The eukaryotic cells possess a skeletal system called cytoskeleton that has got analogous function.

The cytoskeleton is composed of 3 well defined filamentous structurs – microtubules, microfilaments and intermediate filaments with distinct functions .

Each filaments are made of protein subunits held together by weak non covalent bonds.

This type of construction allows rapid assembly and disasembly contolled by cell regulation

MICROTUBULESThey are components of a diverse array of substances including the mitotic spindles of dividing cells and core of flagella and cilia

STRUCTURE AND COMPOSITIONHave an outer diameter of 25nm and a wall thickness of 4nm and may extend across the length and breadth of the cellThe wall of microtubule is composed of globular proteins arranged in longitudinal rows called protofilaments that are allinged parallel to the long axis of the tubule .

When veiwed in cross section they are seen to have 13 protofilaments allinged side by side in a circular pattern

Each protofilament is assembled from dimeric building blocks consisting of one alpha and one beta subunits

The protofilament is asymmetric with alpha subunit on one end and beta on the other

One end of the protofilament is known as the plus end is terminated by a row of beta tubulin units and the mnus end is terminated by the alpha tubulin units

1. ACT AS STRUCTURAL SUPPORT AND ORGANIZERS

They are stiff enough to resist the forces that can bend or compress the fibre

The distribution of microtubules through the cytoplasm of a cell determines the shape of a cell eg: in coloumnar epithelial cells the microtubules are alligned along the axis of the cell

Maintain a key role in the internal organization of a cell

2.ACT AS AGENTS OF INTRACELLULAR MOTILITY

Involved in the movement of

vesicles,proteins,organellsetc across the cytoplasm throught the cell

Eg: AXONAL TRANSPORT: proteins such as neurotransmittors are secreated and packed in membranous vesicles by golgi body and endoplasmic reticulum of the cell body are transported through the axon which consists of a number of of microtubules and motor proteins which takes it down the axon

MOTOR PROTEINS : they convert chemical energy into mechanical

energy that is used to generate force or move the Types of cargo include vesicles,chromosomes , mitochondria, proteins etc

They can be classified into 3 types mainly : kinesins and dyneins that move along the microtubules and myosin that move along microfilaments

The binding of ATP and its hydrolysis provides energy to them to travel

cargo attached to the motor

KINESINS: is a tetramer constructed by 2 identical heavy chains and 2 identical light chains, has a globular head that binds ATP ,a neck a stalk and a fan shaped tail that binds to the cargo to be transported

DYENEINS: it is a huge protein composed of two identical heavy chains and a variety of intermediate and light chains. They are responsible for the movement of cilia and flagella .move towards the minus end of the microtubule. They act as:

As a force-generating agent in positioning the spindle and moving chromosomes during mitosis

As a minus end–directed microtubular motor with a role in positioning the centrosome and Golgi complex and moving organelles,vesicles,and particles through the cytoplasm

MICROTUBULE ORGANIZING CENTRE

1. CENTROSOME: In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring.

• the centroles are surrounded by an electron rich pericentriolar matrix

Microtubules are major components of spindle fibre used to pull apart chromosomes during cell division

2. BASAL BODIES the outer microtubules in a cillia or flagella

arise from basal bodies attached to the base of cillia or flagella

Microtubules are the central structural supports in cilia and flagella.Both can move unicellular and small

multicellular organisms by propelling water past the organism.

If these structures are anchored in a large structure, they move fluid over a surface.

For example, cilia sweep mucus carrying trapped debris from the lungs.

A flagellum has an undulatory movement.Force is generated parallel to the flagellum’s

axis.

Fig. 7.23a

Fig. 7.23b

Cilia move more like oars with alternating power and recovery strokes.They generate force perpendicular to the cilia’s

axis.

In spite of their differences, both cilia and flagella have the same ultrastructure.Both have a core of microtubules sheathed by

the plasma membrane.Nine doublets of microtubules arranged around

a pair at the center, the “9 + 2” pattern.Flexible “wheels” of proteins connect outer

doublets to each other and to the core.The outer doublets are also connected by motor

proteins.The cilium or flagellum is anchored in the cell

by a basal body, whose structure is identical to a centriole.

The bending of cilia and flagella is driven by the arms of a motor protein, dynein.Addition to dynein of a phosphate group from

ATP and its removal causes conformation changes in the protein.

Dynein arms alternately grab, move, and release the outer microtubules.

Protein cross-links limit sliding and the force is expressed as bending.

Fig. 7.25

microfilaments the thinnest class of the cytoskeletal

fibers, are solid rods of the globular protein actin.An actin microfilament consists of a twisted

double chain of actin subunits.Microfilaments are designed to resist

tension.With other proteins, they form a three-

dimensional network just inside the plasma membrane.

In muscle cells, thousands of actin filaments are arranged parallel to one another.

Thicker filaments, composed of a motor protein, myosin, interdigitate with the thinner actin fibers.Myosin molecules walk along the actin filament,

pulling stacks of actin fibers together and shortening the cell.

Fig. 7.21a

In other cells, these actin-myosin aggregates are less organized but still cause localized contraction.A contracting belt of microfilaments divides the

cytoplasm of animals cells during cell division.Localized contraction also drives amoeboid

movement. Pseudopodia, cellular extensions, extend and

contract through the reversible assembly and contraction of actin subunits into microfilaments.

Fig. 7.21b

In plant cells (and others), actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming.This creates a circular flow of cytoplasm in the

cell.This speeds the distribution of materials within

the cell.

Fig. 7.21c