what is the composition of pyrenoids????

Pyrenoids are the spherical protein structure found within chloroplasts of certain algae and hornworts. They contain protein besides starch. Each pyrenoid has a cenral protein called ‘pyreno – crystal’ and a surrounding starch sheath. They serve the purpose of storing starch.

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 Usually a single, centrally located pyrenoid is present in the chloroplasts of C.

pyrenoidosa. Unlike the pyrenoids of diatoms described by Drum & Pankratz (1964)
and Holdsworth (1968) there are no specialized membranes found around the pyrenoids
of C. pyrenoidosa. Starch is commonly present surrounding the pyrenoid matrix
(Figs. 1-5). In the vast majority of cells we examined the pyrenoid matrix appeared to
be granular and homogeneous.
Occasionally we observed a crystalline structure of the pyrenoid that seems to be
present throughout the matrix (Fig. 1). This crystalline structure appears as parallel
lines with a centre-to-centre spacing of approximately 8-o nm. At certain points 2 sets
of parallel lines intersect, giving a criss-crossed appearance to some areas of the pyrenoid matrix (Fig. 3). The angle of intersection is approximately 80°. Measurements
were made on an enlargement of Fig. 3, and the values stated are averages of the total
number of measurements we made in each case.
The two types of lattices in the pyrenoid have been shown to be dependent on the
orientation of the crystal with reference to the cutting plane (Holdsworth, 1968). The
angle of intersection of the 2 sets of parallel lines and the centre-to-centre spacing of
the parallel lines in the crystalline lattice seen in Fig. 3 fall within the range of values
reported by Holdsworth (1968).
In one of the cells we found a crystal-like body in the pyrenoid (Fig. 2). This observation is of particular interest as it suggests that the entire pyrenoid matrix need not
be crystalline in nature. It is reasonable to assume that there are one or more crystalline areas in the pyrenoid matrix. This may answer the question of why the crystalline
nature of a pyrenoid is not observed in all cases. The recognition of the crystalline
structure will depend on the portion of the pyrenoid that is sectioned, as well as the
plane of sectioning. Holdsworth (1968) has estimated from serial sections that there
may be as many as 10-15 crystalline and/or non-crystalline regions composing the
3-dimensional structure of a pyrenoid from Achnanthes breviceps.Fine structure of Chlorella pyrenoids 625
It is also reasonable to assume that the crystal-like body in the pyrenoid matrix is
made up of protein. The presence of protein crystals in plastids of higher plants has
been reported by different investigators (Perner, 1963; Manton, 1966a; Newcomb,
1967; Shumway, Weier & Stocking, 1967). It has been suggested by Sager & Palade
(1957) that the function of pyrenoids in the lower plants is taken over by non-specialized
regions of the chloroplasts in higher plants. Assuming the pyrenoids as sites of
starch and/or protein storage, this suggestion seems reasonable in light of our knowledge
of chloroplast and pyrenoid structure. The significance of the close association of the
crystalline body with the hylakoid lamellae which traverse the pyrenoid is not clear.
Frequently a single thylakoid lamella or a pair of closely apposed lamellae traverses
the pyrenoids (Fig. 4). It is not uncommon, especially among other algae, to find
multiple lamellae in the pyrenoid matrix (Manton, 1966a). In one cell we have
observed 2 pairs of lamellae in the pyrenoid (Fig. 5). Although they run separately
through the pyrenoid matrix, they are actually continuous with one another outside
the pyrenoid. The significance, if any, of this is presently not understood. Nevertheless,
it suggests the possibility that the multiple lamellae that are occasionally seen in
pyrenoids of C. pyrenoidosa may actually be sections of a single lamella resulting from
the plane of sectioning.
Although much more evidence is needed to propose that at least some portions of all
pyrenoids contain a crystalline lattice, recent investigations of pyrenoid ultrastructure
in 3 different groups of algae suggest such a possibility. However, the physiological
state of the organism at any given time should also be considered as a criterion for the
presence or absence of a crystalline lattice in the pyrenoid matrix. There is also evidence from selective staining and fluorescent microscopy that pyrenoids contain
protein (Bose, 1941). The crystalline structure of the pyrenoid matrix observed by us
and other investigators (Holdsworth, 1968; Kowallik, 1969) further supports this
evidence. Alack of understanding of the exact function or functions of pyrenoids makes
a more rational interpretation of the pyrenoid ultrastructure difficult at the present
time. It has been aptly expressed by Manton (1966a) when she said 'it is perhaps a
valuable indicator of our ignorance about many matters in which a cytologist must
turn to an experimental biochemist for guidance. Granted that photosynthesis is the
flywheel of the whole organic world, we need to know not only how a chloroplast is
constructed and how it works photosynthetically but also much more than we know
at present of the nature of its distribution products, and of the distribution system or
systems which convey these from the factory to construction sites elsewhere in the
cell.'
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