P&P - Hydrogels, latex, resins

PLANTS AND PEOPLE - Botany 328 - LECTURE NOTES - Spring 1999
Hugh D. Wilson - rm. 306, Butler Hall

Hydrogels, Elastic Latexes, and Resins

As opposed to prior topics, the plant products overviewed here, these have no comonality with regard to associated plant part, biological relevance (for the plant), or biochemistry. They are more or less 'sticky' substances that are exuded or extracted from many different plants.

1. HYDROGELS - or hydrocolloids are water-modifying substances. They alter the behavior of water in a way similar to that of soap, i.e., charge relationships. Where soap is composed of bipolar molecules that both attract and repulse water molecular - hydrogels attract and combine with water to make it more viscous. Typically complex polymers. Three types:

a. GUMS - polysaccharides of acid salts of sugars other than glucose - metal ions of the salts cause the gum molecules to associate with water - naturally produced as a wound response in plants - exuded and not produced or contained in a specific plant part - they result from the breakdown of compounds in injured cells and may function to seal wounds against entry of pathgens - initially [still in some places] harvested by wounding and scraping - now usually extracted on a large scale or synthetically produced from cellulose.

Gums are inert in terms of nutrition - used to add 'body' to foods (makes toothpaste a paste or gel), stablilize emulsions, thicken liquids, suspend particles, prevent ice crystal formation (ice cream). Also used in medicine and industry (sizing [fill open spaces, make smooth/rigid]) in paper production, oil drilling (bit lubrication, emulsify water used in secondary recovery).

Exudate gum: (exuded from wounds): Acacia senegal - Fabaceae - small tree of western Africa - known as 'gum arabic' because it was shipped to Europe from Arabian ports (name of the 'Turkey', a native American bird - associated with trade routes and shipment to medieval Europe) - 90% of the world's supply collected from wild trees. - In many commercial products - beer (stabilize foam), postage stamps, ink, paint, etc.)

Extractive gum: Guar gum (Cyamopsis tetragonolobus - Fabaceae) - known only as a cultivated plant, possibly domesticated in India although all wild relatives are African - almost all guar gum produced in the U.S. is grown in Texas and Oklahoma (ca. 100k acres) where it is known as 'cluster bean' - gum is derived from dried, ground contents of the seed which, after extraction, is a good (high protein) livestock feed..

b. PECTINS - plant polysaccharides (non-glucose) located between cells and as components of the primary cell wall - associated with calcium, very high molecular weight - non-digestable by humans - pectins tend for form gels under specific conditions - used primarily in the manufacture of jams and jellies (75%) - derived as a by product of Citrus (Rutaceae)and apple (Rosaceae) processing.

c. STARCH - starch molecule complex polymer of glucose - found in plants as a grain - sometimes size and shape of grain distictive - grain is composed of two molecular forms AMYLOSE [straight or coiled] and AMYLOPECTIN [branched] - ground starch grains [flour] - heat allows these molecules to complex with water - gravy - has food value - although over 60% of processed starch is used for making cardboard - increases strength.

2. LATEX: any mixture of organic compounds that are produced is special plant cells LATICIFERS. These form tubes or canals or networks in various plant organs - known only from angiosperms - mostly dicots. Complex compounds - many properties - mostly hydrocarbons and not soluable in water. Functions of latex for the plant are a mystery - antipredaton vs. toxic waste site. Latex with elastic properties = RUBBER.

Chicle - chewing 'gum' - not a true gum (see above), but a latex from Manilkara zapota (Sapotaceae) - Santa Anna exiled president of Mexico ripped off (see Box 11-1, p. 371) in New York - left behind a large (2 ton) chunk of chicle [used as a chewing gum by the Aztec] - exploited by an American entrepreneur (Thomas Adams - sweetened paraffin, chewing tobacco) and eventually developed into the Amerian Chicle Company - thus, smoking tobacco and chewing gum are native American cultural traits from native American plants. Chewing gum does not 'bounce back ' - not elastic.

Latex with elastic properties = RUBBER.

Early 1500s - explorers noted that native americans of the amazon basin dipped their feet in the sap taken from a large canopy tree of the rain forest - Hevea brasiliensis - Euphorbiaceae and holding them in smoke [coagulated the latex] to produce a water resistant surface- spanish adoped the practice with their hats and cloaks.
1823 - Charles Macintosh discovered that Hevea rubber was soluable in hexane - allowed a more controlled application to fabric ('Macintosh'), but cracked when cold and was sticky in hot weather - a bit of a problem.
1839 - Charles Goodyear - ran across the process of vulcanization - adding sulfur to rubber - cross-links molecules
Major industry, but plantations were difficult to establish within the native range (Amazon basin) - native pests [leaf blight]. Until 1890 - all rubber was harvested from wild stands in the Amazon basin [controversy in Brasil involves rubber collectors - Chico Mendez - 'chiclero' or rubber tapper - murdered by ranchers - trying to maintain tall forest in Amazonia (Brazil) - a symbol of Rain Forest destruction]
1876 - seeds taken from Kew Garden to Asia - plantations established beyond the pest - began producing in 1890 - by WW II - 90% of the native rubber was produced in Asia.
WWII - break in supply lines to Asian plantations - development of synthetic rubber from petroleum products - essentially replaced natural rubber.
Oil shortage, new demands (radial tires) - increased demand for natural rubber. Other options:
Parthenium argentatium - Asteraceae - 'guayule' - shrub native to mexico - studied during WWII - forgotten after the war - the topic of some research during the 1980s - level of interest on these types of plants is concordant with the price of petroleum.

3. RESINS: Resins are activity produced by the plant and secreted into specialized canals or ducts, usually positioned in the conductive tissues or bark of many gymnosperms and some dicots. Their function in nature appears to center on biochemical defenses against herbivory and possibly as general antibiotics. Chemically heterogeneous, they usually are are mixture of polymerized terpenes and volitle oils. Ancient human connections with plant resins include incense (frankincence [Boswellia sacra] and myrrh [Commiphora abyssinica] - both Burseraceae), embalming (same taxa among others). Pine (Pinus - Pinaceae) resin products, initally termed 'naval stores' due to importance in ship building and maintenance, include pitch, turpentine, and rosin. (rosin precipitates from raw pine resin and the remaining liquid is distilled to produce turpentine). Development of linoleum involved mixtures of linseed oil (Linum), resins (Agathis - Araucariaceae), and cork (Quercus) particles and, like many modern applications, has been replaced by synthetics compounds that are based on the original natural product. Also, the only jewel of plant origin - amber - fossilized resins - Jurassic Park

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