Cellular Respiration and Fermentation

2D says, “We Monarchs need lots of energy to live our lives. As adult butterflies, we siphon nectar from flowers. That is high in sugar, which our mitochondria ‘burn for fuel,’ via the process of cellular respiration, so our bodies can use the energy for other things, including flying all the way to Mexico to spend the winter. While we are ‘cold-blooded,’ you might actually see us using cellular respiration to warm up before we begin to fly. To you humans, that might look like we’re ‘shivering,’ but by rapidly twitching our flight muscles, thereby stimulating the mitochondria to do cellular respiration, we’re able to warm up those muscles to prepare for flight.”

Background Information:

Structure of ATP
Structure of ATP
As was mentioned on the Photosynthesis Web Page, cells store energy as potential energy in the bonds of a chemical called ATP (adenosine triphosphate). Note that “ATP” is not a chemical name/formula, but is an abbreviation for “adenosine triphosphate.” This chemical is made of the nucleic acid adenine (also found in DNA) bonded to ribose, a 5-carbon sugar also found in DNA, to make the nucleoside adenosine. As in our DNA, when adenosine is bonded to a phosphate group (-O-PO3-3, often symbolized as ), that makes adenosine monophosphate or “AMP.” Thus, ATP is very similar to the building blocks for our DNA.

AMP may be joined to another phosphate group by dehydration synthesis to form adenosine diphosphate or “ADP.” That, in turn, may be joined to a third phosphate group, again via dehydration synthesis, to form ATP. The energy may be released, again, via hydrolysis, so this chemical reaction can go either direction, and thus may be written as
ADP +  ↔ ATP + H2O

The bonds that hold the last two phosphate groups, especially the last one, onto the molecule are “high-energy” bonds. That means it takes a lot of energy to create the bond, thus a lot of energy is “stored in the bond,” that bond is very unstable, and when that bond is broken, a lot of energy will be released. Frequently, those high-energy bonds are symbolized by a “~,” so for example, AMP~~. Cells use those high-energy bonds in ATP to store the energy harvested via cellular respiration and “transport” that energy by moving the ATP molecule to somewhere else in the cell.

Frequently the energy is transferred to some other molecule by transferring the to that other molecule. In that case, the molecule to which the is attached is said to be phosphorylated. Typically, once that molecule has done whatever chemical reaction for which that energy was needed, the is released.

(For more information on adenine, adenosine, and AMP, see the DNA Web Page. For more information on dehydration synthesis, see the Carbon Compounds Web Page.)

As was mentioned on the Cells and Organelles Web Page, Mitochondria are found in nearly all eukaryotic cells, usually several or many per cell. Note, by the way, that “mitochondrion” is singular and “mitochondria” is the plural form of the word. Mitochondria are semi-autonomous: they grow and divide on their own, they can move around within the cell and change shape, and they contain their own DNA and thus are able to direct some of their own protein synthesis independent of the cell nucleus (although they do depend on other molecules which are provided by the cell). Consequently, it is thought that mitochondria (and chloroplasts) may have originally arisen from a prokaryote that “invaded” and “parasitized” a host cell.

Muscle and brain (nerve) cells that need a lot of energy to do their jobs contain an especially high number of mitochondria. That’s because mitochondria are the “engines” of the cell: they burn sugar for fuel in the process of cellular respiration and harvest the energy from the sugar, transferring it to ATP so the cell can use that energy for other purposes.

Mitochondria consist of a smooth outer membrane and a convoluted inner membrane separated by an intermembrane space. The convolutions of the inner membrane are called cristae and the space inside the inner membrane is the mitochondrial matrix. The chemicals which make up the electron transport chain (described below) are built into the inner membrane. As sugar is burned for fuel, a mitochondrion shunts electrons, hydrogen ions, and a few other chemicals across the inner membrane (matrix to/from intermembrane space).

Not directly related to cellular respiration, but there have been some very interesting studies done on mitochondrial DNA (mt-DNA) in humans. In humans (and other mammals), when a sperm fertilizes an egg to create a new baby, the only thing from the sperm cell that enters the egg cell is the sperm’s chromosomes. None of the sperm’s mitochondria (or other organelles) enter the egg. Thus, while we get half of the DNA/chromosomes in the nucleus of each of our cells from our mother and half from our father, we get 100% of our mitochondria, and therefore mt-DNA, from our mother. Studying and comparing mt-DNA has enabled researchers to construct a “giant family tree” for humans, as well as things like being able to tell you whether your maternal ancestors came from Ireland or Germany or were Native American, etc.

