Metabolism Fueling Cell Growth

■ n the 1850s, Louis Pasteur, a chemist, enthusiastically

■ accepted the challenge of studying how alcohol arises from JL grape juice. Biologists had already observed that when grape juice is held in large vats, alcohol and carbon dioxide are produced and the number of yeast cells increases. They argued that the multiplying yeast cells convert the sugar in the juice to alcohol and carbon dioxide. Pasteur agreed, but he could not convince two very powerful and influential German chemists, Justus von Liebig and Friedrich Wähler, who refused to believe that microorganisms caused the breakdown of sugar. Both men lampooned the hypothesis and tried to discredit it by publishing pictures of yeast cells looking like miniature animals taking in grape juice through one orifice and eliminating carbon dioxide and alcohol through the other.

Pasteur studied the relationship between yeast and alcohol production using a strategy commonly employed by scientists today—that is, simplifying the experimental system so that relationships can be more easily identified. First, he prepared a clear solution of sugar, ammonia, mineral salts, and trace elements. He then added a few yeast cells. As the yeast grew, the sugar level decreased and the alcohol level increased, indicating that the sugar was being converted to alcohol as the cells multiplied. This strongly suggested that living cells caused the chemical transformation. Liebig, however, still would not believe the process was actually occurring inside microorganisms. To convince him, Pasteur tried to extract something from inside the yeast cells that would convert sugar into alcohol and carbon dioxide. He failed, like many others before him.

In 1897, Eduard Buchner, a German chemist, showed that crushed yeast cells could convert sugar to ethanol and CO2. We now know that the active ingredients of the crushed cells that carried out this transformation were enzymes. For these pioneering studies, Buchner was awarded the Nobel Prize in 1907. He was the first of many investigators who received Nobel Prizes for studies on the processes by which cells degrade sugars.

—A Glimpse of History

TO GROW, ALL CELLS MUST ACCOMPLISH TWO fundamental tasks. They must continually synthesize new components including cell walls, membranes, ribosomes, nucleic acids, and surface structures such as flagella. These allow the cell to enlarge and eventually divide. In addition,

Wine—a beverage produced using microbial metabolism

cells need to harvest energy and convert it to a form that is usable to power biosynthetic reactions, transport nutrients and other molecules, and in some cases, move. The sum total of chemical reactions used for biosynthetic and energy-harvesting processes is called metabolism.

Bacterial metabolism is important to humans for a number of reasons. Many bacterial products are commercially or medically important. For example, the metabolic waste products of Clostridium acetobutylicum are the solvents, acetone and butanol. Cheese-makers intentionally add Lactococcus and Lactobacillus species to milk because the metabolic wastes of these bacteria contribute to the flavor and texture of various cheeses. Yet those same products contribute to tooth decay when related bacteria are growing on teeth. Microbial metabolism is also important in the laboratory, because products that are characteristic of a specific group of microorganisms can be used as identifying markers. The metabolic end products of Escherichia coli distinguish it from related Gram-negative rods such as Klebsiella and Enterobacter species. In addition, the metabolic pathways of organisms such as E. coli have served as an invaluable model for studying analogous processes in eukaryotic cells, including those of humans. Metabolic processes unique to prokaryotes are potential targets for antimicrobial drugs.

Chapter 6 Metabolism: Fueling Cell Growth

6.1 Principles of Metabolism

Metabolism can be viewed as having two components—catab-olism and anabolism (figure 6.1). Catabolism encompasses processes that harvest energy released during the disassembly or breakdown of compounds such as glucose, using it to synthesize ATP, the energy currency of all cells. In contrast, anabolism, or biosynthesis, includes processes that utilize energy stored in ATP to synthesize and assemble subunits of macromolecules making up the cell. These subunits include amino acids, nucleotides, and lipids.

Although catabolism and anabolism are often discussed separately, they are intimately linked. As mentioned, ATP generated during catabolism is used in anabolism. In addition, some of the compounds produced in steps of the catabolic processes can be diverted by the cell and used as precursors of subunits employed in anabolic processes. ■ ATR p. 24

Harvesting Energy

Energy is defined as the capacity to do work. It can exist as potential energy, which is stored energy, and kinetic energy, which is energy of motion (figure 6.2). Potential energy can be stored in a variety of forms including chemical bonds, a rock on a hill, or water behind a dam. The fundamental principles of energy relationships are embodied in the laws of thermodynam-

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