verted to the phosphate-containing compound fruc­tose 1,6 bisphosphate. This conversion requires an in­put of energy so that two ATP molecules are con­sumed in the reactions that bring about this conver­sion. The energy that has been transferred to fructose 1,6 bisphosphate subsequently is used to form ATP. Fructose 1,6 bisphosphate, which like glucose has 6- carbon atoms, is broken down into molecules that have three carbon atoms. This results in the produc­tion of two molecules of pyruvate (see FIG. 6-10). It also results in the net production of two molecules of NADH and the net synthesis of two ATP molecules so that the overall equation for the Embden-Meyer- hof pathway of glycolysis can be written as:

The net reaction of metabolism through the Krebs cycle, starting with the pyruvate generated from glu­cose, can be written as:

At the end of the Krebs cycle, the cell has con­verted all of the substrate carbon of the glucose mol­ecule to carbon dioxide. There also has been a net synthesis of four ATP molecules—the production of ten reduced coenzyme molecules as NADH and the generation of two reduced coenzyme molecules as FADH₂.

In the Krebs cycle, intermediary metabolites, such

as pyruvate, are completely oxidized to C0₂ with

the production of reduced coenzymes and some

ATP.

The Krebs cycle and glycolytic pathways have im­portant roles within the overall respiratory genera­tion of ATP. They also occupy a central place in the flow of carbon through the cell. As a result of its func­tion of supplying small biochemical molecules for biosynthetic pathways, the Krebs cycle is rarely com­pleted. Some of the intermediates are siphoned out of the cycle, especially into amino acid biosynthesis, and so some of the intermediary metabolites of this pathway must be continuously resynthesized to con­tinue the Krebs cycle. In many microorganisms, only part of the substrate is completely oxidized for driv­ing the synthesis of ATP, and the remainder is used for biosynthesis. Similarly, the reduced coenzymes generated in this pathway can be used for generating ATP or for the synthesis of the reduced coenzyme NADPH (reduced nicotinamide adenine dinucleo­tide phosphate) for use in biosynthesis.

Electron Transport Chain and Chemiosmotic Generation of ATP

The reduced coenzyme molecules that are generated during glycolysis and the Krebs cycle can be reoxi­dized and the energy stored in them used to generate additional ATP (FIG. 6-12). The energy-requiring synthesis of ATP from ADP in respiration is driven largely by chemiosmosis. As electrons pass down an ! electron transport chain, some of their carrier mole­cules extrude protons from one side of the membrane to the other. Some of the compounds in the electron transport chain can accept and transfer whole hydro­gen atoms (that is, a proton and an electron), whereas others can accept only electrons. When only electrons are accepted, the protons (hydrogen ions) must go I somewhere. In bacteria they are extruded to the out­side of the plasma membrane. Since the phospho-1 lipid portions of plasma membranes are normally, impermeable to protons, this pumping establishes a

In the Embden-Meyerhof pathway of glycolysis, glu­cose is partially broken down into pyruvate, two nadh molecules are formed, and the energy re­leased leads to a net yield of two atp.

Krebs Cycle

The second phase of respiratory metabolism occurs when the pyruvate generated by glycolysis feeds into the Krebs cycle, which is also known as the tricar­boxylic acid or citric acid cycle (FIG. 6-11). In prokaryotic cells, the Krebs cycle occurs within the cytoplasm. In eukaryotic cells, this metabolic process occurs within mitochondria. In the series of chemical reactions that make up the Krebs cycle, the potential chemical energy stored in intermediate compounds derived from pyruvate is released in a series of oxi- dation-reduction reactions. As a result of the reac­tions of the Krebs cycle the pyruvate molecules formed during glycolysis are oxidized to form car­bon dioxide. Thus at the end of the Krebs cycle, six carbon dioxide molecules are produced for each 6-carbon glucose molecule metabolized.

