Christian de Duve observed in 1955 that cells released the enzyme acid phosphatase in larger amounts when frozen and thawed before centrifugation. To explain this, de Duve suggested the enzyme must be enclosed in a membrane-bound organelle. After estimating the organelle's probable size, he was able to identify it as lysosomes in electron microscope images. Lysosomes are membrane-bound sacs containing digestive enzymes that can break down macromolecules and are involved in intracellular digestion, defense against pathogens, and cellular waste disposal. De Duve's discovery of lysosomes established them as an important cellular organelle.

2. In 1955, Belgian scientist Christian de Duve observed that
the cells released an enzyme called acid phosphatase in
much larger amounts when they were repeatedly frozen and
thawed before centrifugation
de Duve C (1975) Exploring cells with a centrifuge. Science 189, 186-194

3.  To explain this phenomenon, de Duve
suggested that the digestive enzyme
must have been encased in some sort
of membrane-bound organelle within
the cell.
 After estimating the probable size of
the lysosomes, he was able to identify
the organelle in images produced with
an electron microscope

4.  A lysosome is a membranous sac of digestive enzymes called hydrolytic
enzymes that can digest all major classes of macromolecules synthesized
by Golgi apparatus only in animal cells.
 These organelles are simple, spherical membrane bound surrounded by a
single membrane. They range in size from 25nm to 1μm in diameter.
 Lysosomes digest food and worn out cell parts that are no longer used by
the cell and are sometimes considered “Suicide Sacs” and plays a role in
waste disposal.
 With the exception of erythrocytes, lysosomes are found in all eukaryotic
cell types
old organelles
go to die!
 Lysosome = lyso (dissolving), some (body)

5. i. Primary lysosomes :contain digestive
enzymes
ii. Secondary lysosomes: contain
digestive enzymes + the digested
material.
iii. Residual bodies- undigested material
inside these bodies
iv. Lipofuscin pigments
Primary lysosome
Phagosome
Types of lysosomes and their formation
Phagosome
Residual BodiesLipofuscin pigments
Autophgaic vacoule
Residual Bodies

6.  Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and
nucleic acids via. action of catabolic enzymes like proteases, lipases,
carbohydrases and nucleases respectively.
 Enzymes present in the lysosomes are synthesized in rough endoplasmic
reticulum and are then transported through the cytoplasm by transport
vesicles into Golgi apparatus through cis-Golgi or forming face.
 These enzymes are further processed in Golgi apparatus and then budded
off from trans-Golgi or maturing face of Golgi apparatus in the form of
primary lysosome.

7.  They are formed when primary lysosomes fuses with phagocytic vesicles,
thus exposing the vesicles contents to lysosomal enzymes. The enzymes
present in the primary lysosomes digest the food present and the soluble
substances are diffused into the cytoplasm of the cell. Undigested material
containing vacuole known as residual body is expelled out by exocytosis.
Other forms of Lysosomes:
 Phagosomes: When the primary lysosome fuses with a specialized white
blood cell, the phagocyte, an activated phaogsome or phagocytic vesicles
formed. They fight against pathogen by engulfing them very rapidly than
the ordinary phagocytes.
 Autophagic vacuoles: During starvation or after the destruction of cell
components especially the liver cells and cells destroyed during
metamorphosis fuse with the primary lysosome to form autophagic
vacuoles or cytolysosomes.

8.  A typical lysosome contains at least 50 different hydrolytic enzymes
produced in the rough ER and targeted to these organelles.
 Important enzymes present in the lysosome are acid hydrolases, proteases,
lipases and acid phosphatases. These enzymes are capable of digesting
organic molecules like lipids, proteins nucleic acids and polysaccharides
under acidic conditions. So these are called “Suicidal Bags"
 They share an important property: all have their optimal activity at an acid
pH and thus are acid hydrolases and works best in the acidic (pH=4.6-5)
inside the lysosome
 Lysosomal enzymes trafficked to lysosome via mannose-6-phosphate
receptor pathway