As cells do cellular respiration, they need a way to store and transport electrons. There are two main chemicals that are used to do this. One is an ion called nicotinamide adenine dinucleotide or NAD+. The other is flavin adenine dinucleotide or FAD (which is not an ion). NAD+ can combine with the proton from one and the electrons from two hydrogen atoms, leaving behind the “other” hydrogen proton as an ion. These hydrogen atoms (remember a hydrogen atom = 1 proton + 1 electron) come from the chemical reactions that are taking place during cellular respiration (described below). FAD can combine with two hydrogen protons and two electrons (so two “whole” hydrogen atoms). The chemical equations for these reactions are:
NAD+ + 2 H+ + 2 e → NADH + H+
FAD + 2 H+ + 2 e → FADH2

In many of the chemical reactions that are part of cellular respiration, the molecules involved are gaining or losing electrons: either receiving electrons from another molecule, or giving electrons to another molecule. When a substance loses electrons, we say is is oxidized, and when a substance gains electrons, it is reduced. When I was in school, I struggled to try to remember which is which, until one of my chemistry professors shared with our class a really easy way to remember which is which:
LEO the lion says GER
where “LEO” stands for “loss of electrons is oxidation” and “GER” stands for “gain of electrons is reduction.” Typically, in this type of chemical reaction, both reduction and oxidation will happen simultaneously because one molecule is losing electrons as it gives them to another molecule which, therefore, is gaining them. Because of that, chemists and biologists call these kinds of reactions redox reactions (“red-” from “reduction” and “ox” from “oxidation”).

Actually, you have probably heard these words used in your daily life: for example, “Vitamin C is good for you because it is an antioxidant.” What that means is that vitamin C, itself, is very easily oxidized (LEO). Thus, when “free radicals” are cruising around in your cells looking for electrons to grab (GER), vitamin C will gladly give up some of its electrons. That’s a “good” thing because it prevents other molecules that you need from losing electrons, from being oxidized. That’s why vitamin C is called an “antioxidant.” It is also called a “reducing agent” because when it gives up its electrons (LEO), it thereby causes some other molecule to be reduced (GER). We will see numerous examples of redox reactions, below: for example:
4 H+ + 4 e + 2 NAD+ → 2 NADH + 2 H+
O2 + 4 H+ + 4 e → 2 H2O


There are two important ways a cell can harvest energy from food: fermentation and cellular respiration, also known as aerobic respiration because oxygen is used as a “final electron acceptor.” Technically, a third way would be anaerobic respiration, in which a cell (mostly only in some kinds of bacteria) harvests energy from food by using the same chemical pathways as in aerobic respiration, except that some other chemical, such as sulfur, is used as the final electron acceptor. Typically, the kinds of bacteria that use anaerobic respiration live in extremely-low-oxygen environments such as deep in soil or deep underwater, and not only do they not use oxygen, but they are actually poisoned/killed if exposed to it.

Both cellular respiration and fermentation start with the same first step: the process of glycolysis which is the breakdown or splitting of glucose (6 carbons) into two 3-carbon molecules called pyruvic acid. The energy from other sugars, such as fructose, is also harvested using this process.

Glycolysis is probably the oldest known way of producing ATP. There is evidence that the process of glycolysis predates both the existence of oxygen (O2) in the Earth’s atmosphere and organelles in cells. The Earth’s early atmosphere, in which living organisms first evolved, did not contain O2. Those first organisms obtained the energy they needed via fermentation or anaerobic respiration. Eventually, organisms evolved (predecessors to plants) that were able to harvest energy from sunlight via photosynthesis, but that process generated a toxic waste product: oxygen! As those organisms became more numerous and released “toxic” oxygen into the atmosphere, the oxygen level gradually increased, and somewhere along the line, some other organisms evolved that were not killed by the oxygen, and were also able to use it to their advantage by being able to do cellular respiration. Consider that:

Glycolysis is a process that includes a number of steps/chemical reactions, each of which needs special enzymes to make it happen. In the third step in the process, glucose is converted to fructose. Thus, any fructose from the organism’s food can be “plugged into” the process at that point. Similarly, there are ways to chemically react other dietary sugars, and even proteins and lipids, such that their energy can be harvested. However, keep in mind that not all nutrients are used as “fuel” — many are used, instead, as building blocks. What is shown below are net totals. For ATP, for example, two of the early steps actually use 2 ATPs, while two of the later steps make a total of 4 ATPs, and thus the net total is 2 ATPs. None of the steps in glycolysis use or need oxygen, so as mentioned, it can be the first stage in either aerobic or anaerobic processes.

