Cell Biology: Golgi Apparatus Functions

Golgi Apparatus

• A cup-shaped organelle near the nucleus
• Found in many cell types
• Composed of smooth cisternae (flattened membranous sacs/vesicles)
• Cisternae are stacked in parallel rows
• Surrounded by membrane-bound vesicles
• Vesicles transport proteins to and from the Golgi
• At least three distinct cisternae classes: Cis, Median, and Trans Golgi
• Protein movement: Rough ER → Cis Golgi → Median Golgi → Trans Golgi → Secretory Vesicles → Cortical Granules

Golgi Functions

• Packages secretory materials
• Synthesizes some polysaccharides and glycolipids
• Forms the acrosome of spermatozoa
  • Part of the acrosome membrane involved in sperm recognition and binding to the egg during fertilization

Acrosome

• Derived from the Golgi complex of spermatids
• Contains a protein matrix core
• Contains hydrolytic and glycolytic enzymes crucial for fertilization
• Key enzymes: proacrosin-acrosin, hyaluronidase, β-galactosidase, proteinases, neuraminidases, esterases, arylsulfatase, phospholipases A & C, phosphatases, and regulatory enzymes/proteins

Endoplasmic Reticulum (ER)

• A system of membranous tubules and sacs, acting as an internal transport system
• ER quantity varies depending on cell activity
• Rough ER (RER): studded with 80S ribosomes; site of protein synthesis
  • Extension of nuclear envelope outer membrane , allowing mRNA transport to 80S ribosomes for translation.
• Smooth ER (SER): site of polypeptide conversion to functional proteins and protein preparation for secretion
  • Site for lipid and steroid synthesis
  • Involved in calcium regulation in muscle cells; toxin breakdown in liver cells

Ribosomes

• Dense, rounded, granular, and smallest known electron microscopic ribonucleoprotein (RNP) particles
• Site of protein synthesis
• Free in cytoplasm (prokaryotes) or attached to RER (eukaryotes)
• Eukaryotic ribosomes are 80S (composed of 40S and 60S subunits)
• Prokaryotic ribosomes are 70S (composed of 30S and 50S subunits)
• Groups of 80S ribosomes working together are known as polysomes

Lysosomes

• Tiny, spherical or irregular-shaped membrane-bound vesicles in the cytoplasm
• Digest material taken in by endocytosis
• Highly acidic medium (pH 5): maintained by ATP-dependent proton pumps in the lysosomal membrane
• Exhibits polymorphism:
  • Primary lysosomes (storage granules)
  • Secondary lysosomes (digestive vacuoles)
  • Residual bodies
• Functions
  • Contain digestive enzymes to degrade macromolecules/organelles
  • Enzymes originate in the ER but are activated in the Golgi
  • Important in phagocytosis (macrophages degrading acquired particles) and autophagy (degrading organelles)

Cell Death

• Necrosis: Results from damage by an external agent (e.g., infection, injury); causes cell rupturing and leakage of contents into surrounding tissues; triggers non-specific immune response and inflammation
• Apoptosis: A controlled, continuous process maintains cell numbers by balancing production with death. It doesn't cause leakage, resulting in no inflammation. Essential for processes like embryological development, childhood development, and maintaining a balanced cell count in adults.
  • Critical in human health and is linked to diseases like, Cancer (cells losing ability to respond to apoptosis signals; leading to uncontrolled growth and proliferation) and neurodegenerative disorders (apoptosis causing progressive loss of neurons)