Cellular Respiration: Harvesting Chemical Energy

Important words and concepts from Chapter 9, Campbell & Reece, 2002 (1/26/2005):

by Stephen T. Abedon (abedon.1@osu.edu) for Biology 113 at the Ohio State University

Course-external links are in brackets

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(1) Chapter Title: Cellular Respiration: Harvesting Chemical Energy

(a) Found at this site are additional pages of possibly related interest including: [energetics of life] [glycolysis and fermentation] [glycolysis in detail] [cellular respiration]

(b) Cellular respiration links include: [index]

(i) [cellular respiration: harvesting chemical energy, cellular respiration (Google Search)]

(ii) [metabolic pathways in biochemistry (Karl J. Miller)]

(iii) [metabolism problem set (The Biology Project)]

(2) Oxidation of organic compounds

(a) The complete oxidation of an organic compound looks like this:

(i) Organic compound + O₂ à CO₂ + H₂O + energy (plus other components if more than C, H, and O are present in the original compound)

(b) Note how the reaction consists basically of the combination (chemical reaction) of the organic compound with some amount of oxygen

(c) This is followed by the chemical conversion of all carbons to carbon dioxide (CO₂) and all hydrogens from the organic compound to water (H₂O)

(d) Though activation energy is, of course, required to get this reaction started, ultimately there is a net gain in energy from the reaction… it is an exergonic reaction

(e) In biological systems, the steps involved in the oxidation of organic compounds are not nearly this simple, but ultimately the result is the same though with one major difference: the energy liberated during the oxidation of organic compounds by organisms is employed to generate ATP

(f) See Figure 9.3, Methane combustion as an energy-yielding redox reaction

(g) [oxidation of organic compounds (Google Search)] [index]

(3) Oxidation of glucose (complete)

(a) Glucose (whose molecular formula is C₆H₁₂O₆) is the organic compound typically employed to illustrate the oxidation of organic compounds in biological systems

(b) C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + energy

(d) Note that each carbon of glucose is converted to a CO₂

(e) Note that each hydrogen of glucose is converted to ½ H₂O

(f) (for the sake of completeness, note that each oxygen atom of glucose liberated during cellular respiration ultimately is found in CO₂ and which are liberated just prior to (1/3) and then during (2/3) the Krebs citric acid cycle)

(g) [oxidation of glucose (Google Search)] [index]

(4) Oxidation of carbon

(a) Recall that carbon may be oxidized in subsequent steps characterized (or exemplified) by the following succession:

(i) CH₄ (a.k.a., methane)

(ii) CH₃OH (a.k.a., methanol)

(iii) CH₂O (a.k.a., formaldehyde)

(iv) CHOOH (a.k.a., formic acid)

(v) CO₂ (a.k.a., carbon dioxide)

(b) Keep in mind in the above succession that all that is being considered is the oxidation of carbon (i.e., the oxidation of hydrogens in the above examples were ignored)

(i) The above succession’s hydrogen count goes: 4, 4, 2, 2, 0

(ii) However, the number of hydrogens bound directly to carbon decreases with each step: 4, 3, 2, 1, 0

(d) Note that as carbon is increasingly oxidized, more and more oxygens are bound to carbon

(i) The above succession’s oxygen count goes: 0, 1, 1, 2, 2

(ii) However, the number of oxygen-to-carbon bonds increases with each step: 0, 1, 2, 3, 4

(e) [oxidation of carbon (Google Search)] [index]

(5) Oxidation

(a) Oxidation is the movement of electrons away from an atom (or, more precisely, away from an atom’s nucleus)

(b) Thus, when oxygen oxidizes something, it pulls the electrons away from that something and towards itself (in the process, oxygen, serving as an oxidizing agent, is itself reduced)

(d) [oxidation (Google Search)] [index]

(6) Reduction

(a) The movement of electrons towards an atom (or, more precisely, towards an atom’s nucleus)

(b) Because carbon is not terribly electronegative, the reduction of (donation of electrons to) carbon tends to result in the formation of energy-rich bonds (e.g., C-C and C-H bonds)