To summarize, overall, in the process of glycolysis:

Glucose (C6H12O6)

Pyruvic AcidPyruvic Acid
2 Pyruvic Acid (written as either C3H4O3 or CH3COCOOH) molecules
+ Four Hydrogens
4 H+ + 4 e

and then 2 of those electrons and one hydrogen ion combine with a NAD+ molecule to form NADH, leaving behind a hydrogen ion (H+), × 2 for the whole glucose molecule, so

NAD+ + 2 H
+ NAD+ + 2 H

+ NADH + H+
4 H+ + 4 e + 2 NAD+ → 2 NADH + 2 H+
+ energy which is stored by making a net total of 2 ATP molecules, so:

ADP and Phosphate ADP and Phosphate
(2 ADP + 2 → 2 ATP + 2 H2O)
So, overall:
C6H12O6 + 2 NAD+ + 2 AMP-PO3H2 + 2 H3PO4
2 C3H4O3 + 2 NADH + 2 H+ + 2 AMP-PO3H-PO3H2 + 2 H2O

(Note that, to make it easier to see what’s going on here, these molecules have all been shown with all of their hydrogens attached. However, for those of you who have had some chemistry, technically, in “real life,” just like HCl is really H+ + Cl, so also some of the hydrogen protons on the pyruvic acid, the lactic acid (below), and the ATP are really detached as H+ ions, leaving their electrons behind. If you haven’t had chemistry, don’t get stressed out over this technicality.)

From there, the cell has to “do something” with the pyruvic acid and with the NADH that was formed. As mentioned above, that is accomplished via either fermentation or cellular respiration. Note, by the way, that cellular respiration is not “breathing,” but is a process whereby sugar is burned for fuel. Coincidentally, in humans, the oxygen needed for cellular respiration is inhaled via breathing before being distributed throughout the body via the blood. For many other organisms, however, other means of obtaining and distributing oxygen are used.


In fermentation the pyruvic acid molecules are turned into some “waste product,” and no more energy is produced. Note that a net total of only two ATP molecules per molecule of glucose are produced in glycolysis, and no more is produced during fermentation. Out of many possible types of fermentation processes, two of the most common types are lactic acid fermentation and alcohol fermentation (other types of fermentation such as methanol fermentation and acetone fermentation also exist). The various fermentation processes are all anaerobic — none of them use oxygen.

In lactic-acid fermentation:   In alcohol fermentation:
Pyruvic Acid + NADH + H+
Pyruvic Acid (C3H4O3) + H+ + NADH
arrow   arrow
Lactic Acid
NAD+ + 2 H
Lactic Acid (C3H6O3) + NAD+
 or  Ethanol
Carbon Dioxide
NAD+ + 2 H
Ethanol (C2H5OH) +
Carbon Dioxide (CO2) + NAD+

Notice that these values are per pyruvic acid, so since there are two molecules of pyruvic acid formed from each glucose, these numbers would be doubled per glucose. Notice that no more ATP is formed, so the various fermentation processes are only able to harvest 2 ATPs’-worth of energy from a glucose molecule (as compared with 30-ish ATPs’-worth harvested via cellular respiration, as described below).

Lactic acid fermentation is done by some fungi, some bacteria like the Lactobacillus acidophilus. in yogurt, and sometimes by our muscles. Normally our muscles do cellular respiration like the rest of our bodies, using O2 supplied via our lungs and blood. However, under greater exertion when the oxygen supplied by the lungs and blood system can’t get there fast enough to keep up with the muscles’ needs, our muscles can switch over and do lactic acid fermentation. In the process of lactic acid fermentation, the 3-carbon pyruvic acid molecules are turned into lactic acid. It is the presence of lactic acid in yogurt that gives it its sour taste, lactic acid produced by the bacteria and/or fungi in many cheeses that gives those cheeses their characteristic flavors, and it is the presence of lactic acid in our muscles “the morning after” that makes them so sore. Once our muscles form lactic acid, they can’t do anything else with it, so until it is gradually washed away by the blood stream and carried to the liver (which is able to deal with it), our over-exerted muscles feel stiff and achy even if they haven’t been physically injured.

