Remember ! In previous lectures you have known that there are two types of cellular organelles1- Membranous organelles2- Non-membranous organelles
Here we are going to talk about n non-membranous organelles First, you should ask yourself question what does this namemean ?
They are living , permanent and active structures present in the cytoplasm with special functions, and they are not surrounded by membranes.Non-membranous organelles include :1-Ribosomes 2-Microtubules 3-Filaments( Microtubules and filaments form what is called cytoskeleton 4-centrioles
They are small particles formed of ribo- nucleoprotein particles (RNA)&protein both are formed into the nucleolus * consist of two subunits large and small one (Department book 2021) They are macromolecular machines which assemble polypeptides from amino acids on molecules of transfer RNA (tRNA) in sequence specified by mRNA mRNA spicified assemblepolypeptides from amino acids on molecules oftransfer RNA (tRNA ( (Jouqueir's basic histology )
Types of ribosomes: 1-Free ribosomes Involved in synthesis of proteins utilized by the cell growth and differentiation . single ribosomes are not involved in protein synthesize and have to be organized in the form of polysomes by the aid of mRNA. 2-Attached ribosomes These ribosomes are attached to the outer surface of RER they are concerned with synthesis of secretory proteins exported outside the cell.
Components of cytoskeleton: 1-Microtubules : Within the cytoplasm of all eukaryotic cells are fine tubular structures known as microtubules . They are thin rigid tubules 25nm in diameter and formed of protein tubulin organized in thread like protofilaments.- The wall of each microtubule is formed of 13 protofilaments .-Microtubules assembly occurs in the microtubule organizing center near the centriole .-Their length can change rapidly by the process of polymerization and depolymerization of tubulin subunits.
Functions 1-Displacement of macromolecules and organelles through the cytoplasm . 2-Mechincal support of the cell and determine its shape.3-Form the mitotic spindles during cell division .4- They constitute important structures in the cell e.g. centrioles , cilia and flagella .Notice that microtubules are invisible L.M. and needs special staining methods.
Filaments They are thread like non membranous organelles od three types thin or microfilaments , intermediate filaments and thick (myosin) filaments.1-Microfilaments:Are composed of actin subunits and allow motility and most contractile activity in cells present under the plasma membrane forming a network . Its responsible for muscle contraction with myosin.
2-Intermediate filaments : They are many types according to their chemical composition .1-Keratin: Forming the tonofilaments in stratified epithelia (acidic and basic cytokeratin)* 2-Desmin: present in smooth muscle cells . 3-Vimentin: present in fibroblasts 4-Neurofilaments: present in nerve cells5-Gilal filaments: present in glial cells 3- Thick (myosin) filaments : They are filaments present in skeletal and cardiac muscle cells .
Functions of filaments 1-Mechanical support of cell2-Distrbute the tensile force on the cell3-Involved in muscle contraction4-play a role in cell division
Centrioles L.M:Appears as 2 dots close to the nucleus with special stain e.g. iron Hx E.M:Each one appears as hollow cylinder the wall of which is formed of triplet (3) microtubules
Functions 1-They are important in cell division2-They are duplicated during G2* to become 2 pairs3-Each pair migrates to one pole of the cells and form mitotic spindles in the microtubule organizing centre4-They serve as the basal body of cilia and flagella
NOTES 1-ribosomes proteins are formed in the cytoplasm then transported into nucleus where they associate newly synthesized rRNa to form new ribosomes .2-you should notice that actin and myosin filaments only found on skeletal and cardiac muscles .3-this is the chemical components of keratin filaments4- they are supporting cells found in the nervous system5-one stage of three forming what is called interphase a part of the cell cycle (will be discussed in lecture 9)