Terry M. Bella
Cellular respiration is critical content to cover in biology. This is a wonderful opportunity for a teacher to weave in instruction about enzymes in order to provide a relevant context for students.
If you were to query your students at the beginning of a unit they will know that sugar provides energy for humans. They will know that we need to ingest the sugar to get the energy that it stores. They will also know that we must inhale oxygen and that we exhale carbon dioxide. This is of course an important understanding, but severely limited. At this point we can ask our students what questions they have about the process, starting with a "Chalk Talk" around the word respiration. I use "Chalk Talks" throughout the year to focus the class and to help them realize what they may or may not know about a topic. This effective at the beginning of a unit, but is also very effective at the summation of a unit.
Chalk Talk
For a "Chalk Talk" the teacher chooses a word or phrase and writes it on the board. Then the students are expected to make connections and associations with the word. The rules are simple: no talking and students are free to get up and write on the board without asking permission. You want to encourage them to keep what they write brief because it will be discussed when the writing period is over. The no talking rule achieves two ends, number one it helps maintain order and number two it reduces or eliminates judging. It is important that the students feel safe to write whatever is on their mind to connect to the topic and not be judged in the process. After the energy has simmered out and the ideas are not flowing anymore the teacher takes the lead again and discusses each and every idea. It is important that with each idea the teacher offers the author a chance to explain but also that the teacher asks the class how the idea ties to the topic. Some students will not want to talk about their ideas; what matters is that they were given the chance to share and be a part of the class in a way that was comfortable for them.
The "Chalk Talk" will generate many questions because it demonstrates to the students that not only do they already know about the topic but that they also have associations with the topic that they may not be able to make much sense of. After discussing each idea, it is time to generate a list of questions with the students. Typically we pull on the main themes of the discussion and generate a list of questions around this.
This approach is very popular with my students. The first time is a little difficult because they are generally not used to having such freedom. Subsequent "Chalk Talks" always run much more smoothly. I find that it is not only easy and quick, but really helps to focus the students on the topic and give them some freedom to ask questions. Most importantly this allows the students a time to get up and move around during class.
Questions: Cellular Respiration
Cellular respiration is the process of breaking down glucose, or other substrates, in order to generate energy in the form of adenosine triphosphate (ATP). This is complex process involving three separate pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation. Each of these separate pathways involves multiple enzymes specific to the intermediates produced and used within the pathways.
The equation for cellular respiration is: C
6
H
12
O
6
+ 6 O
2
--> 6 CO
2
+ 6 H
2
0 + Energy
Our cells are taking in glucose and oxygen and releasing carbon dioxide and water constantly in the effort to keep the body alive and functioning. The process of gylcolysis, one of the three pathways involved, requires ten different enzymes to convert one molecule of glucose into two molecules of pyruvate. This also yields a net gain of two ATP's for the cell.
The pyruvate is the substrate for the citric acid cycle which also yields two ATP per glucose. The citric acid cycle uses a multienzyme complex, the pyruvate dehydrogenase complex, to achieve the process in a fast and efficient manner.
Both glycolysis and the citric acid cycle also reduce molecules of NAD
+
and FAD to NADH and FADH
2
that will supply electrons to the third process, oxidative phosphorylation. Through an electron transport chain and chemiosmosis this third pathway will generate between 32 and 34 ATP per glucose for the cell.
Again, provided is just a brief synopsis of the content in order to discuss the topic of inspiring students to generate questions by providing a context for them to learn the abstract concepts of enzymes.
Teaching enzymes through cellular respiration the teacher has numerous ways to tie relevance and meaning into the unit. As the body needs more energy there are mechanisms that will ramp up the process and vice versa if the body does not need the energy and is storing instead. This can be linked to instruction about enzyme activation and inhibition.
The intent is to motivate students to ask questions about the impacts of enzymes on the entire process of cellular respiration both as a whole and on a specific level. Students can pose questions about the impact of each enzyme on the process. Students may pose questions about the loss of a particular enzyme and how that will impact the process. Students may pose questions about the effect of temperature on the different enzymes. Students can pose questions about how the multienzyme complex involved in the citric acid cycle facilitates a more efficient process. Students can ask what happens when not enough glucose is available or if there is too much. Students can investigate how the body removes the waste products of carbon dioxide and water.