(c) Why “reduction”? Think of reduction as the reduction in electrical charge (increasing negativity) an atom or molecule (or ion) experiences as it gains electrons

(d) Note, however, that reduction does not always result in a decline in electrical charge since protons often are donated to a compound simultaneously with the donation of electrons

(7) Redox reactions (oxidation-reduction reactions)

(a) In order for electrons to move away from something, they invariably move towards something else

(b) Consequently, oxidation and reduction tend to be coupled

(c) We abbreviate this coupling using the phrase “redox reaction” meaning chemical reactions in which both oxidation and reduction occur

(d) See Figure 9.3, Methane combustion as an energy-yielding redox reaction

(e) [The rusting of metals, the process involved in photography, the way living systems produce and utilize energy, and the operation of a car battery, are but a few examples of a very common and important type of chemical reaction. These chemical changes are all classified as "electron-transfer" or oxidation-reduction reactions. The term, oxidation , was derived from the observation that almost all elements reacted with oxygen to form compounds called, oxides. A typical example is the corrosion or rusting of iron… Reduction, was the term originally used to describe the removal of oxygen from metal ores, which "reduced" the metal ore to pure metal… Based on the two examples above, oxidation can be defined very simply as, the "addition" of oxygen; and reduction, as the "removal" of oxygen. But there is a lot more to "oxidation-reduction"… (Internet Chemistry)]

(f) [Oxidation-reduction reactions always involve a change in the oxidation state of the atoms or ions involved. This change in oxidation state is due to the "loss" or "gain" of electrons. The loss of electrons from an atom produces a positive oxidation state, while the gain of electrons results in negative oxidation states. (Internet Chemistry)]

(g) Oxidation/reduction links: [index]

(i) [oxidation-reduction, redox (Google Search)]

(ii) [oxidation/reduction (Internet Chemistry)]

(iii) [oxidation/reduction (Online Biology Book)]

(iv) [oxidation and reduction in organic chemistry (The Australian National University)]

(8) Oxidizing agent

(a) A substance capable of stealing electrons (i.e., oxidizing another substance) is called an oxidizing agent

(b) Oxygen atoms tend to be good oxidizing agents

(9) Reducing agent

(a) A substance that allows its electrons to be stolen (i.e., thereby reducing another substance) is called a reducing agent (i.e., reducing agents tend to donate there electrons to other substances)

(b) Substances with carbon-to-carbon and carbon-to-hydrogen bonds tend to be good reducing agents (well, good at reducing molecular oxygen, at least)

(c) For example, these substances can react with O₂ thereby reducing oxygen (i.e., donating electrons to oxygen—in the process the reducing agent is oxidized, e.g., the carbon or the hydrogens)

(d) The more C-C or C-H bonds a substance has, the more that substance may be successively oxidized, e.g., upon reaction with oxygen

(e) [reducing agent (Google Search)] [index]

(10) Energy in bonds

(a) Recall that the farther an electron exists from the atoms it is associated with, the more energy that electron possesses (a generalization, sure, but let’s attempt to avoid complicating things more than we have to)

(b) Highly electronegative atoms hold electrons more tightly to themselves than do less electronegative atoms

(c) Consequently, the electrons held by more-electronegative atoms/nuclei store less energy than the electrons that are held by less-electronegative atoms

(d) By the second law of thermodynamics, the transfer of an electron from a less electronegative atom to a more electronegative atom therefore must result in some transfer of energy from that electron to the surrounding environment

(e) Thus, the transfer of an electron from a C-H or C-C bond to a C-O or H-O bond must release energy

(f) That energy may be harnessed to do work, e.g.,

(i) ADP + Pi + energy à ATP +H₂O

(g) FAQ: What do you mean by "Energy in bonds"? When electrons are locked into chemical bonds, then there is a certain amount of energy associated with those electrons. This is the (chemically available) energy that exists within, for example, the food you eat is associated with electrons locked into chemical bonds. Recall that the farther an electron is from the atomic nucleus, the more energy the electron contains (indeed, must contain). This distance from an atomic nucleus can be locked into an electron when that electron is locked into a chemical bond. Indeed, one can think of the energy required to drive forward the endergonic dehydration synthesis reaction as energy that becomes trapped in chemical bonds and is associated with electrons that are now farther from atomic nuclei than they otherwise might have been (in fact, were). Finally, note that all else held constant, an electron that is shared between two atoms possessing relatively equal electronegativity will be trapped at a further distance from the two atomic nuclei than an atom locked between two atoms having dissimilar electronegativities. For example, an electron found between H and O will be much closer to an atomic nuclei (i.e., that of O) than an electron found between C and C, or even O and O.

