biology7 cell structrue..................

cell structure

1. Mr. Mugtaba A. A. A. CELL STRUCTURE

2. Cytoskeleton: is a network of fibers extending throughout the cytoplasm. Functions: 1/determine the shapes of cells 2/play a role in movement of organelles and cytoplasmic vesicles, hence the entire cell movement. 3/ organizes the structures and activities of the cell (regulation). 4/ Fibers act like a geodesic dome to stabilize and balance opposing forces There are three main types of fibers in the cytoskeleton: microtubules , microfilaments , and intermediate filaments

3. The cytoskeleton interacts with motor proteins. In cilia and flagella motor proteins pull components of the cytoskeleton past each other. This is also true in muscle cells

4. Motor molecules also carry vesicles or organelles to various destinations along “monorails’ provided by the cytoskeleton. Interactions of motor proteins and the cytoskeleton circulates materials within a cell via streaming. Recently, evidence is accumulating that the cytoskeleton may transmit mechanical signals that rearrange the nucleoli and other structures

5. Microtubules: The thickest fibers, are hollow rods about 25 microns in diameter. Microtubule fibers are constructed of the globular protein, tubulin, and they grow or shrink as more tubulin molecules are added or removed. They move chromosomes during cell division. Another function is as tracks that guide motor proteins carrying organelles to their destination In animal cells, the centrosome has a pair of centrioles , each with nine triplets of microtubules arranged in a ring. During cell division the centrioles replicate.

6. 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.

7. 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.

8. 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.

9. Intermediate filaments : intermediate in size at 8 - 12 nanometers, are specialized for bearing tension. Intermediate filaments are built from a diverse class of subunits from a family of proteins called keratins. Intermediate filaments are more permanent fixtures of the cytoskeleton than are the other two classes. They reinforce cell shape and fix organelle location.

10. THE END THANKS