The endomembrane system

• Endomembrane system


  Regulates protein traffic and performs metabolic functions in the cell
• Components of the endomembrane system


  • Nuclear envelope
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Vacuoles
  • Plasma membrane
• The components of the endomembrane system are either continuous or connected via transfer by vesicles
• Endoplasmic reticulum (ER)


  • Accounts for more than half of the total membrane in many eukaryotic cells
  • Membrane is continuous with the nuclear envelope
  • Has two distinct regions: smooth ER (lacks ribosomes) and rough ER (surface is studded with ribosomes)
• Rough ER


  • Has bound ribosomes, which secrete glycoproteins
  • Distributes transport vesicles, proteins surrounded by membranes
  • Is a membrane factory for the cell
• Functions of smooth ER


  1. Produces enzymes important in the synthesis of lipids, oils, phospholipids, and steroids

  2. Helps process drugs, alcohol, and other potentially harmful substances

  3. Helps store calcium ions
• Golgi apparatus


  • Consists of flattened membranous sacs called cisternae
  • Serves as a molecular warehouse and finishing factory for products manufactured by the ER
  • One side functions as a receiving dock for the product and the other as a shipping dock
  • Products are modified as they go from one side to the other and travel in vesicles to other sites
• Lysosome


  • A membranous sac of hydrolytic enzymes that can digest proteins, fats, polysaccharides, and nucleic acids
  • Lysosomal enzymes work best in the acidic environment inside the lysosome
• Phagocytosis


  1. A cell can engulf another cell, forming a food vacuole

  2. A lysosome fuses with the food vacuole and digests the molecules
• Autophagy


  Lysosomes use enzymes to recycle the cell's own organelles and macromolecules
• Vacuoles


  • A plant cell or fungal cell may have one or several vacuoles, derived from endoplasmic reticulum and Golgi apparatus
  • Food vacuoles are formed by phagocytosis
  • Contractile vacuoles, found in many freshwater protists, pump excess water out of cells
  • Central vacuoles, found in many mature plant cells, hold organic compounds and water