Cell Organization: Eukaryotic Cells and Ribosomes

• The topic is organization of cells.

Eukaryotic Cells

• Animals, plants, protists(unicellular eukaryotes), fungi (yeast, mold, mushroom) are eukaryotic cells.
• Protists that are not green or red algae are unicellular eukaryotes.
• Red and green algae can be unicellular or multicellular.

Ribosomes

• All living cells contain ribosomes
• Ribosomes are the unit of protein synthesis
• Ribosomes are non-membrane bounded
• Ribosomes can be found free in cytoplasm
• Ribosomes can be membrane-bounded (attached to GER)

Classes of Ribosomes

• Two classes: 70S and 80S.
• Both classes have nearly the same structure and function, but differ in size
• 70S ribosomes: Found in prokaryotic organisms (bacteria) and eukaryotic cell organelles (mitochondria and chloroplast).
• 80S ribosomes: Found in eukaryotic cells
• 70S ribosomes are made of a small(30S) and large subunit (50S)
• The 50S subunit: made of 5S rRNA (120 nts), 23S rRNA (2900 nts), and 34 proteins (L1-L34)
• The 30S subunit: made of 16S rRNA (1540 nts) and 21 proteins (S1-S21)
• 80S ribosomes are made of a small(40S) and large subunit (60S)
• The 60S subunit: 5S rRNA (120 nts), 5. 8S rRNA (160 nts), 28S rRNA (4700 nts), and about 49 proteins
• The 40S subunit: made of 18S rRNA (1900 nts) and ~33 proteins
• Ribosomes bind mRNA and make protein synthesis when they come together

Ribosome Biogenesis

• This is the process of ribosome making
• In prokaryotic cells: occurs in the cytoplasm
• In eukaryotes: occurs in the cell cytoplasm and in the nucleolus of eukaryotic cells.
• Ribosomes exist in the cell as poly-ribosomes and mono-ribosomes

Functional Sites on a Ribosome

• Aminoacyl-site (A site): Location where incoming tRNA molecules charged with an amino acid bind
• Peptidyl-site (Psite): Location where the tRNA molecule linked to the growing end of the polypeptide chain binds
• Exit site (E site): Location from which deacylated tRNA is released.
• m-RNA -binding site: Site where mRNA binds

Ribosome Sets

• Free Ribosomes usually make proteins that will function in the cytosol
  • Structural proteins of the cell which may remain in the cytoplasm or be targeted to particular organelles are made here, such as the enzymes of glycolysis, tubulins, actin, histons, transcription factors, ...mitochondrial or nuclear structural proteins.
• Attached Ribosomes to the membranes of GER usually make proteins that are exported or included in the cell's membranes.
  • These are usually secretory proteins (collagen, peptide hormones), lysosomal enzymes, and glycoproteins of plasma membrane

Ribosomes in Cytoplasm

• Free ribosomes are found as individual granules or in clusters
• Clusters are called polysomes or polyribosomes
• Polysomes consist of a group of ribosomes connected by a strand of mRNA during translation
• m-RNA runs successively through the several ribosomes in a cluster to produce multiple copies of a protein

Endomembrane System

• Regulates protein traffic, and performs metabolic functions.
• Includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and the plasma membrane.
• Carries out: synthesis of proteins, transport of proteins into membranes and organelles, metabolism and movement of lipids, and detoxification of poisons.

Endoplasmic Reticulum

• Extensive network of folded membranes that extend from the nuclear envelope and connects to the cytoplasm
• Found in all eukaryotic cells
• Constitutes more than half of the total membrane of an average animal cell
• A network of membranous tubules and sacs called cisternae
• Has two different regions, which are distinct in structure and function

Endoplasmic Reticulum: Smooth ER

• Outer surface lacks ribosomes.

Endoplasmic Reticulum: Rough ER

• Studded with ribosomes on the outer surface of the membrane.
• Appears rough on the electron microscope

Endoplasmic Reticulum: Basic Kinds

• Two basic kinds, Rough (granular) ER and Smooth (agranular) ER
• Rough (granular) ER: involved mainly in protein production
• Smooth (agranular) ER: involved mainly in lipid metabolism
• The surface of the RER is studded with ribosomes giving it a "rough" appearance
• The quantity of RER and SER in a cell can vary from one type to the other, depending on changing metabolic needs

Smooth ER (SER)

• Regions of ER that lack bound ribosomes are called smooth endoplasmic reticulum
• Not involved in protein synthesis
• Responsible for: synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions (muscle cell)

Smooth ER Enzymes

• Play a role in the synthesis of lipids such as oils, steroids (sex hormones of vertebrates, steroid hormones), and new membrane phospholipids.
• Help detoxify drugs and poisons in liver cells.
  • Detoxification works by adding hydroxyl groups to drug molecules, which makes them more water-soluble and easier to flush from the body.
  • Example: the sedative phenobarbital (drug) is metabolized in this manner by smooth ER in liver cells.
• In muscle cells, Ca2+ is stored in the lumen of the ER by SER
  • A muscle cell is stimulated by a nerve impulse, which results in calcium ions being transferred from lumen into the cytosol to trigger the contraction of the muscle cell.

Rough ER

• Involved in protein synthesis
• Functions in producing secreted proteins
• Example: Insulin produced in ribosomes in RER and secreted into the bloodstream
• Secretory proteins leave the ER, are enclosed in membrane-bound vesicles that bud off like bubbles from a specialized region called the transitional ER

Protein Folding

• Only properly folded proteins are transported from the GER to the Golgi
• Abnormally folded or unfolded proteins are exported from ER
  • They are then degraded in the ubiquitin-proteasome pathway in the cytosol
• Otherwise improperly folded proteins are retained in the GER and result in ER stress

Golgi Apparatus

• After leaving the ER, many transport vesicles travel here
• This organelle is where products are stored, modified, and then sent to destinations
• The Golgi complex (apparatus, body) was first described by Camillo Golgi in 1897, and named as 'Golgi apparatus' (1898)
• Acts as a site for receiving, sorting, shipping, and manufacturing
• The organelle is especially extensive in cells specialized for secretion
• Due to its large and regular structure, it was one of the first organelles described by Light Microscopists.
• With the development of Electron Microscope, its structure has been observed
• Located at one side of the nucleus and close to the centrosome