The cytoskeleton: (Fig.4.19 page 76) Cell shape, movement, and internal transport all require structural support provided by the cytoskeleton. The protein components of the cytoskeleton interconnect and extend from the nucleus to the plasma membrane in eukaryotic cells. Contains 3 types of elements: a. actin filaments, b. intermediate filaments, c. and microtubules. Function in: a. Maintain cell shape. b. Allow the cell and its organelles to move. It is dynamic, and can rearrange its protein components as necessary in response to changes in internal and external environments.

A number of different mechanisms appear to regulate this process, including: Protein phosphatases which remove phosphates from proteins and bring about assembly. Protein kinases which phosphorylate proteins and lead to disassembly.

a. Actin filaments:

b. Intermediate filaments:

c. Microtubules:

There are 3 motor molecules: i. Myosin:- is associated with actin. ii. Kinesin:- is associated with microtubules. iii. Dynein:- is associated with microtubules. The following organelles are made of microtubules: i. Centriole ii. Cilia and flagella

i. Centriole: (Fig.4.20 page 77). Are short cylinders. Have a ‘9+0’ of microtubules triplets. Nine sets of triplets are arranged in an outer ring, and the center of a centriole does not contain a microtubule. Found in animal cells and most protists in a centrosome. Centrosome, which is the major MTOC for the cell, contains 2 centrioles lying at right angles to each other. Centrioles might be involved in the process by which microtubules assemble and disassemble. Plant and fungal cells have the equivalent of a centrosome, but this structure does not contain centrioles.

A basal body is an organelle that lies at the base of cilia and flagella and may direct the organization of microtubules within these structures. A basal body may do for a cilium or flagellum what the centrosome does for the cell. In cells with cilia and flagella, centrioles are believed to give rise to basal bodies which have the same circular arrangement of microtubule triplets ‘9+0’ as centrioles.

Fig.4.20 page 77. Centrioles

ii. Cilia and flagella: (Fig.4.21 page 78) Are hairlike projections that function in movement of cells. In free cells, cilia (or flagella) move the cell through liquid. If the cell is attached to other cells, cilia (or flagella) are capable of moving liquid over the cell. Both are membrane-bounded cylinders enclosing a matrix. Both have ‘9+2’ pattern of microtubules doublets. Examples:- Paramecium has cilia - Cell lining the upper respiratory tract has cilia - Euglena has flagella - Human’s sperms have flagella

Each cilium and flagellum has a basal body lying in the cytoplasm at its base. Basal bodies have the same circular arrangement of microtubule triplets ‘9+0’ as centrioles and are believed to be derived from them.

Structure of a Flagellum

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