(h) [energy in bonds (Google Search)] [ATP links (MicroDude)] [index]

(11) Hydrogen atoms

(a) H (hydrogen) = e^- + H⁺

(b) In biological systems, electrons typically move around in conjunction with protons, i.e., electrons move around (or are moved around) as free hydrogen atoms

(12) NAD⁺ reduction (NADH)

(a) NAD⁺ + 2e^- + 2H⁺ à NADH + H⁺

(b) NAD⁺ (a.k.a., nicotinamide adenine dinucleotide) is a biologically important oxidizing agent

(d) NAD⁺ receives electrons in pairs (see above reaction)

(e) NAD⁺ receives one proton while it receives its electrons

(f) The reduced form of NAD⁺ is NADH + H⁺ (note that both NADH and H⁺ are products of this reduction reaction, and that both protons are accounted for)

(g) See Figure 9.4, NAD⁺ as an electron shuttle

(h) [NAD reduction, nicotinimide adenine dinucleotide, nicotinimide adenine dinucleotide reduction (Google Search)] [NAD⁺/NADH image (Anabolism/Photosynthesis – Biology for Engineers)] [index]

(13) Dehydrogenase

(a) Enzymes called dehydrogenases are involved in these pair-wise oxidations of organic molecules

(b) For example: H-C-OH + NAD⁺ + dehydrogenase à C=O + NADH + H⁺ + dehydrogenase

(i) Note the loss of 2 H’s from H-C-OH

(ii) Note that, as shown, neither the structure H-C-OH nor C=O are complete molecules, i.e., two H’s are missing from each

(iii) Note that the complete reaction between two one-carbon compounds would be: CH₃OH + NAD⁺ + dehydrogenase à CH₂O + NADH + H⁺ + dehydrogenase (that is, note the removal of two hydrogen atoms and their acceptance by NAD⁺)

(iv) Note that dehydrogenase is found on both sides of the equation; that is, it is rejuvenated in the course of the reaction and thus, like all enzymes, truly is a catalyst in this regard

(c) The additional electrons now associated with NADH may be harnessed in subsequent reactions of cellular respiration to generate ATP

(d) See Figure 9.5, An introduction to electron transport chains

(e) [dehydrogenase (Google Search)] [index]

(14) Cellular respiration, overview

(a) See Figure 9.6, An overview of cellular respiration

(b) Cellular respiration is a series of chemical and physical processes which together serve to remove potential energy-containing electrons from organic compounds, use the energy thus liberated to generate ATP, and then donate these now energy-spent electrons to oxygen

(i) Glycolysis

(ii) Pyruvate oxidation (acetyl CoA production)

(iii) Krebs (citric acid) cycle

(iv) Electron transport

(v) Chemiosmosis

(d) ATP is generated by two types of processes:

(i) Substrate-level phosphorylation

(ii) Oxidative phosphorylation

(iii) By far, in humans, oxidative phosphorylation is the more important of these processes in terms of total ATPs directly generated

(e) [cellular respiration (Google Search)] [cellular respiration links (MicroDude)] [index]

(15) Oxidative phosphorylation (1)

(a) Oxidative phosphorylation is the phosphorylation of ADP using a mechanism powered by reduced electrons which, once their potential energy has been removed, are ultimately donated to atoms of oxygen

(b) See Figure 9.5, An introduction to electron transport chains

(16) Substrate-level phosphorylation

(a) Substrate-level phosphorylation is the donation of a phosphate directly to ADP from a phosphorylated organic intermediate