Note that, as mentioned, it is the hemoglobin in our red blood cells (RBCs) that carries oxygen from the lungs to all the cells in the body so they can do cellular respiration (as described below). However, if someone is anemic (technically, there are several different kinds of anemia), that typically means that person doesn’t have enough RBCs/hemoglobin to carry enough oxygen to the cells so they can perform as much cellular respiration as is needed to harvest all the energy they require. In that case, the muscles will more frequently rely on lactic acid fermentation to obtain at least a little bit of energy to keep moving. Thus an anemic person may be more likely to have stiff, achy muscles. This can be a concern for people with cancer if either their cancer and/or their chemotherapy has supressed production of RBCs. There are also some rare genetic disorders in which a person’s body lacks one of the enzymes needed to do cellular respiration, and those people’s muscles will also depend on lactic acid fermentation to obtain energy.

A related bit of information: while most textbooks focus on the role of cellular respiration in our muscles’ ability to contract and their ability to use lactic acid fermentation when they’re short on oxygen, little mention is made of the role of cellular respiration in our other cells’ ability to “do their jobs” properly. One example of another very important use for cellular respiration is that our nervous system’s ability to function properly depends on cellular respiration. All of the nerve cells (neurons) in our brain and the rest of our nervous system depend on cellular respiration in their mitochondria to provide them with the energy they need to think and to process other incoming and outgoing sensory messages. One extremely important difference between nerve cells and muscle cells is that nerve cells cannot switch to lactic acid fermentation if oxygen is low, but rather, our nervous system is minute-to-minute, second-to-second, totally dependent on the oxygen delivered by the blood. That means that if someone is anemic, therefore lacking the hemoglobin needed to transport oxygen, therefore lacking proper oxygen levels in his/her brain, his/her brain function will be impaired. That person will think more slowly and perhaps be unable to follow conversation occuring at a normal speed. Also, his/her reaction time will be slowed, possibly to the point where it might be dangerous to drive. That person may seem to do everything in “slow motion,” and take a long time between each step in a task to “think about it” and process information before (s)he is able to go on to the next step: for example, opening the refrigerator door, then just standing there, “staring at it” for a couple minutes before being able to remove the desired item(s). (Also note that while muscles can also store some of the sugar they need for fuel, the brain cannot store that, either, and so is also totally dependent on the sugar delivered by the blood. Thus, someone who is both anemic and hypoglycemic could, potentially, have significant problems with brain function.)

The muscles of diving mammals such as whales, porpoises, and seals make an interesting use of lactic acid fermentation. Their bodies have several adaptations to allow them to hold their breath for a long time while they’re underwater. They are able to concentrate their blood in the center of their bodies, and because their blood is in a smaller area of their bodies, their heart rates can be slower and their hearts do not have to work as hard. They have more of a special oxygen-storage molecule called myoglobin in their muscles, enabling them to store a lot of O2 before a dive. So that the gases in their blood don’t come out of solution during a dive and so that lungs full of air don’t make them more bouyant, many of these diving animals exhale before a dive and depend on circulation and metabolism to provide the needed oxygen. While submerged, diving animals use lactic acid fermentation to harvest the energy needed to swim, and then once they surface and breathe in O2, they are able to re-convert the lactic acid that has built up in their muscles back into pyruvic acid, which is then sent through the rest of the cellular respiration process to harvest more energy and form more ATP.

Alcohol fermentation is done by yeast and some kinds of bacteria. The “waste” products of this process are ethanol and carbon dioxide (CO2). Humans have long taken advantage of this process in making bread, beer, and wine. In bread making, it is the CO2 which forms and is trapped between the gluten (a long protein in wheat, rye, and barley) molecules that causes the bread to rise, and the ethanol (often abbreviated as EtOH – do you remember how to draw it?) evaporating that gives bread its wonderful smell while baking.