To begin the Krebs cycle, pyruvate produced by glycolysis is split and a fragment of it is attached to coenzyme A (CoA). The combined molecule is called acetyl-CoA. Acetyl-CoA then enters the Krebs cycle, initiating a series of reactions that release electrons and protons (H⁺). NADH is generated during several reactions of the Krebs cycle. Another coenzyme, flavin adenine dinucleotide (FAD), is also reduced to FADH₂. One of the reactions of the Krebs cycle is also directly coupled with the substrate-level generation of a high-energy phosphate-containing compound called guanidine triphosphate (GTP). GTP can be converted to ATP, and, for accounting purposes, the GTP generated in this reaction is counted as ATP in determining the net synthesis of ATP during respira­tion.

proton gradient, that is, there is a higher concentra­tion outside the membrane than inside. The protons on the outside of the membrane can be transported inward by the enzyme adenosine triphosphatase (ATPase), which is located in the membrane. As this movement occurs, energy is released from the pro­tonmotive force and used by the ATPase to convert ADP to ATP.

Chemiosmosis is based on pumping protons across a membrane and using the energy released by their diffusive return (protonmotive force) to gener­ate ATP.

Through chemiosmosis, energy contained in re­duced coenzyme molecules is used to drive ATP syn­thesis. The hydrogen ion gradient (protonmotive force) across a membrane and chemiosmosis drives the formation of ATP. The reduced coenzyme NADH contains more stored chemical energy than the re­duced coenzyme FADH₂. For each NADH molecule, three ATP molecules can be synthesized during ox­idative phosphorylation, compared to only two ATP molecules for each FADH₂. The ten NADH molecules generated during glycolysis and the Krebs cycle, therefore, can be converted to 30 ATP molecules dur­ing oxidative phosphorylation. The two FADH₂ mol­ecules generated during the Krebs cycle can generate

four ATP molecules. This ATP is in addition to that j formed during glycolysis and the Krebs cycle, so that a total of 38 ATP molecules may be produced from the respiratory metabolism of each glucose molecule.

Prokaryotic and eukaryotic cells use chemiosmo- : sis for ATP production in both oxidative phosphory- I lation and photophosphorylation. Electron transport carriers and ATPase are located in membranes, either j the plasma membrane of prokaryotes, the inner mi­tochondrial membrane, or the thylakoid membrane of photosynthetic cells, such as those found in chloro- plasts.

Aerobic Respiration

When oxygen (0₂) serves as the terminal electron ас- I ceptor, as in the above example, the respiratory me- [ tabolism is called aerobic respiration. The oxygen is I reduced to form water in this process. The overall re- і action for the aerobic respiratory metabolism of glu­cose can be written as:

Aerobic respiration (respiration in which molecu­lar oxygen serves as the terminal electron acceptor) is remarkably efficient. The initial breakdown of glu­cose by glycolysis yields only two ATP molecules per

170 CHAPTER 6 CELLULAR METABOLISM

Various microorganisms produce extracellular en­zymes, called lipases, that break down lipids (fats) into their fatty acid and glycerol components. These components enter the cell, where they are further me­tabolized (FIG. 6-14). Many microorganisms convert glycerol into dihydroxyacetone phosphate, one of the intermediates formed during glycolysis. The dihy­droxyacetone phosphate enters the glycolytic path­way and is further metabolized to C0₂. Fatty acids are catabolized by beta-oxidation. In this process, car­bon fragments of long chains of fatty acids are re­moved two at a time and acetyl-CoA is formed. A

fatty acid with sixteen carbons yields eight molecules of acetyl-CoA in seven cleavage steps. As the mole­cules of acetyl-CoA form, they enter the Krebs cycle, as do the acetyl-CoA molecules formed by the oxida­tion of pyruvate. In this process, reduced coenzymes are produced and their subsequent reoxidation re­sults in the chemiosmotic generation of ATP.

Microorganisms also produce extracellular en­zymes, called proteases. Proteases break down pro­teins into short polypeptides and amino acids (FIG. 6-15). These small subunits enter the cell and are fur­ther attacked. Amino acids can be enzymatically

FIG. 6-15 Proteins are broken down by proteases into peptides and amino acids.

172 CHAPTER 6 CELLULAR METABOLISM