Pectic acid plant tissues

Pectic substances are the glues to plant cells and exist predominately in middle lamella between cell walls. The American Chemical Society defined the terms protopectin, pectinic acid and pectic acid (21). The native protopectin is a long chain, alpha-1, 4-D-galacturonic acid polymer, interspersed with alpha-1,2-L-rhamnose residues. Shorter polymers, galactans and arabinans exist and are bonded covalently with the main chain (22). Many types and different amounts of side chains exist in various plant tissues. 

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. 

Certain plants accumulate897 very high levels of nickel in their tissues Hybanthus floribundus contains up to 1.3% nickel on a dry mass basis, while Psychotria douarrei has up to 44% nickel in the plant ash. AUysum is also known as a nickel accumulator. A large part of the nickel in these plants is water soluble, and in the last case is associated with malic and malonic acids.898 In Hybanthus floribundus, the nickel is largely accumulated in the leaves, where it is associated with pectic carbohydrates. 

A better understanding of this subject is possible when the location and function of the pectic substances in plants are reviewed. The movement of water and plant fluids to the rapidly growing fruits and the retention of form and firmness of fruits are functions of pectin. This intercellular substance in plants is similar in action to the intercellular substance of the vertebrates—collagen (the precursor of gelatin). Protopectin, the water-insoluble precursor of pectin, is abundant in immature fruit tissues. Ripening processes involve hydrolytic changes of protopectin to form pectin and later, as maturity is passed, enzymic demethylation and depolymerization of pectin to form pectates and eventually soluble sugars and acids. 

Pectic Substances 46, 77). An important group of substances commonly called pectins is found in the primary cell wall and intercellular layers of all land plants, and, in some ways, seems to correspond to the hyaluronic acid of the ground substance of much animal tissue. Because of the immense gelling power of pectic substances, they are widely used for the gelation of fruit juices to form jellies. 

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