinogenes , Desulfovibrio , E. coli
DMSO DMS Campylobacter , Escherichia
TMAO TMA Escherichia coli
Chlorobenzoate Benzoate Desulfomonile


Use of nitrate as an electron acceptor 

Many bacteria anaerobic contain the enzymes nitrate reductases that catalyze the reduction of nitrate to nitrite :

However, the resulting product (nitrite) is very toxic so some species of Pseudomonas and Bacillus can reduce nitrate beyond the level of nitrite, to molecular nitrogen :

The end result, nitrogen, is an inert and non-toxic gas . This process is known as denitrification which, if produced in the soil is considered harmful to agriculture as it causes the loss of nitrates, necessary for the growth of plants.

Nitrate-reducing bacteria are facultative anaerobes since the use of nitrates and nitrites as electron acceptors are alternative processes that can be used by these bacteria to grow in the absence of oxygen. In the presence of it, although nitrate is present, respiration proceeds entirely through the aerobic chain of electron transport.

Use of sulfate as an electron acceptor 

Diagram of the corrosion in anaerobic conditions caused by bacteria of the genus Desulfovibrio.

The use of sulfate as an electron acceptor is a rare ability, restricted to the genus Desulfovibrio and some species of Clostridium . All these bacteria are strictly anaerobic , so the reduction of sulfate is not an alternative of their metabolism, as is the reduction of nitrate. The reaction is as follows:

SO 2- + 8e  + 8H + ? S 2- + 4H 2 O

Sulphate-reducing bacteria attack only a few organic compounds , with lactic acid and 4- carbon dicarboxylic acids being its main substrates.

Use of carbon dioxide as an electron acceptor 

A small group of strict anaerobic prokaryotes , methane-producing archaea , use carbon dioxide as an electron acceptor; the reduction leads to methane (CH 4 ). The simplest case is the oxidation of molecular hydrogen , energy-producing reaction:

4H 2 + CO 2 ? CH 4 + 2H 2 O

Hydrogen is not a common gas in the biosphere , so that these microorganisms inhabit very specific places as in anaerobic sediments of lakes and marshes , or in the digestive tract of ruminants , where other microorganisms produce the free H 2 they need .

Use of ferric ion as an electron acceptor 

Ferric ion (Fe 3+ ) can be used by several bacteria as an electron acceptor, reducing it to ferrous ion (Fe 2+ ); this process is carried out by many microorganisms that reduce nitrate. The ferric ion is found in soil and rocks, often forming ferric hydroxide (Fe (OH) 3 ) insoluble; under anaerobic conditions, these bacteria can reduce it to the ferrous state. The ferrous ion is much more soluble than the ferric, whereby the iron is mobilized, this being an important first step in the formation of a type of mineral deposit called marsh iron .


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