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Life Science: Session 8

Material Cycles

What is a material cycle?

Sometimes called nutrient cycles, material cycles describe the flow of matter from the nonliving to the living world and back again. As this happens, matter can be stored, transformed into different molecules, transferred from organism to organism, and returned to its initial configuration. The implications of material cycles are profound. There is essentially a finite amount of matter on Earth (with some input from meteors and other astronomical objects). Life uses certain types of matter (the SPONCH CaFe) by removing and concentrating it in living bodies. What would happen if this matter weren’t recycled? Fortunately, material cycles avert this situation.

What are some examples of material cycles?

The carbon cycle The first material cycle described in the video is the carbon cycle, which is also a cycle for oxygen. The carbon cycle involves two chemical reactions that are also important in energy flow: photosynthesis and cell respiration. Both of these reactions are described in detail in “A Closer Look” for Session 7. If you compare the two reactions, you can see that one is the reverse of the other in terms of matter. Carbon enters the living world as carbon dioxide gas, which is “fixed” (made useful to life) into sugar molecules. Carbon is recycled to the nonliving world as carbon dioxide gas; oxygen is recycled as oxygen gas. Oxygen from oxygen gas is never “fixed” in the way carbon is. It is used to “burn” sugar for energy.

Photosynthesis:

light energy + CO2 + H2O -> CH2O + O 2

light energy + carbon dioxide + water -> sugar + oxygen

Cell Respiration:

CH2O + O2 -> CO2 + H2O + energy

sugar + oxygen -> carbon dioxide + water + energy

nitrogen cycle

The nitrogen cycle The second material cycle, described by Dr. Nicky Sheats in the video, is the nitrogen cycle. The main reservoir of nitrogen in the nonliving world is the atmosphere, where it exists as nitrogen gas. Plants, animals, and most other life forms cannot use nitrogen gas. Certain bacteria, however, use nitrogen gas as a nutrient. In a series of steps involving bacteria, nitrogen is fixed into a form that can be used by plants and other producers. Like carbon, nitrogen may travel through the links of a food chain in food. Ultimately, however, nitrogen resides in the wastes of consumers and the dead bodies of all life forms. Another group of bacteria uses this matter for food, and in doing so, releases nitrogen gas as a by-product.

What about other material cycles?

All of the elements in the SPONCH CaFe are recycled, although some take longer than others. A generalized material cycle can be described for most of these elements. Most elements have a reservoir in bedrock. As bedrock decays, these elements, in various molecules, enter the soil. As soil nutrients, these molecules are taken into plants, where they are processed to become part of a plant’s body. From there, these nutrients may travel along food chains, but ultimately become solid or liquid wastes or dead bodies. In using this matter for food, the decomposers return these nutrients back to the soil.

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