9.  It delivers the degradative enzymes or lysosomal proteins and co-factors to
the lysosome through mannose-6-phosphate receptor
 The process requires the sequential action of two enzymes,
phosphotransferase and diesterase.
 In the Golgi apparatus, mannose-6-phosphate(M-6-P) is covalently linked
to soluble enzymes via N-linked oligosaccharides destined for lysosomes.
 N-linked oligosaccharides helps in targeting lysosomal enzymes to
lysosomes and prevent their secretion.
 Clathrin-coated vesicles transport lysosomal proteins from the trans-Golgi
network to maturing endosomes
 M-6-P is then recognised by M-6-P receptors(M-6-PR) in trans-golgi and
delivers them to late endosome

10.  M-6-P marker separates glycoproteins destined for the lysosome from
secretory glycoproteins
 Failure to acquire this marker results in mistargeting of lysosomal
enzymes; they will not enter the lysosome and substrate breakdown will
not occur
 The receptor–protein complex then moves to the late endosome where
lower pH causes dissociation
 M-6-P receptor then retrieved in late endosome and trafficked for re-use in
trans-golgi (recognised via C-terminal tail).
 The hydrolase moves on into the lysosome and the receptor is recycled
either to the Golgi to pick up another ligand, or to the plasma membrane.
 The final steps in the maturation of the lysosomal enzyme include
proteolysis, folding and aggregation

11. Adaptins bridge
the M6P receptor
to clathrin.
Hydrolases are
transported to the late
endosome which later
matures into a lysosome.
Acidic pH causes
hydrolase to dissociate
from the receptor.

12. ENZYME SUBSTRATE
Acid phosphatase Phosphate esters
Acid ribonuclease RNA
Acid deoxyribonuclease DNA
Glycosidases Polysaccharides
Protease Proteins and peptides
Lipase Lipids
Phospholipase Phospholipids

13. • They hydrolyse proteins, fats, polysaccharides, and nucleic acids.
• Can destroy the cell by autodigestion (autophagy).
• Can fuse with food vacuoles to digest food, (when a food item is brought
into the cell by phagocytosis).
• Can also fuse with another organelle or part of the cytosol. This process of
autophagy called recycling which renews the cell
• They digest unwanted particles
• They help white blood cells to destroy bacteria
Function of Lysosomal enzymes

14.  Lysosomal membrane performs important
functions:
 Sequesters potentially destructive hydrolytic
enzymes from the cytosol
 High internal proton concentration is
maintained by a proton transporter (H+-
ATPase) on the lysosomal membrane (Arai
et al.,1993)
 Maintains the optimal acidic environment for
enzyme activity by pumping H+s inward
from the cytosol to the lumen
 Membrane prevents digestive enzymes
leaking out and potentially destroying vital
cell components

15.  Lysosomal membranes contain a variety of highly glycosylated integral
proteins whose carbohydrate chains are thought to form a protective lining
that shields the membrane from attack by the enclosed enzymes.
 Mannose 6-phosphate residues in lysosomal enzymes act as an
“address” for delivery of these proteins to lysosomes
 Many single-celled organisms ingest food particles, which are then
enzymatically disassembled in a lysosome. The resulting nutrients pass
through the lysosomal membrane into the cytosol.

16.  Extracellular (large):
phagocytosis (cellular eating)
 Extracellular (small):
◦ pinocytosis (cellular
drinking)
◦ receptor-mediated
endocytosis
 Intracellular: autophagy

17.  Within the acidic compartments, the endosomes and lysosomes,
macromolecules, complex lipids, and oligosaccharides are degraded into
their building blocks by hydrolytic enzymes. The resulting catabolites are
exported from the lysosome and reused in cellular metabolism (Luzio et
al., 2007).
 This export is mediated by transport proteins present in the limiting
lysosomal membrane. These transporters make use of the energy conserved
in the proton gradient along the membrane and co-transport small
molecules or ions together with protons
◦ e.g. transport proteins for hexoses (GLUT-8) (Schmidt et al., 2009)
◦ Cobalamin, essential for degradation of branched chain amino acids, odd
chain fatty acids, and C1-metabolism delivered to the cell via the
lysosomes, which illustrates the role of lysosomes for cellular nutrition
 Other metal ions like iron ions which are essential for many cellular
processes from oxygen-transport to respiration are delivered or recycled
through the lysosomal compartment (Rutsch et al.,2009).

