Sieve-tube pores

Callose is a morphologically distinctive polysaccharide that occurs in granular form deposited around sieve plates and on the side of sieve-tube pores. Callose is a 3-1,3-linked D-glucan with no detectable branching (Aspinall, 1980). In higher plants, callose is deposited as a response to wounding or infection by microorganisms (Bacic et al., 1988). 

Sieve cells More primitive than sieve tube members and occur in nonflowering plants sieve cells contain clusters of pores, which are narrow and uniform in structure Translocation of sugars and other organic nutrients... 

Path and Sink Features. In the translocation path (e.g., stems and petioles), assimilates and solutes move in mass flow through the cylindrical sieve tubes which have open sieve pores. The ability of a chemical to leak across the membrane from the sieve tube during transit will affect its ability to be transported through the entire pathway. 

In the current case, slightly more than half of the hydrostatic pressure drop along the phloem is necessary to overcome the resistance of the sieve plate pores. When the end walls are steeply inclined to the axis of the sieve element, the pores of the sieve plate can occupy an area that is greater than the cross section of the sieve tube. This causes Jv in the pores to be less than in the lumen and tends to reduce the resistance to flow in the phloem. 

A notable feature of the development and differentiation of the cell walls in phloem tissues is the formation of pores connecting the adjacent sieve-tubes to each other and to the companion cells. At an early stage of development, walls of the sieve tubes are marked by parts of endoplasmic reticulum on both sides where the pore is to be formed. As the sieve plate develops and the wall thickens, normal materials are deposited on the cell wall, except in the areas below the endoplasmic reticulum these areas grow, instead, by the... 

In the development of phloem, the plasma membranes ends of the cells are joined by elaborate plasmodesmata, which make contact through channels in the cell wall. Callose is deposited around these and the sieve tube is the much specialised chain of cells that results, with residual, functional cytoplasm still present, connected across the pores of the sieve plates. Connections to companion cells also remain though the lateral walls and these must serve to sustain the c5hoplasm of the sieve tubes. 

The transport system of the sieve tubes is also a multiplet of generalized transport units (pTU) with the sieve plate pores as organismal capillaries. Though the sieve tubes are thin as such, the dependence of the hydraulic conductivity, on the fourth power of the radius, makes the distinction between them and the pores essential. The effects of occlusion on gw are difficult to calculate but the general features of the sieve tube system permit one to surmise that it has been made to cope with adverse hydrostatic gradients of variable but often large magnitudes. 

Inside forest trees under water stress, the plugging of sieve-plate pores with P-protein is an almost instantaneous reaction to pressure release in active sieve tubes. P-protein plugs, or slime plugs, are now thought of as the first line of defense against the loss of assimilates. Wound callose further strengthens the cell s defenses but at variable rates (44 cf.. Sect. 5.1.3.4). 

In CGE the capillary is filled with a gel containing cross-linked or linear polymers. The gel thereby acts as molecular sieve. Traditionally, cross-linked polyacrylamides and agarose have been utilized in the slab and tube format. Polyacrylamides when cross-linked have smaller pore sizes and are used for protein separations. The larger pore sizes of agarose gels are more suitable for DNA separation. Polyacrylamides yield very viscous gels. Therefore, polymerization is usually done on column, which has a lot of practical problems. 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Recently, carbon molecular sieves have been fabricated in the form of planar membranes and hollow tubes by the pyrolysis of polyacrylonitrile in suitable forms (12-16). Very high separation selectivities have been reported with these materials. Their pore sizes are in the range from 3 to 5.2A. Selectivities of greater than 100 1 are observed between molecules which differ by as little as 0.2A in their critical dimensions. Kinetics of adsorption on these materials have been determined (2.,ii,l ) -... 

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