Water Surrounding Biological Units

Many BU are bathed in water or a water-based medium. Most tissue or organ BU are supplied with this medium by other systems in the organism. The brain, for instance, is bathed in cerebrospinal fluid originating from the blood plasma. Population or biome BU are supplied with water from the Earth s environment, such as from a river or from rain. 

There are some BU that must struggle to acquire enough water to survive. These are desert BU, and they cannot waste precious water. Desert plants have a very impenetrable surface cuticle that conserves water. Stomata in their leaves close during midday, when water stress is highest. Desert animals excrete very concentrated urine, and move to shady locations during hot periods. 

Tardigrades, also called water bears, are tiny animals found all over the world, and can tolerate extreme drying. They can survive in the extreme vacuum of space for days. 

Because water is one product of carbohydrate aerobic metabolism (Section 3.9), water that is produced as a metabolic byproduct can supply the water needs of BU that produce the water, or the needs of nearby BU that are close enough to use excess moisture. One reason that spoilage of foods or grains is so difficult to stop once it begins is that the moisture that results from metabolism modifies and enhances the environment for the growth of additional spoilage microbes. 

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