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Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be described as a set of metabolic reactions and processes that take place in the
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
s of organisms to convert
chemical energy Chemical energy is the energy of chemical substances that is released when they undergo a chemical reaction and transform into other substances. Some examples of storage media of chemical energy include batteries, Schmidt-Rohr, K. (2018). "How ...
from nutrients into
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
(ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a
combustion reaction Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
, it is an unusual one because of the slow, controlled release of energy from the series of reactions. Nutrients that are commonly used by animal and plant cells in respiration include sugar,
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
and fatty acids, and the most common oxidizing agent is molecular oxygen (O2). The chemical energy stored in ATP (the bond of its third phosphate group to the rest of the molecule can be broken allowing more stable products to form, thereby releasing energy for use by the cell) can then be used to drive processes requiring energy, including
biosynthesis Biosynthesis is a multi-step, enzyme- catalyzed process where substrates are converted into more complex products in living organisms. In biosynthesis, simple compounds are modified, converted into other compounds, or joined to form macromolecule ...
, locomotion or transport of molecules across cell membranes.


Aerobic respiration

''Aerobic respiration'' requires oxygen (O2) in order to create ATP. Although carbohydrates,
fat In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food. The term often refers specifically to triglycerides (triple est ...
s, and proteins are consumed as reactants, aerobic respiration is the preferred method of pyruvate breakdown in
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) ...
, and requires pyruvate to the mitochondria in order to be fully oxidized by the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protei ...
. The products of this process are carbon dioxide and water, and the energy transferred is used to break bonds in ADP to add a third phosphate group to form ATP (
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms of ...
), by
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group att ...
,
NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
and FADH2 The negative ΔG indicates that the reaction can occur spontaneously. The potential of NADH and FADH2 is converted to more ATP through an electron transport chain with oxygen and protons (hydrogen) as the "terminal electron acceptors". Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. The energy released is used to create a
chemiosmotic potential An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and t ...
by pumping protons across a membrane. This potential is then used to drive ATP synthase and produce ATP from
ADP Adp or ADP may refer to: Aviation * Aéroports de Paris, airport authority for the Parisian region in France * Aeropuertos del Perú, airport operator for airports in northern Peru * SLAF Anuradhapura, an airport in Sri Lanka * Ampara Airp ...
and a phosphate group. Biology textbooks often state that 38 ATP molecules can be made per oxidized glucose molecule during cellular respiration (2 from glycolysis, 2 from the
Krebs cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protei ...
, and about 34 from the electron transport system). However, this maximum yield is never quite reached because of losses due to 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 up to 15 times more efficient than anaerobic metabolism (which yields 2 molecules ATP per 1 molecule glucose). However, some anaerobic organisms, such as methanogens are able to continue with
anaerobic respiration Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing r ...
, yielding more ATP by using inorganic molecules other than oxygen as final electron acceptors in the electron transport chain. They share the initial pathway of
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) ...
but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post-glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
in
prokaryotic cell A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
s. Although plants are net consumers of carbon dioxide and producers of oxygen via photosynthesis, plant respiration accounts for about half of the CO2 generated annually by terrestrial ecosystems.


Glycolysis

Glycolysis is a metabolic pathway that takes place in the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrio ...
of cells in all living organisms. Glycolysis can be literally translated as "sugar splitting", and occurs with or without the presence of oxygen. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced, but two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two pyruvate molecules by the enzyme
aldolase Fructose-bisphosphate aldolase (), often just aldolase, is an enzyme catalyzing a reversible reaction that splits the aldol, fructose 1,6-bisphosphate, into the triose phosphates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate ( ...
. During the pay-off phase of glycolysis, four phosphate groups are transferred to ADP by
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group att ...
to make four ATP, and two NADH are produced when the pyruvate is oxidized. The overall reaction can be expressed this way: :Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2 pyruvate + 2 H+ + 2 NADH + 2 ATP + 2 H+ + 2 H2O + energy Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. Glycogen can be converted into glucose 6-phosphate as well with the help of glycogen phosphorylase. During energy metabolism, glucose 6-phosphate becomes fructose 6-phosphate. An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-bisphosphate by the help of phosphofructokinase. Fructose 1,6-biphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.


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 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.


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) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized fo ...
is produced from the pyruvate molecules created from glycolysis. Once
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized fo ...
is formed, aerobic or anaerobic respiration can occur. 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 is produced, the molecule then enters the
citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protei ...
(Krebs cycle) inside the mitochondrial matrix, and is oxidized to CO2 while at the same time reducing NAD to
NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
.
NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
can be used by the electron transport chain to create further ATP 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 low-energy waste products, H2O and CO2, are created during this cycle. The citric acid cycle is an 8-step process involving 18 different enzymes and co-enzymes. During the cycle, acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) yields citrate (6 carbons), which is rearranged to a more reactive form called isocitrate (6 carbons). Isocitrate is modified to become α-ketoglutarate (5 carbons), succinyl-CoA, succinate, fumarate, malate, and, finally, oxaloacetate. The net gain from one cycle is 3 NADH and 1 FADH2 as hydrogen- (proton plus electron)-carrying compounds and 1 high-energy GTP, which may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH2, and 2 ATP.


