Synthesis of sucrose and polisaharidov. Starch and cellulose synthesis

you are: Synthesis of sucrose and polisaharidov

Long polymeric chains of starch and cellulose are constructed of the same elementary links - the rests of glucose only connected differently. This structural distinction causes that two considered polymers of glucose (gljukany) essentially differ by the nature; starch, for example, easily is digested in a human body, and cellulose is not digested at all. their main distinction consists that 1st and 4th carbon atoms of two next rests of glucose are connected at starch by a-communications, and at cellulose b-communications. Starch is presented by two forms: linear polymer, or amilozoj, not containing any other communications, except a-1,4-glikozidnyh, and the branched out polymer, or amilopektinom in which along with a-1,4-gli-kozidnymi communications there are also 1,6-communications. Distinction in character of communications defines also an unequal spatial arrangement of polymeric chains. Starch - main spare polisaharid plants. It is insoluble in water and otlagaetsja a layer behind a layer in the starched grains containing in hloroplastah (fig. 2.20 see) or in the deprived chlorophyll lejkoplastah reserving fabrics of a stalk, roots and seeds. Sometimes cages of a reserving fabric appear are hammered by starched grains which are easy for revealing in Them as they are capable to be painted iodom in dark blue colour literally. Being it is insoluble in water, starch unlike sucrose and from geksoz does not cause in cages of osmotic effect (see gl. 6). Therefore starch formation in sheet cages during the periods of intensive photosynthesis prevents the suppression of the last resulting accumulation of products of photosynthesis. In the dark starch gradually is again hydrolyzed with formation gljukozofosfata which then turns to the sucrose transported in other parts of a plant.

As the Initial product for starch synthesis serves adenozindifosfogljukoza (ADPG), formed of ATR and G-1-P:

The starch Molecule is under construction by gradual addition of one glucosic rest behind another in reaction ADPG with the preformed glucosic chain:

At the low maintenance of sucrose starch is split and, translated in sucrose. however at first it is split to the rests of glucose and the rest joins each of them, phosphoric acid, i.e. It is formed G-1-P, than it is provided, communication conservation of energy:

This G-1-P can then be used for synthesis of sucrose which we have described above. In seeds and in some.

Other bodies to which simultaneously there is a splitting of considerable quantities of starch, it breaks up to disaharida maltozy (G-G) under action a-amilazy. Maltoza then breaks up to glucose from which (for transport) sucrose again is synthesised. On this second way unlike the first energy of communication does not remain, therefore here for glucose transformation in gljukozo-6-R it is required ATR.

Cellulose, the most widespread on the Earth carbohydrates, serves as the main component of a primary cellular wall. Its molecules are under construction just as starch molecules, with that are under construction, however, difference that the role of the donor of glucose plays another nukleotidnoe derivative monosaharida - guanozin-difosfogljukoza (GDPG) - and that communication between monomeasured links belongs not to and - and to b-type.

In certain cases the donor of glucose for cellulose synthesis can be and UDPG. In an organism of the higher plants cellulose is split seldom (short of the disintegration caused by activity of microbes). Two known exceptions of this rule concern cages in otdelitelnoj to the zone of sheet formed before sbrasyvaniem of leaves, and vessels ksilemy at which cross-section walls are dissolved. In otdelitelnoj to a zone of sheet enzyme shchelljulaza destroys cellular walls, splitting cellulose containing in them to separate monomeasured units, i.e. To glucose. The cellular walls weakened by this process, eventually are broken off, and the sheet is dumped.

Cellulose mikrofibrilly in a cellular wall are fastened with the help matriksa from mixed polisaharidnyh chains, mainly ksilogljukanov and arabinogalaktanov (fig. 2.31 see). (Ksiloza and arabinoza - five-carbon sugar (pentozy), and galaktoza - geksoza, related to glucose.) these polisaharidy are synthesised also from predecessors, nukleotidsaharov, mainly in diktiosomah. otshnurovyvajushchiesja from diktiosom vials eventually merge with plazmalemmoj and such way transfer the contents to a formed cellular wall.

So, all polisaharidy easily pass one in another, but their synthesis always goes through a stage nukleotidsaharov whereas disintegration is made by more direct way.