Starch, biological function structure

Starch Biology, Structure and Functionality A. Huber W. Praznik... 

Saccharides are present in food raw materials in quantities ranging from about 1% in meats and fish, to about 4.5% in milk, 18% in potatoes, and 15-20% in sugar beets, to about 70% in cereal grains. Polysaccharides participate in the formation of structures in plants. They are also stored in plants as starch and in muscles as glycogen. Other saccharides are dissolved in tissue fluids or perform different biological functions in free nucleotides, as components of nucleic acids, or bound to proteins and lipids. 

Polysaccharides, i.e., linear or branched heteropolymers of sugars and derived components several are polyelectrolytes. The degree of polymerization is mostly 103 to 104. The main biological functions are nutritional (primarily starch in plants, glycogen in animals) and building material (in plants). The latter are called structural polysaccharides, which occur in a great variety of types and mostly form mixed and highly complex structures, especially in cell walls. 

Carbohydrates, also referred to as saccharides, are polyhydroxyaldehydes or polyhydroxyketones or compounds that can be hydrolyzed to or derived from polyhydroxyaldehydes or polyhydroxyketones. Carbohydrates have many important biological functions— to store and generate energy in animals and plants (glucose, starch, glycogen), as supportive structures in plants (cellulose) and crustaceans (chitin), and as components of cell membranes and nucleic acids. 

Fig. 5.9. Diagram summarising the path of carbon assimilation in chloroplasts (see also eqn. 49). The assimilation can be divided into three phases. Phase I The phosphorylation of ribulose monophosphate (Ru-5-P) to ribulose diphosphate (RuDP) which then accepts a molecule of COj and is cleaved to 2 molecules of PGA (eqn. 49 (g) and (a)). Phase II PGA is reduced to triose phosphate (eqn. 49 (b)). Phase III Triose phosphate acts as the precursor of both Ru-5-P and of hexose and starch. The reactions are driven by ATP and reduced NAD. The assimilation of 1 mole of COj requires 3 moles of ATP and 2 moles of reduced NADP. (After D. I. Amon, in Biological Structure and Function, edited by T. W. Goodwin and O. Lindberg, Academic Press, New York, 1961.)...

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