cell and what the the different part elaboration

1. CELL JUNCTIONS
Specialized cell junctions occur at points of cell-cell and
cell-matrix contact in all tissues and they are particularly
plentiful in epithelia.
 Cell junctions may be classified into three functional
groups.
http://biology.kenyon.edu/edwards/project/greg/pd.htm

2. Occluding junctions:
I. Tight junctions (found only in vertebrates)
II. Septate junctions (invertebrates mainly)
Anchoring junctions:
Actin filaments attachment sites:
I. Cell-cell junctions (adherens junctions)
II. Cell-matrix junctions (focal adhesions)
Intermediate filaments attachment sites:
I. Cell-cell junctions (desmosomes)
II. Cell-matrix junctions (heterodesmosomes)
Communicating junctions:
I. Gap junctions
II. Chemical synapses
III. Plasmodesmata (Plants only)
Functional classification of cell junctions

4. Plasmodesmata perform many of the functions as
Gap junctions
 The tissues of the plants are organized on different principles from
those of an animal. This is because the plant cells are imprisoned
between rigid cell walls composed of an extra cellular matrix rich in
cellulose and other polysaccharides.
 The cell walls of adjacent cells are firmly cemented to those of their
neighbours which eliminates the need for the anchoring junctions to
hold the cell in place. But a need for direct cell-to –cell interaction
remains.
 Thus, plants have only one intercellular junctions called
plasmodesmata (singular: plasmodesma). Like gap junctions they
directly connect the cytoplasm of the adjacent cells.
 In plants however, the cell wall between a typical pair of adjacent cells
is atleast 0.1 μm thick and so a structure very different from gap
junctions is required to mediate communication across it.

5.  Plasmodesmata solve this problem.
 With a few specialized exceptions every cell in a higher plant is
connected to its neighbor by these structures which form fine
cytoplasmic channels through the intervening cell walls.

6. https://www.youtube.com/watch?v=dXsXSIjR1G0

7.  It has been suggested that plasmodesmata transports gibberellic
acid in algae.
 Electric coupling between cells has been associated with the density
of plasmodesmata between the cells, and electrical signals play a
large role in the initiation of a wound response in plants.
 Plasmodesmata have been shown to be sensitive to various levels of
calcium, pH, and light, demonstrating that the mechanisms of
plasmodesmata transport are more complex than previously
thought.
 Plasmodesmata tend to close when there is a significant pressure
difference between adjacent cells, again speaking to a role of
plasmodesmata in a wound response upon a loss of turgor pressure.

8.  The permanent opening or closing of plasmodesmata serve to
signify symplastic domains in plants, often compartmentalizing
various plant functions (a sort of division of labor), such as in guard
cells or in the sieve element-companion cell complex in stems.
 Plasmodesmata serve as directors of plant growth and development
and may help to determine a program of cell differentiation, such as
sealing off root and stem epidermal cells from the rest of the plant.
This notion has been supported by a study in which RNA encoding
the transcription factor KNOTTED1 (KN1) was localized to all cell
layers of the developing maize meristem except the outermost, but
the KN1 protein was found in all cell layers (reviewed by
Zambryski, 1995). Thus, KN1 is most likely being transported to the
outer layer through plasmodesmata, providing a channel through
which this and other important signalling and regulatory molecules
can pass into other cells.