WATER SOLUBLE WASTES
Metabolically active organisms produce waste materials. These wastes are either the end product of the break down of their food, or are poisonous materials ingested with their food.
Earlier we stated that plant life stores energy for later utilization. We used a simplified structure of glucose sugar as an example. Carbohydrates such as glucose and starch are good examples of stored food ~ energy. However, lipids are also important as a stored food, as are proteins and nucleic acids.
Besides acting as a stored food, all of the above types of compounds also have a structural function. They serve as "building blocks" of the living material. During photosynthesis, organisms use sunlight to form these compounds. The simplified diagram below exemplifies these processes.
Notice that oxygen is produced in this process. This oxygen must be eliminated from the plant tissue or it acts as a poison. One can only maintain plant growth in an environment with an oxygen level below a certain threshold.
Non photosynthetic organisms and plants in the dark do the opposite, as shown below.
Oxygen is used to break the food down to its constituent parts, carbon dioxide and water. Energy is released during this process and is used to maintain life. In this case carbon dioxide is a waste product, and if the concentration is above a certain level, is lethal.
Another waste product not mentioned yet is nitrogen. Proteins and nucleic acids contain nitrogen and for the breakdown of those compounds, it would be more accurate to change the above diagram for those food stuffs.
The symbol NH3 stands for ammonia. Ammonia is highly toxic. Fortunately it is also highly water soluble.
Organisms living in an aquatic environment have little problem excreting ammonia. It is a more difficult proposition for terrestrial organisms. They must conserve water or they will desiccate. Unless they urinate constantly they must convert the ammonia to something less toxic. Most mammals convert the ammonia to urea. Urea is also very water soluble, but can be stored for short periods of time, and can be voided at convenient intervals.
Some animals, such as birds and insects, must conserve water. Therefore they must convert ammonia to a water insoluble compound such as uric acid. Consider a chicken, for example. The developing chick must stay inside a hard shelled egg for 21 days. This developing chick cannot drink water nor eliminate its nitrogen wastes during that time. The chicken solve this problem by storing the uric acid next to the shell and walks away from both upon hatching. Spiders and many insects do something similar.
Microorganisms have similar problems with waste materials. Bread yeast and sourdough bacteria, for example, produce carbon dioxide when oxygen is present. This released carbon dioxide gas causes the bread to rise, but is a metabolic poison to the microorganisms.
Under anaerobic conditions or under low oxygen conditions, some yeasts break down their food to carbon dioxide and ethyl alcohol and/or acetic acid and/or lactic acid.
Any of these waste products, in high concentration, slow and then stop yeast metabolism.
To summarize, low oxygen levels plus sugar plus yeast produces energy for the yeast, carbon dioxide gas (bubbles) and ethyl alcohol (booze).
If the oxygen level remains high, sugar plus yeast produces energy, carbon dioxide gas and acetic acid (vinegar) and/or lactic acid.
Other microorganisms produce lactic acid (sour kraut and pickles). Some, in milk, make yogurt, sour cream, or kefir. These organic acids inhibit bacterial and yeast growth in high concentrations. Now you can reason why uncorked wine sours, and why mankind has made pickled fruits, vegetables, and meats for centuries. These high acid and/or alcohol concentrations inhibit other disease producing organisms and keep food from spoiling. Man's tolerance to these acids is greater than the yeasts.
Waste products not taken into the cell structure of organisms are eliminated by regurgitation (Hydra and dogs, for example) or defecated (most of the rest of us).