Cell Organelles

CELL ORGANELLES

Cell: The structural and functional unit of life (10microM – 0.2microM). organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane.

Endoplasmic reticulum-first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in 1945 network of tubules, vesicles and sacs (flattened cisternae) that are interconnected. The Ribosomes sit on the outer surfaces of the sacs (or cisternae). They resemble small beads sitting in rosettes or in a linear pattern. rough endoplasmic reticulum which gets its name from the presence of ribosomes on its surface. smooth endoplasmic reticulum

specialized functions: •protein translation •folding and transport of proteins to be used in the cell membrane •or to be secreted (exocytosed) from the cell •sequestration of calcium •production and storage of glycogen, steroids, and other macromolecules (SER) Basic structure and composition of the ER membrane is similar to the plasma membrane RER - Rough endoplasmic reticulum bears the ribosomes during protein synthesis. The newly synthesized proteins are sequestered in sacs, called cisternae . The system then sends the proteins via small vesicles to the Golgi Complex , or, in the case of membrane proteins, it inserts them into the membrane.

Rough endoplasmic reticulum The surface of the rough endoplasmic reticulum (RER) is studded with proteinmanufacturing ribosomes The ribosomes only bind to the ER once it begins to synthesize a protein The membrane of the RER is continuous with the outer layer of the nuclear envelope Synthesis of all types of protein Smooth endoplasmic reticulum synthesis of lipids and steroids, metabolism of carbohydrates, regulation of calcium concentration, drug detoxification, attachment of receptors on cell membrane proteins, and steroid metabolism Sarcoplasmic reticulum The sarcoplasmic reticulum (SR), is a special type of smooth ER found in smooth and striated muscle the SR stores and pumps calcium ions. The SR contains large stores of calcium, which it sequesters and then releases when the muscle cell is stimulated

Ribosomes - ribonucleoprotein factories in which proteins are synthesized. 70S Ribosomes •In prokaryotes •Sedimentation Coefficient Size - 64S-72S (average 69S) Subunits - Small 30S and large 50S •RNA - 16S RNA in 30S subunit, 23S and 58 RNA in 50S subunit 80S Ribosome - In eukaryotes (algae, fungi, higher plants and animals) - 79-85S in fungi, 80S in mammals. - 4 molecules of RN A :16S-18S RNA in 40S subunit. - 25S-29S, 5.8S and 5S RNA in 60S subunit.

Ribosomes catalyze the joining of the amino acid monomers directed by the mRNA sequence messenger RNA (mRNA) codons are read by the anticodons of adaptor, transfer RNAs (tRNAs) that carry codon-specific amino acids Amino-acyl tRNA synthetases attach amino acids to the appropriate tRNAs These amino acids are added to a growing protein chain by peptide bond formation in the heart of the ribosome.

Initiation

The

initiator tRNA is attracted to the region (carrying a methionine. It binds to the triplet code AUG. This then attracts the large ribosomal subunit which will bind to the small subunit. it has an A site and a P site. These are different binding sites for the tRNAs.