Lysosomes: Structure, Functions, Diagram

Lysosomes are membrane-bound, dense granular structures containing hydrolytic enzymes responsible mainly for intracellular and extracellular digestion.

• The word “lysosome” is made up of two words “lysis” meaning breakdown and “soma” meaning body.
• It is an important cell organelle responsible for the inter and extracellular breakdown of substances.
• They are more commonly found in animal cells while only in some lower plant groups ( slime molds and saprophytic fungi).
• Lysosomes occur freely in the cytoplasm. In animals, found in almost all cells except in the RBCs.
• They are found in most abundant numbers in cells related to enzymatic reactions such as liver cells, pancreatic cells, kidney cells, spleen cells, leucocytes, macrophages, etc.

Structure of Lysosomes

• Lysosomes are without any characteristic shape or structure i.e. they are pleomorphic
• They are mostly globular or granular in appearance.
• It is 0.2-0.5 μm in size and is surrounded by a single lipoprotein membrane unique in composition.
• The membrane contains highly glycosylated lysosomal associated membrane proteins (LAMP) and Lysosomal integral membrane proteins (LIMP).
• LAMPs and LIMPs form a coat on the inner surface of the membrane
• They protect the membrane from attack by the numerous hydrolytic enzymes retained inside.
• The lysosomal membrane has a hydrogen proton pump which is responsible for maintaining pH conditions of the enzyme The acidic medium maintained by the proton pump that pumps H+ inside the lumen, ensures the functionality of the lysosomal enzymes.
• Inside the membrane, the organelle contains enzymes in the crystalline form.

Lysosomal Enzymes

For degradation of extra and intracellular material, lysosomes filled with enzymes called hydrolases. It contains about 40 varieties of enzymes which are classified into the following main types, namely:

• Proteases, which digest proteins
• Lipases, which digests lipids
• Amylase, which digests carbohydrates
• Nucleases, which digest nucleic acids
• Phosphoric acid monoesters

Collectively the group of enzymes is called hydrolases which cause cleavage of substrates by the addition of water molecules. Most of the lysosomal enzymes function in the acidic medium.

 Lysosomal Membrane

• As compared to the mitochondria, it is slightly thicker.
• Sialic acid is present in it.
• Since the lysosomal membrane protein is highly glycosylated, it protects from the action of the lysosomal proteases.
• The lysosomal membrane can fuse with the other membranes of the cell which is the unique property.
• When the lysosomal membrane ruptures, lysosomal enzymes are released.
• It can be caused by the destabilizing influence of the surface-active agents and the steroid sex hormones.
• The lysosomal membrane is stabilized by cortisone and hydrocortisone.
• On the tissue, they possess an anti-inflammatory effect. 
• Within the lysosome all the process of digestion takes place.
• For the action of the lysosomal enzyme, the medium should be acidic.
• To maintain the acidic condition inside the organelle, there should be an accumulation of the H+.
• It is maintained by the proton pump which is ATP-dependent.
• Transport protein is also present in the lysosomal membrane.
• When the macromolecules get digested, the final products can be transported by these proteins.
• After the transportation, they can be further utilized by the cell or be excreted.

Types of Lysosomes

• Polymorphism can be seen in the morphology of the lysosome.
• There are four types of lysosomes. They are:
  • Primary lysosome
  • Heterophagosomes
  • Autophagososomes
  • Residual Bodies

A. Primary Lysosomes

• They are also called:
  • Storage granules
  • Protolysosomes
  • Virgin lysosomes
• The primary lysosome is bounded by a single membrane.
• It has a diameter of 100nm.
• A digestive enzyme is present in it which has not taken part in the digestion.
• One type of enzyme or another is present in it.
• Only in the secondary lysosome, there is the presence of the full complement of acid hydrolases.

B. Heterophagosomes

• They are also called:
  • heterophagic vacuoles
  • heterolysosomes 
  • Phagolysosomes
• When the primary lysosome fuse with the cytoplasmic vacuoles, a heterophagosome is formed.
• Extracellular substances are present in the cytoplasmic vacuoles.
• Different endocytic processes like pinocytosis, phagocytosis, or receptor-mediated endocytosis help in bringing such extracellular substances into the cell.
• In these secondary lysosomes, there is the presence of the hydrolytic enzymes which digest the engulfed substances.
• After digestion, such particles will pass across the membrane of the lysosome.
• Then it will become part of the matrix.

C. Autophagosomes

• They are also called:
  • Autophagic vacuole
  • cytolysosomes 
  • Autolysosomes
• Digestion of the different intracellular structures like mitochondria, ribosome, peroxisome, and glycogen granules can be done by the primary lysosome.
• Autophagy is called autodigestion.
• During cell growth and repair, autophagy is a normal event.
• It is prevalent in the differentiation, the dedifferentiation of the tissues and tissue under stress.

