At some later stage of evolution, unicellular organisms found it advantageous to cluster together, thereby acquiring greater motility, efficiency, or reproductive success than their free-living single-celled competitors. Further evolution of such clustered organisms led to permanent associations among individual cells and eventually to specialization within the colony – to cellular differentiation.
The advantages of
cellular specialization led to the evolution of ever more complex and
highly differentiated organisms, in which some cells carried out the
sensory functions, others the digestive, photosynthetic, or reproductive
functions. Many modern multicellular organisms contain hundreds of
different cell types, each specialized for some function that supports
the entire organism. Fundamental mechanisms that evolved early have been
further refined and embellished through evolution. The simple mechanism
responsible for the motion of myosin along actin filaments in slime
molds has been conserved and elaborated in vertebrate muscle cells,
which are literally filled with actin, myosin, and associated proteins
that regulate muscle contraction. The same basic structure and mechanism
that underlie the beating motion of cilia in Paramecium and flagella in Chlamydomonas
are employed by the highly differentiated vertebrate sperm cell. Figure
2–25 illustrates the range of cellular specializations encountered in
Figure 2–26 Three types of junctions between cells.
(a) Tight junctions produce a seal between adjacent cells. (b) Desmosomes, typical of plant cells, weld adjacent cells together and are reinforced by various cytoskeletal elements. (c) Gap junctions allow ions and electric currents to flow between adjacent cells.
Lehninger-Nelson-Cox: Principles of Biochemistry, 49.o.
2017. november 23., csütörtök
2017. november 10., péntek
"Living cells are chemical engines that function at constant temperature."
Lehninger-Nelson-Cox: Principles of Biochemistry, 9.o.
2017. november 6., hétfő
Eddig azt hittem, hogy a Lehninger-könyv 4. oldalán van az első utalás a biokémia és az informatika összefüggésére - de nem: már a 3. oldalon, az első fejezet címében is ott van !
2017. november 4., szombat
Figure 22-15 (a) Location of the hypothalamus and pituitary gland. (b) Details of the hypothalamus-pituitary system. Signals arriving from connecting neurons stimulate the hypothalamus to secrete hormones destined for the anterior pituitary into a special blood vessel, which carries the hormones directly to a capillary network in the anterior pituitary. In response to each hypothalamic hormone, the anterior pituitary releases its appropriate hormone into the general circulation. Posterior pituitary hormones are made in neurons arising in the hypothalamus, transported in axons to nerve endings in the posterior pituitary, and stored there until released into the blood in response to a neuronal signal.
The hypothalamic hormones pass directly to the nearby pituitary gland through special blood vessels and neurons that connect the two glands (Fig. 22-15b). The pituitary gland has two functionally distinct parts. The anterior pituitary responds to hypothalamic hormones carried in the blood, by producing six tropic hormones or tropins (from the Greek tropos, meaning "turn"), relatively long polypeptides that activate the next rank of endocrine glands (Fig. 22-14). Adrenocorticotropic hormone (ACTH, also called corticotropin; Mr 4,500) stimulates the adrenal cortex; thyroid-stimulating hormone (TSH, also called thyrotropin; Mr 28,000) acts on the thyroid gland; follicle-stimulating hormone (FSH; Mr 34,000) and luteinizing hormone (LH; Mr 20,500) act on the gonads; and growth hormone (GH, also called somatotropin; Mr 21,500) stimulates the liver to produce several growth factors.
Lehninger-Nelson-Cox: Principles of Biochemistry, 752.o.
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