On a somewhat-related note, more and more people, these days, are finding out that they are gluten-intolerant, and thus, cannot eat wheat, barley, or rye. Since it is the gluten in “regular” bread that holds in the CO2, allowing bread to rise, manufacturers of (expensive) gluten-free bread have experimented with using other, perhaps questionable health-wise, additives to trap the CO2. Also, unfortunately, those manufactured breads often also have relatively large amounts of simple starches and sugars added, making them unsuitable for people who have hypoglycemia or diabetes, or who are avoiding consumption of simple carbohydrates for other health reasons.

The adverse effects of the ethanol in beer and wine on the human nervous system are something with which many college students are familiar (sometimes too familiar?), and it is the CO2 produced by the process of fermentation that makes these beverages effervescent. Interestingly, since beer and bread contain pretty-much the same ingredients: yeast, sugar, and grains, in the past, beer has been referred to as “liquid bread.” However, “healthy” whole-grain bread is considerably higher in protein and lower in alcohol than beer!

Dr. Fankhauser has a number of fermentation-related recipes online, complete with photographs:

Cellular Respiration:

Comparison of Cell and Car
Comparison of Cell and Car
An analogy can be drawn between the process of cellular respiration in our cells and a car. The mitochondria are the engines of our cells where sugar is burned for fuel and the exhaust is CO2 and H2O. Note that in a car that burned fuel perfectly, the only exhaust should theoretically be CO2 and H2O also.

There are three steps in the process of cellular respiration: a) glycolysis (as described above), b) the Krebs cycle, named after Hans Krebs who figured it out and also known as the citric acid cycle, and c) the electron transport chain. In contrast to fermentation, in the process of cellular respiration, the pyruvic acid molecules are broken down completely to CO2 and a lot more energy is released.

In eukaryotic cells that contain mitochondria, the Krebs cycle and electron transport chain occur within the mitochondria. Thus, for the process of cellular respiration to continue, the pyruvate, NADH, and H+ formed in glycolysis must enter a mitochondrion. Because NADH cannot cross the mitochondrial membranes by itself, the cell must use active transport to get NADH into the mitochondria, and that “costs” the cell one ATP’s-worth of energy per NADH. Thus, since 2 molecules of NADH were formed during glycolysis, that means 2 of the 4 ATPs formed during glycolysis must give up their energy by being turned back into ADP. Prokaryotes such as bacteria, do not contain mitochondria, and the chemicals needed to do cellular respiration are built into other cell membranes. Because of that, they don’t need to use energy from ATP to actively transport NADH.

In cellular respiration, there is an intermediate step in between glycolysis and the Krebs cycle. In this step, the “end” carbon of each of the pyruvic acid molecules (the carbon that’s attached to two oxygens), along with those oxygens, is removed to form a molecule of CO2, while the remaining 2-carbon molecule, called an acetyl group, is attached to another molecule called coenzyme A, usually referred to as CoA. The “business end” of CoA has a sulfhydryl group (-SH) attached to it. That hydrogen comes off and the 2-carbon acetyl group attaches there, minus its hydrogen, to form acetyl coenzyme A or acetyl CoA. Those two hydrogens and an NAD+ react to form NADH and an H+. The acetyl CoA then carries the acetyl group into the Krebs cycle.

Pyruvic Acid  +  Coenzyme A (CoA)
Pyruvic Acid (C3H4O3) + CoA-SH

Acetyl CoA  +  Carbon Dioxide  +  Two Hydrogens
Acetyl CoA + Carbon Dioxide (CO2) + 2 H+ + 2 e

and then those 2 electrons and one hydrogen ion combine with a NAD+ molecule to form NADH, leaving behind a hydrogen ion (H+), so

NAD+ + 2 HNADH + H+
2 H+ + 2 e + NAD+ → NADH + H+

Again, keep in mind that this is for “only” one pyruvate, so for the whole glucose molecule, these numbers would be doubled.

So, overall, we now have:
C6H12O6 + 4 NAD+ + 2 AMP-PO3H2 + 2 HO-PO4H2 + 2 CoA-SH →
2 CoA-S-C2H3O + 2 CO2 + 4 NADH + 4 H+ + 2 AMP-PO3H-PO3H2 + 2 H2O
(minus 2 AMP-PO3H-PO3H2 used to transport NADH into the mitochondria of eukaryotes)

Krebs Cycle:

