Geoscience Reference
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(Figures 4.3-4.7). During these cycles, the chemical elements may
change state several times (i.e. mineral, nonliving organic or living
organic) and their compounds may change phase (i.e. solid, viscous,
liquid or gaseous).
Different natural cycles can take place at very different spatial and
temporal scales. For example, a biochemical cycle may take place
within an organelle of a living cell, within a matter of seconds. The
cell cycle involves a whole cell or several cells of the same population
or tissue, and takes place over several hours or days. Finally, a
biogeochemical cycle involves the whole planet and requires much
more time to complete than a biochemical cycle or the cell cycle, as
explained afterwards. Despite the differences in spatial and temporal
scales, all these cycles reflect two fundamental laws of nature. On the
one hand, the progressive dissipation of energy in the Universe (i.e.
the conversion of different forms of energy - solar, geothermal,
nuclear, etc. - into heat) generates flows (movements) of energy and
matter, which occur at all spatial scales. On the other hand, as matter
“is neither lost nor created”, the chemical elements must be recycled
at some point or another. The combination of flows and recycling of
matter produces cycles at all scales in nature, including
biogeochemical cycles.
Figure 4.2. Illustration of the cell cycle whose different phases are: G 0 (Gap 0,
resting phase), G 1 (Gap 1, cell growth), S (synthesis, DNA replication), G 2 (Gap 2,
cell growth), M (Mitosis, cell division) (from https://upload.wikimedia.
org/wikipedia/commons/c/c4/Cell_cycle.png?uselang=fr, original figure downloaded
from this site by Magnus Manske) (see color section)
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