It has been duly observed that there exist a good number of‘reserve materials’
located in the prokaryotic cells and are invariably known as thegranular cytoplasmic inclusions. Thethree most vital and important organic cellular reserve materials present in the prokaryotes are namely
Salient Features.The salient features of the organic cellular reserve materials present in the
prokaryotesare as stated under :
(1)Poly-β-hydroxybutyric acid. It is found exclusively in the prokaryotes and invariably caters
as an equivalent of lipoidal content duly stored in the eukaryotic cells. It is observed in
several species ofAzotobacter, bacilli, and pseudomonads. Interestingly, certain specific organisms
viz.,purple bacteria has the ability to synthesize even two types of reserve materials
(e.g., glycogen and poly-β-hydroxybutyrate) simultaneously.
(a) Visibility — These organic cellular reserve materials are found to be deposited almost
uniformly very much within the cytoplasm ; however, they may not be detected under a
light microscope unless and until these are duly stained.
(b) Cellular content — The actively ‘growing cells’ do have these reserve materials present
in rather small quantum in the cellular content ; whereas, they get usually accumulated
exclusively in theC-rich culture medium under the influence of restricted amounts of
(c) Availability — These reserve materials may sometimes represent even upto 50% of the
total cellular content on dry weight basis.
(d) Utility — These reserve materials are fully utilized when the prevailing cells are adequately
provided with a suitable source of N and the growth is resumed subsequently.
(2)Glycogen and Starch — It has been duly established that the synthesis of glycogen and
starch is usually accomplishedvia a proven mechanism for storing C in a form which isosmotically inert ; whereas, in the particular instance of poly-β-hydroxybutyric acid it precisely
designates a method ofneutralizing an acidic metabolite.
(3)Cyanophycine (a copolymer of arginine and aspartic acid) :
In general,prokaryotes fail to store particularly the organic nitrogenous materials, but the
blue-green bacteriais expected which essentially accumulate a nitrogenous reserve material
termed ascyanophycine. It invariably represents as much as 8% of cellular dry weight; and
may be regarded as a copolymer of arginine and aspartic acid.
(4)Volutin (metachromatic) Granules. A plethora of prokaryotes acquire more and more of
volutingranules that may be stained meticulously with a ‘basic dye’, for instance : methylene
blue.In fact, these prokaryotes appear as red on being stained with a ‘blue-dye’. Importantly,
the prevailing metachromatic nature of the ensuing ‘red complex’ is on account of the
very presence of a substantial quantum of‘inorganic phosphates’. Evidently, the actual
accumulation of these substances in the prokaryotes takes place under critical parameters of
starvation specifically during‘sulphate starvation’. It has been observed that these instantly
generated volutin granules disappear as soon as the cells are adequately made available with
a‘sulphur source’, and subsequently the phosphate moiety [PO4
3–] is incorporated strategically
into the nucleic acidsi.e., DNA and RNA. From the above statement of facts one may
vividly infer that the‘volutin granules’ definitely represent particularly the ‘intracellular
phosphate reserve’when the desired nucleic acid synthesis fails to materialize.
(5)Sulphur Bacteria [e.g., photosynthetic purple sulphur bacteria ; and filamentous nonphotosynthetic
bacteria(viz., Baggiatoa and Thiothrix)]. The aforementioned two sulphur
bacteria specifically help in the accumulation of‘Sulphur’ transiently in the course of hydrogensulphide [H
(6)Thylakoids. These are solely present in the blue-green bacteria and are intimately involved
in the phenomenon of photosynthesis. Besides, there arethree prominent structures, namely :
gas vesicles, chlorobium vesicles,and carboxysomes, that are critically bound by non-unit
membranes have been reported to be present in certainphotosynthetic organisms.
(7)Ribs. There are several aquatic prokaryotes essentially containing gas vacuoles that are
intimately engaged in counter-balancing the prevailing gravitational pull appreciably. On
being examined under a ‘light microscope’ the ensuing gas vacuoles do look likedense
refractile structurehaving a distinct irregular peripheral boundary. Importantly, with a certain
surge in the hydrostatic built-up pressure the existing gas vacuoles collapse thereby the
cells lose their buoyancy eventually. Precisely, each gas vesicle more or less has an appearance
very much akin to a‘hollow cylinder’ having an approximate diameter of 75 nm with
distinct conical ends, and a length ranging between 200 and 1000 nm. These conglomerates
of gas vesicles are usually surrounded by a layer of protein approx. 2 mm thick. These
structures do possess several bands consisting of regular rows of subunits that almost run
perpendicular to the axis, and are termed as‘ribs’. The ribs are found to be impermeable to water.
(8)Photosynthetic Apparatus. The photosynthetic apparatus present specifically in the photosynthetic
green bacteria(chlorobium) possesses a distinct strategic intracellular location.
It is usually bound by a series ofcigar-shaped vesicles arranged meticulously in a
corticle-layer which immediately underlies the cell membrane as illustrated in Fig. 2.17.
Interestingly, these structures have a width nearly 50 nm, length varying between 100–150
nm and are delicating enclosed within a single layered membrane of thickness ranging between
3–5 nm. They essentially and invariably contain the‘photosynthetic pigments’.
(9)Carboxysomes. It has been amply demonstrated that a good number of photosynthetic and
chemolithotrophicorganisms, namely : blue-green bacteria, purple bacteria, and thiobacilli
essentially comprise of polyhedral structures having a width of 50–500 nm and carefully
surrounded by a single layer of membrane having a thickness of 3.5 nm approximately.
These characteristic structures are known ascarboxysomes. They are found to consist of
certain key enzymes that are closely associated with and intimately involved in the critical
fixation of carbon dioxide [CO2], such as : carboxy dismutase ; and thus, represent the
precise and most probable site of CO2 fixation in the photosynthetic as well aschemolithotrophic organisms.