Chapter  16:  Cellular Energetics: Glycolysis, Aerobic Oxidation, and Photosynthesis

The most important molecule for capturing and transferring free energy in biological systems is adenosine triphosphate, or ATP (see Figure 2-24). Under standard conditions, hydrolysis of the terminal high-energy phosphoanhydride bond in ATP to yield adenosine diphosphate (ADP) and inorganic phosphate (P_i) releases 7.3 kcal/mol of free energy. Cells can use the energy released during this reaction to power many otherwise energetically unfavorable processes, such as the transport of molecules against a concentration gradient by ATP-powered pumps (Chapter 15), the movement (beating) of cilia (Chapter 19), the contraction of muscle (Chapter 18), and the synthesis of proteins from amino acids (Chapter 4) and of nucleic acids from nucleotides (Chapter 12). Although other high-energy molecules occur in cells, ATP is the universal “currency” of chemical energy; it is found in all types of organisms and must have occurred in the earliest life-forms.

This chapter focuses on how cells generate the high-energy phosphoanhydride bond of ATP from ADP and P_i. This endergonic reaction, which is the reverse of ATP hydrolysis and requires an input of 7.3 kcal/mol to proceed, can be written as

where P_i²⁻ represents inorganic phosphate (HPO₄²⁻). The energy to drive this reaction is produced primarily by two main processes — aerobic oxidation, which occurs in nearly all cells, and photosynthesis, which occurs only in leaf cells of plants and certain single-celled organisms.

In aerobic oxidation, fatty acids and sugars, principally glucose, are metabolized to CO₂ and H₂O, and the released energy is converted to the chemical energy of phosphoanhydride bonds in ATP. In animal cells and most other nonphotosynthetic cells, ATP is generated mainly by this process. The initial steps in the oxidation of glucose, called glycolysis, occur in the cytosol in both eukaryotes and prokaryotes and do not require O₂. The final steps, which require O₂, generate most of the ATP. In eukaryotes, the later stages of aerobic oxidation occur in mitochondria, whereas in prokaryotes, which lack mitochondria, many of the final steps occur on the plasma membrane. The final stages of fatty acid metabolism sometimes occur in mitochondria and generate ATP; in most eukaryotic cells, however, fatty acids are metabolized in peroxisomes without production of ATP.

In photosynthesis, light energy is converted to the chemical energy of phosphoanhydride bonds in ATP and stored in the chemical bonds of carbohydrates (primarily sucrose and starch). Oxygen also is formed during photosynthesis. In plants and eukaryotic single-celled algae, photosynthesis occurs in chloroplasts. Although they lack chloroplasts, several prokaryotes also carry out photosynthesis by a mechanism similar to that in chloroplasts. The oxygen generated during photosynthesis is the source of virtually all the oxygen in the air, and the carbohydrates produced are the ultimate source of energy for virtually all nonphotosynthetic organisms.^*

At first glance, photosynthesis and aerobic oxidation appear to have little in common. However, a revolutionary discovery in cell biology is that bacteria, mitochondria, and chloroplasts all use the same (or very nearly the same) process, called chemiosmosis (or chemiosmotic coupling), to generate ATP from ADP and P_i (Figure 16-1). The immediate energy sources that power ATP synthesis are the transmembrane proton concentration gradient and electric potential (voltage gradient), collectively termed the proton-motive force. The proton-motive force is generated by the stepwise movement of electrons from higher to lower energy states via membrane-bound electron carriers. In mitochondria and nonphotosynthetic bacterial cells, electrons from NADH (produced during the metabolism of sugars, fatty acids, and other substances) are transferred to O₂, the ultimate electron acceptor. In the thylakoid membrane of chloroplasts, energy absorbed from light strips electrons from water (forming O₂) and powers their movement to other electron carriers, particularly NADP⁺; eventually these electrons are donated to CO₂ to synthesize carbohydrates. All these systems, however, contain some similar carriers that couple electron transport to the pumping of protons (always from the cytosolic face to the exoplasmic face of the membrane), thereby generating the proton-motive force (Figure 16-2).

Moreover, all cells utilize essentially the same kind of membrane protein, the F₀F₁ complex, to synthesize ATP. The F₀F₁ complex, also called ATP synthase and F₀F₁ ATPase, is a member of the F class of ATP-powered proton pumps (see Table 15-2). In all cases, the F₀F₁ complex is positioned with the globular F₁ segment, which catalyzes ATP synthesis, on the cytosolic face of the membrane, so ATP is always formed on the cytosolic face of the membrane (see Figure 16-2). Protons always flow through the F₀F₁ complex from the exoplasmic to the cytosolic face of the membrane, driven by a combination of the proton concentration gradient (exoplasmic face > cytosolic face) and the membrane electric potential (exoplasmic face positive with respect to the cytosolic face).

In addition to powering ATP synthesis, the proton-motive force can supply energy for the transport of small molecules across a membrane against a concentration gradient (see Figure 16-1). For example, the uptake of lactose by certain bacteria is catalyzed by a H⁺/sugar symport protein, and the accumulation of ions and sucrose by plant vacuoles is catalyzed by proton-driven antiporters (see Figure 15-22). The rotation of bacterial flagella is also powered by the proton-motive force; in contrast, the beating of eukaryotic cilia is powered by ATP hydrolysis. Conversely, hydrolysis of ATP by V-class ATP-powered proton pumps, which are similar in structure to P-class pumps (see Figure 15-10), provides the energy for transporting protons against a concentration gradient. Chemiosmotic coupling thus illustrates an important principle introduced in our discussion of active transport in Chapter 15: the membrane potential, the concentration gradients of protons (and other ions) across a membrane, and the phosphoanhydride bonds in ATP are equivalent and interconvertible forms of chemical potential energy.

In this brief overview, we’ve seen that oxygen and carbohydrates are produced during photosynthesis, whereas they are consumed during aerobic oxidation. In both processes, the flow of electrons creates a H⁺ electrochemical gradient, or proton-motive force, that powers ATP synthesis. As we examine these two processes at the molecular level, focusing first on aerobic oxidation and then on photosynthesis, the striking parallels between them will become evident.