5. Describe ways in which cells die. 
For every cell, there is a time to live and a time to die. 

There are two ways in which cells die: 

  • they are killed by injurious agents 
  • they are induced to commit suicide

  • Death by injury

    Cells that are damaged by injury, such as by mechanical damage or exposure to toxic chemicals undergo a characteristic series of changes: 

  • they (and their organelles like mitochondria) swell (because the ability of the plasma membrane to control the passage of ions and water is disrupted) 
  • the cell contents leak out, leading to inflammation of surrounding tissues

  • Death by suicide

    Cells that are induced to commit suicide show the following characters: 

  • they shrink 
  • have their mitochondria break down with the release of

  • cytochrome c 
  • develop bubble-like blebs on their surface 
  • have the chromatin (DNA and protein) in their nucleus degraded 
  • break into small, membrane-wrapped, fragments 
  • the phospholipid phosphatidylserine, which is normally hidden within the plasma membrane is exposed on the surface. This is bound by receptors on phagocytic cells like macrophages and dendritic cells which then engulf the cell fragments. 
  • The phagocytic cells secrete cytokines that inhibit inflammation.
  • The pattern of events in death by suicide is so orderly that the process is often called programmed cell death or PCD. The cellular machinery of programmed cell death turns out to be as intrinsic to the cell as, say, mitosis. Programmed cell death is also called apoptosis. 
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