Sources of Biological Energy

Dr. Linden Higgins, Education for Critical Thinking

Dr. Elizabeth Dolci, Department of Environmental and Health Sciences, Johnson State College

College students and older high school students, in the midst of a major period of identity development as adults, are exposed to a great deal of information about weight management, obesity, and diet. For them to accurately interpret information readily available in social networks and other media requires a solid understanding the nutritional value of foods, particularly around energy content, calories, and carbohydrates. Simultaneously, a great deal of advertising emphasizes energy drinks as a way to increase one’s focus and activity level, without admission that this “energy boost” is through caffeine stimulation rather than biological energy (carbohydrates). There is increasing concern that high consumption of energy drinks by young adults can have adverse health effects. In 2014, the Centers for Disease Control and Prevention (CDC) called for increased education around the health issues associated with over-consumption of these drinks (CDC bulletin 3/05/2014).

The confusion around energy vs. stimulation reflects a fundamental misunderstanding of the nature of chemical energy that has been much discussed in the physics and chemistry education literature (e.g., Reissig, Strain, and Griffiths 2009). Many students are unaware that the stimulation they feel after consuming a caffeinated drink is not due to an increase in available energy. This misconception likely contributes to over-use of stimulant energy drinks, which commonly contain more caffeine than recommended daily consumption and can have serious adverse effects (reviewed in Reissig, Strain, and Griffiths 2009). These misunderstandings of the nature of chemical energy may also contribute to a range of other health effects related to obesity, weight management, and diet (Nelson et al. 2009).

This common misunderstanding of the difference between energy and stimulants gives us an opportunity to guide students in an experimental test and, more importantly, provide them with the experience of having a well-designed experiment produce unexpected results, often contradictions their predictions. In this activity, students test whether substances of their own choosing provide energy to a living organism.

In this activity students gain a clearer understanding of the biological process of fermentation and, more foundationally, the difference between biological energy and stimulation. They learn that experimental results do not always agree with predictions and that unexpected results can be much more interesting than the “right” answer. With an improved understanding of the distinction between stimulants and energy sources, students can better connect their personal experiences with caffeinated drinks, sugar intake, and body weight to the chemical nature of food.


Courses Into Which This Activity Could Fit

This activity is well suited to pre-service teacher training and to introductory biology courses for non-majors (general education) or majors. Specifically, it is appropriate for the cell and molecular biology portion of a general (introductory) biology course, for science courses designed for pre-service teachers, and high school biology (aligning with Next Generation Science Standard (NGSS) Dimension 1: Scientific and engineering practice, developing and using models and NGSS Dimension 2: Cross-cutting concepts, cause and effect, and energy and matter.)


Scientific Concepts Addressed and Related Civic Issues

The concept of energy in general and biological energy in particular can be very difficult for students to grasp. Differentiating between foods that provide calories and foods that stimulate is particularly challenging, but this difference is key in helping students comprehend the dietary bases for the “epidemic” of obesity and subsequent health issues.

Equally challenging for many students is the reality that scientific experiments are far more fluid and dynamic in design and execution than the cookbook approach used in most laboratory manuals. That an experiment might be well designed and properly executed and yet provide an unexpected result is foreign to them: students generally take unexpected results to mean that they failed, not that they have more to learn. This misconception can lead to misunderstandings of why scientists disagree, why scientific conclusions change, and why research needs to continue even in fields where scientists appear to have reached consensus. Countering this misconception is the key civic issue addressed by this activity, a self-design experiment.


The Activity

Students will engage in semi-independent design of an experiment to test the fermentation that results from the stored energy in a beverage or soft food of their choice. In this lab, we expose students to two important concepts. Allowing students to design experiments exploring the energy content of beverages of their choice exposes them to real inquiry. And because of the confusion between stimulation and biological energy, many students will choose caffeinated beverages when instructed to choose a beverage or food that serves as an energy source for testing. Brainstorming predictions prior to the lab enables students to make explicit their understanding of the nature of energy (and any existing confusion with stimulation), which is reinforced with requiring them to write the predictions down as part of their pre-lab work.

