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life and death
of a cell
an
excerpt from The Machinery of Life
by
David S. Goodsell
2nd
ed., Springer, 2009 |
The
major discovery made by the earliest cells is the ability to
maintain order when challenged by the inevitable decay into
equilibrium. We might imagine two ways to face this great challenge:
immortality or planned obsolescence. An immortal organism would be
completely resistant to environmental forces, or have powerful
repair mechanisms to undo damage as it occurs. This obviously was
too much to ask for the earliest cells, which were built from
fragile organic materials. Instead, they developed along a path of
planned obsolescence (see below). Molecules, cells, and
organisms are all born perfect and new, they live for a minute, a
year, or a century, and then they die ... but not before reproducing
to create a new molecule, cell, or organism. |
Cells
that are gravely injured make a mess when they die. They swell and
burst, and the contents of the cell spill out, get into inconvenient
places and release their destructive components. The organism
responds with immune cells that struggle to clean up the mess
without damaging the healthy surrounding too much. To avoid this
messy problem, our cells are programmed with a method to commit
suicide, quickly and cleanly: programmed cell death.
All
of our cells are preprogrammed with a method to die on command, if
necessary. When a cell enters into programmed cell death, it
disassem- bles its own molecular machinery in a safe and orderly
manner and notify the immune system that it is ready to be recycled.
In the illustration on the right (magnification is about one million
times), a cytotosic T-cell at the top has given the signal for the
cell at the bottom to die. Proteins on the T-cell surface are
recognized by death receptors (A) on the cell surface,
beginning the process that leads to death. As part of the process,
the BID protein (B) forms a pore in the surface of
mitochondria (shown at the bottom), releasing cytochrome (C)
into the cytoplasm. This is the signal to assemble an apoptosome (D),
which then activates initiator caspases (E), the ultimate
executioners of programmed cell death. These then activate more
caspases (F), launching an orderly campaign to attack key
proteins throughout the cell. For instance, caspases make a cut in
the protein gelsolin (G), converting it into an active form
that disassem- bles actin filaments. |
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