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