3 Cycles For Life
Your assignment: On your own sheet of paper, draw and label the three cycles discussed in class. The rest of this document is help in case you missed something.



THE WATER CYCLE
If we start with a drop of water in a cloud, we know it can't stay there forever. The water falls to earth, either as rain, sleet, snow or hail. These are forms of precipitation. While it may rain anywhere in the world (even the oceans don't stay dry) we'll go to the mountains for this one. The rainwater collects and begins to flow downhill. This is known as run-off. The little streams run into bigger and bigger ones until we have rivers and the whole mess (usually) winds up back in the ocean. If these little water molecules have the good fortune of being near the top, they are warmed by the sun's rays and leave the ocean as evaporation. All of this evaporated water starts to cool in the atmosphere. When the droplets cool, they bunch up into clouds. This is called condensation. When there is enough bunching, the droplets get heavy enough to fall, starting the whole thing over again.



CARBON AND OXYGEN CYCLES
As the name implies, this is actually two cycles. Let us begin with carbon. Carbon exists in the atmosphere as the molecule CO₂, or carbon dioxide. Plants take in this molecule through their leaves and combine it with water to make sugars. Sugar is stored in the plant, which is a food source for animals. Animals will eat the plants and the carbon enters their system. While animals use some carbon to live, the extras are disposed of. Some is excreted, but most of it is breathed out in the form of carbon dioxide.

Oxygen goes through a similar process. The air is about 27% oxygen. Animals breathe this in and excrete CO₂ and water. Plants use these molecules to make sugar, but some of the oxygen is left over. Plants "breathe out" oxygen as a waste product. It goes back into the air.



NITROGEN CYCLE
I'm betting you're less familiar with this cycle. Anyway, all living things need nitrogen to survive. Conveniently, the atmosphere is about 70% nitrogen. The only problem is that it's unusable in that form. First, nitrogen must get into the ground. Lightning, among other things, starts this process off. Once the nitrogen is in the ground, tiny bacteria in the soil and at plant roots changes the nitrogen into a plant-friendly form. The plant can then put nitrogen into its structure. For an animal to get nitrogen, they must eat the plants. Some nitrogen is excreted as waste, and some is stored in the body. When wastes (or bodies) decompose, bacteria are ready to either send it back to the plants or into the air.