The acetyl group and a molecule of water are then transferred to the Krebs cycle and bonded onto another molecule that is part of that cycle, forming citric acid, hence the other name for the cycle, “citric acid cycle.” Simultaneously, the hydrogen of the sulfhydryl group on the CoA is replaced. The CoA is then free to go find another pyruvate with which to react. As the molecule containing the remainder of the acetyl group goes through the eight steps of the Krebs cycle, it gets changed, successively, into a number of different molecules, until finally re-forming the original molecule that first picked up the acetyl group. At two points in the process, a CO2 molecule is given off, thereby finishing the conversion of all of the carbons in the original glucose to CO2. Also, along the way, some NADH and ATP are formed, along with another electron carrier called FADH2 that is similar in function to NADH. At each step in the cycle where NADH or FADH2 is formed, since the remains of the original molecule is losing electrons, it is, thus, being oxidized (LEO), while the NAD+ or FAD is gaining those electrons, and thus, is being reduced (GER).

Thus, overall, what goes into the Krebs cycle per acetyl group (so totals would be doubled if considered per glucose) is:
(Several molecules that enter the cycle in one step, react, then in the next step turn back into the same thing and leave again are not included in these net totals.)

Acetyl CoA
ADP and Phosphate
acetyl CoA (CoA-S-CH3CO) + 3 NAD+ + FAD + 2 H2O + ADP-OH + HO-PO3H2


and what comes out is:

Coenzyme A (CoA)
Carbon Dioxide Carbon Dioxide
CoA-SH + 3 NADH + 3 H+ + FADH2 + 2 CO2 + ADP-OPO3H2 (ATP)

× 2 for whole glucose =

2 CoA-S-CH3CO + 6 NAD+ + 2 FAD + 4 H2O + 2 ADP-OH + 2 HO-PO3H2 →
2 CoA-SH + 6 NADH + 6 H+ + 2 FADH2 + 4 CO2 + 2 ADP-OPO3H2

When those amounts are added to the amounts from glycolysis and the “intermediate” step (above), the net total, so far, is:

C6H12O6 + 10 NAD+ + 2 FAD + 4 AMP-PO3H2 + 4 HO-PO4H2 + 4 H2O →
6 CO2 + 10 NADH + 10 H+ + 2 FADH2 + 4 AMP-PO3H-PO3H2 + 2 H2O

Eliminating “extra” waters that occur on both sides of the equation, this becomes:

C6H12O6 + 10 NAD+ + 2 FAD + 4 AMP-PO3H2 + 4 HO-PO4H2 + 2 H2O →
6 CO2 + 10 NADH + 10 H+ + 2 FADH2 + 4 AMP-PO3H-PO3H2
(minus 2 AMP-PO3H-PO3H2 used to transport NADH into the mitochondria of eukaryotes)

Electron Transport Chain:

Draw Heme Group
(Double-) Click on the this porphyrin ring to see how to draw one.
The electron transport chain is a system of electron carriers which, in eukaryotes, are embedded into the inner membrane of a mitochondrion (in prokaryotes, these are embedded into the plasma membrane). As electrons are passed from one compound to the next in the chain, their energy is harvested and stored by forming ATP.

Many of the compounds that make up the electron transport chain belong to a special group of chemicals called cytochromes. The central structure of a cytochrome is a porphyrin ring like chlorophyll but with iron in the center (remember that chlorophyll has magnesium in the center). A porphyrin with iron in the center is called a heme group, and these are also found in hemoglobin in our blood.

Notice from the last chemical equation, above, that as the glucose went through glycolysis and the Krebs cycle, a total of 10 molecules of NADH and 2 molecules of FADH2 were formed, storing a total of 24 electrons, which will now be used. In the electron transport chain, the NADH and FADH2 will be oxidixed (LEO) back to NAD+ and FAD. Each of the cytochromes and other chemicals that make up the electron transport chain can first gain electrons (GER), then give them up, again (LEO). The first chemical in the chain takes the electrons (GER) from the NADH (LEO), thus re-forming NAD+. Those electrons are then handed off (LEO) to the second chemical (GER), then to the third, etc. Finally, the last molecule in the electron transport chain (LEO) hands the electrons to oxygen as the final electron acceptor (GER), and that, simultaneously deals with an equal amount of H+. Keeping in mind that the oxygen in the air we breathe is O2, this reaction can be written as:
O2 + 4 H+ + 4 e → 2 H2O
so considering all 24 electrons from all the NADH and FADH2, this could be written as:
10 NADH + 10 H+ + 2 FADH2 + 6 O2 → 10 NAD+ + 2 FAD + 12 H2O
Note that in anaerobic respiration something else is used as the final electron acceptor. For example, there is a group of bacteria called the purple sulfur bacteria that use sulfur in place of the oxygen.

