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.'