Eukaryotic cells and Ribosomes

Eukaryotic Cells

• Animal cells are eukaryotic
• Plant cells are eukaryotic
• Protists are unicellular eukaryotes
• Red and green algae are uni and multicellular
• Fungi, like yeast, mold, and mushrooms, are eukaryotic cells

Ribosomes

• All living cells contain ribosomes
• Ribosomes are not membrane-bound
• Ribosomes are the unit of protein synthesis
• Ribosomes exist in the cytoplasm
• Some ribosomes are free
• Other ribosomes are membrane-bound, attached to Granular Endoplasmic Reticulum (GER)

Ribosome Classes

• There are two classes of ribosomes: 70S and 80S
• 70S ribosomes are present in prokaryotic organisms, specifically bacteria
• 70S ribosomes exist in eukaryotic cell organelles, such as mitochondria and chloroplasts
• 80S ribosomes are present in eukaryotic cells
• Both ribosome classes share nearly identical structure and function
• The two ribosome classes are different sizes

Ribosome Components

• Both ribosome classes have small and large subunits
• Bacterial Ribosome-70S
  • Contains a small subunit with 16S rRNA (1540 nts) and ~21 proteins (S1-S21)
  • Contains a large subunit with 5S rRNA (120 nts), 23S rRNA (2900 nts) and ~34 proteins (L1-L34)
• Eukaryotic Ribosome-80S
  • Contains a small subunit with 18S rRNA (1900 nts) and ~33 proteins
  • Contains a large subunit with 5S rRNA (120 nts), 5.8S rRNA (160 nts), 28S rRNA (4700 nts) and ~49 proteins

Ribosome Biogenesis

• Ribosome biogenesis refers to the ribosome-making process
• In prokaryotic cells, ribosome biogenesis occurs in the cytoplasm
• In eukaryotic cells, ribosome biogenesis takes place in the cytoplasm and nucleolus
• Ribosomes exist in cells as poly-ribosomes and mono-ribosomes

Functional Ribosome Sites

• There are 4 functional sites in the ribosome
• Aminoacyl-site (A site) holds the incoming tRNA molecule charged with an amino acid
• Peptidyl-site (P site) holds the tRNA molecule linked to the growing end of the polypeptide chain
• Exit site (E site) facilitates the release of deacylated tRNA from ribosomes
• m-RNA is a binding-site

Major Ribosome Sets

• There are two major kinds of ribosome
• Free ribosomes usually make proteins that function in the cytosol
• Structural proteins of the cell, like the enzymes of glycolysis, tubulins, actin, histones, and transcription factors made by free ribosomes
• Attached ribosomes are bound to the membranes of the Granular Endoplasmic Reticulum (GER)
• Attached ribosomes usually make proteins that are exported or included in the cell's membranes
• Attached ribosomes synthesize secretory proteins like collagen, peptide hormones, lysosomal enzymes and glycoproteins of the plasma membrane

Free Ribosomes

• Free ribosomes are found in cell cytoplasm, either as individual granules or in clusters, called polysomes or polyribosomes
• Polysomes consist of several ribosomes connected by a strand of mRNA during translation
• Usually, a single m-RNA runs successively through several ribosomes in a cluster, producing multiple protein copies

Endomembrane System

• The endomembrane system regulates protein traffic and performs metabolic functions
• Elements of the endomembrane system include the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and plasma membrane
• The endomembrane system carries out tasks in the cell, including synthesizing and transporting proteins into membranes and organelles, lipid metabolism and movement, and detoxification of poisons

Endoplasmic Reticulum

• The endoplasmic reticulum is an extensive network of folded membranes extending from the nuclear envelope and connecting to the cytoplasm
• Endoplasmic reticulum is present in all eukaryotic cells
• The endoplasmic reticulum constitutes more than half of the total membrane of an average animal cell
• The endoplasmic reticulum consists of a network of membranous tubules and sacs, called cisternae
• The endoplasmic reticulum has two different regions: smooth ER and rough ER, each with distinct structure and function

Smooth ER

• Smooth endoplasmic reticulum's (smooth ER) outer surface lacks ribosomes
• Synthesis of lipids
• Metabolism of carbohydrates
• Detoxification of drugs and poisons
• Storage of calcium ions in muscle cells are all functions of smooth ER
• Steroids, oils, sex hormones of vertebrates and new membrane phospholipids are synthesized by smooth ER
• Enzymes in smooth ER detoxify drugs and poisons in liver cells, adding hydroxyl groups to them to make them more water-soluble
• Example: the sedative phenobarbital (drug) is metabolized this way
• Calcium Ca2+ is stored in the lumen of the ER by Smooth ER in muscle cells
• Calcium ions are transferred from lumen into the cytosol when muscle cells are stimulated triggering muscle cell contraction

Rough ER

• Rough endoplasmic reticulum (RER) is studded with ribosomes on its outer surface
• Studded ribosomes give the ER its "rough" appearance when viewed with an electron microscope
• Rough Endoplasmic Reticulum (RER) is involved in protein synthesis
• Rough is also known as granular ER
• The surface of rough is studded with ribosomes
• Rough ER produces secreted proteins
• Example: Insulin produced in ribosomes in RER and secreted into the bloodstream

GER and the Golgi

• Only properly folded proteins are transported from the Granular Endoplasmic Reticulum (GER) to the Golgi apparatus
• Abnormally folded or unfolded proteins are exported from ER
• Abnormally proteins are degraded in the ubiquitin-proteasome pathway in the cytosol
• Otherwise, improperly folded proteins are retained in the GER, resulting in ER stress

Golgi Apparatus

• After leaving the ER, transport vesicles travel to the Golgi apparatus
• The Golgi apparatus is a site for receiving, sorting, shipping, and manufacturing proteins
• Products from the ER are modified, stored, and sent to other locations from the Golgi apparatus
• The Golgi apparatus are extensive in specialized secretion cells

Golgi Complex

• Camillo Golgi first described the Golgi complex (apparatus, body) in 1897, naming it 'Golgi apparatus' in 1898
• The Golgi complex has been detected in many cells
• The large and regular structure has enabled the Golgi to be one of the first organelles described by Light Microscopists
• The development of the Electron Microscope enabled the structure to be observed
• The Golgi complex is on one side of the nucleus and close to the centrosome