- Introduction to Lysosomes
- Structure of Lysosomes
- Lysosomal Enzymes
- Types of Lysosomes
- Functions of Lysosomes
Introduction to Lysosomes
- Lysosomes exhibit a membrane-bound, dense granular structure and house hydrolytic enzymes crucial for intracellular and extracellular digestion.
- The term "lysosome" originates from the combination of "lysis," indicating breakdown, and "soma," signifying body. This organelle plays a pivotal role in both inter and extracellular breakdown processes.
- While more prevalent in animal cells, lysosomes are selectively present in specific lower plant groups like slime molds and saprophytic fungi.
- Freely distributed in the cytoplasm, lysosomes are ubiquitous in animal cells, with the exception of red blood cells (RBCs).
- Abundantly found in cells associated with enzymatic reactions, lysosomes are particularly prevalent in liver cells, pancreatic cells, kidney cells, spleen cells, leukocytes, macrophages, among others.
Structure of Lysosomes
- Lysosomes lack a distinct or characteristic shape, displaying pleomorphism. Typically, they exhibit a globular or granular appearance.
- Ranging in size from 0.2 to 0.5 μm, lysosomes are encased in a singular lipoprotein membrane with a unique composition.
- The membrane is equipped with highly glycosylated proteins known as Lysosomal Associated Membrane Proteins (LAMP) and Lysosomal Integral Membrane Proteins (LIMP), forming a protective coat on its inner surface.
- This protective coat, comprised of LAMPs and LIMPs, serves to safeguard the membrane against potential damage from the numerous hydrolytic enzymes housed within.
- Within the lysosomal membrane, there exists a hydrogen proton pump responsible for maintaining optimal pH conditions for enzyme activity. This pump actively transports H+ ions into the lumen, creating an acidic environment crucial for the efficacy of lysosomal enzymes.
- The organelle's interior contains enzymes in a crystalline form, contributing to its functional capacity.
Lysosomal Enzymes
Lysosomes are organelles responsible for breaking down both extracellular and intracellular materials, utilizing a diverse array of enzymes known as hydrolases. This enzyme collection comprises approximately 40 different types, primarily falling into the following categories:
- Proteases: These enzymes specialize in the digestion of proteins.
- Lipases: Their role is to break down lipids.
- Amylase: This enzyme targets the digestion of carbohydrates.
- Nucleases: Responsible for breaking down nucleic acids.
- Phosphoric Acid Monoesters: Enzymes within this category facilitate the cleavage of substrates through the addition of water molecules.
Collectively referred to as hydrolases, these enzymes catalyze substrate cleavage through hydrolysis. Notably, most lysosomal enzymes demonstrate optimal activity in acidic environments.
Lysosomal Membrane
- Lysosomal membrane, slightly thicker than mitochondria, contains sialic acid.
- High glycosylation of lysosomal membrane proteins protects against lysosomal proteases.
- Unique property: lysosomal membrane can fuse with other cellular membranes.
- Lysosomal membrane rupture releases lysosomal enzymes, influenced by surface-active agents and steroid sex hormones.
- Stability maintained by cortisone and hydrocortisone, contributing to anti-inflammatory effects on tissues.
- Lysosomal digestion processes occur within the lysosome, reliant on an acidic medium for optimal enzyme activity.
- Acidic conditions are upheld by the ATP-dependent lysosomal proton pump, accumulating H+ ions.
- Transport proteins present in the lysosomal membrane facilitate the movement of final digestion products.
- Transported products can be utilized by the cell or excreted after digestion.
Types of Lysosomes
Morphological variability is observed in lysosomes, resulting in four distinct types:
A. Primary Lysosomes
- Also known as Storage granules, Protolysosomes, or Virgin lysosomes.
- Bounded by a single membrane, with a 100nm diameter.
- Contains digestive enzymes that have not participated in digestion yet.
- Various types of enzymes may be present.
- Full complement of acid hydrolases is found only in secondary lysosomes.
B. Heterophagosomes
- Also referred to as heterophagic vacuoles or phagolysosomes.
- Formed when primary lysosomes fuse with cytoplasmic vacuoles containing extracellular substances.
- Endocytic processes like pinocytosis, phagocytosis, or receptor-mediated endocytosis bring in extracellular substances.
- Secondary lysosomes with hydrolytic enzymes digest engulfed substances, and digested particles become part of the lysosomal matrix.
C. Autophagosomes
- Also known as Autophagic vacuole or Autolysosomes.
- Responsible for digesting intracellular structures like mitochondria, ribosomes, peroxisomes, and glycogen granules.
- Autophagy, a normal event during cell growth, repair, differentiation, dedifferentiation, and tissue stress.
- Autophagy occurs through fusion of lysosomes and formation of vesicles that fuse with primary lysosomes.
D. Residual Bodies
- Also called Telolysosome or Dense bodies.
- Formed due to incomplete digestion, often when certain lysosomal enzymes are absent.
- Undigested residue remains inside digestive vacuoles and may take different forms.
- Residual bodies are larger and irregular in shape, eliminated through defecation in some organisms.
- Prolonged presence of residual bodies in certain cells may contribute to aging.
Functions of Lysosomes:
Lysosomes serve two primary functions:
1. Intracellular Digestion:
- The lysosomal membrane fuses with the membrane of a food vacuole, releasing enzymes inside for digestion.
- Digested food diffuses through the vacuole membrane, entering the cell for energy and growth.
2. Autolytic Action:
- Autophagy involves covering cell organelles with vesicles or vacuoles, forming autophagosomes.
- Lysosomal enzymes destroy the autophagosome, facilitating cellular cleanup.
Processes Involving Lysosomal Roles:
a. Heterophagy:
- The cell takes in exogenous material through phagocytosis or pinocytosis.
- Fusion of the vacuole with a lysosome leads to the digestion of ingested material.
b. Autophagy:
- A physiological process ensuring cell destruction for homeostasis.
- Essential for normal functioning, involving protein degradation and turnover of cell organelles.
c. Extracellular Digestion:
- Primary lysosomes secrete hydrolases via exocytosis, degrading extracellular materials.
- Example: Saprophytic fungi.
d. Autolysis:
- Breakdown of the lysosomal membrane results in the killing of a set of cells.
- Occurs during amphibian and insect metamorphosis.
e. Fertilization:
- The acrosome of the sperm head, a giant lysosome, ruptures, releasing enzymes on the egg surface.
- Enables sperm entry into the egg by digesting the egg membrane.
f. Janitors of the Cell:
- Lysosomes function as cellular 'janitors,' removing accumulated 'junk' to prevent diseases.
Distribution of Lysosomes:
In Animal Tissues:
Lysosomes are found in various animal tissues, including:
- Liver
- Kidney
- Nerve cells
- Brain
- Intestinal epithelium
- Lung epithelium
- Macrophages (spleen, bone marrow, liver, and connective tissue)
- Thyroid gland
- Adrenal gland
- Bone
- Urinary bladder
- Prostate
- Uterus
- Ovaries
In Protozoa:
Lysosomes are present in certain protozoa, such as:
- Leucocytes
- Amoeba
- Tetrahymena
- Paramecium
- Euglena
In Plants:
Lysosomes can be found in specific plant structures:
- Onion seeds
- Barley seeds
- Corn seedlings
- Yeast
- Neurospora
In Tissue Culture Cells:
Lysosomes are identified in various cultured cells, including:
- HeLa cells
- Fibroblasts
- Chick cells
- Lymphocytes
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