Oxidative phosphorylation

In eukaryotes, oxidative phosphorylation occurs in the mitochondrial
crista A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...
e. It comprises the electron transport chain that establishes a
proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and th ...
(chemiosmotic potential) across the boundary of the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized 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.


Efficiency of ATP production

The table below describes the reactions involved when one glucose molecule is fully oxidized into carbon dioxide. It is assumed that all the reduced
coenzyme A cofactor is a non- protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that as ...
s are oxidized by the electron transport chain and used for oxidative phosphorylation. Although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, such conditions are generally not realized because of 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 utilize the stored energy in the proton
electrochemical gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and ...
. * Pyruvate is taken up by a specific, low ''K''m transporter to bring it into the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex. * The phosphate carrier (PiC) mediates the electroneutral exchange (
antiport An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membr ...
) of phosphate (H2PO4; Pi) for OH or
symport A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...
of phosphate and protons (H+) across the inner membrane, and the driving force for moving phosphate ions into the mitochondria is the
proton motive force Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...
. * The
ATP-ADP translocase Adenine nucleotide translocator (ANT), also known as the ADP/ATP translocase (ANT), ADP/ATP carrier protein (AAC) or mitochondrial ADP/ATP carrier, exchanges free ATP with free ADP across the inner mitochondrial membrane. ANT is the most abund ...
(also called adenine nucleotide translocase, ANT) is an
antiporter An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma membr ...
and exchanges ADP and ATP across the inner 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 because the inner membrane of the mitochondria is slightly leaky to protons. Other factors may also dissipate the proton gradient creating an apparently leaky mitochondria. An uncoupling protein known as
thermogenin Thermogenin (called uncoupling protein by its discoverers and now known as uncoupling protein 1, or UCP1) is a mitochondrial carrier protein found in brown adipose tissue (BAT). It is used to generate heat by non-shivering thermogenesis, and mak ...
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 and
ATP synthesis ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...
. 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 (BAT) or brown fat makes up the adipose organ together with white adipose tissue (or white fat). Brown adipose tissue is found in almost all mammals. Classification of brown fat refers to two distinct cell populations with si ...
thermogenesis of newborn and hibernating mammals. According to some newer sources, the ATP yield during aerobic respiration is not 36–38, but only about 30–32 ATP molecules / 1 molecule of glucose , because: * ATP : NADH+H+ and ATP : FADH2 ratios during the oxidative phosphorylation appear to be not 3 and 2, but 2.5 and 1.5 respectively. Unlike in the
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group att ...
, the
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...
here is difficult to establish. **
ATP synthase ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...
produces 1 ATP / 3 H+. However the exchange of matrix ATP for cytosolic ADP and Pi (antiport with OH or symport with H+) mediated by
ATP–ADP translocase Adenine nucleotide translocator (ANT), also known as the ADP/ATP translocase (ANT), ADP/ATP carrier protein (AAC) or mitochondrial ADP/ATP carrier, exchanges free ATP with free ADP across the inner mitochondrial membrane. ANT is the most abund ...
and phosphate carrier consumes 1 H+ / 1 ATP as a result of regeneration of the transmembrane potential changed during this transfer, so the net ratio is 1 ATP : 4 H+. ** The mitochondrial electron transport chain proton pump transfers across the inner membrane 10 H+ / 1 NADH+H+ (4 + 2 + 4) or 6 H+ / 1 FADH2 (2 + 4). :So the final stoichiometry is :1 NADH+H+ : 10 H+ : 10/4 ATP = 1 NADH+H+ : 2.5 ATP :1 FADH2 : 6 H+ : 6/4 ATP = 1 FADH2 : 1.5 ATP * ATP : NADH+H+ coming from glycolysis ratio during the oxidative phosphorylation is ** 1.5, as for FADH2, if hydrogen atoms (2H++2e) are transferred from cytosolic NADH+H+ to mitochondrial FAD by the
glycerol phosphate shuttle The glycerol-3-phosphate shuttle is a mechanism that regenerates NAD+ from NADH, a by-product of glycolysis. The shuttle consists of the sequential activity of two proteins: GPD1 which transfers an electron pair from NADH to dihydroxyacetone phosph ...
located in the inner mitochondrial membrane. ** 2.5 in case of
malate-aspartate shuttle The malate-aspartate shuttle (sometimes simply the malate shuttle) is a biochemical system for translocating electrons produced during glycolysis across the semipermeable inner membrane of the mitochondrion for oxidative phosphorylation in eukary ...
transferring hydrogen atoms from cytosolic NADH+H+ to mitochondrial NAD+ So finally we have, per molecule of glucose *
Substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group att ...
: 2 ATP from
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) ...
+ 2 ATP (directly GTP) from
Krebs cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protei ...
* Oxidative phosphorylation ** 2 NADH+H+ from glycolysis: 2 × 1.5 ATP (if glycerol phosphate shuttle transfers hydrogen atoms) or 2 × 2.5 ATP (malate-aspartate shuttle) ** 2 NADH+H+ from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle: 8 × 2.5 ATP ** 2 FADH2 from the Krebs cycle: 2 × 1.5 ATP Altogether this gives 4 + 3 (or 5) + 20 + 3 = 30 (or 32) ATP per molecule of glucose These figures may still require further tweaking as new structural details become available. The above value of 3 H+/ATP for the synthase assumes that the synthase translocates 9 protons, and produces 3 ATP, per rotation. The number of protons depends on the number of c subunits in the Fo c-ring, and it is now known that this is 10 in yeast Fo and 8 for vertebrates. Including one H+ for the transport reactions, this means that synthesis of one ATP requires 1+10/3=4.33 protons in yeast and 1+8/3 = 3.67 in vertebrates. This would imply that in human mitochondria the 10 protons from oxidizing NADH would produce 2.72 ATP (instead of 2.5) and the 6 protons from oxidizing succinate or ubiquinol would produce 1.64 ATP (instead of 1.5). This is consistent with experimental results within the margin of error described in a recent review. The total ATP yield in ethanol or lactic acid fermentation is only 2 molecules coming from
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH) ...
, because pyruvate is not transferred to the mitochondrion and finally oxidized to the carbon dioxide (CO2), but reduced to ethanol or lactic acid in the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
.