Autophagy occurs in different forms:

a. By fusion of the lysosome:

• It is enclosed inside the double membrane sac.
• Then there is the breakdown of the inner membrane.
• Penetration of the enzyme can occur to the enclosed organelle.

b. Formation of a vesicle and fusion with primary lysosome:

• Vesicle is formed when the smooth endoplasmic reticulum encases the organelle which needs to be digested.

Microautophagy also takes place. When the digestion process proceeds then it becomes difficult to identify the type of secondary lysosome if it is heterophagosome or autophagosome. So, in this stage, it is said as the digestive vacuole.

D. Residual Bodies

• They are also called:
  • Telolysosome
  • Dense bodies
• Incomplete digestion results in the formation of the residual bodies.
• When some lysosomal enzymes are absent, incomplete digestion may occur.
• Inside the digestive vacuoles, the undigested food remains as the residue.
• Then they make take different forms.
• Residue body is larger and irregular in shape.
• By the defecation, residual bodies are eliminated in the case of Amoeba and some other protozoa.
• In some cells, for a longer period, the residual body may stay which may cause aging.

Functions of Lysosomes

Lysosomes serve two major functions:

1. Intracellular Digestion

• To digest food, the lysosome membrane fuses with the membrane of food vacuole and squirts the enzymes inside.
• The digested food then diffuses through the vacuole membrane and enters the cell to be used for energy and growth.

2. Autolytic Action

• Cell organelles that need to be ridden are covered by vesicles or vacuoles by the process of autophagy to form autophagosome.
• The autophagosome is then destroyed by the action of lysosomal enzymes.

Processes in which lysosomes play crucial roles include:

a. Heterophagy

The taking into the cell of exogenous material by phagocytosis or pinocytosis and the digestion of the ingested material after fusion of the newly formed vacuole with a lysosome.

b. Autophagy

A normal physiological process that deals with the destruction of cells in the body. It is essential for maintaining homeostasis, for normal functioning by protein degradation, turnover of destroyed cell organelles for new cell formation

c. Extracellular Digestion

Primary lysosomes secrete hydrolases outside by exocytosis resulting in degradation of extracellular materials.

Eg. Saprophytic fungi

d. Autolysis

It refers to the killing of an entire set of cells by the breakdown of the lysosomal membrane. It occurs during amphibian and insect metamorphosis.

e. Fertilization

The acrosome of the sperm head is a giant lysosome that ruptures and releases enzymes on the surface of the egg. This provides the way for sperm entry into the egg by digesting the egg membrane.

f. As Janitors of the Cell

Lysosomes remove ‘junk’ that may accumulate in the cell helping to prevent diseases.

Presence of Lysosomes

Presence of Lysosomes in the Animal tissue

• Liver
• Kidney
• Nerve cells
• Brain
• Intestinal epithelium
• Lung epithelium
• Macrophages(of the spleen, bone marrow, liver, and connective tissue)
• Thyroid gland
• Adrenal gland
• Bone
• Urinary bladder
• Prostate
• Uterus
• Ovaries

Presence of lysosome in the Protozoa

• Leucocytes
• Amoeba
• Tetrahymena
• Paramecium
• Euglena

Presence of lysosome in Plants

• Onion seeds
• Barley seeds
• Corn seedlings
• Yeast
• Neurospora

Presence of lysosome in Tissue culture cells

• HeLa cells
• Fibroblasts
• Chick cells
• Lymphocytes

Reference

1. Verma, P.S. and Agrawal, V..K. (1991). Cell biology, Genetics, Molecular Biology, Evolution and Ecology, S.Chand Company.
2. Kar,D.K. and halder,S. (2015). Cell biology genetics and molecular biology.Kolkata, New central book agency.
3. Perera, R. M., & Zoncu, R. (2016). The Lysosome as a Regulatory Hub. Annual Review of Cell and Developmental Biology, 32, 223–253. https://doi.org/10.1146/annurev-cellbio-111315-125125 
4. Jezegou A, Llinares E, Anne C, Kieffer-Jaquinod S, O’Regan S, et al. 2012. Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy. PNAS109:E3434–43
5. Liu B, Du H, Rutkowski R, Gartner A, Wang X. 2012. LAAT-1 is the lysosomal lysine/arginine transporter that maintains amino acid homeostasis. Science 337:351–54
6. Sagne C, Agulhon C, Ravassard P, Darmon M, Hamon M, et al. 2001. Identification and characterization of a lysosomal transporter for small neutral amino acids. PNAS 98:7206–11.
7. R and suwal,S.N. (2010).Human Anatomy and physiology. Kathmandu, vidyarthi prakashan (p.) ltd.
8. https://study.com/academy/lesson/lysosome-definition-function-quiz.html
9. https://biologydictionary.net/lysosome/
10. https://biologywise.com/lysosome-structure