Cellular Transport and Plasma Membrane Structures

Posted on Jun 19, 2025 in Biotechnology

Macrotransport: Cellular Uptake and Release

Characteristics of Macrotransport

  • Transport of large molecules/particles into and out of the cell
  • Involving deformation/folding of the plasma membrane

Types of Macrotransport

  1. Endocytosis: Transport into the cell
    • Pinocytosis (“cell drinking”)
    • Phagocytosis (“cell eating”)
  2. Exocytosis: Transport out of the cell

Endocytosis

Phagocytosis vs. Pinocytosis: Key Differences

  1. Size of the material engulfed:
    • Phagocytosis: > 250 nm vacuoles
    • Pinocytosis: < 150 nm vesicles
  2. Size of the membrane elements generated:
    • Phagocytosis: Irregular, content is electron-dense (generally)
    • Pinocytosis: Round, content is electron-lucent (generally)
  3. Cells that carry out the process:
    • Pinocytosis: Most cells (constitutively)
    • Phagocytosis: Only specialized cells (e.g., macrophages, neutrophils)
  4. Function:
    • Pinocytosis: Uptake of beneficial substances
    • Phagocytosis: Destruction of harmful particles and debris

Similarities between Phagocytosis and Pinocytosis

In both cases, the uptaken material is directed to the lysosomes.

Transcytosis

Vesicles formed on one surface of the cell move to the opposite surface, where they fuse with the membrane and release their contents outside the cell.

Receptor-Mediated Endocytosis

  • High-affinity binding of ligands to their specific receptors
  • It is a faster process
  • Depends on specific cytoplasmic proteins, known as coat proteins:
    • Clathrin is the major one
    • Caveolin (found in endothelial cells)

Structure of the Clathrin Coat

  • Units: Triskelions
  • Network: 12 pentagons and 6 hexagons

Exocytosis: Transport Out of the Cell

Secretion

The process of exocytosis of macromolecules made by cells, occurring via either of two pathways:

  • Constitutive Secretion:
    • For products that are released continuously, as soon as synthesis is completed.
    • Involves the synthesis of new components of the plasma membrane, glycocalyx molecules, extracellular matrix components, etc.
  • Regulated Secretion:
    • Only in cells specialized in secretion, in response to signals coming to the cells.
    • Involves the secretion of hormones, neurotransmitters, and enzymes.

Plasma Membrane Specializations and Functions

Introduction to Plasma Membrane Specializations

Regions of the plasma membrane that exhibit special structural modifications to carry out specific functions.

Types of Plasma Membrane Specializations

  • Apical Domain: Microvilli and Cilia
  • Basal Domain: Basal Infoldings
  • Lateral Domain: (In contact with the opposed lateral domains of neighboring cells) Cell Junctions

Microvilli

  • Finger-like cytoplasmic projections on the apical surface of most epithelial cells.
  • Function: Microvilli increase the surface area of the apical cell domain, improving absorption.
  • In addition, there is a very developed glycocalyx in microvilli (with filter function).

Appearance of Microvilli

  • In Transmission Electron Microscopy (TEM), microvilli vary in appearance.
  • In general, the number, length, and shape of the microvilli of a given cell type correlate with the cell’s absorptive capacity.
  • Among fluid-transporting epithelia (e.g., intestine and kidney tubule cells), a distinctive border of vertical striations at the apical surface of the cell is easily seen in Light Microscopy (LM), especially when stained with Periodic Acid-Schiff (PAS).

Basal Infoldings

  • Interdigitations of the basal cell membrane, especially in cells that transport fluid.
  • Function: To increase the surface area of the basal cell domain, improving osmotic regulation.
  • High numbers of mitochondria are found in the infoldings.

Cell Junctions

  • Epithelial cell junctions found between cells or between a cell and the extracellular matrix.
  • Types:
    1. Interdigitation
    2. Tight Junction
    3. Adherent Junction
    4. Desmosome
    5. Hemidesmosome
    6. Gap Junction

Interdigitations

  • Cell processes (of various dimensions) extending between similar processes of neighboring cells.
  • To reinforce cell union.
  • Most frequent in epithelial tissues.

Tight Junction (Zonula Occludens)

  • Function: Controls the passage of molecules between adjacent cells.
  • Major Transmembrane Link Proteins: Occludins, Claudins.
  • Studies with different techniques:
    • Conventional Electron Microscopy (EM)
    • Studies with tracer molecules
    • Freeze fracture

Terminology for Cell Junctions

  • According to the extension:
    • Zonula: Circumferential belt around the apical region of the cell.
    • Macula: Localized spot-like junction (around 1 mm²).
  • According to the distance between cells:
    • Occludens: Localized sealing of the plasma membranes of adjacent cells.
    • Adherens: The membranes of adjacent cells are separated.

Cell Junctions: Classification by Function

  • Anchoring Junctions:
    • Strengthen binding between cells and cell-extracellular matrix.
    • Types: Adherent Junction, Desmosome, and Hemidesmosome.
  • Occluding Junctions:
    • Control the passage of molecules between adjacent cells (paracellular barrier function).
    • Type: Tight Junction.
  • Channel-Forming Junctions:
    • Allow the passage of small molecules and ions from one cell to another.
    • Type: Gap Junction.