iklan 2
Plasmodesmata
Plasmodesmata-Neighboring cells are
normally connected by plasmodesmata thrusting through the cell walls. The
plasmodesmata allow mostly the passage of mollecules up to a molecular mass of
about 800 to 900 Dalton. They are permeable to the various intermediates of
metabolism such as soluble sugars. Amino acids, and free nucleotides. A single
plant cell may contain from 1000 to more than 10000 plasmodesmata. These
plasmodesmata connect many plant cell to form a single large metanolic
compartmen where the metabolites in the cytosol can move between the various
cells by diffution. This continuous compartment formed by different plant cells
is called the symplast. In contrast, the space between cells, which are often
continuous, are termed the extracellular space of the apoplast.
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| Plasmodesmata |
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| Plasmodesmata |
The figure aboveshows a
diagram of plasmodesm. The tubelike opening through the cell wall is lined by
the plasma membrane, which is continuous between the neighboring cells. In the
interior of this tube there is another tubelike membrane structure, which is
part of he endoplasmatic reticulum (ER) of the adjacent cells. In this way the
ER system of the entire symplast represents a continuity. The space between the
plasma membrane and the ER membrane forms the diffution pathway between the
cytosol of adjacent cells. Protein particles, which are connected to each
other, are attached to the outer tube formed by the plasma membrane and the ER
membrane. It is assume that the free space between these protein particles
determines the aparture of the plasmodesm. A number of plant viruses, including
the Tobacco mosaic virus, cause the
synthesis of virus movement proteins, which can alter the plasmodesmata to such
an extent that viral nucleic acids bound to the movement protein can slip
through. Thus, after infecting a single cell, a virus can spread over the
entire symplast. In the widening process of the plasmodesmata by virus movement
proteins, the cytoskeleton appears to be involved. There are indications that
this represents a general transport process of which the virus take advantage.
It is presumed that the cell’s own movement proteins, upon the consumption of
ATP, facilitate the transfer of macromolecules, such as RNA and proteins, from
one cell to the next via the plasmodesmata. In this way, for example,
transcription factors might be distribute as signals in a regulated mode via
the symplast.
The plan cell wall can
be lysed by cellulose and pactin hydrolyzing enzymes obtain from microorganisms.
When leaf pieces are incubated with these enzymes, plant cells that have lose
their surrounding cell wall can be obtained. These naked cells are called
protoplast. Protoplast, however, are stable only in an isotonic medium in which
the osmotic pressure corresponds to the osmotic pressure of the cell fluid. In
pure water the protoplast, as they have no cell wall, swell so much that they
burst. In appropriate media, the protoplasts of some plants are viable, they
can be propagated in cell culture, and they can be stimulated to form a cell
wall and even to regenerate a whole new plant.
iklan 3
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