Ca(2+) is a universal carrier of biological information: it controls cell life 
from its origin at fertilization to its end in the process of programmed cell 
death. Ca(2+) is a conventional diffusible second messenger released inside 
cells by the interaction of first messengers with plasma membrane receptors. 
However, it can also penetrate directly into cells to deliver information 
without the intermediation of first or second messengers. Even more 
distinctively, Ca(2+) can act as a first messenger, by interacting with a plasma 
membrane receptor to set in motion intracellular signaling pathways that involve 
Ca(2+) itself. Perhaps the most distinctive property of the Ca(2+) signal is its 
ambivalence: while essential to the correct functioning of cells, Ca(2+) becomes 
an agent that mediates cell distress, or even (toxic) cell death, if its 
concentration and movements inside cells are not carefully tuned. Ca(2+) is 
controlled by reversible complexation to specific proteins, which could be pure 
Ca(2+) buffers, or which, in addition to buffering Ca(2+), also decode its 
signal to pass it on to targets. The most important actors in the buffering of 
cell Ca(2+) are proteins that transport it across the plasma membrane and the 
membrane of the organelles: some have high Ca(2+) affinity and low transport 
capacity (e.g., Ca(2+) pumps), others have opposite properties (e.g., the Ca(2+) 
uptake system of mitochondria). Between the initial event of fertilization, and 
the terminal event of programmed cell death, the Ca(2+) signal regulates the 
most important activities of the cell, from the expression of genes, to heart 
and muscle contraction and other motility processes, to diverse metabolic 
pathways involved in the generation of cell fuels.