7. Cytoskeleton


– It is a network of filamentous proteinaceous structures present in the cytoplasm.

– It provides mechanical support, motility, maintenance of the shape of the cell etc.

8. Cilia and Flagella


– They are hair-like outgrowths of the cell membrane.

– Cilia: Small structures which work like oars. Causes the movement of the cell or surrounding fluid.

– Flagella: Longer. Responsible for cell movement. Flagella of prokaryotes and eukaryotes are structurally different.

– Cilium and flagellum are covered with plasma membrane.

Their core (axoneme) has many microtubules running parallel to the long axis.

– The axoneme has 9 pairs of doublets of radially arranged peripheral microtubules and a pair of central microtubules.

This is called 9+2 array.

– The central tubules are connected by bridges and are enclosed by a central sheath.

-It is connected to one of the tubules of each peripheral doublet by a radial spoke.

-Thus, there are 9 radial spokes. The peripheral doublets are also interconnected by linkers.

– Cilium and flagellum emerge from centriole-like structure called the basal bodies.

9. Centrosome and Centrioles


– Centrosome is an organelle usually containing two non-membrane bound cylindrical structures called centrioles.

– They are surrounded by pericentriolar materials. – The centrioles lie perpendicular to each other.

-They are made up of 9 evenly spaced peripheral fibrils of tubulin.

– Each of the peripheral fibril is a triplet. The adjacent triplets are also linked.

– The central part of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.

– The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

10. Nucleus

– Nucleus was first described by Robert Brown (1831).

– The material of the nucleus stained by the basic dyes (acetocarmine) was given the name chromatin by Flemming.

-Normally, a cell has only one nucleus. Some cells have more than one called multinucleate (syncytial in case of animals and coenocytic in case of plants)

Some mature cells lack nucleus (enucleate). E.g. mammalian RBC and sieve tube cells of vascular plants.

– The interphase nucleus contains


o Nuclear envelope:

-Double layered membrane with a space between (10 – 50 nm) called perinuclear space.

-It is a barrier between the materials present in the nucleus & cytoplasm.

-Outer membrane usually remains continuous with the ER and also bears ribosomes on it.


-Nuclear envelope has minute pores formed by the fusion of its two membranes. These are the passages for the movement of RNA and protein between the nucleus and cytoplasm.

o Nuclear matrix (nucleoplasm):

-gelly like substance, a reservoir of nucleotides, nucleosides, histones, and non-histone

o Chromatin:

-A network of nucleoprotein fibres. It contains DNA and basic proteins (histones), non-histone proteins and RNA.

-During cell division, chromatins condense to form chromosomes.


o Nucleolus:

-One or more non-membranous spherical bodies.

It is continuous with the nucleoplasm.

-It is a site for ribosomal RNA synthesis.


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