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THE CHITIN SYSTEM 总被引:3,自引:0,他引:3
K. M. RUDALL W. KENCHINGTON 《Biological reviews of the Cambridge Philosophical Society》1973,48(4):597-633
- 1 The view is supported that chitin is not found in Deuterostomia because of the absence of chitin synthetase, and is not found in higher plants because of the absence of glucosamine. In Fungi, control mechanisms are present affecting the synthesis of glucosamine; chitin is often present, but when it is absent this probably results from a failure to synthesize glucosamine.
- 2 A review of conformation maps for cellulose and chitin indicates the possibility of a slightly right-handed twist in small groups of chitin chains.
- 3 The occurrence of α, β and γ-forms of chitin in the peritrophic membranes of various insects is described. Gamma chitin seems to be the commonest form.
- 4 In several beetles, optical and electron-microscope studies trace the formation of chitinous cocoon fibres from larval peritrophic membrane and define the discrete ribbon-like nature of the, β chitin produced in the mid-gut.
- 5 By studying apodemes it is found that orthopteroid insects are most varied, different molecular structures being present in levator, depressor and pretarsal tendons. By contrast, Hymenoptera and Coleoptera show very similar structures in all three apodemes as well as in other parts of the cuticle. Apodemes are regarded as sampling the cuticle at their varying points of origin; they provide especially favour able material for diffraction studies.
- 6 In arthropod cuticles there is evidence for the widespread occurrence of α chitin micelles which are three chains thick in the direction of the c axis. This is compared with the structure of γ chitin where the chains repeat in groups of three along the c axis.
- 7 Changes in the diffraction pattern are related to the series of proteins defined by Hackman. The chitin-protein complex is not affected by water or neutral salt extrac tion, but is disrupted by treatment in urea.
- 8 Electron microscopy defines the unit of structure as a composite microfibril: a core of chitin surrounded by adsorbed proteins. This consists of ‘primary’ protein (often repeating as regular units along the fibrils) and a quantity of ‘satellite’ protein which obscures the imaging of the regularly arranged ‘primary’ protein. There are apparent ‘bridges' between the microfibrils.
- 9 New diffraction data give information about the size and arrangement of micro-fibrils. These fibrils may be arranged in layers of ‘rods’, or as an hexagonal arrange ment of ‘rods’.
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THE EARLY SOLAR SYSTEM 总被引:1,自引:0,他引:1
Life arose on an early Earth which was the product of the conditions present, and processes operating, during formation of the solar system. The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet. 相似文献
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NERVOUS SYSTEM SPECIFIC PROTEINS 总被引:10,自引:7,他引:3
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George Wald 《The Journal of general physiology》1939,22(6):775-794
1. In the rods of fresh-water and some anadromous fishes, rhodopsin is replaced by the purple photolabile pigment porphyropsin. This participates in a retinal cycle identical in form with that of rhodopsin, but in which new carotenoids replace retinene and vitamin A. 2. Porphyropsin possesses a broad absorption maximum at 522 ± 2 mµ, and perhaps a minimum at about 430 mµ. The vitamin A-analogue, vitamin A2, possesses a maximum in chloroform at 355 mµ and yields with antimony trichloride a deep blue color due to a band at 696 mµ. The retinene-analogue, retinene2, absorbs maximally in chloroform at 405 mµ and possesses an antimony chloride maximum at 706 mµ. 3. Its non-diffusibility through a semi-permeable membrane, salting-out properties, and sensitivity to chemical denaturants and to heat, characterize porphyropsin as a conjugated carotenoid-protein. 4. The porphyropsin cycle may be formulated: porphyropsin See PDF for Structure. retinene2-protein (2)
→ vitamin A2-protein (3)
→ porphyropsin. Isolation of the retina cuts this cycle at (3); denaturation procedures or extraction of porphyropsin into aqueous solution eliminate in addition (1) and (2). 5. The primary difference between the rhodopsin and porphyropsin systems appears to be the possession by the latter of an added ethylenic group in the polyene chain. 相似文献
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Charles Yanofsky 《Microbiological reviews》1960,24(2):221-245
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四爪陆龟消化、呼吸系统的解剖 总被引:3,自引:1,他引:3
四爪陆龟(Testudohorsfieldi)的消化管总长度为体长的4.5—4.8倍。喙缘锋利,硬腭侧缘具细齿,舌不伸出口,食道扩展性强,胃呈囊状,被肝叶覆盖,小肠较长,盲肠发达。肝较大,重18.3g,分左叶、中叶和右叶,绿色胆囊位于右肝两小叶间。胰腺长条形,分布于十二指肠内。肺长囊形,紧贴在背甲的内表面,位于肩带和腰带之间。气管较短由15-20个软骨环组成,支气管较长由70个左右的软骨环组成。 相似文献
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