Nutrient Cycles


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Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th Ed.


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Nutrient Cycles

Water Cycle

All living things are composed mainly of water, but most of the water on Earth lies in the environment (e.g. in lakes, oceans, streams, and the air). The movement of water from the physical environment, and through the biological environment is driven by the sun.

In the physical environment, the sun radiates the Earth's surface with heat, evaporating the water, slowly turning liquid water to water vapor, gaseous water. As a gas, water rises from the ground, the surface of streams, lakes, but water mainly rises from our oceans.

Once in the air, air currents moves the gaseous water around the Earth. But when molecules of water hits a particle, or when temperatures and pressures reach the point where water liquefied or even solidifies, water begins to condense. As water condenses, it forms into rain or snow, thus the water begins to fall to the ground as it is now too heavy to remain in the air.

As the water strikes the ground, some of it seeps into the soil, gathering within water tables under the earth. But the majority of the water runs across the ground, slowly collecting until streams, then rivers form. Where the ground has large divots, lakes form. But eventually, most of the water again reaches the oceans.

But the biological environment lives along side of the physical environment. Along this route, organisms live in, bath in, and drink the water, taking water into their bodies. This is important for many organisms, but it is especially important to those who live on land. Terrestrial organisms generally lose tremendous amounts of water, as it evaporates from their bodies. All organisms lose some water as they remove waste from their bodies.