Lysosomes and Peroxisomes
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Transport of Golgi vesicles to the membrane-bound bags of digestive enzymes known as lysosomes has been well described and provides a model for Golgi sorting and transport to other destinations. Lysosomes are filled with more than 40 different acid hydrolases, which are capable of digesting organic molecules, including proteins, nucleotides, fats, and carbohydrates.
Lysosomes obtain the materials they digest from three main pathways. The first is the pathway used to digest products absorbed by endocytosis. In this pathway, endocytotic vesicles bud off from the plasma membrane to fuse with endosomes. Endosomes mature into lysosomes as the Golgi delivers lysosomal enzymes to them; the pH inside the lysosome acidifies, and active digestion occurs.
The second pathway is autophagy, whereby damaged and obsolete parts of the cell itself are destroyed. Unwanted cellular structures are enclosed by a membrane from the ER, which then fuses with the lysosome, leading to autodigestion of the cellular components. Autophagy also may occur during cell starvation or disuse, leading to a process called atrophy, in which the cells become smaller and more energy efficient. The third pathway providing materials to the lysosomes is present only in specialized phagocytic cells. White blood cells (WBCs), for example, are capable of ingesting large particles, which then form a phagosome capable of Cytoplasmic filament Cytoplasmic ring Spoke ring
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Outer membrane
Inner membrane
Lumen Nuclear ring
Basket filament
Terminal ring
Nuclear basket
Nuclear envelope
Nucleus
Nucleolus
Outer nuclear membrane
Endoplasmic reticulum
Nuclear pores
Inner nuclear membrane
Nuclear envelope