Cells use endocytosis and exocytosis to move big stuff in and out. Endocytosis brings things like nutrients and pathogens into the cell, while exocytosis releases hormones and other important molecules to the outside world.

These processes are crucial for cellular transport and communication. Endocytosis has different types, like phagocytosis for eating large particles, while exocytosis relies on vesicles fusing with the cell membrane to release their contents.

Bulk Transport Mechanisms

Endocytosis and exocytosis in transport

• Endocytosis involves the internalization of substances from the extracellular environment by invaginating the plasma membrane to form vesicles containing the ingested material (nutrients, signaling molecules, pathogens)
• Exocytosis releases substances to the extracellular environment through the fusion of intracellular vesicles with the plasma membrane, expelling their contents (hormones, neurotransmitters, extracellular matrix components)
• Both processes play crucial roles in cellular transport, enabling cells to take up essential substances and secrete molecules for communication and regulation of physiological processes

Types of endocytosis

• Phagocytosis or "cell eating" ingests large particles (>0.5 μm) such as bacteria, cell debris, or apoptotic cells using actin-driven membrane protrusions that engulf the particle to form a phagosome, primarily performed by specialized cells (macrophages, neutrophils)
• Pinocytosis or "cell drinking" uptakes small particles and dissolved substances from the extracellular fluid by forming small vesicles (<0.2 μm) that pinch off from the plasma membrane, further classified into macropinocytosis and micropinocytosis based on vesicle size
• Receptor-mediated endocytosis specifically uptakes macromolecules by binding ligands to specific receptors on the cell surface, causing ligand-receptor complexes to cluster in coated pits that invaginate to form coated vesicles, mainly composed of clathrin protein forming a polyhedral lattice

Process and importance of exocytosis

• Exocytosis process:

1. Secretory vesicles derived from the Golgi apparatus contain substances to be released
2. Fusion of secretory vesicles with the plasma membrane is mediated by SNARE proteins
   • v-SNAREs on the vesicle membrane and t-SNAREs on the target (plasma) membrane interact to facilitate fusion

• Importance of exocytosis:
  • Enables secretion of hormones (insulin, growth hormone), neurotransmitters (acetylcholine, dopamine), and digestive enzymes (pepsin, trypsin) for communication between cells and regulation of physiological processes
  • Releases extracellular matrix components (collagen, proteoglycans) crucial for maintaining tissue structure and function
  • Incorporates newly synthesized membrane proteins and lipids into the plasma membrane, allowing for renewal and modification of the cell surface

Examples of endo-exocytosis transport

• Substances transported via endocytosis:
  • Nutrients (proteins, lipids, carbohydrates)
  • Signaling molecules (hormones, growth factors, cytokines)
  • Pathogens (viruses, bacteria during phagocytosis)
  • Plasma membrane components (receptors, lipids during membrane recycling)
• Substances transported via exocytosis:
  • Hormones (insulin, growth hormone, thyroid hormones)
  • Neurotransmitters (acetylcholine, dopamine, serotonin)
  • Digestive enzymes (pepsin, trypsin, lipase)
  • Extracellular matrix components (collagen, fibronectin, proteoglycans)