Chemical Reactions in Living Things

Overview

All organisms need matter and energy to survive. Students will explore where that matter and energy comes from. They will build a model that explains how food can be transformed -via chemical reactions- to provide the matter and energy necessary for organisms to survive.   

Transition in from Continent Formation: Now that the Earth has masses of land poking above the oceans, we return to thinking about life on the precipice of diversification. What challenges do living things face in Earth's shallow seas as well as those they face as they move onto the land?
 
This model is the first in a series of models (the Red Models Sequence, i.e. "matter and energy in organisms") 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 biology and entering a 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 Models Sequence. We decide to first focus on one important input, food. In exploring food, we come to realize that it essentially gives us the matter (the “stuff”) and energy (the “doing”) we need, but we still have questions about how this works. 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: 8-10 days