Chapter 2 – Orientation for the Bio-Curious  43

processes that achieve this include glycolysis as well as fermentation (in plant cells and some

prokaryotes), but the principle ATP manufacturing route, generating over 80% of cellular

ATP, is via the tricarboxylic acid (TCA) cycle (biologists also refer to this variously as the

citric acid, Krebs, or the Szent–​Györgyi–​Krebs cycle), which is a complex series of chemical

reactions in which an intermediate breakdown product of glucose (and also ultimately of fats

and proteins) called “acetyl-​CoA” is combined with the chemical acetate and then converted

in a cyclic series of steps into different organic acids (all characterized as having three –​

COOH groups, hence the preferred name of the process).

Three of these steps are coupled to a process, which involves the transfer of an electron (in

the form of atomic hydrogen H as a bound H^+​ proton and an electron) to the nucleoside nico­

tinamide adenine dinucleotide (NAD^+​), or which ultimately forms the hydrogenated com­

pound NADH, with one of the steps using a similar electron-​carrier protein or flavin adenine

dinucleotide (FAD^+​), which is hydrogenated to make the compound FADH (Figure 2.8).

The TCA cycle is composed of reversible reactions, but is driven in the direction shown

in Figure 2.8 by a relatively high concentration of acetyl-​CoA maintained by reactions that

breakdown glucose.

Prokaryotes and eukaryotes differ in how they ultimately perform the biochemical

processes of manufacturing ATP, known generally as oxidative phosphorylation (OXPHOS),

but all use proteins integrated into a phospholipid membrane, either of the cell membrane

(prokaryotes) or in the inner membrane of mitochondria (eukaryotes). The electron-​carrier

proteins in effect contain one or more electrons with a high electrostatic potential energy.

They then enter the electron transport chain (ETC) and transfer the high-​energy electrons

to/​from a series of different electron-​carrier proteins via quantum tunneling (biologists also

refer to these electron-​carrier proteins as dehydrogenases, since they are enzymes that cata­

lyze the removal of hydrogen). Lower-​energy electrons, at the end of the series of ETCs, are

ultimately transferred to molecular oxygen in most organisms, which then react with protons

to produce water; some bacteria are anaerobic and so do not utilize oxygen, and in these

instances an terminal electron acceptor of either sulfur or nitrogen is typically used.

Chemists treat electron gain and electron loss as reduction and oxidation reactions,

respectively, and so such a series of sequential electron transfer reactions are also called

FIGURE 2.8  Schematic of the tricarboxylic acid or Krebs citric acid cycle.