4.4 The Endomembrane System and Proteins

Learning Objectives

• List the components of the endomembrane system
• Recognize the relationship between the endomembrane system and its functions

Introduction

• The endomembrane system is a group of membranes and organelles in eukaryotic cells.
• Functions include modifying, packaging, and transporting lipids and proteins.
• Key components: nuclear envelope, lysosomes, vesicles, endoplasmic reticulum (ER), Golgi apparatus.
• The plasma membrane is also part of the system.

The Endoplasmic Reticulum

• Rough ER (RER):

  • Studded with ribosomes; involved in protein synthesis and modification.
  • Proteins enter RER lumen, undergo structural modifications.
  • Involved in phospholipid synthesis.
  • RER is abundant in cells that secrete proteins (e.g., liver cells).

• Smooth ER (SER):

  • Lacks ribosomes; involved in synthesis of carbohydrates, lipids, and steroid hormones.
  • Detoxifies medications and poisons, stores calcium ions.
  • Sarcoplasmic reticulum in muscle cells stores calcium.

The Golgi Apparatus

• Lipids and proteins from ER are sorted, packaged, and tagged.
• Structure: Flattened membranous sacs.
• Cis Face: Side closer to ER; receives vesicles from ER.
• Trans Face: Opposite side; vesicles leave to transport contents.
• Frequent modifications: Addition of sugar molecule chains.
• Cells with high secretory activity have abundant Golgi apparatus.
• In plant cells, synthesizes polysaccharides for the cell wall.

Lysosomes

• Function as the digestive component and organelle-recycling facility in animal cells.
• Use hydrolytic enzymes to destroy pathogens.
• Involved in phagocytosis (e.g., in macrophages).

Visual Connection

• Membrane and secretory proteins synthesized in the RER.
• Modified by carbohydrates in the ER and further in the Golgi apparatus.
• Vesicles transport proteins to the plasma membrane or other destinations.

Career Connection

• Cardiologist:

  • Heart disease relates to malfunctioning endoplasmic reticula in cardiac muscle cells.
  • Leads to insufficient calcium ions for muscle contraction.
  • Diagnosed via physical exams, ECGs, chest X-rays.
  • Treatment includes medications, diet, exercise.

• Geneticist:

  • Study genetic mutations causing diseases (e.g., Lowe disease).
  • Lowe disease affects enzyme in the Golgi apparatus.
  • Diagnosed via prenatal testing.
  • Geneticists conduct research and counsel on genetic conditions.

Links and Additional Resources

• Animation of the endomembrane system available for further learning.

Back to Section 4.3 Eukaryotic Cells Next Section: The Cytoskeleton