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6.4.4.2. Macromolecule transport

Transport of macromolecules and particles across the plasma membrane

This transport is always with energy input, either for or against the gradient. In addition to the membrane, other organelles are involved.

There are three main types of mechanisms:

Endocytosis

It is the mechanism by which the cell can encompass high molecular weight particles (macromolecules, cell fragments, viruses, bacteria, etc.) from the environment in which it lives through vesicles that have been formed from membrane invaginations. Depending on the nature and size of the particles involved, several types of endocytosis are distinguished: phagocytosispinocytosis, and receptor-mediated endocytosis.

By Mariana Ruiz Villarrealtrabajo derivado: Gregor_0492 (Endocytosis_types.svg) [CC0], via Wikimedia Commons

    • PhagocytosisLarge-sized particles enter cells, being enclosed in invaginations of the membrane. These membranes constrict and form vesicles with ingested material. Later, the  lysosomes will bind to them to digest it.
    • Pinocytosis. It occurs when the incorporated material is liquid or contains small dissolved solid particles.
    • Receptor-mediated endocytosisIt allows the selective entry of specific macromolecules, called ligands, for which there is a corresponding receptor on the membrane. These receptors accumulate in depressions coated with clathrin (protein microfilaments). When the ligand has bound to the receptor, a vesicle is formed by endocytosis. This process is the one that uses insulin, cholesterol, or iron .

Exocytosis

It is the mechanism by which cells can expel macromolecules contained in vesicles. Thus, they expel substances synthesized by the cell or eliminate waste substances. For it to be expelled, it is necessary for the membrane of the vesicle and the plasma membrane to fuse, generating a pore through which the contents of the exocytosis vesicle can exit.

When the exocytosis vesicle fuses with the plasma membrane to expel its contents, the inner surface of the exocytosis vesicle membrane becomes the outer surface of the plasma membrane, while the outer surface of the secretory vesicle membrane it will form part of the inner surface of the plasma membrane.

As in endocytosis the membrane is lost, and in exocytosis it is gained, it is necessary to have a balance between the two to maintain the plasma membrane surface and cell volume.

By OpenStax [CC BY 4.0], via Wikimedia Commons

Transcytosis

It is a transport system that allows a substance to pass through the entire cell cytoplasm from one side of the cell to the other. Endocytosis and exocytosis occurs. It is a typical mechanism of the endothelial cells that form the blood capillaries, where substances are transported from the blood to the tissues that surround the capillaries.

Transcitosis
Transcitosis.png (Imagen PNG, 500 × 501 píxeles) - Escalado (0 %). (s. f.). Recuperado a partir de https://upload.wikimedia.org/wikipedia/commons/1/12/Transcitosis.png

Fundamental ideas about macromolecule transport

Types of transport of macromolecules through the membrane:

  • Endocytosis.
    • Invaginacioens occur in the membrane.
    • Types of endocytosis:
      • Phagocytosis: vesicles with large particles (phagosomes) are formed to which lysosomes will bind  to digest them.
      • Pinocytosis: for liquids and small dissolved solid particles. 
      • Receptor-mediated endocytosis: for specific macromolecules, called ligands, for which there is a corresponding receptor on the membrane.
  • Exocytosis.
    • Mechanism by which the molecules contained in cytoplasmic vesicles are transported from the interior of the cell to the membranes to be expelled to the extracellular environment.
  • Transcytosis . Transport system by which a substance can pass through the entire cytoplasm from one side (endocytosis) to the other (exocytosis).