They measure rates of fermentation by yeast, in effect determining whether a particular beverage truly serves as a source of energy. Fermentation by yeast is thus a model system for understanding biological sources of chemical energy (NGSS Appendix F), allowing instructors to also discuss the role of model biological systems and scientific inference of general principles from specific contexts. Class-wide discussion of results, whether the results support or refute their predictions, and interpretation of their results in light of the processes of fermentation provide an opportunity for students individually and collectively to come to understand the differences that they had previously confounded of stimulation versus energy.

In this wet-lab experiment, students use an established protocol to test how well yeast can ferment particular beverages, in order to determine whether a beverage serves as a source of energy. They design an experiment that compares the accumulation of carbon dioxide in yeast cultures fed one of two (or more) different substances. They work in pairs to develop and present their protocol and predicted results, execute the experiment, record the results, and develop and present their results with an explanation based upon the chemistry of fermentation (which is covered in class synchronously with this lab).

  • Laboratory 1: Students are given the core yeast fermentation protocol, and they use the form below to discuss anaerobic fermentation, brainstorm different beverages they want to test, why they are interested in those beverages, and what they expect to find. We discuss dependent and independent variables, review the basic biochemistry of fermentation in yeast, and preview the physical set-up of the lab. Note that students choosing to use carbonated beverages should be guided to consider the confounding impact of carbonation in the beverage and CO2 produced by fermentation.

During this lab, students fill out and hand in this form:

  1. What are the basic chemical steps of anaerobic fermentation?
  2. What is a question you have about resources that yeast can use for energy in fermentation?
  3. Select an energy resource to test. Your understanding of fermentation should guide your choice. Choose liquids and not solid foods (such as apples or bananas).
  4. Design a protocol. Consider each of the following before you do this, writing everything dwn in your lab notebook.

a) Manipulation:

b) Measurement:

c) Other variables that could impact fermentation:

d) Variables you need to control for:

Write out your protocol. Include explicit descriptions of what you are manipulating, what you are measuring, and how you are controlling for other variables.

  1. Make predictions. If your understanding of fermentation is correct (#1), what do you expect to see happen (#4b) when you do your experiment (#4a)?


  • Laboratory 2: Students conduct the experiment, following this basic protocol:
  1. They put 5 ml of the substance to be tested (their selected energy source(s) or the sugar solution) in each fermentation chamber. They add the 1% yeast solution to fill each chamber, tipping the tube to mix and remove air bubbles from the closed side-arm (see Mark Garcia video,; note that we provide pre-mixed yeast solution, and do not put the chambers in an incubator as they do fine on the desktop and students gain by watching the fermentation process develop (or not)). They then cover the mouth of each chamber with parafilm, poke a small hole in the film, and note the time at which each fermentation chamber is filled.
  2. For the remainder of the lab period, they make observations and measurements according to their protocol. We keep pH paper or a pH meter available, as acidity is a common source of unexpected results and is almost never controlled for in their designs.


  • Laboratory 3: Students present their results to the class and brainstorm interpretations and the broader implications of their findings. As appropriate to the group of students in the class, we sometimes require that they design a subsequent experiment to answer a question arising from their results. For example, when students find a significant difference between two fruit drinks that we suspect differ in pH as well as sugar content, we brainstorm ways to either manipulate or control for that difference. Soft drinks often produce very unexpected results, and we ask them to investigate online the ingredients for the drinks they used and consider how those ingredients might have affected fermentation. Online research into the source of “energy” in beverages is an additional opportunity for a conversation around the difference between stimulant and biochemical energy.