(b) See Figure 9.7, Substrate-level phosphorylation

(i) In oxidative phosphorylation the energy associated with electrons follows this path:

· Substrate (e.g., glyceraldehyde phosphate—Figure 9.9, A closer look at glycolysis)

· NADH + H⁺

· Electron transport system

· Chemiosmosis

· ATP

(ii) In substrate-level phosphorylation the energy associated with electrons follows this path:

· Phosphorylated substrate (e.g., 1, 3-biphosophoglycerate—Figure 9.9, A closer look at glycolysis)

· ATP

(d) Substrate-level phosphorylation occurs twice per glucose during glycolysis and again twice per glucose during the Krebs cycle

(e) [substrate-level phosphorylation (Google Search)] [index]

(17) Glycolysis

(a) The reactions of glycolysis are the first reactions of cellular respiration

(b) Glycolysis occurs in the cytosol of eukaryotic cells (i.e., does not occur in the mitochondrial matrix)

(d) ATP is generated in glycolysis only by substrate-level phosphorylation

(e) In cellular respiration, the NADH + H⁺ and pyruvic acid (a.k.a., pyruvate) are subsequently oxidized to generate additional ATPs; this subsequent oxidation occurs in the mitochondria

(f) [glycolysis (Google Search)] [glycolysis links (MicroDude)] [index]

(18) Glycolysis, overview

(a) The following steps (and level of detail) of glycolysis I expect you to know (note that I use “P” to imply phosphate group and that lots of detail is ignored):

(i) C₆ (a.k.a., glucose) + ATP à C₆-P + ADP

(ii) C₆-P + ATP à P-C₆-P + ADP

(iii) P-C₆-P à 2C₃-P (this is the sugar-splitting step)

(iv) (note: the stoichiometry of all of the following are 2 for every one glucose)

(v) C₃-P + NAD⁺ + Pi à P-C₃-P + NADH + H⁺

(vi) P-C₃-P + ADP à C₃-P + ATP

(vii) C₃-P + ADP à C₃ (a.k.a., pyruvate) + ATP

(b) For the above to have any meaning to you (i.e., to avoid rote memorization and instead to try giving understanding the process a chance, See Figure 9.9, A closer look at glycolysis

(d) The net ATP produced is two per glucose, i.e., 2 ATP are hydrolyzed and 4 ATP are produced per glucose per round of glycolysis

(e) Notice that all of the ATPs generated directly by glycolysis are generated via substrate-level phosphorylation

(f) See Figure 9.8, The energy input and output of glycolysis

(g) Don’t forget that the primary “purpose” of glycolysis is the generation of ATP, NADH + H⁺, and pyruvate from glucose (i.e., from carbohydrate / from food)

(19) Mitochondrion

(a) The first post-glycolysis step of cellular respiration is the movement of pyruvic acid (pyruvate) from the cytosol into the matrix of the mitochondria

(b) See Figure 9.6, An overview of cellular respiration

(c) See Figure 9.10, Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle

(d)

(e) [mitochondria or mitochondrion (Google Search)] [index]

(20) Acetyl CoA (pyruvate oxidation)

(a) Once in the mitochondria, the pyruvate is converted to acetyl CoA

(b) CoA is short for Coenzyme A, a compound that holds a two-carbon acetyl group (-CO-CH₃) in a reactive state

(c) This conversion of pyruvate to acetyl CoA generates one CO₂ and reduces one NAD⁺ (i.e., makes one NADH + H⁺) per pyruvate (i.e., twice per glucose)

(d) See Figure 9.10, Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the Krebs cycle

(e) This is the first step of cellular respiration that generates a CO₂

(f) CO₂’s are waste products of cellular respiration and represent the elimination as waste of one C and in the above reaction two O’s from pyruvate

(g) [acetyl coA, pyruvate oxidation (Google Search)] [index]

(21) Glucose stoichiometry

(a) Keep in mind that there are two pyruvates per glucose and therefore two acetyl CoA's generated per glucose as well as two CO₂'s generated at this step

(b) All subsequence steps also have their stoichiometry doubled when considered on a per-glucose basis

