TR

The Endomembrane System

Introduction

  • The 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
  • These components are either continuous or connected via transfer by vesicles

Nucleus

  • Chromosomes: composed of DNA and proteins = chromatin
  • Nuclear matrix: filamentous network; organizes chromosomes
  • Ribosome assembly occurs in the nucleolus
  • Inner nuclear membrane: specific proteins that act as binding sites for chromatin and for the nuclear lamina
  • Outer nuclear membrane: continuous with ER membrane; studded with ribosomes
  • Perinuclear space: continuous with ER lumen

Endoplasmic Reticulum

  • Endoplasmic reticulum (ER): a system of membranous channels and saccules.
  • Rough ER: studded with ribosomes and is the site of protein synthesis and processing.
    • Protein synthesis on ribosomes attached to ER membrane:
    • Secreted proteins
    • Integral membrane proteins
    • Soluble proteins destined for life in a membrane bound compartment 
  • Smooth ER: synthesize lipids; metabolizes carbohydrates; detoxifies poison; stores calcium (Sarcoplasmic reticulum)

Golgi Apparatus

  • Center for 
    • Modifying products of ER
    • Manufacturing
    • Warehousing
    • Sorting
    • Shipping
  • Consist of five to eight cup-shaped, membrane-covered sacs called cisternae
    • Cis face: receiver (of vesicles)
    • Trans face: shipper (of vesicles)
  • The Golgi modifies proteins and lipids that arrive from the endoplasmic reticulum
    • Prepares the proteins and lipids for export outside of the cell or for transport to other locations in the cell. 
    • Enzymes present in the Golgi lumen modify the carbohydrate portion of glycoproteins by adding or subtracting individual sugar monomers.
    • Manufactures molecules such as polysaccharides.

Lysosomes

  • Spherical organelles containing a single layer membrane
  • Involved in enzymatic breakdown of macromolecules and organelles
    • Phagocytosis
    • Autophagy
  • Contain hydrolytic enzymes 
    • (over 30)

Peroxisomes

  • Surrounded by single membrane
  • All of their proteins must be imported
  • Contain oxidative enzymes which remove hydrogens from specific substrates that in turn produces H2O2
    • Catalase then converts H2O2 into H2O + O2
  • Site of β-oxidation of fatty acids
  • Photorespiration in leaves
  • Synthesis of bile acid