Cellular Breakdown and Storage Disorders: Exploring Lysosomes, Peroxisomes, and Glyoxisomes

The term "lyso" comes from the Greek word for splitting, and "soma" means body, so lysosomes are cellular structures responsible for breaking down major macromolecules. They were first discovered by De Duve in 1949 and are present in almost all animal cells. Lysosomes are roughly spherical structures enclosed by a single membrane, and their size can vary.

Lysosomes

 

The contents of lysosomes are sacs or vesicles that contain hydrolytic enzymes, which break down proteins, nucleic acids, lipids, and carbohydrates, among other cellular components. The lysosomal enzymes are synthesized in the rough endoplasmic reticulum (RER) and then transported to the Golgi apparatus, where they are enclosed in membranes to form Golgi vesicles. These vesicles are known as primary lysosomes. Once a lysosome has fused with a vesicle containing material to be digested, it is referred to as a secondary lysosome.

Lysosomes have several important functions in the cell, including phagocytosis, where foreign substances within the cell are engulfed by lysosomes and broken down into digestible molecules. This process is crucial for mammalian white blood cells to engulf and destroy bacteria and other cells. Lysosomes are also involved in autophagy, which is the process of destroying a cell's own cytoplasmic contents. Autophagy occurs in structures called autophagic vacuoles, which are a type of secondary lysosome. Additionally, lysosomes are responsible for extracellular digestion, where they break down worn-out cellular components to make way for new ones and recycle the materials within the old components.

Examples of lysosomal functions include the replacement of mitochondria in some tissues every ten days, with lysosomes digesting the old mitochondria as new ones are produced. During the metamorphosis of a tadpole into a frog, the tail is gradually absorbed, and the tail cells, which are rich in lysosomes, die and their remnants are used for the growth of new cells in the developing frog.

Once the digestive process is complete, secondary lysosomes are referred to as residual bodies. In protozoa, residual bodies are eliminated through exocytosis. However, in vertebrate cells, there appears to be no mechanism for elimination of residual bodies, leading to their accumulation within the cytoplasm. These remnants of lysosomal activity are often called lipofuscin granules and increase in number as the individual grows in size.

Storage diseases, also known as congenital diseases, can occur when certain substances accumulate within the cell due to mutations affecting lysosomal enzymes. For example, in glycogen storage disease type II, the liver and muscles appear filled with glycogen within membrane-bound organelles due to the absence of an enzyme that degrades glycogen to glucose. Other examples include Tay-Sachs disease, a congenital disorder caused by a faulty gene that leads to the progressive degeneration of nerve cells in the brain and spinal cord, resulting in mental retardation, blindness, and paralysis. This disease is caused by the absence of an enzyme involved in lipid catabolism, leading to the accumulation of lipids in brain cells.

Peroxisomes are single-membrane enclosed cytoplasmic organelles that are found in both animal and plant cells. They contain hydrogen peroxide (H₂O₂) producing oxidase and catalase enzymes. These organelles are approximately 0.5 micrometers in diameter and are also present in protozoa, yeast, and many cell types of higher plants.

Glyoxisomes, on the other hand, are organelles found specifically in plants. In addition to glycolic acid oxidase and catalase, glyoxisomes contain a number of enzymes that are not found in animal cells. These organelles are more abundant in plant seedlings, which rely on saturated fatty acids to provide them with energy and materials to begin the formation of a new plant. During germination, stored fatty acids are converted to carbohydrates through a cycle called the glyoxylate cycle, and the enzymes involved in this process are located in the glyoxisomes.

Storage Diseases are congenital diseases that result from the accumulation or storage of certain substances within cells, such as glycogen or glycolipids. These substances accumulate due to the absence or dysfunction of specific lysosomal enzymes involved in their catabolism. For example, in glycogen storage disease type II, the liver and muscles appear filled with glycogen within membrane-bound organelles due to the absence of an enzyme that degrades glycogen to glucose. Another example is Tay-Sachs disease, which is caused by a faulty gene and results in the absence of an enzyme involved in lipid catabolism. Accumulation of lipids in brain cells leads to mental retardation and can even result in death.

Lysosomes are cellular structures that contain hydrolytic enzymes and are responsible for the breakdown of major macromolecules in the cell. They are involved in processes such as phagocytosis, autophagy, and extracellular digestion. Peroxisomes are organelles involved in the formation and decomposition of hydrogen peroxide, while glyoxisomes are plant-specific organelles involved in the metabolism of fatty acids during germination. Storage diseases are congenital disorders that result from the accumulation of certain substances within cells due to the absence or dysfunction of specific lysosomal enzymes.