(22) Krebs cycle

(a) In the next step of cellular respiration acetyl CoA donates the two-carbon acetyl group to oxaloacetic acid to enter the Krebs cycle

(b) The product of the above reaction is citric acid (acetyl CoA + oxaloacetate à citric acid/citrate + CoA-SH)

(c) See Figure 9.11, A closer look at the Krebs cycle

(d) This first/last step of the Krebs cycle involves the conversion of a four-carbon bicarboxylic acid to a six-carbon tricarboxylic acid, citric acid:

C – COOH

HO - C – COOH

C – COOH

(e) The Krebs cycle is a mechanism whereby the acetyl group is oxidized to

(i) two CO₂’s (one for each carbon of the acetyl group)

(ii) three NADH + H⁺

(iii) one FADH₂

(iv) one ATP (via substrate-level phosphorylation )

(f) See Figure 9.12, A summary of the Krebs cycle

(g) [Kreb cycle (Google Search)] [Krebs citric acid cycle: one turn of the cycle (2-D version) (3-D version) (start with the former, though the latter is very cool – try manipulating the molecules with your mouse) (Metabolic Pathways in Biochemistry)] [Krebs cycle (Caduceus MCAT Review)] [index]

(23) FAD reduction (FADH₂)

(a) FAD (flavin adenine dinucleotide) works similarly to NAD⁺

(i) FAD + 2e^- + 2H⁺ à FADH₂

(b) A major difference is that FAD binds to electrons that hold less energy than the electrons that NAD⁺ binds

(24) Not-yet-complete oxidation of glucose

(a) Following the oxidation of glucose to CO₂’s, cellular respiration has generated a net total of 4 ATP

(b) This is not yet a complete oxidation of glucose, however, since, although all electrons associated with glucose have been removed, these electrons have not yet been donated to oxygen to form water (i.e., what I mean by not-yet-complete oxidation is that not all of the original glucose’s energy has been dissipated as heat or used to phosphorylate ADP)

(i) Two NAD⁺ were reduced during glycolysis

(ii) Two NAD⁺ were reduced during the oxidation of pyruvate to acetyl CoA, each, and

(iii) An additional three NAD⁺ were reduced during the Krebs cycle per pyruvic acid

(d) In order for glycolysis , generation of acetyl CoA, or Krebs cycle to continue, there must be a regeneration of NAD⁺

(e) In addition, the electrons stored in NADH + H⁺ and FADH₂ can still provide quite a bit of energy capable of being harnessed to phosphorlyate ATP

(25) Electron transport chain (electron transport system, ETS)

(a) The electrons carried by NADH + H⁺ or FADH₂ are utilized via their donation to an electron transport chain (ETS)

(b) See Figure 9.5, An introduction to electron transport chains

(d) Large numbers of copies of these compounds are accommodated in the inner membrane of the mitochondria via surface area-increasing infoldings called cristae (i.e., more folds allow more membrane allowing more ETS)

(e) The ETS is a series of these compounds which remove energy from these electrons in a series of oxidation-reduction reactions

(f) See Figure 9.13, Free-energy change during electron transport

(g) Note how FADH₂ donates lower-energy electrons to this chain (i.e., farther down on the chain) than does NADH + H⁺; this is because FADH₂’s electrons are less energetic than those associated with NADH

(h) [chain or system "electron transport" (Google Search)] [index]

(26) Pumping hydrogen ions (pumping protons)

(a) Some of the compounds in the ETS accept electrons along with the hydrogen ions associated with these electrons

(b) Other compounds of the ETS, on the other hand, accept electrons without the associated hydrogen ion

(d) During donation from a compound that accepts hydrogen ions to one which does not accept hydrogen ions, something has to be done with the excess hydrogen ions

(e) These are released into the intermembrane space of the mitochondria

(f) Since hydrogen ions are moved during this process from the mitochondrial matrix to the mitochondrial intermembrane space, the ETS serves as a oxidation-reduction driven hydrogen ion (proton) pump

(g) This pumping of protons generates a hydrogen-ion electrochemical gradient

(h) See Figure 9.15, Chemiosmosis couples the electron transport chain to ATP synthesis

