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Cellular respiration is the set of
reactions and processes that take place in the
to convert
(ATP), and then release waste products. The reactions involved in respiration are , which break large molecules into smaller ones, releasing energy in the process as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell gains useful energy to fuel cellular activity. Cellular respiration is considered an
which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although, technically, cellular respiration is a , it clearly does not resemble one when it occurs in a living cell due to slow release of energy from the series of reactions.
Nutrients that are commonly used by animal and plant cells in respiration include ,
and , and the most common
() is molecular
(O2). The chemical energy stored in ATP (its third phosphate group is weakly bonded to the rest of the molecule and is cheaply broken allowing stronger bonds to form, thereby transferring energy for use by the cell) can then be used to drive processes requiring energy, including ,
or transportation of molecules across .
Aerobic respiration (red arrows) is the main means by which both fungi and plants utilize chemical energy in the form of organic compounds that were previously created through
(green arrow).
Aerobic respiration requires
in order to generate . Although , , and
are consumed as reactants, it is the preferred method of
breakdown in
and requires that pyruvate enter the
in order to be fully oxidized by the . The products of this process are carbon dioxide and water, but the energy transferred is used to break strong bonds in ADP as the third phosphate group is added to form ATP (), by ,
Simplified reaction:
C6H12O6 (s) + 6 O2 (g) → 6 CO2 (g) + 6 H2O (l) + heat
ΔG = -2880 kJ per mol of C6H12O6
The negative ΔG indicates that the reaction can occur spontaneously.
The potential of NADH and FADH2 is converted to more ATP through an
with oxygen as the "terminal electron acceptor". Most of the ATP produced by aerobic cellular respiration is made by . This works by the energy released in the consumption of pyruvate being used to create a
by pumping
across a membrane. This potential is then used to drive ATP synthase and produce ATP from
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 up to 15 times more efficient than anaerobic metabolism (which yields 2 molecules ATP per 1 molecule glucose). However some anaerobic organisms, such as
are able to continue with , yielding more ATP by using other inorganic molecules (not oxygen) as final electron acceptors in the electron transport chain. They share the initial pathway of
but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post-glycolytic reactions take place in the mitochondria in , and in the
Out of the cytoplasm it goes into the Krebs cycle with the acetyl CoA. It then mixes with CO2 and makes 2 ATP, NADH, and FADH. From there the NADH and FADH go into the NADH reductase, which produces the enzyme. The NADH pulls the enzyme's electrons to send through the electron transport chain. The electron transport chain pulls H+ ions through the chain. From the electron transport chain, the released hydrogen ions make ADP for an end result of 32 ATP. 02 attracts itself to the left over electron to make water. Lastly, ATP leaves through the ATP channel and out of the mitochondria.
Main article:
Glycolysis is a
that takes place in the
of cells in all living organisms. This pathway can function with or without the presence of oxygen. In humans, aerobic conditions produce
and anaerobic conditions produce . In aerobic conditions, the process converts one molecule of
into two molecules of
(pyruvic acid), generating energy in the form of two net molecules of . Four molecules of ATP per glucose are actually produced, however, two are consumed as part of the . The initial
of glucose is required to increase the reactivity (decrease its stability) in order for the molecule to be cleaved into two
molecules by the enzyme . During the
of glycolysis, four
groups are transferred to ADP by
to make four ATP, and two NADH are produced when the
are oxidized. The overall reaction can be expressed this way:
Glucose + 2 NAD+ + 2 Pi + 2 ADP → 2
+ 2 NADH + 2 ATP + 2 H+ + 2 H2O + heat
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-disphosphate by the help of phosphofructokinase. Fructose 1,6-diphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.
Main article:
Pyruvate is oxidized to acetyl-CoA and CO2 by the
(PDC). The PDC contains multiple copies of three enzymes and is located in 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.
Main article:
This is also called the Krebs cycle or the tricarboxylic acid cycle. When oxygen is present,
is produced from the pyruvate molecules created from glycolysis. 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
is produced, the molecule then enters the
(Krebs cycle) inside the mitochondrial matrix, and gets oxidized to
while at the same time reducing
can be used by the
to create further
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 , H2O and CO2, are created during this cycle.