Additional Activities with which this can fit

This activity could fit with other investigations of diet and nutrition, particularly activities asking students to consider their own dietary habits. Walsh (2013) presents a nutritional label–analysis project and both Davis and Rice (2009) and Heidemann and Urquhart (2005) describe case study exercises examining the difference between “energy drinks” and chemical energy.


Laboratory 1 (60 minutes or longer, depending upon the background and prior exposure to experimental design): Students brainstorm different beverages they want to test and what they expect to find, review the basic biochemistry of fermentation in yeast, and preview the physical set-up of the lab.

Laboratory 2 (90 minutes): Students conduct the experiment.

Laboratory 3 (90 minutes): Students present results to class and interpret their data.

Prior knowledge required

This laboratory assumes knowledge of the basic hypothesis (sugars feed into yeast fermentation; the biochemistry of yeast fermentation), but the execution assumes that students do not really grasp the difference between sugars and chemical stimulants. More advanced students will still benefit from the experience of designing their own experiment, but are perhaps less likely to make the foundational error of using a chemical stimulant rather than a sugar source.

Students should have a working knowledge of the general concepts of controlled experimental design and deductive reasoning, and be able to identify reputable online sources, search for background information from food industry websites and other sources, and understand the foundational processes of cellular respiration and fermentation.

Materials needed

  • Fermentation chambers. These vary in size from quite small to quite large. If fermentation chambers are not available, an alternative is to use 15 ml plastic conical tubes with graduations. After a hole is poked in the cap, they are filled completely, mixed, and inverted over a tray. The graduations serve to measure the CO2 produced by fermentation. Another alternative is to put the mixture into test tubes with snug-fitting balloons to collect the CO2.
  • 5% sugar solution (by weight)
  • 1% baker’s yeast solution (by weight)
  • Rulers
  • Parafilm squares
  • Graduated cylinders
  • Thermometers
  • pH paper
  • Test substances (which we require students to bring)

Context and Concepts for Instructors

  • Beverage types and positive controls

Often, students will bring in stimulant beverages rather than sweet beverages and compare different concentrations or across beverages (green versus black tea or caffeinated versus uncaffeinated coffee).

It is important to have all students run a positive control of a 1% sugar solution so that they have one treatment that does show fermentation. We are careful during discussion of the protocols not to indicate at all what the actual outcomes will likely be. As the experiment proceeds, it often becomes obvious which beverages are not allowing fermentation, and we encourage students to use the pH paper and online searches for ingredients’ lists to explore possible explanations. In addition to consideration of sugar concentrations, this also gives us the opportunity to discuss confounding factors (what is the full suite of ingredients that differ between Pepsi and Coke? what are the likely consequence of using carbonated beverages as energy sources?), differences among types of sugars (fructose and sucrose ferment at different rates), and the need to do background reading prior to designing an experiment.

  • Grading rubrics

Instructors will find it helpful to have rubrics for guiding and grading student presentations and written products (if required). Our rubrics are below.

In oral presentations, we require students to ask at least one question of a classmate’s project and include the quality of the question and response as part of their grade. This can then form the basis of a partial or complete lab report. We tend to place this lab early in the semester and use it for a partial lab report: protocol, results, and interpretation.

(1) Sample Protocol Grading Rubric

Score: /60 points

Overall presentation (10)

Full sentences, good paragraph structure, writing is logical and evidence-based, evidence is accurate according to materials available in class and in the laboratory documents.

Hypothesis (10)

Explicit testable hypothesis describing the process of fermentation and the role of “energy sources”

Protocol (20)

Appropriate design: identifies accurately manipulated variable(s), factors being controlled for, and factors being measured

Sufficient detail:detail is sufficient that the protocol could be handed to another student to be implemented

Predictions (20)

Aligned with hypothesis and relevant to protocol: identifies correctly the outcomes of the experiment under the conditions of the protocol


Sample Yeast Final Report Grading Rubric

Score: /100

Overall presentation ( /20)

Full sentences, good paragraph structure, writing is logical and evidence-based, evidence is accurate according to materials available in class and in the laboratory documents

Hypothesis ( /20)

Explicit testable hypothesis describing the process of fermentation and the role of “energy sources,” modified from protocol to reflect any feedback provided prior to the experiment being run

Protocol (/30)

Appropriate design: identifies accurately manipulated variable(s), factors being controlled for, and factors being measure. Reflects what actually happened and written in the past tense.

