The Earth's Cycles
In addition to energy, organisms
require a supply of materials to live. These chemical
elements are found in the biosphere but must be constantly
recycled in order to maintain their availability. Following
is an account of some of these cycles.
What are the ''biogeochemical'' cycles?
Biogeochemical cycles (bio meaning 'life', geo for 'earth'
and chemical for the changing of matter from one form
to another) are natural processes that recycle elements
in various chemical forms from the environment, to organisms,
and then back to the environment. Water, carbon, oxygen,
nitrogen, phosphorous, and other elements pass through
these cycles, connecting the living and non-living components
of the Earth. The element cycles are one of the basic
life support systems of the biosphere.
2 What is the water cycle?
The water cycle (or hydrological cycle)
is the circulation of the waters of the Earth between land,
freshwater lakes and rivers, the salt seas and oceans, and the
atmosphere. This cycle collects, purifies, and distributes the
Earth's fixed supply of water. This cycle comprises several
- Through evaporation, water
on the land and in the oceans is converted by solar energy
into water vapor. Winds then move masses of water vapor
around the Earth.
- Through condensation, water
vapor is turned into water droplets, which form clouds or
- Through precipitation, water
returns to the Earth in the form of dew, rain, hail, or
- Through transpiration, water
is absorbed by the roots of plants, passed through their
stems and other structures, and released from their leaves
as water vapor.
- Through run-off, water moves
from the land to the sea, or else from the land into the
ground where it is stored (from which it eventually returns
to the surface or to lakes, streams, and oceans).
In what ways do human beings affect the water cycle?
deplete groundwater supplies, causing water shortages and subsidence
(sinking of land when groundwater is withdrawn). We clear vegetation
from land areas. When vegetation is removed, water flows over
the ground more quickly, allowing less time for it to sink into
the soil. This results in groundwater depletion. The increased
surface run-off also results in accelerated soil erosion.
What is the carbon cycle?
is an essential component of all living things. It exists mostly
as carbon dioxide in the atmosphere, oceans, and in fossil fuels
stored beneath the Earth's surface. The major steps of the carbon
cycle are the following:
What is the oxygen cycle?
Carbon dioxide in the atmosphere is absorbed by plants and
converted into sugar, by the process of photosynthesis.
Animals eat plants, breaking down the sugars and releasing
carbon into the atmosphere, oceans or soil.
Other organisms break down dead plant and animal matter,
returning carbon to the non-living environment.
Carbon is also exchanged between the oceans and the atmosphere.
This occurs in both directions at the interface between
the air and water. When the level of carbon dioxide in the
oceans (atmosphere) exceeds a certain level, it will flow
from the higher concentration in the oceans (atmosphere)
to the lower concentration in the atmosphere (oceans), maintaining
Oxygen composes nearly 21 percent
of the atmosphere. It combines chemically with a multitude of
other elements to form important substances such as water, carbon
dioxide, plant nutrients and organic substances. The steps of
the oxygen cycle are the following:
Plants release oxygen into the atmosphere as a by-product
Animals take in oxygen and, by the process of respiration,
use it to break down sugars obtained from food.
Carbon dioxide is released by animals and used by plants
Oxygen is also cycled between the oceans and the atmosphere,
maintaining oxygen equilibrium.
The Primer on Environmental Citizenship.
Copyright © 2000 Tierramérica.
Todos los Derechos Reservados
currents. /Claudio Contreras