Cell Biology Course (Bio-1101) Lect. 5: Cell Organelles I PDF

Cell Biology Course (Bio-1101) // Lect. 5: Cell Organelles I Assoc. Prof. / Reda M. Mansour Animal cell * Every cell has a cell membrane (= plasma membrane, plasmalemma) that surrounds the protoplasm which is divided into: cytoplasm and nucleoplasm. So, Cell = Cell membrane + Pr...

Cell Biology Course (Bio-1101) // Lect. 5: Cell Organelles I Assoc. Prof. / Reda M. Mansour Animal cell * Every cell has a cell membrane (= plasma membrane, plasmalemma) that surrounds the protoplasm which is divided into: cytoplasm and nucleoplasm. So, Cell = Cell membrane + Protoplasm (= Cytoplasm + Nucleoplasm). * Cytoplasm is located between cell membrane & nuclear envelope and is divided into: A- Cytosol (composed mostly of water enriched by inorganic and organic constituents). B- Organelles & inclusions. Organelles are divided into: 1- Non-membrane bound e.g. Cytoskeleton, Ribosome. 2- Single-membrane bound e.g. Endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles. 3- Double-membrane bound e.g. Nucleus, Mitochondria, Cholroplast. * Nucleoplasm is housed within nuclear envelope-bound nucleus. Definitions: 1- Cytoplasmic inclusions are not surrounded by any membrane. They are basically granules of starch, glycogen,..etc. and they can store energy. 2- Cytoplasm: The cytoplasm is made of 70% – 80% water and is usually colorless. It contains proteins, carbohydrates, salts, sugars, amino acids, nucleotides, waste products and other biomolecules. It is a colloidal solution. Organelles are metabolically active units of the cell. Cell organelles can be classified into three categories 1 2 3 Organelles without Single membrane- Double membrane- membrane: bound organelles bound organelles: Ribosomes, and :Vacuole, Lysosome, Nucleus, mitochondria Cytoskeleton Golgi Apparatus, and chloroplast Endoplasmic Reticulum  Cell Organelles I- Non-membranous organelles 1- Cytoskeleton 2- Ribosomes  1- Cytoskeleton  Fluorescence micrograph of Cytoskeleton of cell Actin Filaments of Cell Intermediate Filaments of Cell  1- Ribosomes Bacterial ribosomes are composed of 65% rRNA and 35% ribosomal proteins while Eukaryotic ribosomes are about with an rRNA-to-protein ratio that is close to 1. Bacterial ribosomes 70S ribosomes have a small (30S) and a large (50S) subunit. 1- 30S subunit has 16S rRNA and 21 proteins. 2- 50S subunit has 5S, 23S rRNA and 31 proteins. Bacterial ribosomes 80S ribosomes have a small (40S) and large (60S) subunit. 1- 40S subunit has an 18S rRNA and 33 proteins. 2- 60S subunit has a 5S RNA, 28S RNA, a 5.8S rRNA & 46 proteins. Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. Schematic model with mRNA and tRNA. A tRNA fits into the A site when its anticodon base-pairs with an mRNA codon. P site holds the tRNA attached to the growing polypeptide. A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNAs leave from the E site. The polypeptide grows at its carboxyl end. A polyribosome (polysome) is a cluster of ribosomes along a single strand mRNA where every ribosome is concurrently engaged in protein synthesis.  Function of ribosomes Ribosomes are organelles on which mRNA is translated into protein. 1- Free polyribosomes (not attached to the endoplasmic reticulum) synthesize cytosolic and cytoskeletal proteins and proteins for import into the nucleus, mitochondria, and peroxisomes. 2- Bound polyribosomes (attached to Endoplasmic reticulum) synthesize proteins for transport through Golgi apparatus. These proteins enter into membranes, stored in lysosomes, or secreted from the cell (secretory, membrane, and lysosomal). Proper folding of new proteins takes place by protein chaperones. Denatured proteins or those that cannot be refolded properly are conjugated with ubiquitin protein and degraded proteasomes  Proteasome 1.It is the major site for the degradation of unwanted, damaged, malformed, or incorrectly folded proteins. 2.It is a cylindrical complex of non-lysosomal proteases. 3. Before a protein enters the proteasome, it must be tagged with ubiquitin (a relatively small protein 76 amino acids long). This process is called ubiquitination that delivers proteins to the proteasome, where they are broken down to small peptides(7 to 8 amino acids long). * Proteosome structure: - Each proteasome is 26S nonmembranous & cylindrical organelle with a hollow center. - It is composed of three subunits, a 20S core particle and two regulatory particles (each is called 19S). - The core particle is composed of four ring-shaped units, named α, β, β, α. - The two α subunits bind the regulatory particles, whereas the luminal aspects of the two β subunits function as proteolytic enzymes.