Cellular respiration

Cellular respiration

Overview

Cellular respiration is the set of the metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 reactions and processes that take place in the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s to convert biochemical energy from nutrients into adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

 that involve the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reaction (oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of one molecule and the reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of another). Respiration is one of the key ways a cell gains useful energy to fuel cellular changes

Nutrients that are commonly used by animal and plant cells in respiration include sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

, amino acids and fatty acids, and a common oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 (electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

) is molecular oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (O2).
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Encyclopedia

Cellular respiration is the set of the metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 reactions and processes that take place in the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s to convert biochemical energy from nutrients into adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

 that involve the redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 reaction (oxidation
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of one molecule and the reduction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 of another). Respiration is one of the key ways a cell gains useful energy to fuel cellular changes

Nutrients that are commonly used by animal and plant cells in respiration include sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

, amino acids and fatty acids, and a common oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 (electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

) is molecular oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (O2). Bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

 can also be lithotroph
Lithotroph
A lithotroph is an organism that uses an inorganic substrate to obtain reducing equivalents for use in biosynthesis or energy conservation via aerobic or anaerobic respiration. Known chemolithotrophs are exclusively microbes; No known macrofauna possesses the ability to utilize inorganic...

s and these organisms may respire using a broad range of inorganic molecules as electron donors and acceptors, such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

, metal ions
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

, methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 or hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. Organisms that use oxygen as a final electron acceptor in respiration are described as aerobic
Aerobic organism
An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.Faculitative anaerobes grow and survive in an oxygenated environment and so do aerotolerant anaerobes.-Glucose:...

, while those that do not are referred to as anaerobic
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

.

The energy released in respiration is used to synthesize ATP to store this energy. The energy stored in ATP can then be used to drive processes requiring energy, including biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...

, locomotion or transportation of molecules across cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

s.

Aerobic respiration


Aerobic respiration requires oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 in order to generate energy (ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

). Although carbohydrates, fats
Fats
Fats may refer to:*Fat, a generic term for a class of lipids in biochemistry*Fats Domino, rock and roll pianist*Fats Waller, jazz pianist*Fats Navarro, jazz trumpeter*Singing Fats, master of ceremonies*Obesity...

, and proteins can all be processed and consumed as reactant, it is the preferred method of pyruvate breakdown in glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...

 and requires that pyruvate enter the mitochondrion
Mitochondrion
In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

 in order to be fully oxidized by the Krebs cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

. The product of this process is energy in the form of ATP (Adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

), by substrate-level phosphorylation
Substrate-level phosphorylation
Substrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate or guanosine triphosphate by the direct transfer and donation of a phosphoryl group to adenosine diphosphate or guanosine diphosphate from a phosphorylated reactive...

, NADH and FADH2
Simplified reaction: C6H12O6 (aq) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l)
ΔG = -2880 kJ per mole of C6H12O6


The negative ΔG indicates that the reaction can occur spontaneously.

The reducing potential of NADH and FADH2 is converted to more ATP through an electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 with oxygen as the "terminal electron acceptor". Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...

. This works by the energy released in the consumption of pyruvate being used to create a chemiosmotic potential by pumping proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s across a membrane. This potential is then used to drive ATP synthase and produce ATP from ADP
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

 and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport system). However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix and current estimates range around 29 to 30 ATP per glucose.

Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol ATP per 1 mol glucose). They share the initial pathway of glycolysis
Glycolysis
Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...

 but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cell
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s, and in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

 in prokaryotic cell
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s.

Glycolysis



Glycolysis is a metabolic pathway
Metabolic pathway
In biochemistry, metabolic pathways are series of chemical reactions occurring within a cell. In each pathway, a principal chemical is modified by a series of chemical reactions. Enzymes catalyze these reactions, and often require dietary minerals, vitamins, and other cofactors in order to function...

 that is found in the cytosol
Cytosol
The cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....

 of cells in all living organisms. This pathway does not require oxygen, and can therefore function under anaerobic circumstances. The process converts one molecule of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 into two molecules of pyruvate (pyruvic acid), making energy in the form of two net molecules of ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

. Four molecules of ATP per glucose are actually produced; however, two are consumed for the preparatory phase. The initial phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....

