Biology Today — May 2017

CYTOSKELETAL STRUCTURES

• They are minute, fibrous and tubular structures that form the structural framework inside the cell and maintain shape of cell,
regulate orientation and distribution of cell organelles, intracellular transport and cellular movement.
Cytoskeletal Structures

Actin

monomer

6 nm

Microfilament

Microfilaments

•    Long, narrow protein filaments,

made up of actin, occur in plant

and animal cells.

•    Often associate to form hexagonal

bundles.

•    Generally occur below plasma

membrane and are connected with

spindle fibres, ER, chloroplasts, etc.

•    Associated with cleavage furrow at

the time of cytokinesis in animal

cells.

•    Present in myofibrils of muscle

fibres.

•    Form contractile machinery of

cell which aids in motility like

pseudopodia, plasma membrane

undulations, endocytosis, cytoplasmic

streaming and movement of cell

organelles, microvilli formation,

etc.

8-10 nm

Intermediate filament

Intermediate filaments

•    Unbranched, about 10 nm thick, made up

of variety of proteins often arranged in a

network.

•    They are of four types:

(a) Keratin filaments - Form tonofibrils

of desmosomes and keratin of skin

(b) Neurofilaments - Form lattice with

bundles of microtubules in axons and

dendrons of nerve cells to provide strength

(c) Glial filaments - Found in astrocytes

(d) Heterogeneous filaments - Found in

muscles, around nucleus and connected

to centriole, plasmalemma, etc.

•    Provide support to all biomembranes

including plasma membrane and nuclear

membrane, sarcomeres, desmosomes.

•    Also provide mechanical strength to

axons and dendrons of nerve cells and

astrocytes.

Three types are

Microtubules

•    Unbranched, hollow tubules of protein

tubulin.

•    Present in cytoplasm and is constituent

of spindle fibres, chromosome, centrioles,

basal bodies, flagella, cilia, sperm tail,

etc.

•    Diameter is 25 nm with lateral projection

of 100 – 400A° length.

•    Help in intracellular transport, movement

of nuclei during division, movement

of chromosomes during anaphase and

movement of cilia and flagella.

•    Determine the place of cell plate

formation.

•    Play a vital role during differentiation.

Section of a

microtubule

Protofilaments

a and b tubulin

heterodimers

a-tubulin b-tubulin

FLAGELLA AND CILIA
• They are hair-like microscopic locomotory structures.

• Both are structurally similar and have similar parts:

• Basal body or Kinetosome - It is also called basal granule. Basal body occurs embedded in the outer part of
  the cytoplasm below the plasma membrane. It has nine triplet fibrils present on the periphery without a central fibril,
  though a hub of protein is present. Only sub-fibre A is complete (having 13 protofilaments) while sub-fibres B and C
  are incomplete as they share some of their protofilaments.


• Rootlets - They are striated fibrillar outgrowths which develop from the outer lower part of the basal body and are
  meant for providing support to the basal body. The rootlets are made of bundles of microfilaments.


• Basal plate - It is an area of high density which lies above the basal body at the level of plasma membrane. In the
  region of basal plate, one sub-fibre of each peripheral fibril disappears. The central fibrils develop in this area.


• Shaft - It is the hair-like projecting part of flagellum or cilium. The shaft is covered on the outside by a sheath which
  is the extension of plasma membrane. Internally, it contains a semifluid matrix having an axoneme of 9 peripheral
  doublet fibrils and 2 central singlet fibrils. This arrangement is called 9 + 2 or 11-stranded. The two central singlet fibres
  are covered by a proteinaceous central sheath. They are connected by a double bridge. Each peripheral fibril consists
  of two microtubules or sub-fibres B and A. The sub-fibre A bears two bent arms, the outer one having a hook made
  up of protein dynein with ATPase activity. Movement of flagella or cilia occurs due to sliding motion in which dynein
  arm establishes temporary connection with subtubule B of adjacent doublet fibre. The peripheral doublet fibrils are
  interconnected by A-B linkers of protein nexin between B sub-fibre of one and inner side arm of A sub-fibre of adjacent
  fibril. Each of their A sub-fibres sends a radial proteinaceous column to the centre which is called spoke.