Biochemistry and Membrane Protective Mechanisms
A lysosome contains over 60 types of lysosomal hydrolases—enzymes capable of breaking down biopolymers. Their efficient function requires an acidic environment.
Specialized proton pumps ($H^+$-ATPases) are present in the organelle membrane. Using ATP energy, they exchange intracellular sodium ions ($Na^+$) for hydrogen protons ($H^+$), pumping the latter into the vesicle lumen.
Despite the aggressive internal environment, the lysosome is protected from self-digestion. The inner surface of its membrane is covered by a dense carbohydrate layer due to a high degree of glycosylation of membrane proteins.
Functions of Lysosomes
The main task of the organelle is the intracellular digestion of macromolecules. Depending on the source of the material being digested, two processes are distinguished:
- Autophagy: the process of cellular renewal. Damaged macromolecules and obsolete organelles are degraded, allowing new components to be synthesized.
- Heterophagy: the breakdown of exogenous endocytosis products. These can be nutrients or captured solid particles, including microorganisms, ensuring cell nutrition and defense.
Traditional Classification
Historically, classification was based on the stage of the digestive process, which determines the organelle's morphology.
- Primary lysosomes: newly formed vesicles. On electron micrographs, they have a homogeneous content and contain an initial set of yet-inactive enzymes.
- Secondary lysosomes: formed by the fusion of a primary lysosome with a substrate. They are larger, and their content is heterogeneous.
Among secondary structures, phagolysosomes (containing material from pinocytic or phagocytic vesicles) and autophagosomes (containing the cell's own structures, whose number increases sharply during cell damage) are distinguished.
Modern Concept: The Endosomal Pathway
New terminology describes lysosome maturation through changes in pH levels. Primary lysosomes are now referred to as hydrolase vesicles, and secondary lysosomes as lysosomes proper. Additionally, "proton vesicles" containing exclusively acidification pumps are identified.
Stages of the endosomal cycle:
- Early endosome: forms at the cell periphery from pinocytic vesicles. Its pH is neutral and matches the extracellular environment.
- Late endosome: shifts deeper into the cytoplasm. By fusing with proton and hydrolase vesicles, it is acidified to pH 5.5–6.0, triggering enzyme activation.
- Lysosome: massive influx of proton vesicles reduces the pH to < 5.0. Intensive hydrolysis begins.
Telolysosomes and the Aging Pigment
If enzymes fail to completely degrade the material, the process enters its final stage.
- Telolysosomes (residual bodies): structures with incomplete hydrolysis. Their content becomes condensed, and enzymatic activity is lost.
- Lipofuscin: the so-called "aging pigment." It is a complex, inert protein-lipid conglomerate of undigested residues, possessing a brown color due to lipid peroxidation. It accumulates in long-lived cells incapable of division, such as neurons, muscle fibers, and cardiomyocytes.
In vivo, the work of the lysosomal apparatus is often demonstrated using India ink injection. Whole-mount skin preparations show how macrophages actively phagocytose the dye (blue granules—phagosomes and phagolysosomes—accumulate in the cytoplasm), whereas neighboring fibroblasts do not take up the ink.