Cellular Import and Export Mechanisms

Vesicular Transport

  • Cells transport materials in and out via vesicles.
  • Vesicles are membrane-bound containers.

Endocytosis

  • Bringing substances into the cell by forming a vesicle at the cell surface that travels inward.

Exocytosis

  • Secreting manufactured products (hormones, antibodies, etc.) out of the cell.
Exocytosis Process
  • Molecules destined for secretion are produced in the ER and Golgi apparatus.
  • Packaged in transport vesicles.
  • Vesicles move toward the plasma membrane and fuse with it.
  • Contents are emptied into the extracellular space.

Endocytosis: Bringing Substances Into the Cell

  • Substances from outside are brought into the cell in vesicles called endosomes.
  • Three main forms:
    • Phagocytosis.
    • Pinocytosis.
    • Receptor-mediated endocytosis.

Phagocytosis

  • "Cellular eating."
  • Uptake and breakdown of large particles (e.g., bacteria).
  • Plasma membrane extends around the particle, forming pseudopodia (singular: pseudopodium).
  • Engulfed particle is contained in a phagosome (endosome).
  • The particle inside awaits digestion.
Function
  • Important for immune system cells (e.g., neutrophils) engulfing bacteria.
  • Protects humans from disease-causing microbes.
  • Amoebas use phagocytosis to feed on smaller organisms.

Pinocytosis

  • "Cellular drinking."
  • Uptake of small particles or dissolved substances.
  • Plasma membrane invaginates, forming an endosome.
  • Uptake of extracellular fluid and its contents.
Mechanism
  • Phagocytosis involves pseudopods and cytoskeleton movement.
  • Pinocytosis involves invagination via clathrin molecules on the inside of the plasma membrane.

Receptor-Mediated Endocytosis

  • Specific receptors in the plasma membrane bind to cargo molecules in the extracellular space.
  • Clathrin molecules assemble with the cytosolic part of the receptor on the other side of the membrane.
Process
  • Clathrin forms a sphere, creating a coated vesicle.
  • Once inside, the vesicle loses its clathrin coat and becomes "naked" in the cytosol.
  • The coated pits are coated with receptors on one side and clathrin on the other.
  • Clathrin forms a cage-like structure around the vesicle.
Viral Hijacking
  • Viruses like influenza, HIV, and SARS-CoV-2 can enter cells by hijacking receptor-mediated endocytosis.
Cholesterol Uptake Example
  • Studied through the uptake of cholesterol carried by low-density lipoprotein (LDL).
  • Cholesterol-LDL binds to LDL receptors.
  • Enters via clathrin pathway, forming a naked vesicle containing cholesterol-LDL.
  • Vesicle fuses with/becomes an endosome, and cholesterol-LDL detaches from the receptor.
Recycling
  • LDL receptors are recycled back to the plasma membrane in transport vesicles.
  • Endosome with cholesterol-LDL fuses with a lysosome.

Lysosomes

  • Organelles filled with various enzymes that break up the LDL and free the cholesterol.
  • Packages of destruction that deliver breakdown capability to whatever they fuse with.
  • More acidic inside (pH ≈ 5) than cytosol (pH ≈ 7.2) due to ATP-driven proton pumps. The cell spends energy in the form of ATP to pump protons into the lysosome
  • Contains hydrolytic enzymes that break down biological molecules.
  • Lysosomal enzymes are acid-tolerant.
Function
  • Anything brought into the cell by endocytosis can be processed by lysosomes.
  • Involved in autophagy ("eating itself"):