Chap. 9. The Biosphere 231Enzymes are of significant concern in the practice of green chemistry. One obvious
relationship is that between enzymes and chemicals that are toxic to them. In carrying
out green chemical processes, such chemicals should be avoided wherever possible.
Another obvious relationship has to do with the use of biological processes to perform
chemical operations carried out by enzymes. Because of the benign conditions —
particulary of temperature — under which enzymes operate, biologically mediated
chemical processes are usually done under much milder and environmentally friendly
conditions biologically than chemically. Biochemical processes are all carried out by
enzymes. For example, several enzymes, starting with hexokinase, are involved in the
multistepped biochemical fermentation synthesis of ethyl alcohol from carbohydrate
glucose. With recombinant DNA technology (see Section 9.8) it is now possible to
invest bacteria with enzyme systems from other organisms designed to carry out desired
biochemical processes. Bacteria are much more amenable to handling and usually much
more efficient than the organisms from which the genes for the desired enzyme systems
are taken. Another approach is to use isolated enzymes immobilized on a solid support
to carry out biochemical processes without the direct involvement of an organism.
Nutrients
The raw materials that organisms require for their metabolism are nutrients. Those
required in larger quantities include oxygen, hydrogen, carbon, nitrogen, phosphorus,
sulfur, potassium, calcium, and magnesium and are called macronutrients. Plants
and other autotrophic organisms use these nutrients in the form of simple inorganic
species, such as H 2 O and CO 2 , which they obtain from soil, water, and the atmosphere.
Heterotrophic organisms obtain much of the macronutrients that they need as
carbohydrates, proteins, and lipids (see Chapter 5) from organic food material.
An important consideration in plant nutrition is the provision of fertilizers
consisting of sources of nutrient nitrogen, phosphorus, and potassium. A large segment
of the chemical manufacturing industry is involved with fixing nitrogen from the
atmosphere as ammonia, NH 3 , and converting it to nitrate (NO 3 - ), urea (CON 2 H 4 ), or
other compounds that are applied to the soil as nitrogen fertilizer. Phosphorus is mined
as mineral phosphate that is converted to biologically available phosphate (H 2 PO 4 -
and HPO 42 - ions) by treatment with sulfuric or phosphoric acid. Potassium is mined as
potassium salts and applied directly as fertilizer. The ongoing depletion of sources of
phosphorus and potassium fertilizer is a sustainability issue of significant concern.
Organisms also require very low levels of a number of micronutrients, which are
usually used by essential enzymes that enable metabolic reactions to occur. For plants,
essential micronutrients include the elements boron, chlorine, copper, iron, manganese,
sodium, vanadium, and zinc. The bacteria that fix atmospheric nitrogen required by
plants require trace levels of molybdenum. Animals require in their diet elemental
micronutrients including iron and selenium as well as micronutrient vitamins consisting
of small organic molecules.