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

182 CHAPTER 6 CELLULAR METABOLISM

neighboring pigment when the electron returns to a lower energy level. The high energy electron is trans­ferred to an electron acceptor. The capture of light en­ergy and the transfer of electrons and energy occurs via a system called a photosystem. These systems consist of pigment molecules that absorb light energy

and a series of molecules that alternately accept and donate electrons and protons to form a chain of oxi­dation-reduction reactions through which electrons and protons are passed. The transfers of protons es­tablishes a protonmotive force that is used for the chemiosmotic generation of ATP.

FIG. 6-27 The Calvin, or carbon reduction, cycle is the main metabolic pathway used by autotrophs for the conversion of carbon dioxide to organic carbohydrates. The pathway, which is active in photoautotrophs and chemolithotrophs, requires the input of carbon dioxide, ATP (energy), and NADPH (reducing power).

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Calvin Cycle and C0₂ Fixation

Autotrophic microorganisms carry out a metabolic sequence of reactions known as the Calvin cycle. In the Calvin cycle, carbon dioxide is used to form or­ganic matter. The conversion of C0₂ to organic mat­ter requires a significant input of ATP and reduced coenzyme (NADPH).

The Calvin cycle is a complex series of reactions that actually represents three slightly different but fully integrated metabolic sequences (FIG. 6-27). It ef­fectively takes three turns of the Calvin cycle to syn­thesize one molecule of the organic product of this metabolic pathway, which is glyceraldehyde 3-phos-phate. Because glyceraldehyde 3-phosphate contains three carbon atoms, the Calvin cycle is sometimes re­ferred to as a C₃ pathway. The glyceraldehyde 3-phos­phate molecules that are formed during the Calvin cycle can further react to form glucose and polysac­charides of glucose, such as starch and cellulose. It takes six turns of the Calvin cycle to form a 6-carbon carbohydrate, such as glucose. The net input of en­ergy—as ATP—and reducing power—as NADPH— required for the conversion of carbon dioxide to glu­cose is 18 ATP and 12 NADPH molecules.

In the Calvin cycle, carbon dioxide is reduced to form organic compounds for glucose synthesis.

In photoautotrophs the ATP and NADPH (energy and reducing power) to drive the Calvin cycle come from the light reactions of photosynthesis. In chemolithotrophs (discussed below) the needed ATP and reduced coenzymes come from inorganic com­pounds. The Calvin cycle itself is known as a "light-independent" or "dark reaction" because, although it requires ATP and NADPH, it does not require any light reactions.

Chemoautotrophic Metabolism

Some bacteria evolved the metabolic capacity to use inorganic substances as substrates to generate ATP

for cellular energy (Table 6-4). For example, some bacteria use reduced sulfur compounds, such as iron sulfide, to generate reducing power and cellular en­ergy. Bacteria that obtain energy in this way are called chemoautotrophs or chemolithotrophs, from the Greek, meaning obtaining nourishment from stones. These organisms do not require an organic compound or light as a source of energy. They obtain all their energy by oxidizing an inorganic compound. These bacteria have electron transport chains and es­tablish a protonmotive force across membranes, which is used to generate ATP by chemiosmosis. Only a few genera of bacteria obtain their energy from chemoautotrophic metabolism, and all other living organisms depend on them to provide the con­tinuous cycling of materials that are needed for growth.

Chemoautotrophic metabolism, also called chemolithotrophic metabolism, uses inorganic com­pounds to generate ATP.

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