Phenomenon
Living things must take in stuff, rearrange it, and give it off in a different form in order to survive.
Question
Why do living things need food? How do we get matter and energy from food?
Model Ideas
- Food gives living things matter and energy they need for survival.
- Living things transform matter [leverage idea of conservation of matter here]
- Matter transformations give living things the energy they need for life processes.
- Molecules have potential energy [leverage idea of conservation of energy here]. In a reaction the products and reactants have different amounts of potential energy.
- In a chemical reaction if the products have less potential energy than the reactants, energy is released to the surroundings.
*Conversely, if the products have more energy, energy must be added to get the reaction to occur. We will wait to address this “reverse reaction” in later models. See the notes in the PowerPoint for more information on this topic and other ideas related to coherence in the Red Loop (Matter and Energy).
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Overview
Transition in from Natural Selection: Our natural selection model says that the characteristics of a species are shaped by which individuals in the population survive to reproduce. This leads us to ask the question "What do individual organisms need in order to survive?”
This model is the first in a series of models (the Red Loop) that develops ideas around how organisms obtain what they need for survival and reproduction. In order to survive (and reproduce), organisms must obtain matter and energy. The chemical reaction model is the foundation for understanding how, by rearranging atoms in matter, organisms both obtain and store usable biological energy. There are two distinct stages of developing the model: (1) our bodies are matter transformation machines, and (2) matter transformations (called chemical reactions) can give off energy.
We start the unit by returning to the finch mortality data and ask, “Hmmm, why do we die if we don’t eat?” The question soon leads a broader conversation about what we need to take into our bodies in order to survive. As a class we then generate a rather comprehensive representation of ideas about our matter and energy needs called the “Inputs-Outputs-Uses” diagram. We revise this diagram several times in the first stage of developing the model, and then again in later models in the Red Loop. We first work to clarify our ideas about matter (the “stuff”) and energy (the “doing”) before further exploring their relationship to food. Students learn about the molecular nature of the food we take in as well as the stuff our bodies put out. We pause to recognize that the outputs are made of the same elements as the inputs, but they are different in the molecular arrangement.
We infer that by rearranging these atoms organisms obtain the energy and matter they need to survive. This leads us to ask the question “How does rearranging atoms in our food give us energy?” This idea is explored in part by observing ethanol burning. We invoke conservation of energy to infer that the observable energy released in the reactions was somehow “in” the reactants. Some of the chemical potential energy of the reactants has now been transformed into visible energy. Because we know that energy is conserved, it follows that the products must therefore have less energy than the reactants. The same must happen with our bodies as some inputs are transformed into outputs. The reactants we take in (inputs) have more energy than the products we put out (outputs). In the process of rearranging the molecules, we free up the usable biological energy we need to “do stuff”. We end with a final segment where we take stock of what we know about how we get matter and energy from food.
* Note: A myriad of questions arise as students build the class representation of Inputs-Outputs-Uses. Many of these ideas are necessarily set aside while moving forward to make sense of the questions at hand. However, they are taken up in ensuing models. For example, questions about how plants obtain matter and energy (e.g. Do plants eat? Do plants eat sunshine?) usually arise throughout this segment. They are placed in the parking lot for now but can be used to transition into the photosynthesis model later.
Transition out to Cellular Respiration: Now that we have some understanding that we get the matter and energy we need by rearranging the molecules we take in through a series of chemical reactions, we are ready to further explore those reactions in order to make sense of organismal biology in the coming models.
Overall Time: 7-10 days
Advanced Planning
MATERIALS NEEDED (besides basic lab equipment and supplies):
See Resource "Burning Ethanol Teacher Guide" in Learning Segment Table above for more information. Materials required will depend on your class investigation of burning ethanol, most notably how you will test for the presence of potential products from the reaction.
You should plan to have all these resources on hand. (Most are likely already in your possession or in the possession of teachers at your school. Others are cheap and easy to purchase.)
