Lysosomes: Comprehensive Study Notes
Lysosomes: Structure, Types, and Functions
Lysosomes are membrane-bound organelles with a size ranging from 0.1 \text{ to } 0.8\mu m. In white blood cells (WBCs), their size is about 0.8 \text{ to } 2\mu m.
They contain digestive or hydrolytic enzymes, including acid phosphatase, proteases, lipases, glucosidase, and hexosaminidase, totaling about 40 different types of enzymes.
These enzymes are capable of breaking down every major macromolecule within the cell.
Lysosomal enzymes are synthesized in the Rough Endoplasmic Reticulum (RER), transported to the Golgi complex via the Smooth Endoplasmic Reticulum (SER), modified within the Golgi, and then released from its trans face in the form of vesicles, which are known as lysosomes.
The internal environment of lysosomes is acidic.
In plants and fungi, certain vacuoles perform enzymatic hydrolysis, a function analogous to lysosomes in animal cells.
To perform their function, lysosomes fuse with membrane-bound vesicles originating from pathways such as endocytosis, phagocytosis, and autophagocytosis. These vesicles are referred to as endosomes, phagosomes, and autophagosomes, respectively.
Types of Lysosomes
Primary Lysosomes: These are newly formed lysosomes that have not yet begun their digestive functions.
Secondary Lysosomes: These form when primary lysosomes fuse with an endosome, phagosome, or autophagosome. Within secondary lysosomes, larger biomolecules are broken down into smaller forms like amino acids, monosaccharides, nucleotides, and fatty acids, which are then recycled within the cell.
Residual Body or Tertiary Lysosomes: These are lysosomes containing undigested materials after the absorption of digested nutrients into the cytoplasm. In unicellular organisms, these are expelled from the cell via exocytosis. In multicellular organisms, they may be retained within the cell as lipofuscin granules.
Functions of Lysosomes
Intracellular Digestion: Ingested food by the cell is stored in vesicles called food vacuoles. When a lysosome fuses with a food vacuole, it forms a secondary lysosome where digestion commences. Digested materials are absorbed by the cytoplasm, and the remaining waste-containing vesicle, now often referred to as a contractile vacuole, fuses with the cell membrane (exocytosis) to eliminate undigested wastes. This entire process is known as intracellular digestion.
Autophagy: This is the process by which a cell engulfs and digests its own unwanted structures or organelles within lysosomes. It is often described as a "self-eating" process, and the lysosomes involved are sometimes called autophagosomes. Autophagy occurs during periods of starvation to obtain energy or routinely to control the number of specific organelles. For example, in an individual starting heavy muscular exercise, the number of mitochondria in muscle cells increases; if the exercise ceases, autophagy reduces the number of mitochondria.
Autolysis (Programmed Cell Death): Sometimes, particularly during developmental phases, a specific cell needs to be disintegrated. This committed cell death is termed autolysis, or programmed cell death. Lysosomes burst, dispersing their enzyme contents throughout the cytoplasm, leading to the disintegration of the cell into fragments that are subsequently phagocytosed by other cells. Because of this function, lysosomes are often referred to as "suicidal bags." Examples include the disintegration of the tadpole's tail, the tail of a human embryo, and old cells like WBCs, platelets, and epithelial cells. Lysosomal enzymes also contribute to processes like the creation of gaps between fingers and synapse formation.
Crinophagy: This process involves the digestion of excess hormones in endocrine glands by lysosomes.
Release of Extracellular Enzymes: In certain situations, primary lysosomal enzymes can be released outside the cell. An example is during the conversion of cartilage to bone (ossification), which can occur in response to injury.
Lysosomal Storage Diseases
Definition: These are complications caused by the accumulation of various substances within cells due to the lack or inactivity of specific lysosomal enzymes responsible for digesting those particular substances.
Characteristics: Lysosomal storage diseases are hereditary and congenital, meaning they run in particular families and are present from birth. Most are fatal in early childhood, and more than 20 such diseases have been identified.
Tay-Sachs Disease: This disease is characterized by a missing or inactive lipid-digesting enzyme, eta-Hexosaminidase. The brain becomes impaired due to the accumulation of lipids (gangliosides) in the cells, leading to mental retardation. Symptoms include loss of muscle control, paralysis, blindness, and ultimately death.
Glycogenosis Type II (Pompe's Disease / Glycogen Storage Disease (GSD)): In this condition, glycogen accumulates in the liver and muscle cells because the \alpha-glucosidase enzyme is missing.