Download Lysosome

Lecture 10
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Lysosome
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Lysosome
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Lysosomes are large, spherical organelles that contain
enzymes (acid hydrolases).
They break up food so it is easier to digest.
They are found in animal cells, while in plant cells the
same roles are performed by the vacuole.
They digest excess or worn-out organelles, food
particles, and engulfed viruses or bacteria.
The membrane around a lysosome allows the digestive
enzymes to work at the 4.5 pH they require.
Lysosomes fuse with vacuoles and dispense their
enzymes into the vacuoles, digesting their contents.
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How they are created?
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They are created by the addition of hydrolytic enzymes
to early endosomes from the Golgi apparatus.
The name lysosome derives from the Greek words lysis,
which means dissolution or destruction, and soma, which
means body.
They are frequently nicknamed "suicide-bags" or
"suicide-sacs" by cell biologists due to their role in
autolysis.
Lysosomes were discovered by the Belgian cytologist
Christian de Duve in 1949.
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Lysosome structure
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The size of lysosomes varies from 0.1–1.2 µm.
At pH 4.8, the interior of the lysosomes is acidic compared to the
slightly alkaline cytosol (pH 7.2).
The lysosome maintains this pH differential by pumping protons (H+
ions) from the cytosol across the membrane via proton pumps and
chloride ion channels.
The lysosomal membrane protects the cytosol, and therefore the
rest of the cell, from the degradative enzymes within the lysosome.
The cell is additionally protected from any lysosomal acid
hydrolases that leak into the cytosol as these enzymes are pHsensitive and function less well in the alkaline environment of the
cytosol.
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Primary & secondary lysosomes
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Lysosomes are formed by the Golgi apparatus.
There are primary and secondary lysosomes.
The primary are formed on the rough ER (endoplasmic reticulum).
The secondary lysosomes are formed on the smooth ER by
following the phagocytosis (process of taking solid materials into
cells).
Phagosomes fuse with lysosomes and work as one digestive
vacuole.
Lysosomal enzymes are released into this vacuole in order to digest
the bacteria or other materials.
Small molecules which are the result leave the vacuole through its
membrane and are used to make new molecules.
The indigestible materials are deposited outside the cell.
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Primary & secondary lysosomes
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Residual bodies
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In lysosomal digestion, residual bodies are vesicles
containing indigestible materials.
Residual bodies are either secreted by the cell via
exocytosis (this generally only occures in macrophages),
or they become lipofuscin granules that remain in the
cytosol indefinitely.
Longer-living cells like neurons and muscle cells usually
have a higher concentration of lipofuscin than other more
rapidly-proliferating cells.
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The Functions of Lysosomes
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Lysosomes are the cells' garbage disposal system.
They are used for the digestion of macromolecules from
phagocytosis (ingestion of other dying cells or larger
extracellular material, like foreign invading microbes)
endocytosis (where receptor proteins are recycled from
the cell surface)
And autophagy (wherein old or unneeded organelles or
proteins, or microbes that have invaded the cytoplasm
are delivered to the lysosome).
Autophagy may also lead to autophagic cell death, a
form of programmed self-destruction, or autolysis, of the
cell, which means that the cell is digesting itself.
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Enzymes
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Some important enzymes found within lysosomes
include:
Lipase, which digests lipids
Amylase, which digest carbohydrates (e.g., sugars)
Proteases, which digest proteins
Nucleases, which digest nucleic acids
phosphoric acid monoesters.
Lysosomal enzymes are synthesized in the cytosol and
the endoplasmic reticulum, where they receive a
mannose-6-phosphate tag that targets them for the
lysosome
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Autophagy
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When cells are faced with an inadequate supply of
nutrients in their extracellular fluid (ECF), they may begin
to cannibalize some of their internal organelles (e.g.
mitochondria) for re-use of their components.
This phenomenon, called autophagy, involves: formation
of a double membrane within the cell which
envelops the materials to be degraded into a vesicle
called an autophagosome.
The autophagosome then fuses with a lysosome forming
an autolysosome whose
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Autophagy
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hydrolytic enzymes degrade the materials
Intracellular materials, such as old organelles, are
brought into a lysosome by a process called
autophagy.
For example, when a mitochondrion comes to the
end of its ten-day life, it is engulfed by membrane
derived from the endoplasmic reticulum .
The newly enclosed mitochondrion then fuses with
a lysosome, resulting in its degradation by the acid
hydrolases.
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Genetic disorders
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A group of genetic disorders caused by defective
lysosomal enzymes demonstrates the importance of
lysosomes.
Called lysosomal storage diseases, these disorders are
characterized by the harmful accumulation of undigested
substances.
The accumulated materials impair or kill the affected
cells, resulting in skeletal or muscular defects, mental
retardation, or even death.
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