- marshmallows/skewers or equipment to burn Cheetos
- ignition sources (matches might be ok, but better to have camping lighters with long handles)
- glass petri dishes (enough for however many groups you plan to have work semi-independently (under your supervision)
- 95% or 100% ethanol (95% is cheaper and your chemistry teacher next door likely has some)
- bromothymol blue (from above CO2 experiment)
- plastic containers with enough overhead to suffocate/extinguish the reaction without being exposed to the two-inch flames
- a watchglass or extra glass petri dish (chilled on day of use)
1. After we recall the starvation of the finches in the Galapagos drought, we ask a motivating question, “Why do organisms need food?” We then develop and share initial ideas that address the question “Why do we eat?”
We have recognized that all organisms need to take in stuff to survive.
2. We broaden the conversation about the import of food. “What do organisms need to take in to survive? What do they use it for and what is given off?” Students work to generate a representation of inputs-outputs-uses. After some processing, we notice that everything we’ve listed has to do with matter and energy.
We have realized that food gives us the matter and energy we need, which sort of answers our initial motivating question. We now have a refined driving question, “How do we get the matter and energy we need from food?”
We now have a bigger question, “How do we get the matter and energy we need from food?”
3. We pause to pose the questions, “What is matter? What is energy?” The class generates a common understanding of these ideas before moving forward in exploring their roles in the survival and reproduction of organisms.
We have some basic ideas about matter and energy and a bit of common language. We’ve also discussed the Laws of Conservation of Matter and Energy which we will leverage later.
4. We engage in conversation about what is really in food. After they have described the major components—carbohydrates, fat, and protein, they make a connection to the composition of humans and other organisms. We recognize that “we are what we eat”. We acknowledge the idea of calories but decide to put our questions about energy aside for the moment.
We learned about the composition of food. But what are carbs, fats and proteins made of? We briefly acknowledged calories but we are still uncertain about the exact nature of the energy component of food.
5. We next go deeper to look at the elements that make up molecules of carbs, fats and proteins. Ultimately we will recognize that the same four elements are the primary ones to both go into and come out of our bodies.
We went deeper in our understanding of the molecular composition of food by studying the structure of biomolecules.
6. We now explore the molecular nature of the outputs, setting the stage for the identification of a key phenomenon: What comes out is not exactly the same as what went in. We are matter/molecular transforming machines.
We’ve recognized that in our bodies matter is somehow rearranged.
7. We are now trying to understand why we transform matter. This is further motivated by noticing the majority of the matter we take in is not incorporated. Why bother moving all this matter through our bodies? Through discussion, we come to figure out it’s all about energy.
We’ve recognized that matter transformation must have something to do with energy.
8. We recognize that we still don’t know how we get the energy from matter transformations. Since calories are a measure of the energy in food and we burn calories, we explore what happens when we burn food. We recognize that there is an input-output transformation in burning a Cheeto or marshmallow, but the inputs and outputs are complex. If we want to track matter and energy in a chemical reaction, we need a simpler system.
We’ve developed some ideas around food as fuel and burning as an important chemical reaction.
9. In burning ethanol, we first track the rearrangement of molecules. What are the inputs and outputs? A discussion of burning leads us to the testable idea that oxygen is the other input. Recalling the conservation of matter, we generate ideas about what the outputs might be and test these ideas.
We have a balanced equation for burning ethanol, but we still need to figure out what is happening with the energy.
10. We move on to discuss energy. What does it look like before, during and after the reaction? If energy is conserved, where does it come from? Here we recognize the transformation of energy from potential to other forms through use of an analogy. Students work with a representation of what is happening with energy in chemical reactions like burning. The clear inference is that molecules have different inherent energies. Since energy is released here, the reactants must have had more potential energy than the products.
We know that energy is transformed from potential to visible energy in the burning reaction, and we finally have a model for how energy can be released during chemical reactions.
11. We take stock of (1) what we have figured out and (2) what we are still wondering about, returning to our driving question. We apply our understanding to a Challenge Question about a coma patient.
We’ve finalized our model for chemical reactions and have partially answered our driving question, motivating further exploration of Cellular Respiration.