[Management of time by the cell and by the organism]

Time-dependent regulations of cells and organisms can be analysed at different levels. One of these levels is the periodicity of cell functions such as cell division, metabolic processes (generation of ATP by glycolysis or oxidative mitochondrial processes) and the biosynthesis of cell constituents. Studies carried out on unicellular eukaryotes revealed the periodic, oscillatory nature of most of these processes. Time constants of these reactions vary from nanoseconds to hours-days, necessitating coupling mechanisms. Comparative studies revealed the coupling of the rapid processes (mitochondrial ATP generation) to the slower rhythms of the biosynthetic processes of macromolecules. Adenine nucleotides are involved in the coupling mechanisms between rapid and slow processes ("the slow dance of life to the music of time"). The mechanisms underlying these rhythmic processes involve either key allosteric regulatory enzymes (PFK for glycolysis) or "desensitization" of receptors by phosphorylation-dephosphorylation. At the organismic level the study of rhythmic processes is illustrated by the periodicity of heart beats, shown to exhibit multifractality, following apparently the formalism of deterministic chaos. Another example is the rhythmic oscillatory discharges of neuronal networks. The existence of subrhythmes mostly of epigenetic nature, facilitated probably the progressive adjustment of cells during evolution to the slow increase of day time since the separation of the moon from the earth. We analysed the mechanisms underlying the decline of these processes during aging. Loss of receptors or/and their uncoupling from their transmission pathway appear to be involved in most of these processes of decline. One conclusion of this review is the importance of epigenetic mechanisms both in the genesis and in the decline of these rythmic processes involved in time keeping by the cell.