The citric acid cycle is an 8-step process involving 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 of high-energy compounds from one cycle is 3 NADH, 1 FADH2, and 1 GTP; the GTP 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.
Main articles: , ,
In eukaryotes, oxidative phosphorylation occurs in the mitochondrial . It comprises the electron transport chain that establishes a
(chemiosmotic potential) across the boundary of 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
are oxidized by the electron transport chain and used for oxidative phosphorylation.
coenzyme yield
Source of ATP
Glycolysis preparatory phase
Phosphorylation of glucose and fructose 6-phosphate uses two ATP from the cytoplasm.
Glycolysis pay-off phase
Substrate-level phosphorylation
Oxidative phosphorylation – Each NADH produces net 1.5 ATP (instead of usual 2.5) due to NADH transport over the mitochondrial membrane
Oxidative decarboxylation of pyruvate
Oxidative phosphorylation
Krebs cycle
Substrate-level phosphorylation
Oxidative phosphorylation
Oxidative phosphorylation
Total yield
30  or 32 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 utilize the stored energy in the proton .
Pyruvate is taken up by a specific, low Km transporter to bring it into the mitochondrial matrix for oxidation by the pyruvate dehydrogenase complex.
(PiC) mediates the electroneutral exchange () of phosphate (H2PO4-; Pi) for OH- or
of phosphate and protons (H+) across the inner membrane, and the driving force for moving phosphate ions into the mitochondria is the .
(also called ) is an
and exchanges ADP and ATP across the . 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
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
and . The potential energy from the proton gradient is not used to make ATP but generates heat. This is particularly important in
thermogenesis of newborn and hibernating mammals.
Stoichiometry of
and most known
Numbers in circles indicate counts of carbon atoms in molecules, C6 is
C6H12O6, C1
outer membrane is omitted.
According to some of 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
appear to be not 3 and 2, but 2.5 and 1.5 respectively. Unlike in the , the stoichiometry here is difficult to establish.
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
consumes 1 H+ / 1 ATP due to regeneration of the transmembrane potential changed during this transfer, so the net ratio is 1 ATP : 4 H+.
The mitochondrial
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
located in the inner mitochondrial membrane.
2.5 in case of
transferring hydrogen atoms from cytosolic NADH+H+ to mitochondrial NAD+
So finally we have, per molecule of glucose
: 2 ATP from
+ 2 ATP (directly GTP) from
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
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
The total ATP yield in ethanol or lactic acid
is only 2 molecules coming from , because pyruvate is not transferred to the
and finally oxidized to the carbon dioxide (CO2), but reduced to
Main article:
Without oxygen, pyruvate () 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
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 . This type of fermentation is called . 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
and . This type of fermentation is known as alcoholic or . The ATP generated in this process is made by , 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. This is because the
of fermentation still contain chemical potential energy that can be released by oxidation. , for example, can be burned in an internal combustion engine like gasoline. 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 .
Main article:
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 some microorganisms in which neither oxygen (aerobic respiration) nor pyruvate derivatives (fermentation) is the final electron acceptor. Rather, an inorganic acceptor such as
is used. Such organisms are typically found in unusual places such as underwater caves or near the lava shoots at the bottom of the ocean.
Many high-school biology textbooks[] incorrectly refer to fermentation (e.g., to lactate) as anaerobic respiration.
: maintenance as a functional component of cellular respiration
: research tool to explore cellular respiration
: cellular respiration indicator
: NADH:ubiquinone oxidoreductase
Bailey, Regina. .
Rich, P. R. (2003). "The molecular machinery of Keilin's respiratory chain". Biochemical Society Transactions 31 (Pt 6): . :.  .
Porter, R.; Brand, M. (1 September 1995). . The Biochemical journal (Free full text). 310 ( Pt 2) (Pt 2): 379–382.  .  .  . 
Stryer, Lubert (1995). Biochemistry (fourth ed.). New York – Basingstoke: W. H. Freeman and Company.  .
at Clermont College
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