 of glucose is required to destabilize the molecule for cleavage into two pyruvate. During the pay-off phase of glycolysis, four phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...

 groups are transferred to ADP by substrate-level phosphorylation
Substrate-level phosphorylation
Substrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate or guanosine triphosphate by the direct transfer and donation of a phosphoryl group to adenosine diphosphate or guanosine diphosphate from a phosphorylated reactive...

 to make four ATP, and two NADH are produced when the pyruvate are oxidized. The overall reaction can be expressed this way:
Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O

Oxidative decarboxylation of pyruvate



Pyruvate is oxidized to acetyl-CoA and CO2 by the Pyruvate dehydrogenase complex (PDC). The PDC contains multiple copies of three enzymes and is located in the mitochondria
Mitochondrial matrix
In the mitochondrion, the matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules.The mitochondrial matrix also contains the mitochondria's DNA and ribosomes. The word "matrix" stems from the fact that this space is viscous, compared to the...

 of eukaryotic cells and in the cytosol of prokaryotes. In the conversion of pyruvate to acetyl-CoA one molecule of NADH and one molecule of CO2 is formed. This step is also known as the link reaction or transition step, as it links glycolysis and the Krebs cycle.

Citric acid cycle



This is also called the Krebs cycle or the tricarboxylic acid cycle. When oxygen is present, acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

 is produced from the pyruvate molecules created from glycolysis. Once acetyl-CoA is formed, two processes can occur, aerobic or anaerobic respiration. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA
Acetyl-CoA
Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

 is produced, the molecule then enters the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

 (Krebs cycle) inside the mitochondrial matrix, and gets oxidized to CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 while at the same time reducing NAD
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide, abbreviated NAD, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide.In metabolism, NAD is involved...

 to NADH. NADH can be used by the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 to create further ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two waste products
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...

, H2O and CO2, are created during this cycle.

The citric acid cycle is an 8-step process involving different enzymes and co-enzymes. Throughout the entire cycle, acetyl-CoA(2 carbons) + Oxaloacetate(4 carbons). Citrate(6 carbons) is rearranged to a more reactive form called Isocitrate(6 carbons). Isocitrate(6 carbons) modifies to become α-Ketoglutarate(5 carbons), Succinyl-CoA, Succinate, Fumarate, Malate, and finally, Oxaloacetate. The net energy gain from one cycle is 3 NADH, 1 FADH2, and 1 GTP; the GTP may subsequently be used to produce ATP. Thus, the total energy yield from one whole glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH2, and 2 ATP.

Oxidative phosphorylation



In eukaryotes, oxidative phosphorylation occurs in the mitochondrial crista
Crista
Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...

e. It comprises the electron transport chain that establishes a proton gradient (chemiosmotic potential) across the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesised by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. The electrons are finally transferred to exogenous oxygen and, with the addition of two protons, water is formed.

The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide. It is assumed that all the reduced coenzymes are oxidized by the electron transport chain and used for oxidative phosphorylation.
Step coenzyme yield ATP yield Source of ATP
Glycolysis preparatory phase -2 Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm.
Glycolysis pay-off phase 4 Substrate-level phosphorylation
2 NADH 4-6 Oxidative phosphorylation - Each NADH produces net 3 ATP due to NADH transport over the mitochondrial membrane
Oxidative decarboxylation of pyruvate 2 NADH 6 Oxidative phosphorylation
Krebs cycle 2 Substrate-level phosphorylation
6 NADH 18 Oxidative phosphorylation
2 FADH2 4 Oxidative phosphorylation
Total yield 36-38  ATP From the complete oxidation of one glucose molecule to carbon dioxide and oxidation of all the reduced coenzymes.

Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized due to losses such as the cost of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria. All are actively transported using carriers that utilise the stored energy in the proton electrochemical gradient
Electrochemical gradient
An electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane; that is, a combination of the membrane potential and the pH gradient...

.
  • Pyruvate is taken up by a specific, low km transporter to bring it into the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex.
  • The phosphate translocase is a symporter
    Symporter
    A cotransporter is an integral membrane protein that is involved in secondary active transport. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient....

     and the driving force for moving phosphate ions into the mitochondria is the proton motive force.
  • The adenine nucleotide carrier is an antiporter and exchanges ADP and ATP across the inner membrane
    Inner membrane
    The inner membrane is the biological membrane of an organelle or Gram-negative bacteria that is within an outer membrane....

