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Essential Science for Teachers: Life Science

Material Cycles in Ecosystems Children’s Ideas About Material Cycles in Ecosystems

Below are common ideas children in grades K-6 have about this topic, compiled from research on children’s ideas about science (see the Session 8 Children’s Ideas Bibliography). Consider what evidence might refute this idea, and why a child would be likely to believe this?

1. Living things and nonliving things are not made of the same basic materials.

All living and nonliving things are made of matter, with the most basic unit of matter being the atom. Atoms of different types are elements. The elements that are characteristic of living matter (A Closer Look: The SPONCH CaFe) are also found in the nonliving world, although they exist in very different proportions. Life essentially extracts these elements from the nonliving world into organic molecules, which are unique to life. As children observe living things, they are much more likely to see differences from rather than similarities to the nonliving world. This distinction is typically encouraged in the early grades as children are taught “living vs. nonliving.” Even children who have understandings of matter and its basic units are likely to believe that living and nonliving things are somehow different.

2. Plants obtain the matter used to build their bodies from the soil.

A plant’s body is composed of organic molecules which are built around a framework of carbon atoms. During photosynthesis, carbon dioxide from the air is combined with water from the soil to form sugar. Sugar is the raw material that plants use to build their own organic molecules, with some additional matter being obtained from the soil. The air, not the soil, provides most of the matter in a plant’s body. Because children can’t see matter being absorbed from the air into a plant, it’s a natural conclusion that plants get building materials from the soil. One way to challenge this idea is to grow plants from seed in containers where the soil (and the container) has been weighed ahead of time. This was actually done by a scientist in 1630 to test ideas about where plants get their matter.

3. In terms of gases, plants require only carbon dioxide and animals require only oxygen.

Plants require carbon dioxide gas for photosynthesis, which allows a plant to make its own food — sugar. Plants require oxygen gas to burn this sugar for energy in the process cell respiration. Animals also require oxygen for cell respiration, but since they ingest food rather than making their own, do not require carbon dioxide. Children are often taught that plants and animals are opposites in terms of using and releasing gases. The idea that animals use oxygen to “burn” food for energy, on the other hand, is rarely taught, so it’s not surprising that children are not aware that the same process is carried out in plants. Children are typically taught that plants make food and animals eatfood, but not how they both use food.

4. Plants “breathe out” oxygen so that animals have air to breathe.

Oxygen gas is by-product of photosynthesis. This process went on in plants well before animal life evolved, and continues even when they are absent. Many children have a hard time thinking of plants as independent beings — they don’t
“ do anything.” It seems reasonable that a child would look for some “higher” purpose. This is an example of teleological thinking, where a natural process is thought to have a purpose or design.

5. Food is not recognized as the source of matter that composes the body.

Think about a consumer — for example, an animal. The only input of matter that is used by an animal, other than oxygen or water, is food. This food is broken down into basic subunits, which are then built back up into an animal’s body. Producers must make this food first — then it’s used to build a plant’s body. Children typically learn that food gives them energy. Even if they understand that food also provides nutrients, they may not connect nutrients to the material that makes up their bodies. Children can be encouraged to consider the dual role of food as they are introduced to the differences between matter and energy.

6. Food is converted entirely into energy.

Food is matter that stores energy in chemical bonds. This energy can be released from food, but the atoms that compose food remain even after energy from the bonds that hold them together is released. Children’s first understanding about food is likely to be that it is used for energy. Before children are introduced to food as a source of matter, they are likely to think that it “turns into” energy once eaten.

7. When matter decomposes, it disappears.

One fundamental property of matter is that it is conserved — in any system, the total amount of matter stays the same, even if atoms and molecules are recombined in different ways. Think about cell respiration. In this process, sugar in the presence of oxygen is decomposed into carbon dioxide and water. All of the atoms present in sugar and oxygen can be accounted for in carbon dioxide and water. The products of decomposition are atoms and molecules — with carbon dioxide gas being the primary product. It’s understandable that children see this as matter disappearing.

8. All dead matter decays to form soil.

Think about a forest in the fall, where a thick carpet of leaves has accumulated. If all of this dead matter decayed to form soil, there would be an incredible amount of soil accumulated. Most dead matter is actually decomposed into carbon dioxide gas. During cell respiration, decomposers release huge quantities of this gas into the air. If children have observed decomposition, a natural conclusion is that dead matter is broken down into small pieces that become part of the soil. This seems reasonable because the release of gas through the processes of decay cannot be observed.

9. Microbes arise from dead matter.

Like all life forms, microbes arise from previously existing life. Decomposers like bacteria, for example, reproduce by cell division. Microbes that are already present or are nearby in the environment decompose dead matter. Because microbes that decompose can’t be seen, children may reason that they are somehow generated from dead matter itself. This is actually a belief that was held by many scientists up through 1860.

Children's Ideas Bibliography

The Children’s Ideas listed in this section of the Web site were compiled from the following research:

  • Driver, et al. (1992). “Life and living processes.” Leeds National Curriculum Support Project, Part 2. Leeds City Council and the University of Leeds, UK.
  • Leach, J. et al. (1996) “Children’s ideas about ecology: Ideas found in children aged 5-16 about the interdependency of organisms.” International Journal of Science Education, 18, 19-34.

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Essential Science for Teachers: Life Science


Produced by Harvard-Smithsonian Center for Astrophysics. 2003.
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  • ISBN: 1-57680-730-4