Big Ideas in Evolution Made Simple—Cosmos and Yeast Respiration
Matthew C. Bostick
Upper School Science Instructor
Westchester Country Day School, High Point, NC
Background
Many students understand that living organisms need a constant input of energy to grow, reproduce, and maintain homeostasis. However, evolution by natural selection has produced a diversity of metabolic strategies to meet organisms’ energy needs.
Regardless of the metabolic strategy employed, all cells must preform cellular respiration to transform organic compounds into adenosine triphosphate (ATP) to perform cellular work. So, how did these different strategies evolve? And what were the possible ecological and atmospheric ramifications of these novel metabolic strategies?
Evolution and the Cosmos connection
Big questions such as these provide teachers and students with an opportunity to combine the seemingly disparate ideas around evolutionary biology, bioenergetics, earth science, and astronomy into an inquiry-based lesson that builds enduring scientific understanding.
One strategy is to first introduce students to evidence that supports the highly reduced, oxygen-poor atmosphere found on Earth approximately 3.5 billion years ago, and then ask students to predict metabolic strategies that may be advantageous for organisms in such a harsh environment.
My students absolutely adore "Some of the Things That Molecules Do,” the second episode of the 2014 television series Cosmos: A Spacetime Odyssey. The host, Neil deGrasse Tyson, does a magnificent job at narrating the early events that shaped how life evolved on Earth, explaining evolution by natural selection in the process.
The Oxygen Catastrophe
Tyson introduces the major extinction events that shaped life on Earth. Sometimes called the Oxygen Catastrophe, there is evidence that one such extinction event may have been caused by the ancestors of modern cyanobacteria (blue-green algae).
As free oxygen is toxic to obligate anaerobic organisms, the rising concentration of oxygen produced by ancient photosynthesis may have caused a significant selection against Earth’s early anaerobic inhabitants. However, the formation of oxygen as a byproduct of photosynthesis may have opened up opportunities for aerobic respiration, and therefore paved the way for an explosion in biodiversity that we aerobic organisms still enjoy today.
After watching the video, I encourage students to consider the single-celled fungus Saccharomyces cerevisiae, also known as brewer’s yeast. As a facultative anaerobe, this species is capable of altering its metabolism to survive in either the presence or absence of oxygen. Students cannot directly observe respiration occurring inside living yeast cells, even under a microscope, so they will need to design an experiment to indirectly observe this process.
Experiments in respiration
One method of measuring the effects of cellular respiration is to use a respirometer, such as those found in the Carolina Investigations® for AP® Biology: Cell Respiration Kit. I enjoy helping students assemble the respirometers, often using a guided inquiry approach to help them formulate testable hypotheses concerning the relative rates of respiration in dormant and germinating pea seeds. For good measure, we even test the rate of aerobic respiration using crickets.
While pea seeds and crickets are fun ways to measure relative aerobic respiration rates using oxygen consumption as a dependent variable, instructors may ask students to alter their methods to determine the relative rate of respiration in yeast.
“Demonstrating Cellular Respiration and Fermentation,” an article on Carolina.com, outlines a few experimental methods used to measure both pea seed and yeast respiration with carbon dioxide production as a dependent variable. Using this resource, I encourage teachers to guide students toward designing an experiment that compares respiration rates in different yeast suspensions (students may alter their independent variable to include factors such as temperature, pH, substrate, substrate concentration, oxygen presence, etc.).
Tying it all together
After an investigation, consider the following discussion questions as a means of tying together a few of the new AP Biology curriculum’s Big Ideas:
- How did cellular respiration evolve from anaerobic to aerobic, thus becoming approximately 19 times more efficient (when comparing the ATP yield of glycolysis vs. an aerobic method that utilizes the mitochondrial Krebs cycle and electron transport)?
- What is the evolutionary advantage of being a facultative anaerobe such as yeast?
- If aerobic respiration is more energetically efficient than anaerobic respiration, why do some prokaryotes continue to respire anaerobically?
- If eukaryotes are likely to have evolved from prokaryotic ancestors, how was the origin and evolution of aerobic respiration essential to the origin of modern eukaryotes?
- How critical are mitochondria (or chloroplasts in plants) to eukaryotic life? How does the endosymbiotic theory provide a rationale for the evolution of the eukaryotic cell in response to a changing Earth?
Finally, using the NCBI and NCBI BLAST Web sites, encourage students to compare DNA sequences in an effort to understand evolutionary relationships between yeast and other organisms. You never know what they will find!
Sources
- Carolina Biological Supply Company. 2011. Carolina Investigations® for AP® Biology: Cell Respiration Teacher’s Manual and Student Guide. North Carolina: Carolina Biological Supply Company.
- McAlexander, Shana Lee. “Demonstrating Cellular Respiration and Fermentation.” Carolina Biological Supply Company. Accessed December 17, 2014. www.carolina.com/teacher-resources/Interactive/cellular-respiration-and-fermentation/tr10705.tr.
- Druyan, Ann, and Steven Soter. 2014. “Episode 2: Some of the Things That Molecules Do.” Cosmos: A Spacetime Odyssey. DVD. Directed by Bill Pope. Century City, CA: 20th Century Fox.
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