what is the largest organelle in a cell
Lysosomes are acidic organelles that contain a battery of degrading enzymes
Lysosomes provide an excellent example of the ability of intracellular membranes to form closed compartments, where the composition of the lumen (water-containing part of the compartment) is fundamentally different from the composition of the surrounding cell. Found in animal cells, lysosomes are enclosed by a single membrane and are responsible for the breakdown of several components that have become obsolete to the cell or organism. In some cases, materials taken into the cell by endocytosis or phagocytosis are also degraded in lysosomes. Endocytosis refers to the process by which extracellular materials are absorbed by penetrating a segment of the plasma membrane to form a small membrane-bound vesicle (endosome). During phagocytosis, relatively large particles are enclosed by the plasma membrane and internalized.Lysosomes contain a group of enzymes that break down polymers into their monomeric subunits. For example, nucleases break down RNA and DNA into their mononucleotide building blocks; protease breaks down a variety of proteins and peptides; phosphatase removes phosphate groups from mononucleotides, phospholipids and other compounds; Still other enzymes break down complex polysaccharides and lipids into smaller units. Tay-Sachs disease is caused by a defect in an enzyme that catalyzes a step in the lysosomal breakdown of certain glycolipids called gangliosides, which are abundant in nerve cells – with devastating consequences. Symptoms of this genetic disease usually clear up before the age of 1. Affected children often lose their mind and go blind by the age of 2, and die before their third birthday. Neurons from such children are greatly enlarged with swollen lipid-filled lysosomes. A hydrogen ion pump and a Cl− channel protein in the lysosome membrane maintain the internal pH at ≈4.8. The pump hydrolyzes ATP and uses the released free energy to pump H + ions from the cytosol into the lumen of the lysosome; The Cl− channel allows the entry of Cl− ions. Together they transport HCl (Chapter 15). The acidic pH helps to denature proteins, making them accessible to the activity of lysosomal hydrolases, which are themselves resistant to acid denaturation. Lysosomal enzymes are poorly active at neutral pH values of cells and most extracellular fluids. So, if a lysosome releases its contents into the cell, where there is a pH between 7.0 and 7.3, very little breakdown of the cellular components will take place. In fact, they act as collection sites for various decomposed materials (Figure 5-44a). Primary lysosomes are roughly spherical in shape and contain no obvious granular or membrane fragments. Secondary lysosomes, larger and irregularly shaped, appear to be the result of fusion of primary lysosomes with other membrane organelles; they contain particles or membranes that are in the process of being digested (Figure 5-44b). The process by which a perennial organelle degrades in lysosomes is called autophagy (“cannibalism”). Read more: What does patronage mean in Spanish
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