Cellular Transport Mechanisms and Membrane Specializations

Transport of Macromolecules

Exocytosis: Secretion and Excretion

Exocytosis involves the flow of vesicles leaving the Golgi apparatus and moving toward the cell membrane.

• Constitutive: A continuous flow leaving the Golgi apparatus.
• Inducted: Found in cells specialized in secretory processes.

Pathway: Endoplasmic reticulum → Golgi → Membrane.

Endocytosis: Cellular Ingestion

Endocytosis is the ingestion of substances by vesicles formed within the cell membrane.

• Phagocytosis: Ingestion of large particles by specialized cells called phagocytes.
• Pinocytosis: Ingestion of substances dissolved in small liquid drops that traverse the cell membrane, forming small pinocytic vesicles.
• Receptor-Mediated Endocytosis: Macromolecules bind to transmembrane receptor proteins in coated pits. They enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles. This is a selective concentrating mechanism that increases the efficacy of ligand internalization.

Transcytosis

Transcytosis is a form of transcellular transport involving a combination of endocytosis and exocytosis (e.g., in epithelial cells). Receptors from certain polarized cells can transfer specific macromolecules from one extracellular space to another.

Types of Cell Membrane Differentiations

Microvilli

Located on the apical surface, their primary function is absorption. They increase the cellular surface area without increasing volume and feature a well-developed glycocalyx. A single cell may contain up to 3,000 microvilli to maximize absorption rates.

Lateral Interdigitations

These structures increase cell adhesion and the contact surface area between adjacent cells.

Basal Infoldings

Found on the basal surface of cells, these can be ramified or simple.

Cell Junctions

• Occluding (Tight) Junctions: Membrane-to-membrane contact formed by transmembrane proteins (claudins and occludins) that create a reticular belt or zonular junction.
• Anchoring Junctions: Linked to the cytoskeleton and composed of intracellular anchor proteins and transmembrane adhesion proteins (cadherins and integrins).
  • Cadherins: Involved in adhesion, embryogenesis, and signal transduction.
  • Integrins: Involved in cell adhesion, movement, shape, growth, and differentiation.
• Gap (Communicating) Junctions: Facilitate direct communication between cells. Each connexon is made up of 6 subunits called connexins.

Functional Locations

These mechanisms are vital for the function of organs such as the heart, liver, and intestine.