Biology CELL - THE UNIT OF LIFE

Cytoskeleton

In eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system of membranes. These elements collectively form cytoskeleton of the cell. There are of three types.

# (1) Microtubules :
(i) Discovery : These were first discovered by De Robertis and Franchi (1953) in the axons of medullated nerve fibres and were named neurotubules.
(ii) Position : The microtubules are electron-microscopic structures found only in the eukaryotic cellular structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres, sperms tail, neuraxis of nerve fibres etc. These are absent from amoebae, slime-moulds and prokaryotes.
(iii) Structure : A microtubule is a hollow cylindrical structure of about 250 Å in diameter with about 150 Å luman. Its wall is about 50Å thick. Its walls is formed of 13 parallel, proto-tubules, each being formed of a liner series of globular dimeric protein molecules.
(iv) Chemical composition : These are mainly formed of tubulin protein. A tubulin protein is formed of 2 sub-units : tubulin molecule and tubulin molecule which are alternatively in a helical manner.
(v) Function
(a) These form a part of cytoskeleton and help in cell-shape and mechanical support.
(b) The microtubules of cilia and flagella help in locomotion and feeding.
(c) The microtubules of asters and spindle fibres of the mitotic apparatus help in the movement of chromosomes towards the opposite poles in cell-division.
(d) These help in distribution of pigment in the chromatophores, so help in skin colouration.
(e) These also form micro-circulatory system of the cell which helps in intracellular transport.
(f) These control the orientation of cellulose microfibrils of the cell wall of plants.

# (2) Microfilament
(i) Position : These are electron-microscopic, long, narrow, cylindrical, non-contractile and proteins structures found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc. But these are absent from the prokaryotes. These are also associated with the pseudopodia, plasma membrane of fibroblats, etc. These are either scattered or organized into network or parallel arrays in the cytoplasmic matrix.
(ii) Discovery : These were discovered by Paleviz et. al. (1974).
(iii) Structure : Each microfilament is a solid filament of 50-60 Å diameter and is formed of a helical series of globular protein molecules. These are generally grouped to form bundles.
(iv) Chemical composition : These are mainly formed of actin-protein.
(v) Functions
(a) The microfilaments forms a part of cytoskeleton to support the relatively fluid matrix.
(b) The microfilaments bring about directed movements of particles and organelles along them in the cell.
(c) The microfilaments also produce streaming movements of cytoplasm.
(d) The microfilaments also cause cleavage of animal cells which is brought about by contraction of a ring of microfilaments.
(e) The microfilaments also participate in gliding amoeboid motion shown by amoebae, leucocytes and macrophages.
(f) The microfilaments are also resoponsible for the change in cell shape curing development, motility and division.
(g) Myofilaments bring about muscle contraction.
(h) The microfilaments cause movements of villi to quicken absorption of food.
(i) The microfilaments are responsible for the movement of cell membrane during endocytosis and exocytosis.
(j) The microfilaments cause plasma membrane undulations that enable the firoblasts to move.

# (3) Intermediate filaments
(i) Location : They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells. They are missing in mammalian RBCes and in the prokaryotes.
(ii) Structure : The IFs are somewhat larger than the microfilaments and are about 10 nm thick. They are solid, unbranched and composed of nonmotile structural proteins, such as keratin, desmine, vimentin.
(iii) Functions
(a) They form a part of cytoskeleton that supports the fuild cytosol and maintains the shape of the cell.
(b) They stabilize the epithelia by binding to the spot desmosomes.
(c) They form major structural proteins of skin and hair.
(d) They integrate the muscle cell components into a functional unit.
(e) They provided strength to the axons.
(f) They keep nucleus and other organelles in place.

Cilia and flagella

(1) Discovery : Flagellum presence was first reported by Englemann (1868). Jansen (1887) was first scientist to report the structure of sperm flagellum.

(2) Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly from the basal body and help in movements, locomotion, feeding, circulation etc.

(3) Occurrence : Cilia are found in all the ciliate protozoans e.g., Paramecium, Vorticella etc. flame cells of flat worms; in some larval forms e.g., Trochophore larva of Nereis, Bipinnaria larva of starfish etc.; in some body structures e.g. wind-pipe, fallopian tubes, kidney-nephrons etc.
Flagella are found in all the flagellate protozoans e.g., Euglena, Trichonympha etc., collar cells of sponges; gastrodermal cells of coelenterates; spermatozoa of animals and lower plants; zoospores of algae etc. These are absent in red algae, blue-green algae, angiosperms, nematodes, arthropodes etc.

(4) Flagella are 1 – 4 per cell where as cilia are infinity in number.

(5) Cilia are smaller and flagella are longer in size, 5 – 10 and 150 respectively.

