22. Lysosomes and endosomes.

lysosomes

Lysosomes are membrane-bound organelles involved in intracellular digestion. They range in size from approximately 0.05 to 0.5 micrometers in diameter and contain more than 40 different hydrolytic enzymes capable of breaking down various biomolecules, including proteins, nucleic acids, carbohydrates, and lipids.

key features

• Structure: Lysosomes are spherical vesicles surrounded by a lipid bilayer membrane. This membrane helps maintain an acidic environment (around pH 5) inside the lysosome, which is optimal for the activity of the digestive enzymes they contain.

• Enzyme Content: The lysosomal enzymes are hydrolases, such as proteases, lipases, nucleases, and glycosidases, which are synthesized in the rough endoplasmic reticulum (rER), modified in the Golgi apparatus, and packaged into vesicles that become lysosomes.

• Abundance in Phagocytic Cells: Lysosomes are particularly abundant in cells with high phagocytic activity, such as macrophages and neutrophils, which ingest and degrade foreign material and cellular debris.

Function and Types:

• Primary Lysosomes: These are newly formed lysosomes that contain inactive enzymes. They originate from the Golgi apparatus and are not yet involved in digestion.

• Secondary Lysosomes: Formed when primary lysosomes fuse with vesicles containing materials to be digested, such as phagosomes (vesicles formed through phagocytosis). In this fused state, lysosomes become larger and actively digest the engulfed material. This process is known as endocytosis when external substances are engulfed.

Processes Involving Lysosomes:

• Autophagy (Self-Eating): This is a process by which cells digest their own damaged organelles or excess components. During autophagy, an autophagosome (a double-membrane vesicle) forms around the material to be degraded. The autophagosome then fuses with a lysosome, forming an autolysosome, where the enclosed material is broken down. The resulting molecules, such as amino acids and nucleotides, are released back into the cytoplasm for reuse in cellular metabolism or new synthetic reactions.

• Phagocytosis and Endocytosis: Lysosomes also digest material that is brought into the cell from outside. In phagocytosis, the cell engulfs large particles or microorganisms, forming a phagosome, which then fuses with a lysosome for degradation. In receptor-mediated endocytosis, extracellular molecules are selectively brought into the cell in vesicles that also fuse with lysosomes.

Nobel Prize in Autophagy Research:

Yoshinori Ohsumi, a Japanese cell biologist, was awarded the Nobel Prize in Physiology or Medicine in 2016 for his discoveries related to the mechanisms of autophagy, which is a fundamental process for degrading and recycling cellular components. His research has greatly advanced the understanding of how cells maintain homeostasis and respond to stress conditions, including nutrient deprivation, which can be influenced by intermittent fasting.

endosomes

membrane-bound intracellular sorting organelles that play a critical role in the endocytosis process. Endocytosis is the process by which cells internalize particles, such as nutrients, hormones, and other molecules, from the extracellular environment.

Process Involving Endosomes

• Particle Binding and Membrane Invagination: When a particle or molecule approaches the cell membrane from the extracellular side, it binds to specific receptors on the cell surface. This triggers the cell membrane to invaginate, or fold inward, enclosing the particle.

• Vesicle Formation: The membrane continues to fold until it completely engulfs the particle, forming an endocytic vesicle. This vesicle then pinches off from the cell membrane, bringing the particle into the cell's interior.

• Fusion with Endosomes: The newly formed endocytic vesicle fuses with an early endosome. Early endosomes are essential for sorting the internalized material and determining its fate within the cell.

• Sorting of Absorbed Particles:

  • Transport to Lysosomes: Some internalized particles are transported from the early endosome to lysosomes. Lysosomes are acidic organelles containing digestive enzymes that degrade the internalized material into smaller molecules, which can then be used by the cell.

  • Recycling to the Plasma Membrane: Other particles are directed to recycling endosomes. Recycling endosomes help return receptors and other molecules to the plasma membrane for reuse, maintaining cellular homeostasis.