(i) [pumping hydrogen or proton or protons "electron transport" (Google Search)] [index]

(27) Proton-motive force

(a) Proton-motive force is the name given to the hydrogen ion electrochemical gradient produced by the ETS

(b) [proton motive force (Google Search)] [index]

(28) Reduction of O₂

(a) At the end of the ETS, electrons are now very depleted of energy

(b) The only common substance that will still accept these energy-depleted electrons is molecular oxygen

(c) See Figure 9.15, Chemiosmosis couples the electron transport chain to ATP synthesis

(d) Remember, oxygen atoms are very electronegative so are willing to accept electrons even if they are not very energetic (another way of stating this is that electrons that are bound to very electronegative atoms, in polar bonds, are not very energetic)

(e) We breathe, in part, in order to supply oxygen to our mitochondria (the other part is to remove the CO₂ given off by our mitochondria; we are slaves to our mitochondria; who owns whom?)

(f) ["reduction of oxygen" and "electron transport" (Google Search)] [index]

(29) Final electron acceptor

(a) Molecular oxygen thus serves as the final electron acceptor in cellular respiration

(b) Without oxygen serving as the final electron acceptors, electron transport chains would not have any way of getting rid of their excess, unenergetic electrons, and all of cellular respiration would back up and shut down

(c) See Figure 9.15, Chemiosmosis couples the electron transport chain to ATP synthesis

(d) [final electron acceptor (Google Search)] [index]

(30) Metabolic water

(a) Along with two electrons, at the end of the ETS each oxygen atom also is combined with two hydrogen ions

(b) The net result is water (i.e., 2e^- + 2H⁺ + ½O₂ à H₂O)

(d) Thus, one of the products of the complete oxidation of glucose is water which, of course, is as we expect (e.g., C₆H₁₂O₆ + 6O₂ à 6CO₂ + 6H₂O + energy)

(e) [metabolic water (Google Search)] [index]

(31) Chemiosmosis (ATP synthase)

(a) Note that we’ve done a lot of manipulations starting with NADH + H⁺ and FADH₂, but we still haven’t generated a whole lot of additional ATP

(b) The way the ETS is linked to ATP synthesis is through the generation of the proton-motive force

(c) This proton-motive force drives in reverse an ATP-dependent proton pump, called ATP synthase, that is located in the inner membrane of the mitochondria

(d) See Figure 9.14, ATP synthase, a molecular mill

(e) Thus, protons are allowed back into the mitochondrial matrix, through this pump (running in reverse, i.e., not expending energy to pump protons but instead capturing the energy of the protons rushing through in the opposite direction), and ATP is the byproduct

(f) [chemiosmosis, ATP synthase (Google Search)] [ATP synthase links, ATP links (MicroDude)] [index]

(32) Oxidative phosphorylation (2)

(a) This use of an electron transport chain, coupled with hydrogen ion pumping, coupled with reduction of molecular oxygen, coupled with ATP generation via the harnessing of the hydrogen-ion electrochemical gradient are together termed oxidative phosphorylation

(b) Note how oxidative phosphorylation contrasts mechanistically with substrate-level phosphorylation

(c) In short hand (and it’s anything but short), oxidative phosphorylation = glucose à NADH à ETS à proton-motive force à chemiosmosis à ATP

(d) Substrate-level phosphorylation, instead, simply involves: substrate-P + ADP à substrate + ATP

(e) Oxidative phosphorlyation links: [index]

(i) [oxidative phosphorylation (Google Search)]

(ii) [aerobic respiration (Online Biology Book)]

(iii) [oxidative phosphorylation (graphic) (Metabolic Pathways in Biochemistry)]

(iv) [phosphorylation (Caduceus MCAT Review)]

(33) ATP bookkeeping

(a) On average, note the approximate number of ATP generated by each of the following:

(i) Substrate-level phosphorylation = 1 ATP

(ii) NADH = 3 ATP

(iii) FADH₂ = 2 ATP

(b) Note that it takes one ATP to move each of the glycolysis-generated NADH into the matrix of the mitochondria

