Phenomenon
Living things need to take in matter to survive and get energy for life processes. It seems that matter is somehow rearranged inside our bodies because the outputs are different from the inputs.
Question
How do we get energy from food? How is the energy in food released in a form that can be used by cells in a way that does not harm them?
Model Ideas
As we move through the remainder of the Red Models Sequence 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.
Energy from Food (primary 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.
New Model Ideas
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.
Matter from Food (secondary 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.
New Model Ideas
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).
We explicitly split our continued investigation of how we get the matter and energy we need from food into two different tracks before we continue to develop our model for energy from food.
We’ve separated our continuing investigation of food into two complementary models, “Matter from Food” and “Energy from Food” model. We now turn our focus primarily to energy.
We review our ideas about how we get energy from food and recognize that we haven’t identified the specific reaction(s) that tell us exactly how we get the energy from carbs, fats, and proteins. Now, we ask that question.
We have a new driving question, something like: How do living things do the ‘downhill’ reaction to get energy from food?”
We return to burning as a chemical reaction that can help us understand how we get energy from food. We explore burning in a bit more detail.
We’ve developed some ideas around food as fuel and burning as an important chemical reaction. We recognize that burning food does give energy and that burning organic molecules made of Cs Hs and Os yields CO2 and H2O.
The burning reaction takes in oxygen and gives off CO2, so might the reaction that living things do be similar? What happens under conditions when we need more energy (exercise), do we put off more CO2? We explore these questions through a lab that connects energy needs with CO2 output.
We have further evidence that burning (the reaction of organic carbon with oxygen) is important for living things to obtain energy, and we’ve done a bit of thinking about the relationship between the rate of that reaction and our CO2 production.
Now that we are thinking it IS some kind of burning reaction, what is the specific fuel that reacts with oxygen in our cells? Cellular respiration in most organisms is set-up to preferentially use glucose, the building block of carbohydrates. We can easily balance this equation.
We have a balanced equation for cellular respiration. We now know that glucose is what we’re burning in cellular respiration but that the body can also draw upon proteins and fats as fuel as needed.
We recognize that burning glucose gives a TON of energy per glucose molecule. What happens with this energy in the cell? We develop a refined model for cellular respiration where energy is released step-wise through a series of reactions inside cells and in the mitochondria.
We have a better model for how the energy from CR is available to our cells and have summarized our overall model for explaining how we get Energy from Food.
Do all living things use cellular respiration to obtain energy? (Unity and Diversity connection.) What about life that doesn’t have mitochondria? What about cases where there is little to no oxygen? We explore the ubiquity of CO2 with a lab and also consider the alternative of fermentation.
We’ve recognized that most organisms use cellular respiration as a means for getting energy from food. But we’ve also considered alternative pathways when oxygen is present. All organisms need to acquire energy, but there is some variation in the exact pathway used in different circumstances. In this way, we highlight the unity and diversity of life.