Biology, The Science of Life

Cellular Respiration: Harvesting Chemical Energy
Part 1

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Overview: Today’s lesson will be focused on understanding how respiration breaks down the fuel energy from our foods, generating ATP (energy source). The instructor will lecture for the whole duration of the class (i.e. 50 minutes). This topic is difficult to grasp, therefore the instructor will progress slowly through the material and put great emphasis on the diagrams in the Power Point presentation.

Materials: The teacher will need access to the class computer, screen, and projector to present the Power Point presentation. The Power Point presentation is the media object used as a teaching tool and is available for download above. Students will need pencil and paper or their laptops to take down notes. The Power Point will be made available to the students therefore students must focus on taking notes regarding what is not presented on the slides.

Assessment: The assessment component for this topic will be described in Part 2 (lesson 08).

Extra Notes: Students have prior knowledge of redox reactions and the hydrolysis/synthesis of ATP from their Chemistry class which they are taking co-requisitely.

  • The Stages of Cellular Respiration (10 minutes)
  • Glycolysis (20 minutes)
  • The Citric Acid Cycle (20 minutes)

Explain the following:
The first two stages of cellular respiration, glycolysis and and the citric acid cycle, are the catabolic pathways that break down glucose and other organic fuels. Glycolysis, which occurs in the cytosol, begins the degradation process by breaking down glucose into two molecules of a compound called pyruvate. The citric acid cycle, which takes place in the mitochondrial matrix of eukaryotic cells or simply in the cytosol of prokaryotes, completes the breakdown of glucose by oxidizing a derivative of pyruvate to carbon dioxide. Some of the steps of glycolysis and the citric acid cycle are redox reactions in which dehydrogenases transfer electrons from substrates to NAD+, forming NADH.
Open the Power Point and follow the slides very closely. Use the white board to further emphasize what is happening in the pathway (i.e. emphasize where the formation of ATP and NADH occur).

Starch and glycogen is converted to glucose before the latter is used as a substrate for respiration. Glycogen or starch is convereted to glucose-1-phosphate by phosphorylase. Starch or glycogen may also be first converted to glucose, which is then convereted to glucose-1-phosphate using ATP.
Glucose-1-phosphate enters a series of reactions that constitute glycolysis (details in Power Point). It produces 2 NADH and 2 ATP for each glucose consumed. The production of these ATP is through subsrate level phosphorylation (i.e. without the involvement of the proton motive force). However, if glycogen or starch are converted directly to glucose-1-phosphate, there is a net production of 3 ATP per glucose. The end product of glycolysis is pyruvate.

Pyruvate goes to the mitochondria and is decarboxylated (releasing one CO2) and get converted to acetyl Coenzyme A which, in turn, enters a cycle of reactions called the Kreb cycle. In this cycle, the intermediates are 3- or 4-carbon organic acids and there is a progressive decarboxylation (2 more CO2 released) and released hydrogens are used to reduce NAD+ and FAD. So far, we have seen the removal of 3 CO2; one from pyruvate just after it entered the mitochondrion and 2 during the TCA cycle.

Students are only responsible for knowing steps 1,3, & 10 in Glycolysis in detail. However, they must have a vague idea of the rest of the process. They are also responsible for knowing every step in both Glycolysis and the Kreb cycle of when NADH, ATP, FADH2, and CO2 is used.
Once the material has been covered, advise the students that there is no assignment for today's class but there will be one assigned next class which covers the material from today's class and the next.