Transformation of Sugars in Plants

Abstract

The first sugar of photosynthesis.-Glucose, fructose, sucrose, and starch are formed in green plants as a result of photosynthetic activity and can easily be detected. Because of this, plant physiologists devoted considerable effort for over half a century to prove which of these carbohydrates consti­ tutes the "first sugar of photosynthesis." Their experimental approach was to determine the relative concentrations of these carbohydrates in the plant at different times of the day and seasons of the year and to study the changes caused by respiration and illumination. On the basis of such experiments each of the carbohydrates mentioned, especially sucrose and glucose, has been claimed by various investigators to be the first photosynthetic product. However, they neglected to take into consideration the rapidity with which monosaccharides formed in the process of photosynthesis can be isomerized and polymerized to complex carbohydrates by the enzymes in the leaves, and the speed with which the complex carbohydrates can be broken down to monosaccharides by the reverse reactions. The preponderance of a particular sugar at a certain time cannot be interpreted to mean that it is the first to be formed. The subject of carbohydrate transformation has been excellently summarized by several workers in the field (1). It was established within recent years that the mechanism of the photo­ synthetic process (2) consists of a series of reactions which succeed each other so closely that the intermediate products formed are not detectable by common analytical methods. When a green leaf is illuminated, glucose, fructose, sucrose, and starch rapidly appear; however, light and chlorophyll are not required for their formation. That light is not necessary for the starch synthesis is shown by the fact that it can be formed in vitro in the dark from glucose-1-phosphate (Cori ester) by means of the enzyme phos­ phorylase (3). Likewise, the process of sucrose formation does not depend on light. Hartt (4) demonstrated that synthesis of sucrose from glucose and fructose in detached sugar cane blades takes place in the dark and in the absence of chlorophyll. Similar results were obtained by McCready & Hassid (5) with barley shoots which were infiltrated with monosaccharides, indi­ cating that the synthesis of sucrose is a process distinct from photosynthesis. Since the snow plant, Sarcodes sanguine a, which is devoid of chlorophyll and derives its nutrients from organic matter in the soil; contains a considerable quantity of glucose and fructose (6), it is evident that light is not required for the formation of these monosaccharides. It therefore appears that light is required only for the reaction involving the initial phases of carbon dioxide utilization in the photosynthetic process. When the primary organic mole­ cules are produced, sugars, including monosaccharides, can subsequently be