Organelles

• Endomembrane system


  Unique to eukaryotic cells, composed of the nuclear membrane, the endoplasmic reticulum (including both rough and smooth), the Golgi apparatus and the Lysosomes
• Smooth endoplasmic reticulum (SER)

  • Phospholipid, fat and steroid (including sex hormones) manufacturer
  • Carbohydrate metabolism
  • Breaks down stored glycogen to release glucose in hepatocytes
  • Detoxifies lipid-soluble drugs such as barbiturates
  • Adds charged water-soluble groups such as sulphate or glycuronic acid
• Sarcoplasmic reticulum


  • Network of tubular sacs
  • Sequesters calcium ions from the cytosol
  • The level of intercellular calcium regulates muscle contraction in muscle cells
  • Action potential triggers SR to release calcium into the cytosol
  • Calcium ions bind to tropomysin and troponin leading to a conformational change
  • Myosin now interacts with actin and the muscle contracts
• Rough endoplasmic reticulum (RER)

  • Site of protein synthesis
  • Secreted proteins
  • Glycosylated proteins
  • Lysosomal enzymes
  • Membrane bound proteins
• Protein synthesis in the RER

  1. Begins in the cytosol

  2. Signal peptide sequence targets proteins to the ER

  3. Signal recognition particle (SRP) attaches to the signal peptide and stops translation

  4. SRP docks to SRP receptor on ER membrane and translation starts again

  5. Signal peptide passes through membrane and is cleaved off
• Glycosylation in the RER

  Addition of an oligosaccharide composed of N-acetylglucosamine, mannose and glucose residues
• Golgi apparatus

  • Modifies and sorts proteins from the RER
  • Mediates the flow of proteins from the RER to various destinations
  • Default pathway is to the plasma membrane for secretion
  • Some proteins are tagged for specific destinations like lysosomes
• Mannose 6-phosphate (M6P) pathway

  1. Mannose residues of lysosome enzyme proteins are phosphorylated in the cis Golgi

  2. Mannose 6-phosphate receptor binds these proteins in the trans Golgi and directs them to lysosomes
• Glycosylation in the Golgi

  Essential for protein function, e.g. mucin glycoproteins with long oligosaccharide chains
• Lysosomes


  • Vesicular structures with a single smooth membrane
  • Contain about 60 hydrolytic enzymes
  • Primary lysosomes originate from the trans face of the Golgi
  • Fuse with targets, pump in hydrogen ions to lower pH and activate enzymes
  • Carry out autophagy, phagocytosis, and autolysis during apoptosis
• Lysosomal storage diseases are caused by defective lysosomal enzymes leading to accumulation of partially degraded metabolites
• In Tay-Sachs disease, Hexosaminidase A enzyme deficiency results in accumulation of the lipid ganglioside in nerve cells, leading to death by 2-3 years of age