Lec 14

1. Remember the function and basics of Golgi, Lysosome, endosomes.

• Golgi Complex: The Golgi complex processes, sorts, and packages proteins and lipids received from the ER. It consists of compartments like the cis-Golgi network (CGN), medial cisternae, and trans-Golgi network (TGN), each with specific functions in protein processing and transport.

• Lysosomes: Lysosomes are the principal sites of intracellular digestion. They contain various hydrolases that are active at low pH (acid hydrolases). Lysosomes degrade unwanted proteins and organelles through processes like autophagy.

• Endosomes: Endosomes are involved in sorting and transporting materials to lysosomes. They play a key role in the lysosomal sorting pathway, where materials are delivered to lysosomes for degradation.

2. Be able to state how ER resident proteins stay in the ER lumen despite the continuous flow of materials through ER.

ER resident proteins contain retention and retrieval tags, such as the KDEL sequence (Lys-Asp-Glu-Leu). The Golgi complex has KDEL receptor proteins that bind the KDEL sequence and deliver the protein back to the ER, maintaining the protein composition in the ER through retrograde flow.

3. Understand the role of progressive organelle acidification.

Progressive organelle acidification is achieved by active transport (pumping) of H+ into the lumen. This acidification is crucial for the function of various organelles, such as lysosomes, where the low pH activates acid hydrolases for digestion.

4. Lysosomal targeting signal

The lysosomal targeting signal involves the addition of mannose 6-phosphate (M6P) groups to the precursors of lysosomal enzymes in the Golgi network. M6P-tagged hydrolases are recognized by M6P receptors, which facilitate their transport to endosomes and eventually to lysosomes.

5. Examples of regulated exocytosis

• GLUT4 Translocation: Upon insulin signaling, GLUT4 transporters are rapidly inserted into the plasma membrane of fat or muscle cells, increasing glucose intake.

• Synaptic Vesicle Exocytosis: Upon neuronal depolarization, synaptic vesicles dock at the membrane, and the SNARE complex facilitates their fusion, allowing neurotransmitter release.

6. Remember different endocytic pathways

• Pinocytosis: Uptake of plasma membrane along with extracellular fluid (cellular drinking).

• Macropinocytosis: Uptake of extracellular fluid on a large scale (non-specific).

• Receptor-Mediated Endocytosis: Import of select extracellular macromolecules via specific receptors.

• Phagocytosis: Ingestion of large particles (cellular eating), such as the engulfment of red blood cells by macrophages.