At three specific points in the electron transport chain, as the chemical at that point grabs an electron, it must also simultaneously grab a hydrogen ion (H+), and then as the electron is passed on, the H+ is also released, but there’s an interesting way in which this must occur. At these three points in the chain, the H+ must be taken from the mitochondrial matrix, and can only be released/given up into the intermembrane space (as in the “left” side of the diagram, below). Thus, the energy from the electrons is used to pump H+ against its concentration gradient (going from lesser to greater concentration, which it wouldn’t otherwise do) from the mitochondrial matrix into the intermembrane space. That results in the solution in the intermembrane space being 1 to 2 pH units lower than that in the matrix (reminder: pH = –log[H+], so that means 10 to 100× higher concentration of H+), and thus, has the effect of transferring the energy from the electrons to the H+ concentration gradient.

Because there are so many more H+ in the intermembrane space, they “want” to go back into the matrix to reduce the concentration gradient and even out the concentration on both sides of the inner mitochondrial membrane. This could/would occur via simple passive transport (diffusion) except that the membrane is not permeable to H+. There’s only one way they can get back in: there’s a protein built into the membrane which serves as a passageway to allow them to re-enter. However, they have to do a bit of “let’s make a deal” to get in. That protein that lets the H+ back in is called ATP-synthetase or ATP-ase for short (hopefully, seeing the “-ase” on the end of its name will give you a clue that it’s an enzyme), and the only way the H+ can get back into the matrix is if, at the same time, that enzyme picks up an ADP and a to make an ATP and a water molecule (as in the “right” side of the diagram, below). This method of making ATP, called chemiosmotic synthesis, is different than what we saw in glycolysis and the Krebs cycle, where one molecule just handed a to another molecule. However, there is not a 1-to-1 chemical correspondence between number of electrons put into the chain and the number of ATPs formed. According to an e-mail I received from Dr. Todd Silverstein a chemist from Willamette University, biochemists now estimate the ratio at about 2.3 ATPs formed per NADH, and about 1.4 ATPs formed per FADH2. The ratio for FADH2 is lower because its electrons are not given to the first chemical in the electron transport chain, but enter at a later point on the chain, so not as much energy is harvested from them.

Electron Transport Chain

Thus, not counting the ATPs made by the electron transport chain (since they do not take part in the overall reaction), since we now have the same number of NAD+ and FADH2 regenerated and haven’t gained or lost any, we can now change our overall reaction to:

C6H12O6 + 6 O2 + 4 AMP-PO3H2 + 4 HO-PO4H2 + 2 H2O →
6 CO2 + 12 H2O + 4 AMP-PO3H-PO3H2

but there are still 2 water molecules that can be removed from both sides, so:

C6H12O6 + 6 O2 + 4 AMP-PO3H2 + 4 HO-PO4H2 →
6 CO2 + 10 H2O + 4 AMP-PO3H-PO3H2
C6H12O6 + 6 O2 + 4 AMP-PO3H2 + 4 HO-PO4H2 →
6 CO2 + 6 H2O + 4 H2O + 4 AMP-PO3H-PO3H2

Thus, also “eliminating” equal numbers of atoms involved in making ATP (in glycolysis and the Krebs cycle) and considering that as a “separate” reaction, we can simplify this further to the final “official” form of the overall reaction for the “burning” of glucose via cellular respiration:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

Given current estimates on the number of ATP molecules produced by the electron transport chain (as mentioned above), the overall total number of ATP molecules produced per glucose molecule is around 28 to 30. That’s a lot more energy harvested from one glucose molecule than the 2 ATPs that are able to be made from the energy harvested via fermentation.

Health-Related Issues:

It is also important to note that the rates of many of the reactions involved are controlled by feedback inhibition, which occurs when build-up of a product of a given step serves as an inhibitor for that (or a slightly prior) step. One possible cause of this might be due to a genetic mutation that causes a misshapen (nonfunctional) or missing enzyme needed to catalyze the next reaction. Lacking that enzyme, the product of the previous step “piles up” with “nowhere to go.” As that product builds up, its presence serves to inhibit production of more of it, which in turn, affects the step prior to that, etc., bringing the whole process to a grinding halt.

