Lysosomes and Their Functions

Lysosomes

• Lysosomes are membrane-enclosed organelles
• They are responsible for intracellular digestion
• They are dense rounded bodies
• Approximately 300 lysosomes are found in each cell
• Their size and shape vary according to the material being digested
• Lysosomes are 0.5-1 µm in diameter
• They are abundant in cells specializing in phagocytosis, such as macrophages and neutrophils
• Found in all animal cells except erythrocytes
• Plant cells lack lysosomes but contain large vacuoles related to lysosomes which contain hydrolytic enzymes

Lysosomal Enzymes

• Lysosomes contain about 60 degradative enzymes that hydrolyze proteins, DNA, RNA, polysaccharides, and lipids
• Lysosomal acid hydrolases include proteases, nucleases, glycosidases, lipases, phosphatases, and sulphatases
• These enzymes are active at an acidic pH (pH 4.5-5.0), but not in the neutral pH (pH 7.2) of the cytoplasm.

Lysosomal Functions

• Lysosomes maintain an acidic pH by pumping H+ ions into lysosomes using the energy from ATP hydrolysis

• This results in an acidic pH inside the lysosome that is critical for keeping the enzymes active

• The lysosomal membrane includes a proton pump with ATPase that transports protons into the lysosome.

• The acidic pH provides double protection against uncontrolled digestion in the cytosol

• Lysosomal proteins are unusually glycosylated at their noncytosolic surface

• They form a glycocalyx layer, which protects lysosomal membranes from acid hydrolases

• Lysosomal membranes contain transport proteins for carrying final digestion products like amino acids, sugars, and nucleotides into the cytosol.

• Lysosomal enzymes are resistant to proteases and peptidases

• Glycosylation of lysosomal proteins protects them from degradation

Formation and Types of Lysosomes

• Vesicles containing lysosomes enzymes are called primary lysosomes
• Primary lysosomes fuse with vesicles containing material to be digested (late endosomes) to form secondary lysosomes.
• Primary lysosomes contain inactive digestive enzymes in granules
• They do not undergo digestion, and cannot eliminate content outside the cell
• Secondary lysosomes contain active digestive enzymes
• They undergo digestion
• They can eliminate content outside their cell

Lysosomal Proteins and Recognition

• Lysosomal proteins carry a unique marker, mannose-6-phosphate (M6P)
• In the cis-Golgi network, M6P markers are added to the N-linked oligosaccharides of lysosomal enzymes
• The Golgi complex packages hydrolytic lysosomal enzymes to form primary lysosomes
• M6P receptors in the trans-Golgi network recognize the M6P markers on lysosomal hydrolases
• Receptors bind and help package the hydrolases into coated transport vesicles that fuse with lysosomes
• Acidic pH in the lysosome causes the lysosomal enzymes to dissociate from the M6P receptors, releasing the enzymes into the lumen
• M6P receptors then return to the trans Golgi to be reused

Lysosomal Functions - continued

• Lysosomes mediate a variety of digestive functions
• Breakdown of intra- and extracellular materials
• Production of nutrients from digested material
• Destruction of phagocytosed microorganisms (defense mechanism)

Pathways for Delivering Materials to Lysosomes

• Digestion of macromolecules from extracellular fluid by endocytosis
• Digestion of material taken up by phagocytosis (defense against invading bacteria)
• Digestion of cell's own components (autophagy)

End Products of Lysosomal Digestion

• End products of lysosomal digestion include amino acids, fatty acids glycerol, cholesterol, monosaccharides, nucleotides, and undigested cellular waste products (e.g., lipofuscin)
• These final products are transported to the cytosol.
• Material that cannot be digested (resistant or slowly digestible residues) remain in the vesicle.
• The vesicle is released by exocytosis or forms a membrane-enclosed residual body.

Residual Bodies and Lysosomal Diseases

• Residual bodies are vesicles containing indigestible materials
• Lipofuscin is a type of residual body that accumulates in aging nerve, heart, and liver cells
• The accumulation leads to 'brown degeneration'

Lysosomal Diseases

• Lysosomal diseases arise from defects in lysosomal enzymes
• Undigested substrates accumulate and disrupt cellular functions
• There are more than 30 known lysosomal storage diseases
• These disorders are often inherited in an autosomal recessive pattern

This quiz explores lysosomes, their structure, and the crucial roles they play in intracellular digestion. You'll learn about the enzymes contained within lysosomes and how they function in various cell types. Test your knowledge on the details of lysosomes in both animal and plant cells.