A&P Lecture 1.8: Lysosomes

This is an in depth  explanation of Lysosomes as a part of the cell. This lecture note is linked to A&P Lecture 1: The Cell

ANATOMY AND PHYSIOLOGY LECTURE 1.8

LYSOSOMES

After a phagocytic cell has engulfed the proteins, polysaccharides, and lipids present in a particle of “food” (such as a bacterium), these molecules are still kept isolated from the cytoplasm by the membranes surrounding the food vacuole. The large molecules of proteins, polysaccharides, and lipids must first be digested into their smaller subunits (including amino acids, monosaccharides, and fatty acids) before they can cross the vacuole membrane and enter the cytoplasm.

 The digestive enzymes of a cell are isolated from the cytoplasm and concentrated within membrane-bound organelles called lysosomes, which contain more than 60 different enzymes. A primary lysosome is one that contains only digestive enzymes (about 40 different types) within an environment that is more acidic than the surrounding cytoplasm. A primary lysosome may fuse with a food vacuole (or with another cellular organelle) to form a secondary lysosome in which worn-out organelles and the products of phagocytosis can be digested. Thus, a secondary lysosome contains partially digested remnants of other organelles and ingested organic material. A lysosome that contains undigested wastes is called a residual body. Residual bodies may eliminate their waste by exocytosis, or the wastes may accumulate within the cell as the cell ages.

Partly digested membranes of various organelles and other cellular debris are often observed within secondary lysosomes. This is a result of autophagy, a process that destroys worn-out organelles and proteins in the cytoplasm so that they can be continuously replaced. Lysosomes are thus aptly characterized as the “digestive system” of the cell.

Lysosomes have also been called “suicide bags” because a break in their membranes would release their digestive enzymes and thus destroy the cell. This happens normally in programmed cell death (or apoptosis ), . An example is the loss of tissues that must accompany embryonic development, when earlier structures (such as gill pouches) are remodeled or replaced as the embryo matures

SUMMARY

Lysosomes  are membrane-bound vesicles that pinch off from the Golgi apparatus. They contain a variety of hydrolytic enzymes that function as intracellular digestion systems. Vesicles taken into the cell fuse with the lysosomes to form one vesicle and to expose the endocytized materials to hydrolytic enzymes. Various enzymes within lysosomes digest nucleic acids, proteins, polysaccharides, and lipids. Certain white blood cells have large numbers of lysosomes that contain enzymes to digest phagocytized bacteria. Lysosomes also

digest the organelles of the cell that are no longer functional in a process called autophagia . In other

cells, the lysosomes move to the plasma membrane, and the enzymes are secreted by exocytosis. For example, the normal process of bone remodeling involves the breakdown of bone tissue by specialized bone cells. Enzymes responsible for that degradation are released into the extracellular fluid from lysosomes produced by those cells.