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• Was there a darkly coloured, spherical present in eukaryotic cells. Many of the
or oval, dot-like structure near the functions of such organelles are also
centre of each cell? This structure is performed by poorly organised parts of the
called nucleus. Were there similar cytoplasm (see section 5.2.4). The chlorophyll
structures in onion peel cells? in photosynthetic prokaryotic bacteria is
The nucleus has a double layered covering associated with membranous vesicles (bag
called nuclear membrane. The nuclear like structures) but not with plastids as in
membrane has pores which allow the transfer eukaryotic cells (see section 5.2.5).
of material from inside the nucleus to its
outside, that is, to the cytoplasm (which we
will talk about in section 5.2.4). Plasma Ribosomes
The nucleus contains chromosomes, membrane
which are visible as rod-shaped structures
only when the cell is about to divide. Cell wall
Chromosomes contain information for
inheritance of characters from parents to next
Nucleoid
generation in the form of DNA (Deoxyribo
Nucleic Acid) molecules. Chromosomes are
composed of DNA and protein. DNA molecules
contain the information necessary for
constructing and organising cells. Functional Fig. 5.4: Prokaryotic cell
segments of DNA are called genes. In a cell
which is not dividing, this DNA is present as
part of chromatin material. Chromatin 5.2.4 CYTOPLASM
material is visible as entangled mass of thread
like structures. Whenever the cell is about to When we look at the temporary mounts of
divide, the chromatin material gets organised onion peel as well as human cheek cells, we
into chromosomes. can see a large region of each cell enclosed
by the cell membrane. This region takes up
The nucleus plays a central role in cellular
very little stain. It is called the cytoplasm.
reproduction, the process by which a single
cell divides and forms two new cells. It also The cytoplasm is the fluid content inside the
plays a crucial part, along with the plasma membrane. It also contains many
environment, in determining the way the cell specialised cell organelles. Each of these
will develop and what form it will exhibit at organelles performs a specific function for the
maturity, by directing the chemical activities cell.
of the cell. Cell organelles are enclosed by
In some organisms like bacteria, the membranes. In prokaryotes, beside the
nuclear region of the cell may be poorly absence of a defined nuclear region, the
defined due to the absence of a nuclear membrane-bound cell organelles are also
membrane. Such an undefined nuclear region absent. On the other hand, the eukaryotic
containing only nucleic acids is called a cells have nuclear membrane as well as
nucleoid. Such organisms, whose cells lack membrane-enclosed organelles.
a nuclear membrane, are called prokaryotes The significance of membranes can be
(Pro = primitive or primary; karyote ≈ karyon illustrated with the example of viruses.
= nucleus). Organisms with cells having a Viruses lack any membranes and hence do
nuclear membrane are called eukaryotes. not show characteristics of life until they enter
Prokaryotic cells (see Fig. 5.4) also lack a living body and use its cell machinery to
most of the other cytoplasmic organelles multiply.
54 SCIENCE
Rationalised 2023-24
