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.
