174

14

The Nature of Living Things

Fig. 14.1 Schematic diagram of the major relations in living organisms. The innermost zone con-

cerns processes taking place within the cell. The upper portion indicates processes (a comma ba, b) involved in

multiplication (reproduction); the lower portion indicates processes (k comma l comma mk,l, m) involved in life main-

tenance (homeostasis). The curved arrows moving upward in the central region on the left-hand side

indicate processes (p, q, r) of synthesis; those on the right-hand side (g, h, i, j) indicate processes

of regulation. Exchange with the environment (input and output) takes place: Inputsn 1 comma n 2n1, n2, andn 3n3

could be, respectively, cosmic rays causing DNA mutations, toxicants interfering with the regula-

tion of transcription and translation, and food.ss indicates specific molecular factors ingested from

the environment, such as folic acid providing a source of methyl groups for DNA methylation. The

successive generations (cc) are, of course, released into the environment. Secretion (oo) includes not

only waste products but also highly specific molecules for altering a surface in the vicinity of an

organism, or its outer shell (the set of secreted molecules other than waste is called the secretome)

can separate a fairly fluid fraction, which contains very little apart from a few ions

and small osmolytes like sugars. ¹ The proteins and the rest that are usually called

“cytoplasmic” are mostly bound to macromolecular constructs such as the inner

surface of the outer lipid membrane, internal membranes such as the endoplasmic

reticulum, other polymers such as various filaments (the cytoskeleton) made from

proteins such as actin or tubulin, and polysaccharides. These bound proteins can only

be released if the ultrastructure of the cell is completely disrupted (e.g., by mechan-

1 Kempner and Miller (1968).