Phenomenon
We don't just use our inputs to get energy, we also use the "stuff" to make new biomolecules for repair, replacement and growth.
Question
How do we make new carbohydrates, fats and proteins?
- What happens when we take in more food than our body can use?
- When you lose weight, where does the matter go?
- (Biggest Loser Challenge Question)
Model Ideas
As we move through the remainder of the Red Loop and develop ideas about matter and energy flow in organisms, we explicitly develop two models in parallel. In our work with cellular respiration, we primarily add ideas to a model for Energy from Food.
These models have been building since Chemical Reactions and so represent a fairly lengthy list of ideas. Be sure to track them publicly with your students as you move through the learning segments. Note that the wording of each statement should reflect those of your students. The target here is the idea. Try to honor student language. For more information, see the MBER Essential: Close Enough.
Matter from Food (primary model here):
Model Ideas from Chemical Reactions
Matter is conserved, neither created nor destroyed. Matter is rearranged in chemical reactions.
Food has matter in the form of protein, carbs and fats- the same things we find our bodies are made of. We also take in matter as oxygen and water.
Some of this matter is used in our body, but we take in much more matter than we need to use to grow or maintain body structures.
Some of this matter (especially much of the water but also some indigestible material) basically passes through us.
Model Ideas from Cellular Respiration
Some of this matter is really taken in for energy. It is rearranged to obtain energy in a reaction called cellular respiration. The products are expelled from the body as carbon dioxide and water (CO2 + H2O).
New Model Ideas in this Triangle
- Some of the matter we take in is used to build proteins, fats and complex carbs that form cells of body structures.
- Excess food not used for energy or building materials is converted to fat and stored.
Energy from Food (secondary model here):
Model Ideas from Chemical Reactions
Energy is conserved, neither created nor destroyed. Energy is transformed in chemical reactions.
When the reactants have more potential energy than the products, energy is released in the reaction (“downhill” reaction).
[Electrolysis Classrooms Only: When the products have more potential energy than the reactants, energy must be added to the reaction (“uphill” reaction). ]
Food has energy in the form of calories.
Living things get energy by rearranging food and oxygen molecules.
Model Ideas from Cellular Respiration
Living things rearrange food (specifically glucose - C6H12O6) and O2 into CO2 and H2O.
(C6H12O6 + O2) have higher energy than (CO2 + H2O) so this rearrangement releases energy.
- The rearrangements occur in a series of steps rather than all at once.
- Collectively the reactions are called cellular respiration.
- Usable cellular energy is released in the form of ATP.
UNITY AND DIVERSITY: Other reactions (such as fermentation) can produce biologically usable energy, but they are usually less efficient. We see these reactions in some groups of organisms that have evolved under different environmental conditions and in our own bodies during times when oxygen is not readily available.
New Model Ideas in this Triangle
- Building large molecules (fat, protein, complex carbs) requires energy.
Overview
Transition in: The transition here is to now think about the matter from the food we eat. We know that not all of the food we eat is used for energy. What happens to the food not used for energy? What happens if we take in more food than we can use?
We now switch to thinking about the matter from the food we eat and how it is involved in cellular respiration. We ask, is it used for anything else? We quickly review relevant ideas from past models and readily recognize that the food we eat provides us with the components we need to build new body tissue.
We also review some ideas about digestion—that it breaks food first into carbs, fats and protein molecules, and then into their monomers. But now we explicitly discuss how some of our digested food is used to build new macromolecules and to repair existing or build new body tissue. We also figure out that the excess food that we take in is stored as fat. We then reason that our bodies can use fat for cellular respiration when we run out of glucose. Lastly, we recognize that building macromolecules requires energy or the “uphill” energy diagram. This is presented differently for the ethanol and electrolysis pathways, as this is the first time students who made sense of chemical reactions using ethanol have been exposed to the idea that some reactions require energy input. Just before we return to the Biggest Loser challenge question we discuss the idea of energy and matter cycles in organisms, tying several components of our Matter and Energy from Food models together.