Sufficient detail: detail is sufficient that the protocol could be handed to another student to be implemented. Written in the past tense.

Predicted results: Aligned with hypothesis and relevant to protocol; identifies correctly the outcomes of the experiment under the conditions of the protocol. Written in the past tense.

Results and interpretation (/30)

Data adequately described: Outcome of the experiment is described verbally and is linked explicitly to any graphs or tables

Appropriate presentation: Figures and tables are used as appropriate but always accompanied by verbal descriptions, headings, and labels

Evaluation/interpretation: Data are linked explicitly back to the hypothesis and predictions, and students reflect on unexpected results or problems encountered, propose questions related to improving their understanding

Broader implications: Hypothesis and results are linked to the broader context of energy, diet, or societal issues as appropriate for the course context

Revision or elaboration: Students propose new experiments based on questions that arose from doing this experiment



What students will be able to do

Upon completion of this activity, students will be able to:

  • Design a simple experiment with manipulated (independent) and dependent variables
  • Discuss the importance of identifying and, if possible, controlling for potential confounding variables
  • Understand the difference between a chemical providing energy and a chemical stimulant
  • Recognize the value of having an experiment “fail”
  • Brainstorm protocols
  • Brainstorm interpretation of results
  • Offer and receive constructive criticism from peers

Ways that this activity enriches the engagement of citizens with social and civic problems having underlying scientific issues

This activity connects to several SENCER ideals, including:

  • SENCER robustly connects science and civic engagement by teaching “through” complex, contested, capacious, current, and unresolved public issues “to” basic science. Diet and nutrition are a source of a large amount of press, and the actual link between hype and physiology is generally ignored. On the one hand, students are exposed to a great deal of information about obesity, weight management, and diet. On the other hand, students are exposed to hype about energy drinks. The distinction between nutritional energy sources and stimulants is obscured in the press and becomes obvious in this activity. This distinction is important for students learning to evaluate the quality of their personal diets and may inform their decisions about caffeine intake.


  • SENCER invites students to put scientific knowledge and scientific method to immediate use on matters of immediate interest to students.
  • Students’ justification for their choices of foods to test in this experiment tends to be personal and immediate: they choose foods and beverages they are eating, often in order to wake up or stay awake during the day. They understand at some foundational level that the energy-harvesting processes in yeast are analogous to their own processes, and thus often view their results as important and, when unexpected, really surprising.


  • SENCER seeks to extract from the immediate issues the larger, common lessons about scientific processes and methods.
  • This activity engages students in the actual process of designing, executing, and interpreting an experiment.


  • SENCER locates the responsibilities (the burdens and the pleasures) of discovery as the work of the student.
  • We have never had a student make a prediction in this exercise that was based on preliminary investigation of information available online. We encourage them to not explore likely results of their experiment prior to running the experiment, because we want them going with their intuitive understanding of “energy sources.” The problems of unidentified confounding variables and unexpected results become real rather than academic conversations in class, and the discussions of interpreting results in light of these problems increases their interest in critical reading of media reports about experimental and observational results.


  • SENCER, by focusing on contested issues, encourages student engagement with “multidisciplinary trouble” and with civic questions that require attention now. By doing so, SENCER hopes to help students overcome both unfounded fears and unquestioning awe of science.
  • We have developed the habit of extending this activity with journal reflections on “doing” science. The first week, the journal prompt is, “How do you feel about designing your own experiment?” and the third week, the journal prompt is, “How do you feel about what happened in the yeast experiment?” The majority of students answer the first prompt very negatively, at least in the open-enrollment college where we teach. In contrast, the majority of students answer the third-week prompt very positively, enthusiastic about having “done” science and had it turn out “okay.”