    . The driving force is due to the ATP (-4) having a more negative charge than the ADP (-3) and thus it dissipates some of the electrical component of the proton electrochemical gradient.


The outcome of these transport processes using the proton electrochemical gradient is that more than 3 H+ are needed to make 1 ATP. Obviously this reduces the theoretical efficiency of the whole process and the likely maximum is closer to 28-30 ATP molecules. In practice the efficiency may be even lower due to the inner membrane of the mitochondria being slightly leaky to protons. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as thermogenin
Thermogenin
Thermogenin is an uncoupling protein found in the mitochondria of brown adipose tissue . It is used to generate heat by non-shivering thermogenesis...

 is expressed in some cell types and is a channel that can transport protons. When this protein is active in the inner membrane it short circuits the coupling between the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 and ATP synthesis
ATP synthase
right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...

. The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in brown fat
Brown adipose tissue
Brown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals....

 thermogenesis of newborn and hibernating mammals.

Fermentation



Without oxygen, pyruvate (pyruvic acid) is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...

 that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the build up of NADH in the cytoplasm and provides NAD+ for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

. This type of fermentation is called lactic acid fermentation
Lactic acid fermentation
Lactic acid fermentation is a biological process by which sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic byproduct lactate. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells, in the...

. In yeast, the waste products are ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

. This type of fermentation is known as alcoholic or ethanol fermentation
Ethanol fermentation
Ethanol fermentation, also referred to as alcoholic fermentation, is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products...

. The ATP generated in this process is made by substrate-level phosphorylation
Substrate-level phosphorylation
Substrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate or guanosine triphosphate by the direct transfer and donation of a phosphoryl group to adenosine diphosphate or guanosine diphosphate from a phosphorylated reactive...

, which does not require oxygen.

Fermentation is less efficient at using the energy from glucose since 2 ATP are produced per glucose, compared to the 38 ATP per glucose produced by aerobic respiration. This is because the waste products
Cellular waste product
Cellular waste products are formed as a byproduct of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP...

 of fermentation still contain plenty of energy. Ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, for example, can be used in gasoline (petrol) solutions. Glycolytic ATP, however, is created more quickly. For prokaryotes to continue a rapid growth rate when they are shifted from an aerobic environment to an anaerobic environment, they must increase the rate of the glycolytic reactions. For multicellular organisms, during short bursts of strenuous activity, muscle cells use fermentation to supplement the ATP production from the slower aerobic respiration, so fermentation may be used by a cell even before the oxygen levels are depleted, as is the case in sports that do not require athletes to pace themselves, such as sprinting.

Anaerobic respiration



Anaerobic respiration is used by some microorganisms in which neither oxygen (aerobic respiration) nor pyruvate or a pyruvate derivative (fermentation) is the final electron acceptor. Rather, an inorganic acceptor such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 is used.

See also

  • Tetrazolium chloride
    Tetrazolium chloride
    Triphenyl tetrazolium chloride, TTC, or simply Tetrazolium chloride is a redox indicator commonly used in biochemical experiments especially to indicate cellular respiration....

    : cellular respiration indicator
  • Maintenance respiration
    Maintenance respiration
    Maintenance respiration refers to metabolism occurring in an organism that is needed to maintain that organism in a healthy, living state...

    : maintenance as a functional component of cellular respiration
  • Pasteur point
    Pasteur point
    The Pasteur point is a level of oxygen above which aerobic microorganisms and facultative anaerobes adapt from anaerobic respiration to aerobic respiration. It is also used to mark the level of oxygen in the early atmosphere of the Earth that is believed to have led to major evolutionary changes...

  • Respirometry
    Respirometry
    Respirometry is a general term that encompass a number of techniques for obtaining estimates of the rates of metabolism of vertebrates, invertebrates, plants, tissues, cells, or microorganisms via an indirect measure of heat production ....

    : research tool to explore cellular respiration


Pyruvate is decarboxylated to form acetylaldehyde and then to ethanol.
There can be at the most 2 ATP molecules formed

External links