As knowledge regarding the formation of organic compounds from carbon dioxide and other inorganic materials in green plants accumulates, it becomes increasingly apparent that it is difficult to distinguish which transformations of carbon compounds should be classified as part of the pathway of carbon in photosynthesis and which reactions should be considered as other metabolic processes of the plant. All reactions of a photoautotrophic plant rely ultimately on the energy stored by the photosynthetic process. Therefore, any definition of carbon reduction during photosynthesis based on requirement of energy or equivalents of reducing agents should specify the requirement precisely. Even so, it is questionable whether such a definition can distinguish between carbon reduction reactions of photosynthesis and other metabolic transformations of carbon compounds. It is now believed that energy-carrying compounds such as adenosine triphosphate and reducing agents such as reduced triphosphopyridine nucleotide which are formed during respiratory processes may also be formed directly from products close to the primary photochemical reactions of photosynthesis. Transformations of carbon compounds which require such substances and which take place in the dark may also occur at a greatly accelerated rate during photosynthesis. Furthermore, most, if not all, of the reactions of carbon reduction in photosynthesis are known to occur, although at a diminished rate, in the dark long after the immediate reducing and energy-carrying agents formed from the photochemical reaction have decayed.

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The Path of Carbon in Photosynthesis

This review provides a brief historical account of how microscopical studies of chloroplasts have contributed to our current knowledge of the structural and functional organization of thylakoid membranes. It starts by tracing the origins of the terms plastid, grana, stroma and chloroplasts to light microscopic studies of 19th century German botanists, and then describes how different types of electron microscopical techniques have added to this field. The most notable contributions of thin section electron microscopy include the elucidation of the 3-D organization of thylakoid membranes, the discovery of prolamellar bodies in etioplasts, and the structural changes in thylakoid architecture that accompany the light-dependent transformation of etioplasts into chloroplasts. Attention is then focused on the roles that freeze-fracture and freeze-etch electron microscopy and immuno electron microscopy have played in defining the extent to which the functional complexes of thylakoids are non-randomly distributed between appressed, grana and non-appressed stroma thylakoids. Studies reporting on how this lateral differentiation can be altered experimentally, and how the spatial organization of functional complexes is affected by alterations in the light environment of plants are also included in this discussion. Finally, the review points to the possible uses of electron microscope tomography techniques in future structural studies of thylakoid membranes.

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Chloroplast structure: from chlorophyll granules to supra-molecular architecture of thylakoid membranes

Historic discoveries and key observations related to Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase), from 1947 to 2006, are presented. Currently, around 200 papers describing Rubisco research are published each year and the literature contains more than 5000 manuscripts on the subject. While trying to ensure that all the major events over this period are recorded, this analysis will inevitably be incomplete and will reflect the areas of particular interest to the authors.

Discoveries in Rubisco (Ribulose 1,5-bisphosphate carboxylase/oxygenase):a historical perspective

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1.

The kinetics of 14CO2 incorporation into cellular intermediates was used to determine the primary pathway of carbon fixation by four genetically diverse unicellular blue-green algae. In each case label was first detected in 3-phosphoglycerate and then in compounds of the reductive pentose cycle.




2.

A light to dark transition evoked the same response in all four strains: Immediate cessation of biosynthesis, rapid increase in the concentration of 6-phosphogluconate and changes in the concentrations of sugar mono- and diphosphates. On the other hand, after the first few seconds of dark incubation little or comparatively slow change was noted in the concentrations of 3-phosphoglycerate, phosphoenolpyruvate, citrate, aspartate, and glutamate.




3.

For one strain (aphanocapsa 6308) an experiment using both 32P and 14CO2 as tracers revealed comparatively rapid turnover of metabolites common to the oxidative pentose cycle during dark incubation. Much slower turnover of labeled carbon was found in other metabolites of glycolysis and the biosynthetic portions of the tricarboxylic acid cycle.




4.

During dark incubation of Aphanocapsa 6308 the concentration of adenosine triphosphate decreased to approximately 50% of the photosynthetic value within the first 50 sec. In the same period, adenosine diphosphate nearly doubled in concentration. By 4 min after the beginning of the dark period, the steady-state levels of the two adenylates had been restored to photosynthetic levels.

Photosynthetic and dark carbon metabolism in unicellular blue-green algae.

Metabolism of soybean leaves. IV. Translocation from soybean leaves.

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The path of carbon in photosynthesis. V. Paper chromatography and radioautography of the products

UNIVERSITY OF CALIFORNIA t Arjc :j 1 198: I..IRWAIIY rWL) UOCUMEN 15 SECT ION TWO-WEEK LOAN COPY This is a Library Circulating Copy which may be borrowed for two weeks. BERKELEY, CALIFORNIA

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The Path of Carbon in Photosynthesis II. Amino Acids - eScholarship

Chemical transformations of carbon in photosynthesis.

The path of carbon in photosynthesis. 21. the cyclic regeneration of carbon dioxide acceptor

Author(s): Benson, A.A.; Calvin, M.; Haas, V.A.; Aronoff, S.; Hall, A.G.; Bassham, J.A.; Weigl, J.W.

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A.A. Benson

Papers

The path of carbon in photosynthesis. V. Paper chromatography and radioautography of the products

The Path of Carbon in Photosynthesis II. Amino Acids - eScholarship

Chemical transformations of carbon in photosynthesis.

The path of carbon in photosynthesis. 21. the cyclic regeneration of carbon dioxide acceptor

C14 in Photosynthesis