For example, the last cytochrome complex in the electron transport chain is called cytochrome oxidase (COX for short), and some people have a genetic mutation such that they either don’t have COX or what they do have doesn’t work. Because of that, they are unable to transfer electrons to O2, and thus the whole electron transport chain is ineffective. Thus, their muscles have to rely almost exclusively on lactic acid fermentation to obtain the energy they need to move, and because their brains are so short on energy, they typically have various neurological problems.

A number of drugs and other substances can also inhibit specific enzymes, thereby stopping the process. Cyanide, for example, interferes with COX’s ability to pass electrons to O2, again, thereby stopping the whole process.

Porphyria is a genetic disorder in which the person’s body cannot properly make porphyrin rings. Thus, porphyria would affect both the cytochromes in the electron transport chain in that person’s mitochondria, causing problems with cellular respiration, and the hemoglobin that carries oxygen in the blood. Because there are a number of enzymes and steps involved in forming porphyrin rings, there are a number of possible points in the process where genetic defects could occur. The Merck Manual says there are eight steps in the process of making porphyrin rings, with genetic abnormalities possible in seven of the eight enzymes. My mother once had a friend who had a type of porphyria, and this woman’s symptoms were quite variable. At times, she would appear nearly normal while on other occasions she would have to be hospitalized for temporary paralysis of part of her body or other symptoms. There were a number of foods and drugs she had to avoid because they would trigger or worsen her symptoms. She frequently was in a lot of pain. Because the kind of porphyria she had was a dominant genetic disorder, there was a 50% chance this woman’s daughter would also have porphyria. Thus after the woman was diagnosed with porphyria, a number of tests were also run on the girl, and she was more carefully monitored as she grew up. My mother eventually lost contact with them, so I never heard the end of the story.

Many of the enzymes in the cells of organisms need other helpers to function. These non-protein enzyme helpers are called cofactors and can include substances like iron, zinc, or copper. If a cofactor is an organic molecule, it then is called a coenzyme. Many of the vitamins needed by our bodies are used as coenzymes to help our enzymes to do their jobs. Vitamin B1 (thiamine) is a coenzyme used in removing CO2 from various organic compounds. B2 (riboflavin) is a component of FAD (or FADH2), one of the chemicals used to transport electrons from the Krebs cycle to the electron transport chain. Vitamin B3 (niacin) is a component of NAD+ (or NADH) which is the major transporter of electrons from glycolysis and the Krebs cycle to the electron transport chain. Without enough of these B vitamins, our ability to get the energy out of our food would cease! B6 (pyridoxine), B12 (cobalamin), pantothenic acid, folic acid, and biotin are all other B vitamins which serve as coenzymes at various points in metabolizing our food. Interestingly, B12 has cobalt in it, a mineral which we need in only very minute quantities, but whose absence can cause symptoms of deficiency (and if overabundant, is toxic).

Some Related Topics:

Remember that plants also do cellular respiration and need O2, too. If there is too much water in the soil, a plant’s roots can’t get O2, and the plant “drowns” and dies. Similarly, earthworms need the high humidity of damp soil because they “breathe” through their skin, but they will drown in totally water-logged soil. Thus, in a heavy rain, many earthworms come to the surface so they can get sufficient oxygen. Unfortunately, many of them end up on our sidewalks where they dehydrate if they can’t find a way back into the soil.

Several years ago, Dr. Fankhauser mentioned to me that he heard that an “average” 70 kg (= 154 lb) person makes about 40 kg (= 88 lb) of ATP/day, which would be 57% of that person’s body weight. As we discussed that, the question arose, “What would be the hypothetical maximum amount of ATP that a person could possibly make?” To try to come up with a ballpark answer to that question, I did the following calculations.

I once received an e-mail from a student who asked how long the whole process of cellular respiration takes. While I have never seen any information on that in print, a rough approximation can also be calculated from the above statistic:

Check out Dr. Fankhauser’s pictures of the molecules involved in glycolysis and the Krebs cycle.


Berkow, Robert, ed. 1999. The Merck Manual. 17th Ed. Merck, Sharp & Dohme, Rahway, NJ.

Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.

Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5th Ed.   Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)

Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3rd Ed.   Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)

Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.

Silverstein, Todd. 2010. pers. comm.

Copyright © 1996 by J. Stein Carter. All rights reserved.
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