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Home >>Biology Dictionary >> Centipide - Centriole
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Cellulose.
Fundamental constituent of cell wall in higher plants, many algae and some fungi. Occurs also in Urochordata. Long chain polysaccharide made up of β-glucose units. In primary cell wall long chain cellulose molecules are aligned in parallel and bonded together in group of up to 200 to form crystalline micelles. Micellus and unoriented cellulose molecules form a loose network with pectin filling the interstices.
This organization allows for wall extension in cell growth and more cellulose is laid down between existing micelles.
In secondary wall, micelles associates to form long stands, microfibrils, separated by non-cellulose material and laid down in successive layers in different directions, conferring rigidity on wall. Fibrous nature of cellulose is responsible for its use in textile industries (cotton, linen, artificial silk.).
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Cell Wall
Limiting layer of bacterial and plant cells, formed by and closely investing, the protoplasm; comparatively rigid and giving mechanical support. In bacteria, contains mureins (q.v.). In higher plants, wall is traversed by extremely fine cytoplasmic threads, the plasmodesmata, which form delicate propoplasmic connections between adjacent cells. The walls of newly formed cells are at first very thin but as the cells assume their permanent character the walls thicken.
Cement. Bone-like substance which makes a thin covering to root of vertebrate tooth (below gum level) and in some mammals (e.g. ungulates) covers part of enamel of crown.
Cenospecies. All numbers of an ecospecies related to each other by the ability to breed and exchange genes through hybridization e.g. dogs and wolves.
Centipide.
An anthropod that belongs to class mipriapoda.
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Central Nervous Systems. (C.N.S). A mass of nervous tissue which co-ordinates the activities of an animal. In vertebrates the C.N.S. is the brain and spinal cord, which together are basically a single hollow tube with very thick walls, enclosed within skull and backbone. In most invertebrate the C.N.S. consists of a few solid cords of nervous tissue, often associated with larger masses called ganglia.
In some invertebrates however (Coelentrata, Echinodermata) the place of a C.N.S. in providing co-ordination is taken by a diffuse nerve net (q.v.)
The C.N.S. is in a position to co-ordinate the activities of an animal with each other and with the environment because almost all the messages sent from the sense-organs travel straight into the C.N.S., and almost all impulse received by the muscles, glands, etc., come out from the C.N.S.
The C.N.S. contains many fairly direct nervous pathways between certain sense-organs and certain muscles of glands (see Reflux) which ensure that some stimuli are followed rapidly by their appropriate standardized responses.
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But there is also great opportunity in the C.N.S. for co-ordination of a variety of stimuli by interaction between nerve cells, because of the large number of junctions (synapses) between nerve cells which occur there.
In vertebrates for instance, the great majority of synapses occurs in the C.N.S. The C.N.S., however, is not to be regarded simply as a means of reconciling and co-ordinating the immediate activities of the sense-organs; it has an activity of its own independent of the stimuli of the moment, most obvious in the influence of past experience on present performance. The rest of the nervous system, other than the C.N.S., is the peripheral nervous system.
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Centre. (Nerve). Region of central nervous system with a restricted special function, e.g. respiratory centre in medulla of vertebrates which controls respiratory movements.
Centric fission. The splitting of a chromosome at or near the centromere to produce two chromosomes.
Centric fusion. The joining of together of two accentric chromosomes to produce one metacentric chromosome.
Centrifuge.
A rotating machine that separates liquids from solids, or dispersions of one liquid in another liquid by the action of centrifugal force.
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Centriole.Minute rod-shaped body, 300-500 nm long and 150nm diameter, present in many resting cells, just outside the nuclear membrane. Cylindrical, containing nine short, parallel, peripheral rods each consisting of the beginning of next mitosis the two centrioles move apart and form the pole of the spindle and the centres of the asters when present. Absent. in higher plants. Similar structures, known variously as basal bodies, kineloplasts, kinetosomes, blepharoplasts, are connected to axial filaments of flagella and cilia, e.g. in sperm, zoospores.
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