  • Student reflections. Pre-lab prompt: “Is designing your own laboratory experiment exciting or intimidating, and why?”Responses to this prompt reflect students pre-existing comfort with cookbook laboratories, such as “I’d rather be told what to do than figure it out on my own;” and “I’ll probably just ask someone what they did.” Many students also approach the lab as having only one pre-determined outcome, such as “…the fear that we …may not properly execute this lab correctly;” and “I’m afraid that I’ll mess up…” and that “I’m not exactly sure… what I’m looking for.” A few students approach the lab with a sense of adventure – “designing our own laboratory experiment gives us some hands on feel of what is being taught” and the “idea is exciting enough to test.” The pre-lab exercises did provide some students with confidence, “I know what I’m doing right now…”– Student reflections. Post-lab prompt: “Was doing the lab on fermentation easier or more difficult than you thought it would be, and why?”This reflection was written after the wet lab and the post-lab discussion. The general response was that the lab had been “easy” (nearly all responses; the exceptions were students who spilled or otherwise had “accidents” during the lab). The reasons varied and included both the pre-lab conversations, the actual simplicity of the process, and the in-lab help. That results were unexpected frustrated some students, as in “I just had a lot of unanswered questions on the outcome…”


The College Board’s Enduring Understandings That Connect Most Closely

The Enduring Understandings in biology and environmental science that arise depend on the larger context in which this activity rests. These may include:


Big Idea 2: Biological systems utilize free energy and molecular building blocks to grow, to reproduce and to maintain dynamic homeostasis.

Enduring Understanding 2.A: Growth, reproduction, and maintenance of the organization of living systems require free energy and matter. 

Essential Knowledge 2.A.1.c: Energy-related pathways… note that this experiment shows that not all molecules will feed into an energy-related pathway

Essential Knowledge 2.A.2: Organisms capture and store free energy for use in biological processes.

Additional Resources

Laboratory protocol distributed to students:

Analyzing Energy Sources

Introduction: This is the first of three experiments that you will be responsible for designing, executing, and reporting on. For each of these, we will work together to develop your hypothesis, protocol and predictions, and to interpret your results.

In this lab, you will be testing the ability of different resources to provide energy to a living organism. We are using a small fungus, yeast that has metabolic pathways that are very similar to other eukaryotes (including humans). Yeast, like many eukaryotes, can engage in fermentation: they can convert resources into energy (ATP) while producing alcohol and carbon dioxide (lactic acid produced in animals). Each of you will have access to fermentation chambers and yeast, and will provide energy sources of your choosing to test.

Background: Yeast are single-celled eukaryotes capable of both aerobic and anaerobic respiration. In the absence of oxygen, yeast oxidize glucose to carbon dioxide and ethanol in a process called alcoholic fermentation.

In the absence of oxygen, normal cellular respiration cannot go forward, and ATP synthesis in the mitochondria halts, leaving high levels of NADH in the cytoplasm and a shortage of NAD+. ATP can come from glycolysis, the first step of glucose degradation, but only in the presence of NAD+. Fermentation is a “side-step” that regenerates cytoplasmic NAD+, producing ethanol and CO2 as byproducts. CO2 is released as a gas, which accumulates in the side arm of the fermentation chamber.

Our Moodle website has a link to Mark Garcia’s video about setting up fermentation ( You must view this, and review the class and textbook discussions of fermentation in order to develop reasonable protocols for your experiments!

Experimental design: To design your experiment, you need to answer these questions. Be sure that you are writing everything down in your lab notebook.

