Butanol Fermentation

In yet another pathway, members of the genus Clostridium carry out a butanol fermentation. Differ­ent species of Clostridium form various end products via this fermentation pathway, with pyruvate being converted either to acetone and carbon dioxide, propanol and carbon dioxide, butyrate, or butanol. Several of these products are good solvents. Chaim Weizmann, a Polish-born microbiologist working in Britain, discovered the butanol fermentation path­way in time to allow the British to produce acetone for the manufacture of munitions for World War I. Their ability to do so was instrumental in the success of the Allied forces. Today acetone for nail polish re­mover often is produced by microbial fermentation via this pathway.

Photosynthetic Metabolism

Some early organisms evolved a different way of generating ATP by photoautotrophic metabolism, or photosynthesis. Instead of obtaining energy for ATP synthesis by breaking chemical bonds, these organ­isms, called photoautotrophs, developed the ability to use light as the source of energy that moves pro­tons across membranes of their cells to create a pro-tonmotive force. Photoautotrophs also obtain their carbon from inorganic C0₂.

In photosynthesis, light energy is captured and used to generate ATP.

In photosynthesis, sunlight strikes pigments em­bedded within the membranes of the cell. In this first stage of photosynthesis, electrons within special re-

180 CHAPTER 6 CELLULAR METABOLISM

FIG. 6-24 Micrograph of the cyanobacterium Aphani-zomenon flos-aquae.

action centers of certain pigments are excited, mean­ing that they have absorbed energy from the light. These excited electrons pass from the reaction-center pigments through a series of proteins embedded within the membrane, eventually reaching and acti­vating channels that enable the transport of protons across the membrane. These channels retain the elec­trons and export the protons across the membrane. The protonmotive force generated in this process is used to produce ATP by chemiosmosis. As a result of electron transport, the reduced coenzyme NADPH also is produced; NADPH is later used to produce sugars from carbon dioxide.

Photoautotrophs

Organisms that use light as a source of energy and carbon dioxide as their chief source of carbon are called photoautotrophs. The photoautotrophs in­clude photosynthetic bacteria (green sulfur and pur­ple sulfur bacteria, and cyanobacteria), algae, and green plants (FIG. 6-24). In the photosynthetic reac­tions of cyanobacteria, algae, and plants, the elec­trons of water are used to reduce carbon dioxide, and oxygen gas (0₂) is given off (FIG. 6-25). Because this photosynthetic process produces 0₂, it is sometimes

FIG. 6-25 The Z pathway of oxidative photophosphorylation combines two separate pho-toactivation steps (photosystems) into a unified pathway. These are the excitation of P₆₈₀ to P₅₈₀. and the excitation of P₇₀₀ to P₇₀₀.. The P₆₈₀ has a sufficiently positive reduction potential to use H₂0 as an electron donor. The resulting P₆₈₀. is at a considerably more negative re­duction potential, such that the resulting electrons can "fall down" a potential gradient in which protons are extruded across the membrane and form a proton gradient (protonmo­tive force) to generate ATP. The electrons are passed to the P₇₀₀ complex, which when ex­cited is at a potential more negative than that of the NADPV NADPH redox pair and is thereby capable of reducing NADP⁺ to NADPH. The pathway is called the Z pathway be­cause when these reactions are plotted as a function of reduction potential the resulting fig­ure resembles a Z. The protons originate from the photolysis of water to establish the pro­tonmotive force.

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