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The Proteasome

Protein degradation is as essential to the cell as protein synthesis. For example,

  • to supply amino acids for fresh protein synthesis
  • to remove excess enzymes
  • to remove transcription factors that are no longer needed.

There are two major intracellular


When cells are faced with an inadequate supply of nutrients in their extracellular fluid (ECF), they may begin to cannibalize some of their internal macromolecules (e.g., proteins) and even organelles (e.g. mitochondria) for re-use of their components.

This phenomenon is called autophagy.

Three different mechanisms of autophagy have been discovered.

  • macroautophagy,
  • microautophagy, and
  • chaperone-mediated autophagy.

In macroautophagy,

  • a double membrane forms within the

Cilia and Flagella

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These whiplike appendages extend from the surface of many types of eukaryotic cells.

  • If there are many of them, they are called cilia;
  • if only one, or a few, they are flagella. Flagella also tend to be longer than cilia but are