When we return to our challenge question we spend a bit of time making sure our ideas are consistent with our models. This is scaffolded by a Four Corners activity, several readings, review of key points from the Biggest Loser explanation, then continuing the explanation process through group writes, a gallery walk, group and a whole class discussions. You may want to use some or all of these activities depending on your students’ level of understanding. As their final assessment, students now answer the Biggest Loser Question individually using their class models.
Transition out: We have fairly complete models for how we (and many other organisms) obtain matter and energy from food. But we have a number of lingering questions about plants? Do they eat food? How do they obtain matter and energy? We transition into our exploration of Photosynthesis at this point and ask a Challenge Question that is the reverse of the Biggest Loser: How does a seed grow into a tree? Where did the matter (and energy) come from?
1. We now switch to thinking about the matter from the food we eat and how it is involved in cellular respiration. We ask, is it used for anything else? We quickly review relevant ideas from past models and reason that the food we eat provides us with the components we need to build new body tissue.
1. We’ve explicitly shifted our focus to questions about how we use matter after two triangles of dealing with energy. And we have reviewed what happens to the matter once it enters our bodies.
2. We now go deeper to try to understand what happens to the food we eat beyond what we need for energy, growth and maintenance. We then ask why would we store excess fat, what purpose does it serve? In this way we develop ideas around fat as an energy reserve so if you run out of glucose, then you can draw on your fat for fuel for cellular respiration.
We have figured out that when you take in excess food, it is stored as fat. We also reasoned that this fat provides an energy reserve if your glucose pool runs dry. (Only as a last result would you draw from your amino acid pool for fuel.).
3. We apply the chemical reaction model to the process of building new biomolecules in our cells.
Electrolysis Path: In classrooms that did electrolysis, this can be a simple conversation asking which reaction diagram makes sense for biosynthesis—the uphill reaction or the downhill reaction?
Burning Ethanol Path: In classrooms that have only considered energy-releasing reactions, we need to take some time to consider how energy might be involved in the synthesis of larger biomolecules. We do this in a paper activity by “building a protein” from amino acids to see that synthesis takes work.
We figured out that building biomolecules requires energy, as indicated in our “uphill” reaction diagram. The energy comes from chemical reactions such as cellular respiration.
4. Now that we recognize both energy releasing and energy requiring reactions occur in our cells, we pause for a moment to consider the pairing of these reactions in the body. We apply our models (Chemical Reactions, Energy from Food and Matter from Food) to the case of the coma patient in order to reinforce the idea of our bodies as matter and energy cycling machines.
We now have some concept that energy “cycles” in organisms. These cycles keep us going at our bare minimum, or maintain the basal metabolic rate. If we need to do more than just maintain our body, then we need more fuel.
5. Now we have the revised models we need to tackle the Biggest Loser explanation. We work in groups to consider any new ideas and to evaluate our understanding. This learning segment is heavily scaffolded and depending on your students you may wish to use all of it or very little.
We hope to have a coherent explanation for the Biggest Loser that includes two key points: (1) When we use more food than we eat our body begins breaking down stored fat for cellular respiration. (2) This reaction produces H2O (lost as sweat or urine) and CO2 which is exhaled. H2O input and output are equal,
so the matter he lost is mostly converted to CO2.
6. Optional. We ask students to apply the models to explain how doing a lot of exercise helps the Biggest Loser contestants lose more weight. This short segment can be used as a “ticket out the door”.
Exercise helps with weight loss because exercise requires a lot of energy and the more energy we need, the more cellular respiration must happen to provide the energy. The more respiration, the more glucose (or fat when all glucose is used) needed. Fat is converted to CO2 and leaves the body when we exhale
7. We use our two big models, Energy from Food and Matter from Food to individually write final explanations for the Biggest Loser. This is the summative assessment.
We’ve figured out that when we lose weight we lose matter in the form of carbon dioxide that we breathe out. The final product is a complete, coherent model-based written explanation for the Biggest Loser (see rubric).
Download Resources
| Attachment | Size |
|---|---|
| All MBER-LE Biosynthesis Materials (Download) | 24.41 MB |