Apoptosis

Apoptosis is known as a mechanism for regulated cell death that takes places in lysosomes. Cell death is an extremely important regulatory system of the cell to ensure that any damaged, malfunctioning or unrequired cells are removed from the body to keep a healthy balance. In VCE Biology apoptosis is used as an example of a cellular response to a specific signal used to integrate knowledge about the coordination and regulation mechanisms that occur at a cellular level.

Importance of apoptosis

As described above apoptosis is a normal part of the life of cells and is an important factor in maintaining homeostasis.

Apoptosis is particularly important in:

  • Developmental changes such as in growing embryos where fingers and toes are webbed, and as the embryo develops the cells between the appendages die, freeing up the fingers and toes.
  • Removal of cells that have failed to develop properly
  • Removal of tissues of old, infected or damaged cells.
  • Removal of immune cells that attack self-cells.
  • Removal of cells with sustained DNA damage, to remove the possibility of these cells reproducing and potentially causing cancer.
  • When a cell is infected with a virus, the cell identifies the infection and kills itself before the virus has the opportunity to replicate and spread to other cells.
  • Elimination of unneeded cells, whereby too many cells are produced than needed. To maintain these cells the organism has to expend a lot of energy and materials to keeps these cells alive, therefore it is more efficient to have these excess cells removed.

Apoptosis pathways

Signals from outside the cell- the death receptor pathway

Cell membranes have death receptors on their cell membrane that respond to signals that initiate cell death (as mentioned above). When this signal is received a cascade of events occurs.

  1. The signal is recognised through the activation of different caspases (enzymes that cleave specific proteins at a particular amino acid) in the cells, the message goes out to phagocytes in the same area.
  2. All cells that have received the death receptor signal begin to shrink and develop small blebs  (bumps) on their surface.
  3. Caspases enter through the nuclear pores, the DNA and proteins in the nucleus are degraded, and the mitochondria break down.
  4. Other organelles are generally preserved as the cell breaks into small membrane enclosed fragments. The small fragments bind to the receptors on the phagocytic cells that have responded to the messages of the dying cells, these phagocytes then engulf the fragments. They secrete cytokines which inhibit inflammation to protect neighboring cells.

Signals from inside a cell- the mitochondrial pathway

This pathway is used for damage inside the cell such as DNA damage or malfunction of an oxidative enzyme.

  1.  Proteins on the mitochondria are activated and the mitochondrial membrane breaks down  inducing a series of events in the cell.
  2. Caspases enter the nuclear pores and break DNA in to small pieces. Any other remnants of undesirable parts of the cell are broken into small fragments.
  3. Small fragments bind to receptors of phagocytic cells that have responded to message of dying cell. Phagocytes respond by engulfing fragments and secreting cytokines which inhibit inflammation so surrounding cells are not damaged.

Importance of ensuring appropriate levels of apoptosis occur

The importance of apoptosis cannot be undermined however it has to be regulated to ensure that it occurs within appropriate levels. When too little apoptosis occurs it can promote the onset of cancers and autoimmune diseases. In contrast, too much apoptosis can lead to neurodegenerative diseases such as Alzheimer’s and Huntington’s disease.

See also