FIG. 6-23 The Voges-Proskauer test, which detects the production of acetoin, is useful for differentiating various bacterial species, including Enterobacter aerogenes and Es­cherichia coli. Negative test result for Escherichia coli is on left. Positive test result for Enterobacter aerogenes is on right.

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 pro­tonmotive 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-

Metabolic pathways 179

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

FIG. 6-25 The Z pathway of oxidative photophosphorylation combines two separate pho­toactivation steps (photosystems) into a unified pathway. These are the excitation of P₆₈₀ to Р_ш. 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 NADP⁺/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.

METABOLIC PATHWAYS 181

called oxygenic photosynthesis. The light-trapping pigment required by green plants, algae, and cyanobacteria is the green compound chlorophyll a. It is located in the thylakoids of chloroplasts in algae and green plants and in the thylakoids that form a part of an elaborate internal membrane structure in cyanobacteria.

Algae and cyanobacteria have chlorophyll a, which absorbs light energy.

Oxygen is formed from water during oxygenic pho­tosynthesis.

Besides the cyanobacteria, there are several other families of photosynthetic prokaryotes that are classi­fied according to the way they reduce carbon diox­ide. These bacteria cannot use electrons from water to reduce carbon dioxide and cannot carry on photo­synthesis when oxygen is present, that is, they must have an anaerobic environment (FIG. 6-26). Their photosynthetic process does not produce oxygen gas, and therefore this type of photosynthesis is called anoxygenic photosynthesis.

The green sulfur and purple sulfur bacteria are photoautotrophic microorganisms that carry out anoxygenic photosynthesis. The chlorophyll used by these photosynthetic bacteria is bacteriochlorophyll, which absorbs light of longer wavelengths than that absorbed by chlorophyll a. The green sulfur bacteria use sulfur, sulfur compounds, or hydrogen gas to re­duce carbon dioxide and form organic compounds. Applying the energy from light to bacteriochloro­

phyll and the appropriate enzymes, these bacteria oxidize sulfur to sulfuric acid, hydrogen sulfide to sulfur, or hydrogen gas to water. The purple sulfur bacteria also use sulfur, sulfur compounds, or hydro­gen gas to reduce carbon dioxide. They are distin­guished from the green sulfur bacteria on the basis of their biochemistry and morphology.

Oxygen is not produced during anoxygenic photo­synthesis.

Anaerobic photosynthetic sulfur bacteria oxidize hy­drogen sulfide (H₂S) so that they can reduce carbon dioxide to organic matter.

Photoheterotrophs

Photoheterotrophs use light as a source of energy and can convert C0₂ to sugars, but they also require organic compounds as sources of carbon for growth. Their organic compound sources include alcohols, fatty and other organic acids, and carbohydrates. Among the photoheterotrophs are the green and pur­ple nonsulfur bacteria.

Photosystems and ATP Generation

Chlorophyll and various other colored pigmented compounds are light-trapping pigments that are or­ganized into clusters of 200 to 300 molecules. These compounds harvest or collect light energy in one of their chemical bonds. When this happens, an electron becomes excited and reaches a higher energy level. That extra energy is released and transferred to a