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

Click [index] to access site index

Click here to access text’s website

(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