Fermentation

Without oxygen, pyruvate (
pyruvic acid Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ac ...
) 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 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 buildup 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. This type of fermentation is called lactic acid fermentation. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD+ regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD+ attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by
substrate-level phosphorylation Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP or GTP by the transfer of a phosphate group from a substrate directly to ADP or GDP. Transferring from a higher energy (whether phosphate group att ...
, which does not require oxygen. Fermentation is less efficient at using the energy from glucose: only 2 ATP are produced per glucose, compared to the 38 ATP per glucose nominally produced by aerobic respiration. 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

Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Anaerobic respiration is used by microorganisms either bacteria or archaea in which neither oxygen (aerobic respiration) nor pyruvate derivatives (fermentation) is the final electron acceptor. Rather, an inorganic acceptor such as
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
(SO42-),
nitrate Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insolubl ...
(NO3), or sulfur (S) is used. Such organisms are could be found in unusual places such as underwater caves or near
hydrothermal vents A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
at the bottom of the ocean., as well as in anoxic soils or sediment in wetland ecosystems. In July 2019, a scientific study of Kidd Mine in Canada discovered
sulfur-breathing organisms Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termi ...
which live 7900 feet below the surface, and which breathe sulfur in order to survive. These organisms are also remarkable due to consuming minerals such as
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
as their food source.Strange life-forms found deep in a mine point to vast 'underground Galapagos'
, By Corey S. Powell, Sept. 7, 2019, nbcnews.com.


See also

*
Maintenance respiration Maintenance respiration (or maintenance energy) refers to metabolism occurring in an organism that is needed to maintain that organism in a healthy, living state. Maintenance respiration contrasts with growth respiration, which is responsible for t ...
: maintenance as a functional component of cellular respiration *
Microphysiometry Microphysiometry is the ''in vitro'' measurement of the functions and activities of life or of living matter (as organs, tissues, or cells) and of the physical and chemical phenomena involved on a very small (micrometer) scale. The term microphysiom ...
*
Pasteur point The Pasteur point is a level of oxygen (about 0.3% by volume which is less than 1% of Present Atmospheric Level or PAL) above which facultative aerobic microorganisms and facultative anaerobes adapt from fermentation to aerobic respiration. It is ...
*
Respirometry Respirometry is a general term that encompasses 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 ( calori ...
: research tool to explore cellular respiration *
Tetrazolium chloride Triphenyl tetrazolium chloride, TTC, or simply tetrazolium chloride (with the formula 2,3,5-triphenyl-2H-tetrazolium chloride) is a redox indicator commonly used in biochemical experiments especially to indicate cellular respiration (example: to ...
: cellular respiration indicator * Complex 1: NADH:ubiquinone oxidoreductes


References


External links


A detailed description of respiration vs. fermentation




at Clermont College {{DEFAULTSORT:Cellular Respiration Metabolism Plant physiology