Types, Morphology, Function, Process and Microscopy
What are Lysosomes?
Lysosomes are the main digestive compartment of
the cell. As such, they contain a variety of enzymes capable of degrading
different types of biological material including nucleic acids, lipids and
proteins among others.
They can be found in animal cells and some plant
cells (occurring as vacuoles) and are capable of breaking down various types of
macromolecules brought in to the cell to be degraded. Most of these are either damaged or have completed their life cycle and are no
In addition to these macromolecules, lysosomes
also serve to break down cells once they die. While they can be found in almost
all cells in animals (except red blood cells) they are particularly
abundant in tissues/organs that are involved in high enzymatic reactions.
include such tissues/organs as the liver, kidney, macrophages and pancreas
among a few others. Cells of these tissues/organs contain abundant lysosomes.
* The name lysosome originated from Greek words
Lysis (meaning destroy/dissolve) and Soma (meaning body).
* Animal cells may contain numerous lysosomes
(several hundred) plant and yeast cells typically have a single, large lysosome
Primary lysosomes - are
formed from Golgi apparatus appearing as small vesicles. Although primary
lysosomes are popular on Golgi apparatus, they also occur as granulocytes and
monocytes. These lysosomes are surrounded by a single phospholipid layer and
contain acid hydrolases.
The pH value of the acid in these vesicles is
important in that its changes activate or deactivate the enzymes. Ultimately,
most of the primary granules will fuse with phagosomes, which results in the
formation of secondary lysosomes.
Secondary lysosomes - are formed when primary lysosomes fuse with phagosomes/pinosome (they
are also referred to a endosomes). The fusion also causes the previously
inactive enzymes to be activated and capable of digesting such biomolecules as
nucleic acids and lipids among others.
Compared to primary lysosomes, secondary are larger in size and capable of releasing their content (enzymes)
outside the cells where they degrade foreign material.
A majority of lysosomal enzymes function inside
the acidic environment, which is why they are referred to a acid hydrolases.
They contain about 45 enzymes that are grouped in to six main
Nucleases - Nucleases are important
enzymes that hydrolyze nucleic acids. Nucleases are divided in to
deoxyribonuclease (acts on DNA) and ribonuclease which hydrolyses RNA.
Hydrolysis action on nucleic acids results in the production of sugars,
nitrogen bases as well as phosphates.
Proteases - Proteases includes enzymes
like collagenase and peptidases that acts on proteins converting them to amino
Glycosidases - Glycosidases like beta
galactosidase act on the glycosidic bonds of polysaccharides converting polysaccharides
to monosaccharides. For instance, the enzyme galactosidase acts on such bonds
converting lactose to glucose and galactose.
Phosphatases - Good examples of
Phosphatases are acid phosphodiesterases. These are important enzymes that act
on organic compounds releaing phosphate in the process. However, the compound
has to have a phosphate group.
Lipases - Lipases include
esterases and phospholipiases that act on lipids to produce acids and alcohol
Sulphatases - Sulphatases are enzymes
that act on organic compounds to release sulphates
* Lysosomes cannot digest themselves - Most of
the proteins present in its membrane contain high amounts of
carbohydrate-sugar groups. Because of the present of these groups, digestive enzymes
are unable to digest the proteins present on the membrane.
Lysosomes are membrane-delimited organelles.
This means that they are surrounded by a membrane that prevents its components
from being released. This is particularly important given that uncontrolled
release of the acidic fluid and enzymes can cause damage to the components of the
They also have a high concentration of protons, which results in pH value
of less than 5.
The surrounding membrane is composed of integral proteins as
well as a vacuolar-type H+ ATPase, highly glycosylated proteins and a number of
transporters. Depending on the type of lysosome and their function, they also
greatly vary in size (between 1 micrometer and several microns) and general
They are less defined compared to other types of organelles.
When viewed, they appear as cytoplasmic dense bodies that may be ovoid, spheric
or tubular on occasion.
The manner in which lysosomes function highly
depends on the way the enzymes affect other materials outside and inside the
cell. There are a number of processes through which lysosomes digest material.
Endocytosis is one of the most popular
phenomenon exhibited by cells. In endocytosis, invagination of the plasma
membrane of the cell results in the creation of an endocytic vesicle that
engulfs different types of extracellular molecules.
In phagocytosis (a type of
endocytosis) large molecules/microorganisms like bacteria are engulfed in a
vacuole (phagosome). However, in pinosomes (another type of endocytosis) a
small amount of the surrounding fluid and solute molecules are pinched off as
pinosomes (pinocytic vesicle).
Once these vesicles fuse with primary lysosomes,
secondary lysosomes are formed. Enzymes are then activated
and act on the molecules. This process is commonly referred to as the
* Digested material is typically passed into
cellular component while undigested ones are excreted.
Material uptake- formation of endosome - formation of the phagolysosome
- lysis- diffusion of digested material and exocytosis.
Apart from endocytosis, lysosomes are also
involved in another process referred to as autophagocytosis. This process helps
in the degradation of various cell components that are either worn out or
In addition to simply breaking down cellular components, this
process helps in recycling of these material. Here, autophagic lysosomes
(secondary) release enzymes that digest various cell components as
soon as the cell dies. This is also referred to as autolysis.
takes place during starvation. During starvation periods, lysosomes will start hydrolyzing
organic foods that are stored in cells so as to produce energy.
In plants, vacuoles serve many functions, which make
them multifunctional organelles. However, they also have basic properties
similar to lysosomes found in the cells of animal. As such, they have an acidic
nature and contain various hydrolic enzymes capable of breaking down different
types of molecules.
There are different types of vacuoles that serve different
functions. These include:
Sap vacuoles - these vacuoles
store salt minerals and nutrients
Contractile vacuoles -
involved in osmoregulation and excretion
Food vacuoles - involved in
the digestion of nutrients - this vacuoles contain digestive enzymes that
digest these nutrients
Air vacuoles - serve to store
metabolic gases in prokaryotes
Differences and Similarities between Primary and Secondary Lysosomes
Both have acid
Both have a single
phospholipid membrane surrounding them
Whereas primary lysosomes
are buds that form from Golgi apparatus, the secondary are formed from
fusion of primary lysosomes and phagosomes or pinosomes
Primary lysosomes largely
serve to store material (as storage vacuoles) while secondary largely
serve as digestive vacuoles where lysis occurs through the activity of
While the primary do
not release any of their content, secondary lysosomes can release their content
(enzymes) in to the cell cytoplasm for exocytosis. Exocytosis is where enzymes
are released in to the cytoplasm or outside the cell where they degrade
different types of material.
For most part, primary
lysosomes have inactive hydrolases while secondary have active
lysosomes capable of acting on molecules and other material
Lysosomes are also important in that they act as
recycling centers. Whenever different types of molecules or cell components are
broken down (for instance proteins broken down to amino acids) the amino acids
are then used as building blocks of new proteins. This ensures that some of the
byproducts are re-used in the body.
On the other hand, they help recycle
material that are not easy to excrete. A good example of this is iron. When iron
is released from the breakdown of various cells or cell components (such as red
blood cells), iron is recycled and used for the construction of new organelles.
This allows for a minimized excretion of the by-products and retention of
others to be used in the body.
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