(c) (it makes sense that only the NADH would be moved in, not NADH + H⁺, since leaving the H⁺ outside the mitochondria prevents it from mitigating the proton-motive force —also note that when O₂ is reduced to water then further H⁺ are sequestered within the matrix of the mitochondria, thus further adding to the proton-motive force)

(d) Glucose generates 2 ATP, 2 NADH + H⁺, and 2 pyruvate (net) via glycolysis

(e) Pyruvate generates 1 CO₂, 1 NADH + H⁺, and 1 acetyl CoA upon conversion to acetyl CoA

(f) Acetyl CoA generates 2 CO₂, 3 NADH + H⁺, 1 FADH₂, and 1 ATP per turn of the Krebs cycle

(g) In total (net), then, there are 4 ATP, 10 NADH + H⁺, and 2 FADH₂ generated during the oxidation of glucose to 6 CO₂ less 2 ATP to pump the NADH from glycolysis into the mitochondria:

(i) 4 ATP – 2 ATP + 30 ATP + 4 ATP = 36

(h) See Figure 9.16, Review: how each molecule of glucose yields many ATP molecules during cellular respiration

(i) ["ATP bookkeeping", cellular respiration bookkeeping (Google Search)] [index]

(34) Anaerobic generation of ATP

(a) Without oxygen, cellular respiration backs up and won’t function

(b) Many organisms can still generate ATP from glucose, without oxygen present, by relying entirely on glycolysis

(d) Two ATP, however, are a lot better than none

(e) [anaerobic generation of ATP (Google Search)] [fermentation links (MicroDude)] [index]

(35) Regeneration of NAD⁺

(a) Just as cellular respiration backs up in the absence of oxygen, glycolysis backs up in the absence of NAD⁺ regeneration

(b) In fact, NAD⁺ regeneration is one of the functions of the ETS

(36) Organic final electron acceptor

(a) In the absence of oxygen, NAD⁺ is regenerated via the reduction of a non-oxygen compound

(b) That non-oxygen compound typically is pyruvate

(c) Pyruvate thus serves as the final electron acceptor in the anaerobic generation of ATP that occurs via the glycolytic pathway

(d) [organic final electron acceptor (Google Search)] [index]

(37) Fermentation

(a) Fermentation is the name given to this glycolytic generation of ATP employing an organic final electron acceptor

(b) Fermentation also involves the generation of waste products which typically accumulate in the fermenter’s environment

(d) These waste products include such things as ethanol and carbon dioxide, or lactic acid

(e) See Figure 9.17, Fermentation

(f) {The word fermentation has a number of related meanings, depending on context. For the most part these meanings describe similar processes that involve either energy production in the absence of air or the associated production of byproducts (a.k.a. wastes, but often quite valuable to you or me). Historically (if not actually) all of the following have at their root fermentation as defined in its strictest (i.e., most scientific) sense (see definition numbers 5 & 6, below) (p. 124, Tortora et al., 1995, for definitions 1-5).

· Any process that produces alcoholic beverages or acidic dairy products (general use).

· Any spoilage of food by microorganism (general use).

· Any large-scale microbial process occurring with or without air (common definition used in industry).

· Any energy-releasing metabolic process that takes place only under anaerobic conditions (becoming more scientific).

· All metabolic processes that release energy from a sugar or other organic compound, do not require molecular oxygen or an electron transport system, and use an organic compound as the final electron acceptor.

· “An energy-yielding metabolic pathway that involves no net change in oxidation state.” (p. 449, Christopher K. Mathews & K. E. Van Holde, 1996, Biochemistry, Second Edition, Benjamin/Cummings Publishing Company)

· “A chemical change with effervescence.” (ditto)}

(g) [fermentation links (including links to the fermentation of various alcohol-containing beverages and food-fermentation links) (MicroDude)] [index]

(38) Facultative anaerobes

(a) Organisms that are able to generate ATP via either cellular respiration (when O₂ is around) or via fermentation (when O₂ is not present) are called facultative anaerobes

(b) An example of facultative anaerobes are our own muscles (lactic acid fermentation), many yeasts (alcohol fermentation), and many bacteria (numerous additional kinds of fermentation)