Biomed 05

B- Non-membranous organelles 1- Ribosomes They are found in both prokaryotic and eukaryotic cells but they are larger in eukaryotes. They are synthesized in the nucleolus then pass through the nuclear pores to the cytoplasm. Each ribosome is composed of 2 subunits, a small subunit and a large subunit. Between them there is a small cleft in which a central growing polypeptide chain is present. Chemically, they are consisted of * ribosomal RNA (rRNA) (65%) and * proteins (35%) i.e. ribonucleoprotein. Ribosomes are found in 3 different places or cases in cells which are: 1. Free floating in cytoplasm as individual subunits or dimers. 2. Membrane bound on outer surface of rough ER. 3. Attached to mRNA molecule in a

polysome (polyribosome).

Function of ribosomes: Ribosomes are the site of cellular protein synthesis. They receive amino acids (the building units of protein), grouping them into peptide chains by interaction between transfer RNA (tRNA) which carries the amino acids and messenger RNA (mRNA) which carries the specific genetic code from DNA in the nucleus. The formed protein molecules by ribosomes penetrate the cavity of rough ER where they accumulated in vesicles. These vesicles migrate to the Golgi apparatus to be exported outside the cell. Proteins for use in the nucleus were found to be formed by free floating ribosomes where their products float inwards till they reach the nucleus.

2- Microtubules The microtubule is a long cylindrical structure with a cavity. It is elastic and capable to bend without breaking. Chemically, it is made of dimmers of alpha and beta tubulin (type of protein).

Functions of microtubules: 1- Microtubules form centrioles, cilia, flagella and microvilli. 2- They facilitate the transport of various particles inside the cytoplasm. 3- They share in the formation of cytoskeleton of the cell. Note: The cytoskeleton determines the shape and provides mechanical support to the cell. It is formed from: 1) Microfilaments, 2) Intermediate filaments and 3) Microtubules.

3- Centrioles Centrioles are short hollow cylindrical tubules that found near the nucleus. There are two centrioles at right angles to each other. Each centriole consists of 8 peripheral sets of microtubules arranged in a pinwheel, 3 microtubules (triplet) in each set. Thus, each centriole consists of 27 (3x9) microtubules in the configuration of (9+0).

Functions of centrioles: 1- They play an important role in the process of cell division where they form spindle fibers. 2- They are able to replicate giving identical structures that migrate towards the plasma membrane to form basal bodies from which cilia or flagella. 3- They are involved in the cytoplasmic movement.

Basal bodies:

So, the basal bodies and centrioles are homologous structures with the same configuration (9+0). Each cilium or flagellum has a basal body located at the base.

Flagella and cilia:

Cilia and flagella are hair-like structures projecting from the basal bodies (that found in the cytoplasm) and enclosed (covered) by the plasma membrane. Eukaryotes have 9 doublets (pairs) of microtubules arranged in a circle around 2 central microtubules i.e. (9 + 2). Cilia are being much shorter than cilia. Many unicellular organisms such as Paramecium move by cilia. Many unicellular organisms such as Euglena move by flagella.

The 9+2 arrangement of microtubules in a flagellum or cilium.

The upper respiratory tract have cilia while sperms use flagella to move.

Microvilli They are formed from microtubules covered by cell membrane. They are finger like structures projecting from the surfaces of some cells of intestine or kidney. They increase the surface area for absorption.

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