18. Functions of lysosomes
 The presence within a cell of what is, in essence, a bag of destructive
enzymes suggests a number of possible functions.
 In mammals, phagocytic cells, such as macrophages and neutrophils,
function as scavengers that ingest debris and potentially dangerous
microorganisms. Ingested bacteria are generally inactivated by the low pH
of the lysosome and then digested enzymatically.
 The macromolecules that are degraded in the lysosome arrive by
endocytosis, phagocytosis, or autophagy, receptor-mediated endocytosis.
 They are involved in intracellular and extracellular digestion since they
have enzymes to digest the phagocytosed food particles present in food
vacuoles.

19.  Exo-cytosis: Sometimes enzymes of primary lysosome are released from
the cell. This occurs during the replacement of cartilage by bone during
development. Similarly the matrix of bone may be broken down during the
remodelling of bone that can occur in response to injury, new stresses and
so on.
 Autolysis: It is the self destruction of a cell by release of the contents of
lysosomes within the cell. It is normal event in some differentiation
processes and may occur throughout a tissue, as when a tadpole tail is
reabsorbed during metamorphosis. It also occurs after cells die. Sometimes
it occurs as a result of certain lysosomal diseases or after cell damage.
 Recycling of important constituents: As a result of phagocytosis and
digestion of different components, the lysosomes help in the recycling of
important components of the cytoplasm

20.  They help in nutrition of the cell by digesting food, as they are rich in
various enzymes which enable them to digest almost all major chemical
constituents of the living cell.
 Help in defence by digesting germs, as in white blood cells.
 Help in cleaning up the cell by digesting damaged material of the cell.
 Provide energy during cell starvation by digestion of the cells own parts
(autophagic, auto : self; phagos: eat up).
 Help sperm cells in entering the egg by breaking through (digesting) the
egg membrane.
 In plant cells, mature xylem cells lose all cellular contents by lysosome
activity.
 When cells are old, diseased or injured, lysosomes attack their cell
organelles and digest them. In other words lysosomes are autophagic, i.e.
self devouring.

21.  Phagocytosis – cellular process of ingestion, in which the plasma
membrane engulfs substances and pinches off to form a particle-
containing vacuole
 Lysosomes may fuse with food-filled vacuoles, and their hydrolytic
enzymes digest the food. The food vacuole formed in this way then
fuses with a lysosome, whose enzymes digest the food.
 e.g. Amoebas and many other protists eat by engulfing smaller organisms
or other food particlesby the process of phagocytosis
Some human cells also carry out phagocytosis. Among them are
macrophages, a type of white blood cell that helps defend the body by
engulfing and destroying bacteria and other invaders.

22.  Apoptosis: It is a programmed cell destruction in multi-cellular organisms.
This process is important during metamorphosis and development.
 Lysosomes can be used to kill cells when they are supposed to be
destroyed, some cells have to die for proper development in an organism
e.g.: when a tadpole becomes a frog, lysosomes digest away the cells of the
tail and tail gets re-absorbed
◦ The fingers of a human embryo are at first webbed, but they are free
from one another as a result of lysosomal degradation during fetal
development.
◦ In case of plants, virus infected plant cell auto-destructs and even cells
around it to wall off virus. e.g. Brown spots on leaves
◦ if cell grows improperly this self-destruct mechanism is triggered to
remove damaged cell
◦ cancer over-rides this to enable tumor growth
 There are sensors in the cell that monitor growth. They trigger self-destruct
when they sense processes.
 These all shows Feedback mechanism

23.  Lysosomes also play a key role in organelle turnover, that is, the regulated
destruction of the cell’s own organelles and their replacement
 Lysosomes also use enzymes to recycle the cell’s own organelles, organic
material and macromolecules by the process called autophagy.
 Lysosomes may engulf other cellular organelles or part of the cytosol and
digest them. Resulting monomers are released into the cytosol where they
can be recycled into new macromolecules
◦ e.g. A human liver cell, for example, recycles half of its macromolecules
each weak
Lysosome
Lysosomes: Autophagy
Peroxisome
Mitochondrion
Vesicle
Digestion