(6) Structure : Both cilia flagella are structurally similar and possess similar parts-basal body, rootlets, basal plate and shaft
(i) Basal body : These are also termed as blepharoplast (kinetosome) or basal granule. It is present below the plasma membrane in cytoplasm. The structure is similar to centriole made of 9 triplets of microtubules. Out of the 3 fibrils of a triplet first is A which is round and other two B and C are semi-circular. 9 triplets are connected to the centre by spokes. ‘C’ fibrils disappears as it enters into shaft.
(ii) Rootlets : Made of microfilament and providing support to the basal body. These are striated fibrillar outgrowths.
(iii) Basal plate : Central fibril develop in this area. It is highly dense and lie above plasma-membrane.
(iv) Shaft : It is the hair like projecting part of cilia and flagella which remains outside the cytoplasm. It has 9 duplets of microtubules in radial symmetry. These are called axonema. Each axonema has 11 fibrils, 9 in the periphery and 2 in the centre. The arrangement is called 9 + 2 pattern. Central fibrils are singlet fibrils and covered by a central sheath. 9 pheripheral fibrils are duplet and are present at 10o difference from each other. Inner fibril of duplet is known as subfibre A with two bent arms and the outer one is subfibre-B. Peripheral fibrils are linked with each other by peripheral linkage and with the central fibril by radial linkage.

(7) Chemical composition : Chemically, the central tubules are formed of dynein protein while the peripheral microtubules are formed of tubulin protein. Dynein is the ATPase enzyme which hydrolyses the ATP to provide free energy for ciliary /flagellar beating. The interdoublet linkers are formed of nexin protein. Quantitatively, it is formed of
Proteins = 74 – 84% Lipids = 13 - 23%
Carbohydrates = 1 – 6% Nucleotides = 0.2 – 0.4%

(8) Type of flagella : There are two types of flagella.
(i) Tinsel – type : In this, flagellum has lateral hair-like processes, called flimmers or mastigonemes.
(ii) Whiplash – type : In this, flagellum has no flimmers.

(9) Motion : Cilia beat in coordinated rhythm either simultaneously (synchronus) or one after the other (metachronic rhythm). The cilia
produce a sweeping or pendular stroke. The flagella beat independently, hence produce undulatory motion.

(10) Function
(i) They help in locomotion, respiration, cleaning, circulation, feeding, etc.
(ii) Being protoplasmic structure they can function as sensory organs.
(iii) They show sensitivity to changes in light, temperature and contact.
(iv) Ciliated larvae take part in dispersal of the species.
(v) The cilia of respiratory tract remove solid particles from it. Long term smoking damages the ciliated epithelium, allowing dust and smoke particles to enter the long alveoli.
(vi) The cilia of urinary and genital tracts drive out urine and gametes.

Centrosome /Centrosome

(1) Discovery : Centrosome was first discovered by Van Benden (1887) and structure was given by T. Boveri.

(2) Occurrence : It is found in all the animal cell except mature mammalian RBC’s. It is also found in most of protists and motile plant cells like antherozoids of ferns, zoospores of algae and motile algal forms e.g., Chlamydomonas but is absent in prokaryotes, fungi, gymnosperms and angiosperms.

(3) Structure : Centrosome is without unit membrane structure. It is formed of two darkly stained granules called centrioles, which are collectively called diplosome. These centrioles are surrounded by a transparent cytoplasmic area called centrosphere or Kinetoplasm. Centriole and centrosphere are collectively called centrosome. Before the cell division the centrioles at each pole of the spindle. The two centrioles are situated at to each other. Each centriole is a microtubular structure and is formed of microtubules arranged in 9 + 0 manner (all the 9 microtubules are peripheral in position).
Each microtubule is a triplet and is formed of three subtubules which are called A, B and C. A subtubule is about 45Å thick and is formed of 13 parallel protofilaments while each of B and C subtubule is formed of 10 parallel protofilaments. Each protofilament is formed of a row of , -tubulin dimers. C sub-tubule of each microtubule is linked to A sub-tubule of adjacent microtubule by a dense material (DM) strand called A-C linker, so all the microtubules are tilted at . Each microtubule is about 250Å in diameter.
Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub (about 25Å in diameter) and 9 radial spokes or radial fibres. Each radial spoke ends into a dense material (DM) thickening, called X-body or foot which is further linked to A-subtubule. Between two adjacent X-bodies there is another DM-thickening, called Y-body, which is linked to X-body on either side and to A-C linker on outer side.
Centriole is rich in tubulin and ATPase. Centriole can replicate but has no DNA. Centrioles replicate in phase of interphase of cell cycle but do not initiate cell division.

(4) Chemical composition : Centrosome is lipoproteinaceous structure. The microtubules of centriole are composed of protein tubulin and some lipids. They are rich in ATPase enzyme.

(5) Origin : The daughter centriole is formed from the pre-existing centriole in of interphase so called self-replicating organelle.

(6) Functions
(i) The centrioles help organising the spindle fibres and astral rays during cell division. Therefore, they are called microtubules organising centres. The cells of higher plants lack centrioles and still form a spindle.
(ii) They provide basal bodies which give rise to cilia and flagella.
(iii) The distal centriole of a spermatozoan give rise to the axial filament of the tail.