  1. What are the basic processes of anaerobic fermentation? Why do you need to understand the chemistry of fermentation before starting the deductive process of developing your experiment?
  2. What is a question you have about the resources that yeast can use for energy to carry out fermentation?
  3. Choose a relevant energy resource to test. Your understanding of fermentation should guide your choice. Do not choose solid foods (e.g., apples, bananas).
  4. Design a protocol: Consider each of the following before you do this, writing everything down in your lab notebook.
  5. a) Manipulation:
  6. b) Measurement:
  7. c) Other variables that could impact fermentation:
  8. d) Variables you need to control:

Write out your protocol. Include explicit descriptions of what you are manipulating, what you are measuring, and how you are controlling for other variables.

  1. Make predictions: If your understanding of fermentation is correct (#1), what do you expect to see happen (#4b) when you do your experiment (#4a)?


Materials available:

  • Fermentation chambers. These vary in size from quite small to quite large. You can have up to 3 chambers for your experiment.
  • 1% yeast solution (by weight).
  • 5% glucose solution (by weight).
  • Rulers
  • parafilm squares
  • graduated cylinders
  • thermometers
  • pH paper


General procedure:

Choose a set of yeast chambers that match in size (we have three different sizes available). Determine the volume of your fermentation chamber by filling it with water until the side arm is filled and the water covers the entrance, tipping it back and forth to eliminate bubbles in the side arm. Pour the water into a graduated cylinder, and note the volume. This is the total volume of your chamber.

Put 5 ml of your energy source into each chamber. Add the yeast solution to fill the chamber, tipping the tube to mix and remove air bubbles from the closed side-arm (see video). Cover the mouth with parafilm and poke a small hole. Note the time at which each fermentation chamber is filled. How much of the yeast solution did you use (how can you calculate this from the volume you determined)?

Run the experiment for the remainder of the lab period, making observations and measurements at least every 10 minutes or according to your protocol. Be certain to leave enough time to clean up the chambers and wipe down your table.

Writing up:

After the lab, look at your data and your observations. Write a narrative description of your protocol (past tense first person) and then write a description of your results, including graphical presentation of your data. Be certain to describe your findings in your narrative, referencing your figure. End the description of your results with a comparison of your prediction(s) and your understanding of the resources used in fermentation by yeast. Were your predictions upheld by your data or not? How has this experiment changed your understanding of fermentation? Lastly, briefly describe what experiment you would do next, given the opportunity. You will present your data to the class for discussion, and then will prepare a paper that includes your protocol, your results, and a brief discussion of your results.


You will be evaluated on three things during this laboratory:

  1. Your protocol
  2. Your presentation of your results
  3. Your written paper.

The rubrics for these will be posted to Moodle.


Literature Cited

Centers for Disease Control 2014. CDC Study: Youth perceptions about energy drinks.

Davis, Cheryl D and Nancy A Rice. 2009. “Case Teaching Notes for “Another Can of Bull? Do Energy Drinks Really Provide a Source of Energy?” National Center for Case Study Teaching in Science, University at Buffalo, SUNY.

Heidemann, Merle and Gerald R. Urquhart. 2005. “A Can of Bull? Do Energy Drinks Really Provide a Source of Energy?” Journal of College Science Teaching 35:40-44.

Nelson, Melissa C., Lytle, Leslie A., and Keryn E. Pasch. 2009. Improving literacy about energy-related issues: The need for a better understanding of the concepts behind energy intake and expenditure among adolescents and their parents. Journal of the American Deitetic Association 109:281-287.

Next Generation Science Standards Appendix F: Science and engineering practices in the NGSS. F Science and Engineering Practices in the NGSS – FINAL 060513.pdf.

Reissig, Chad J., Eric C. Strain, and Roland R. Griffiths. 2009. “Caffeinated energy drinks – A growing problem.” Drug and Alcohol Dependence 99:1-10.

Walsh, Joseph A. 2013. “Obesity & the First Law of Thermodynamics.” The American Biology Teacher 75:413-415.