Polysaccharide natural carboxylated

Modified aldoses, of wide natural occurrence, are the uronic acids. In these molecules the terminal CH2OH group of a hexose has been oxidised to carboxyl. Polysaccharides consisting almost entirely of uronic acid units are pectic acid (D-galacturonic acid) (LXXVIII), the main component of pectin found in plant tissues, and alginic acid (D-mannuronic (LXXIX) and L-guluronic acid (LXXX)) synthesised by the brown algae. For a full description of these acids see Chapter II, p. 73. 

In this experiment you will work with a natural polymer called alginic acid that comes from seaweed. Alginic acid is a polysaccharide as are cellulose and starch. Unlike cellulose and starch, however alginic acid as the name implies contains acidic functional groups called carboxylic acids. Salts of alginic acid are used as food additives, especially as a thickener. 

Uronic acids are important components in many naturally occurring polysaccharides. By chemical or enz)matic degradation of these polysaccharides, smaller uronic acid units ranging from monosaccharides to smaller oligosaccharides can be prepared. In this connection, reduction of the carboxyl group can serve both synthetic and analytical purposes. 

When an animal eats more carbohydrate than it uses up, it stores the excess some as the polysaccharide glycogen (Sec. 35.9), but most of it as fats. Fats, we know (Sec. 33.2), are triacylglycerols, esters derived (in most cases) from long straight-chain carboxylic acids containing an even number of carbon atoms. These even numbers, we said, are a natural consequence of the way fats are synthesized in biological systems. 

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