4f. Lysosomes and The Cytoskeleton [Beyond Books

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4f. Lysosomes and The Cytoskeleton
Any factory needs a good maintenance crew to keep everything orderly, to get rid of the trash, and to dismantle and dispose of outmoded machinery. The maintenance crew also functions as a second line of defense. If someone gets past the security guard at the front door, it is usually the maintenance crew who catches the trespassers and chases them out. In a cell, the role of building maintenance crew is filled by the lysosomes.

Lysosomes

Production of lysosome by Golgi apparatus

The word "lysosome" is Latin for "kill body." This is a very colorful description for some of the most unusual organelles in the cell. LYSOSOMES are organelles produced by the Golgi apparatus that contain powerful protein digesting enzymes.

Lysosomes are responsible for the breakdown and absorption of materials taken in by the cell. Often, a cell engulfs a foreign substance through ENDOCYTOSIS, another form of active transport. During endocytosis, the cell membrane puckers up, forms a pouch around materials outside the cell, and pinches off to become a vesicle. If the contents need to be destroyed, lysosomes combine with the vesicle and release their enzymes.

Autolysis: A. Cellular swelling B. Normal cells C. Swelling and rupture (notice holes on membrane)
When a white blood cell engulfs a bacterium, it is the lysosomes that digest the bacteria. Lysosomes are sometimes called "suicide sacks" because they are responsible for AUTOLYSIS, a process in which a cell self-destructs. Autolysis, incidentally, is Greek for "self kill." How does this happen? When a lysosome ruptures in a cell, it causes all of the cell's internal proteins to be digested. This action is not accidental; rather, it is regulated by signals that scientists do not fully understand. Autolysis allows an organism to eliminate worn-out cells.

Maintenance crews do not get the glory of chief executive officers, or even that of the production worker, but they serve a very important purpose.

Without lysosomes, the cell would accumulate too much junk and would not be able to function for very long.

Support Beams

There is another major department in a cell factory, although it usually isn't given a department name in a regular factory. It's the walls, floors, and ceilings of a factory.

Within the cytoplasm of the cell are many protein fibers that act as support structures. These protein fibers, called MICROFIBERS are composed of a specialized protein called ACTIN.

The cytoskeleton supports the cell.

Actin has the ability to link together to form fibers quickly. These fibers make up an invisible support structure inside the cytoplasm called the cytoskeleton. The CYTOSKELETON maintains the cell's shape and can be used to move the cell membrane. Thicker protein structures, called MICROTUBULES, are composed of several microfibers and allow movement within the cytoplasm.

Now, let's take a look at where a cell gets the energy it needs to survive and function.

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