- Precipitation: water returns to earth through precipitation in the form of rain, sleet,
snow or ice (hail). When rain occurs due to precipitation, most of it runs off into lakes
and rivers while a significant portion of it sinks into the ground. - Infiltration: the process through which water sinks into the ground is known as infiltra-
tion and is determined by the soil or rock type through which water moves. During the
process of sinking into the Earth’s surface, water is filtered and purified. Depending on
the soil type and the depth to which the water has sunk, the ground water becomes
increasingly purified: the deeper the water, the cleaner it becomes. - Melting and freezing: some water freezes and is ”locked up in ice, such as in glaciers,
and ice sheets. Similarly, water sometimes melts and is returned to oceans and seas.
The processes involved in the water cycle are shown in Figure 9.19.Figure 9.19: The water cycle.Oxygen cycle DUMMY
Oxygen is one of the main gases found in the air, along with nitrogen. Oxygen is re-cycled
between the air and living organisms in the following ways:- Breathing and respiration: organisms such as animals and plants take in oxygen from
the air, during breathing and gaseous exchange processes. The oxygen is used for
cellular respiration to release energy from organic nutrients such as glucose. - Photosynthesis: during photosynthesis, plants absorb carbon dioxide from the air to
synthesize sugars, and release oxygen. - There is acomplementaryrelationship between photosynthesis and respiration in that
the former produces oxygen and the latter consumes oxygen.
The oxygen cycle is shown in Figure 9.20.282 9.7. Nutrient cycles