Cytoskeleton

      CYTOSKELETON 

👉 An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

       MICROTUBULES

👉 Microtubules are composed of protein, Tubulin [Size 25 nm.]

👉 In plants microtubules often found associated with cell wall. Probably these transport cell wall material from Golgi body to outside of cell. During cell division these microtubules form spindle fibers.

      MICROFILAMENTS

👉 They are composed of contractile protein, Actin which concern with muscle contraction, Microtubules and microfilament are part of cytoskeleton-base of cell. [Size 6-7 nm]

INTERMEDIATE FILAMENT

Intermediate filaments has size/diameter in between microfilaments and microtubules. These fillaments form basket like structure around the nucleus. [Size 8-12 nm]

   CILIA AND FLAGELLA

Cilia (sing: cilium) and flagella (sing. flagellum) are hair-like outgrowths of the cell membrane. Cilla are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement. The bacteria (prokaryotic cell) also possess flagella but these are structurally different from that of the eukaryotic flagella.

The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.(9 doublet +2 singlet)

Arms of A tubules consist of an enzymatic protein dynein similar to myosin of muscle cells. Dynein have ability of hydrolysis of ATP & liberates energy for ciliary or flagellar movement.

The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by radial spoke. Thus there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Both the cilium & flagellum emerge from centriole-like structure called the basal bodies or blepharoplast.

   CENTROSOME & CENTRIOLES

• Centrosome is absent in higher plants.

• Centrosome containing two centrioles (diplosome) located just outside the nucleus and lie at right angle (90°) to each other. Each centriole is surrounded by amorphous pericentriolar materials.

• Centrioles are membraneless cylindrical structure which exhibit cart wheel structure in transverse section. 

• Centriole mainly consist of 9 evenly spaced peripheral triplet fibrils of tubulin. These triplets are linked with the help of A-C linker.

• The central part of the centriole is proteinaceous and called the hub, which is connected with peripheral triplets by radial spokes made of protein. (9+0 arrangement)

• Centrioles are self duplicating units.

Function:-

(i) In animal cells, centrioles play important role in cell division by arranging spindle fibres between two poles of cell. The location of centrioles during cell division decides the plane of division. The plane of division is always at right angle to the spindle.

(ii) Centrioles form the basal body of cilia or flagella.

           MICRO-BODIES

• These are many, membrane bound minute vesicle contain various enzyme that are present in both plant and animal cells.

(1) Peroxisomes or Uricosomes :-

• These are found in both plant and animal cells. Peroxisomes contain catalase enzyme which is concerned with peroxide (water)metabolism. Catalase degrade the (H2O2) into water and oxygen.

• In plants, peroxisomes occurs in cells of green tissues and concerned with photorespiration (glycolate pathway).

• Peroxisomes are also involved in B-oxidation of fatty acids.

(2) Glyoxysomes:-

• Glyoxysomes occurs only in plants especially in fatty seeds (castor seed, ground nut seed etc.).

• Glyoxysomes are considered as a highly specialised peroxisomes. Glyoxylate acid cycle takes place in glycysomes. This cycle convert fats into carbohydrats.