The Life Sciences Recent Progress and Application to Human Affairs The World of Biological Research Requirements for the Future (1970) / Chapter Skim
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Form and Function
Form and Function
Pages 80-91
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From page 80... ...
is required for the synthesis of infectious virus; newly synthesized virus then contains the genome of the transforming virus and the coat protein of the helper virus. FORM AND FUNCTION For the isolated cell, structural form is correlated with its simplest needsto remain alive in the face of adversity, to grow, and to reproduce by fission.
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From page 81... ...
As shown in Figure 22, phase-contrast and electron microscopy revealed the presence in muscle of two quite distinct types of fibers: In skeletal muscle, filaments of about 200 ~ are each surrounded by six filaments about 100 ~ in diameter. When muscle is stretched, the two sets of filaments pull away from each other, and when muscle is contracted, they telescope into each other.
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From page 82... ...
v · · - ~ ~Li 9 htHeavy · · · · · · Meromyosi n Meromyosin ~\ \ \ ~\ \ \ FIGURE 22 Structure and function of skeletal muscle.
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From page 83... ...
However, in close relation to the transverse tubules is a network of extremely tiny tubes, the "sarcoplasmic reticulum," a system that, utilizing the energy of ATP, sequesters the calcium inside the tubules, thereby bringing contraction to a halt and permitting the muscle to relax until the next wave of excitation arrives. Numerous details of this fundamental life process remain to be unraveled.
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From page 84... ...
The latter goes straight down toward the renal medulla, makes a hairpin turn, returns toward the outer surface, and then descends again into a thicker channel, the collecting duct, which drains into the hilum of the kidney. Particularly noteworthy is the fact that the blood in the venule formed by coalescence of the glomerular capillaries also surrounds the ascending tubule and the collecting duct before entering into the larger veins for exit from the kidney.
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From page 85... ...
its solutes are reabsorbed as this filtrate passes along the tubules, for urine output is only about one liter per day. The experimental challenge has been to ascertain the mechanisms by which this versatile organ so alters its behavior as to assure excretion of very dilute urine when water intake has been copious or extremely concentrated urine when water intake has been meager or salt intake excessive, to excrete alkaline or acid urine as may be appropriate to physiological circumstance, and to assure that none of the material in the glomerular filtrate that is valuable to the body, e.g., glucose, is lost.
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From page 86... ...
Other animals, particularly desert-dwellers, are considerably more adept at this task than are we; for example, the kangaroo rat need never drink water and survives by excreting urine 14 times as concentrated as his own plasma! The principal feature that distinguishes the kidney of the kangaroo rat from that of man is that in the kangaroo rat the descending tubule dips much farther down into the cortical and medullary tissue of the kidney; i.e., the tubule is decidedly longer relative to kidney size than is that of man.
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From page 87... ...
1000 400 600 800 1000 1200 [3~1 /30~/ Active Passive Transport Diffusion Na I .> H2O Exchange of Na for K,H, N H4 FIGURE 24 Salt concentrations in various regions of the nephron, basis for the countercurrent concentration device. Summary of passive and active exchanges of water and ions in the nephron in the course of elaboration of hypertonic urine.
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From page 88... ...
This ion is removed in the descending tubule by active transport; the hormone blocks this process and phosphate continues into the urine. Secretion of this hormone, however, is sensitive not to the phosphate concentration but to the calcium concentration of blood plasma.
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From page 89... ...
The actual molecular basis for the influence of aldosterone, antidiuretic hormone, parathormone, and angiotensin on the structures of the kidney and of the adrenal gland remains totally unknown. Each is secreted in response to changes in the internal ionic environment.
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From page 90... ...
From this small group of cells, neuroendocrine secretions, simply termed "releasing factors," are transmitted to the nearby pituitary gland via the very short hypophyseal portal vein. At least two specific releasing factors are involved, causing the pituitary to release, respectively, its follicle-stimulating hormone and its luteinizing hormone.
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From page 91... ...
But cyclic adenylate formation was also discovered as the primary event in the activation of the cells of the adrenal cortex by the trophic hormone of the pituitary; after arrival of the latter hormone, cyclic adenylate is formed, which somehow makes possible the formation and secretion of adrenocortical hormones. The same compound is made when pituitary antidiuretic hormone acts upon the collecting duct of the renal tubule; somehow its presence permits movement of water across that struc ture.
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