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