The Physiology of Flowering Plants - KHAM PHA MOI

used as drugs, e.g. morphine, nicotine and quinine. Plants also containnumerous non-protein amino acids, which are not incorporatedinto normal proteins. There has been much dispute about the possiblephysiological functions of the secondary chemicals. Both thealkaloids and the non-protein amino acids are toxic and often bittertasting; one possible function is protection against herbivores. Inseeds, non-protein amino acids, with a high proportion of N byweight, can act as N storage compounds. Some non-photosyntheticpigments contain N, e.g. betacyanin, the red pigment of beetroot(Beta vulgaris).Sulphur performs an important structural role in proteins wherethe disulphide bridges –S–S– stabilize tertiary protein structures.Sulphydryl groups, –SH, are found in the active sites of manyenzymes. There are also –SH-containing coenzymes, e.g. coenzymeA, whilst glutathione, again with a –SH group, is an important antioxidant.Several iron–sulphur proteins, e.g. ferredoxins, occur in theelectron transfer systems of chloroplasts and mitochondria; theseproteins contain clusters of linked S and Fe atoms at their reactivesites. Membrane sulpholipids are structural molecules which containa sulphate group, found in chloroplast thylakoid membranes.Numerous flowering plants contain pungent secondary S-containingcompounds appreciated as flavours; these are very common in theBrassicaceae (cabbage family) which includes mustard (Sinapis alba).Onions (Allium cepa), garlic (Allium sativum) and related species are alsoflavoured with S-containing chemicals. The presence of such compoundsmay deter some herbivores.Phosphorus is contained in nucleic acids and also in membranephospholipids which make up the bimolecular lipid leafletof biological membranes. As a component of the adenosine phosphates(ATP, ADP and AMP) and related nucleotides, the phosphategroup is involved in ‘energy metabolism’ and intermediary metabolisminvolves many phosphorylated intermediates. In metabolicallyactive cells there is a continuous turnover of phosphate from organiccombination to Pi (inorganic phosphate) and back again.Calcium, as in cells of other kingdoms, contributes to membranestability in plant cells by its association with membrane phospholipids,and it is necessary for the maintenance of the normal permeabilityof the plasmalemma. In plants it also contributes to cell wallstructure as calcium pectate; this is a major component of the middlelamella which cements adjacent cell walls together. The Ca 2+ ion isextremely important in stimulus perception; one of the first effectsin the chain of reactions set off by a stimulus, environmental orhormonal, is very often a change in the cellular concentration ofCa 2+ which is termed a ‘second messenger’ (see Section 7.4.4). Ca 2+further acts as activator to some enzymes – amylases, ATPases andphospholipases.Potassium is something of a mystery element. It is present incells as the free K + ion; it does not enter into organic combination. Itis known to be the activator of some enzymes, but other elementsESSENTIAL ELEMENTS 103