Decomposers | Lesson 5 - Explaining How Decomposers Grow

The four activities in this lesson complete the Explanations Phase of the Decomposers unit. This involves modeling and coaching with the goal of helping students develop atomic-molecular scale accounts of the digestion, and biosynthesis that were the drivers of the macroscopic changes they observed in their Bread Molding Investigation in Lesson 3.

Students will learn that carbohydrates, proteins, and fats are materials that decomposers ingest to give them mass to grow. If students completed the Animals Unit, they know these materials are carbon-based and that they have chemical energy. Students likely cannot trace these substances beyond the decomposer’s body. These substances (which are polymers) are digested outside of the decomposer’s body by digestive enzymes excreted by the decomposer, During digestion, polymers are broken down into monomers. Once they are fully digested into monomers they can be transported across membranes into the fungal hyphae and then carried to all the cells in the body. In the cell, they are rebuilt through various biosynthetic processes back into polymers. The monomers that are the product of digestion can follow different pathways in the body and go through many different processes, but your students need to know a general storyline about what happens in growth: polymers are broken down into monomers through digestion, then rebuilt into polymers that become part of the decomposer’s biomass.

Activity 5.1 is the first part of the Explanations Phase of the instructional model (going down the triangle) for digestion and biosynthesis. Students trace the chemical changes of digestion and biosynthesis in a decomposer on a poster of a mushroom.

Activity 5.2 is a 2-turtle activity appropriate for advanced middle school or high school students and classes. If you decide not to teach 5.2, you can move directly from 5.1 to 5.3. In 5.2, students model the chemical changes of digestion and biosynthesis using paper molecules. This activity introduces and uses the vocabulary of polymer and monomer, as well as the names of specific monomers.

Digestion and biosynthesis are chemically the most complicated processes in the Carbon TIME units. Although we believe that with enough effort many middle school students could understand the chemical details included in Activity 5.2, we do not recommend it for middle school. We feel that middle school students need to understand four key points:

1. Fungi use food for one of two purposes:
   1. Growth (digestion and biosynthesis), OR
   2. As a source of energy (digestion and cellular respiration)
2. The food that fungi digest and absorb and the bodies of fungi are both made of large organic molecules.
3. Digestion breaks large organic molecules into smaller organic molecules.
   1. Digestion occurs OUTSIDE the cells of fungi, when they secrete digestive enzymes into organic materials that are their food source.
   2. Digestion is different from cellular respiration, which combines small organic molecules with oxygen to release energy.
4. Biosynthesis makes small organic molecules into large organic molecules.
   1. Biosynthesis occurs in every cell of a fungus, after the cell gets small organic molecules from its food source or other cells.
   2. Biosynthesis is how cells grow bigger so that they can divide and the fungus can grow.

Activity 5.3 is the second part of the Explanations Phase of the instructional model (going down the triangle) for digestion. Students use the Explanations Tool to construct a final explanation of what happens when decomposers break large organic molecules from their food into small organic molecules. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale. This activity is appropriate for students who did only 5.1 and students who did both 5.1 and 5.2, but the vocabulary used to describe the molecules will be different depending on what activities were taught. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of the chemical change at the atomic-molecular scale.

Activity 5.4 is a continuation of the second part of the Explanations Phase of the instructional model (going down the triangle) for biosynthesis. Students use the Explanations Tool to construct a final explanation of what happens when decomposers use the small organic molecules to grow. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale. This activity is appropriate for students who did only 5.1 and students who did both 5.1 and 5.2, but the vocabulary used to describe the molecules will be different depending on what activities were taught. Ideally, at this phase their explanations will combine evidence from macroscopic-scale observations during the investigation with their new knowledge of chemical change at the atomic-molecular scale.

These basic stories of digestion and biosynthesis omit many complications. Although we hope that students will come to appreciate the vast number and complexity of biomolecules, our emphasis in Carbon TIME is on helping students understand that fungi can build their bodies out of decaying materials through a few basic processes in digestion and biosynthesis.

Some key complexities that you as a teacher may wish to be aware of include the following;

• Fungi often modify monomers before using them for biosynthesis. For example, they have limited abilities to modify fatty acids (e.g., changing unsaturated to saturated fatty acids) and to modify amino acids.

The large organic molecules (sometimes called macromolecules) in plants, animals, and decomposers are actually much larger than the molecules shown in the presentation and during the construction of molecular models:

• Fatty acids typically contain 10 to 25 carbon atoms.
• Proteins can consist of hundreds of amino acids. Humans are typical of many organisms in that our proteins contain 21 different kinds of amino acids.
• Starch and cellulose molecules can be made of hundreds or thousands of monomers. Different kinds of starch and fiber can also include other 5-carbon and 6-carbon sugars besides glucose.

Key Carbon-Transforming Processes: Digestion and Biosynthesis