24.  During this process (autophagy) an organelle, such as the mitochondrion
surrounded by a double membrane to produce a structure called an
autophagosome.
 The outer membrane then fuses with a lysosome to produce an
autophagolysosome in which the enclosed organelle is degraded and the
breakdown products are made available to the cell.
 If a cell is deprived of nutrients, a marked increase in autophagy is
observed. Under these conditions, the cell acquires energy to maintain its
life by cannibalizing its own organelles.
 In recent years, autophagy has also been shown to help protect an organism
against intracellular threats ranging from abnormal protein aggregates to
invading bacteria.

25.  If autophagy is blocked in a particular portion of the brain of a laboratory
animal, that region of the nervous system experiences massive loss of
nerve cells.
 These findings reveal the importance of autophagy in protecting brain cells
from the continuous damage to proteins and organelles that is experienced
by these long-lived cells.
 Once the digestive process in the autophagolysosome has been completed,
the organelle is termed a residual body.
 Depending on the type of cell, the contents of the residual body may be
eliminated from the cell by exocytosis, or they may be retained within the
cytoplasm indefinitely as a lipofuscin granule.
 Lipofuscin granules increase in number as an individual becomes older;
accumulation is particularly evident in long-lived cells such as neurons,
where these granules are considered a major characteristic of the aging
process

26. The steps involved in the autophagic pathway

27.  Diseases characterized by the deficiency of a single lysosomal enzyme and the
corresponding accumulation of undegraded substrate are called lysosomal
storage disorders.
 Lack of a specific lysosomal enzymes causes substrate accumulation which
interferes with lysosomal metabolism and other cellular functions
 Lysosomal dysfunction has a profound impact on cell homeostasis, resulting in
manifold pathological situations, including infectious diseases, neuro-
degeneration and aging.
 They can be classified according to the stored substances, as sphingolipidoses,
mucopolysaccharidoses, mucolipidoses, glycoprotein and glycogen storage
diseases
 Most of these diseases result from deficiencies in single lysosomal
enzymes

28.  Tay-Sachs disease: brain impairment by accumulation of lipids. a
metabolic disorder involving a missing or inactive lysosomal enzyme in
nerve cells. In these cases, the lysosomes fill to capacity with
macromolecules that cannot be broken down. The nerve cells become so
full of these lysosomes that the child dies.
 Pompe’s disease: In 1965, H.G. Hers explained the absence of an
unimportant lysosomal enzyme, α-glucosidase that breaks down glycogen -
glycogen accumulation which damages the liver and could lead to the
development a fatal inherited condition known as Pompe’s disease. In the
absence of α-glucosidase, undigested glycogen accumulated in lysosomes,
causing swelling of the organelles and irreversible damage to the cells and
tissues.
 Niemann-Pick disease: It leads to the accumulation not only of
cholesterol, but also of sphingomyelin, glycosphingolipids, sphingosine,
and others in multilamellar storage bodies

29.  Gaucher’s disease: Most common of lysosomal storage diseases. It is a
genetic disorder where there is accumulation of lipids in cells and organs. It
results from a mutation in the gene that encodes a lysosomal enzyme
required for the breakdown of glycolipids.
 Signs & Symptoms:
◦ Enlarged liver and spleen
◦ Low number of red blood cells (anemia)
◦ bone abnormalities such as bone pain, fractures, and arthritis.
◦ Problems with central nervous system

30.  LSD lysosomal defects give rise to swollen lysosomes, developmental and
degenerative defects with varying involvement of the nervous system due
to ‘storage’ of material in the lysosome.
 LSDs cause death in childhood (generally) after normal infancy
 LSDs are essentially incurable, but some are treatable to varying degrees
 Model organisms are helping to define the biology of the LSDs, in
particular the ‘pathogenic cascade’