Lesson Ten: Aerobic Respiration

• Outcome
• Student Misconceptions
• Concept Mapping
• Main activity (25 minutes)
• Glycolysis
• Kreb's Cycle
• Electron Transport Chain
• Group work
• Assessment questions

Outcome

• Demonstrate the process of cellular respiration.
• Describe the three main stages of aerobic respiration.
• Determine the yield of energy carrier molecules produced by glycolysis.
• Identify the products of Glycolysis, the Krebs cycle, and Electron transport chain.
• Determine the efficiency of aerobic respiration by calculating the total production of ATP.

Suggested teaching Methodology: - Lecture method and Group work

Student Misconceptions

Students may be confused by terms that have familiar, everyday meanings distinct from their    biological definitions. The term respiration is particularly confusing, because it is an everyday term with two biological definitions, both in cellular respiration and in breathing. Students may have considerable difficulty distinguishing breathing from cellular cellular respiration.

Although most students recognize that plants respire, they may not fully understand that cellular respiration plays the same role in all aerobically respiring organisms. Many students do not appreciate the relationship between photosynthesis and respiration in plants.

Let us consider some common misconceptions:

• Photosynthesis is the plant’s form of cellular respiration.
• Plants respire only when they don’t photosynthesize.
• Cellular respiration takes place only in plant roots, not throughout the plant.
• Fermentation is misunderstood by many students. You may be surprised to know that many students do not realize that yeast is a living organism.
• Many students have the mistaken view that baking yeast is dead or think of yeast as a set of enzymes rather than living cells.

Focus Question: How do living cells carry out energy releasing reactions

The teacher may begin this class with the Essential Questions.

Essential Questions

• Why do many organisms require oxygen, and food to stay alive?
• For what purpose oxygen is used in our cells?
• What is the source of energy in our body cells? Some more questions might be posed to begin a lesson so that the students can engage themselves to seek answers the questions raised by the teacher. These activities aim to engage students’ minds actively in the learning process, so that they remain “on task” and focused.

Concept Mapping

The teacher is expected to lead students so as to create a concept map regarding cellular respiration by giving hint.  Based on the previous knowledge and readings, students should be encouraged to create a concept map to illustrate how glucose and oxygen is used in aerobic cellular respiration to produce ATP.

As cellular respiration is a complex process, it can be difficult for students to visualize how glucose is broken up to form energy.  Therefore, the teacher is expected to use different strategies that enable students understand the concept of cellular respiration.  

Main activity (25 minutes)

Lecture and Group work

The teacher is expected to write the essential question on the board. Also, the teacher will carry out demonstration of food burning if possible. Teacher guides students through the process of identifying reactants, products, and other essential elements in the demonstration. Students share observations and make inferences on the process, the specific need for oxygen, to burn food.

He /she will construct a concept map based on the observations and process. Finally, the teacher will explain the process of aerobic respiration from the concept map.

How breathing is differs from cellular respiration?

When you hear the word respiration, you might think of breathing. However, cellular respiration is different from breathing. Cellular respiration is a biochemical process that involves series of enzymatic reactions and electron transfer system in cells. In eukaryotic cells, such as plant and animal cells, cellular respiration takes place in structures called mitochondria.

Recall that to get energy, cells must break down glucose. During cellular respiration, glucose is broken down into carbon dioxide (CO₂) and water (H₂O), and energy is released. This energy is stored in a molecule called ATP (Adenosine Triphosphate). The equation of cellular respiration is as follows:

C6H12O6 + 6O₂ ==> 6CO₂ + 6H20 + energy (heat and ATP)

This reaction shows aerobic respiration. In aerobic respiration, food is completely oxidized; carbon dioxide and water are formed. In aerobic respiration, mitochondria are involved.

Mitochondria are spherical or rod-shaped organelles found within the cytoplasm of eukaryotic cells, and are referred to as the “powerhouse” of the cell since they act as the site for the production of high-energy compounds, such as ATP, which are a vital energy source for several cellular processes.

They produce large amounts of energy through oxidative phosphorylation of organic molecules during cellular respiration. That is, they are capable of using glucose and oxygen to produce energy (and releasing carbon dioxide and water in the process) for use in many metabolic processes. Thus, it is not surprising to find several mitochondria in high energy-requiring cells, such as muscle cells.

Aerobic respiration takes place in a series of three reactions. These are:

• Glycolysis
• Kreb's Cycle
• Electron Transport Chain

Glycolysis

The teacher is expected to pose essential questions like, what is glycolysis? Where does it take place? Does it require oxygen? What are the products of glycolysis? Etc.

Glycolysis is the first phase of oxidation of glucose. It involves series of steps catalyzed by different enzymes. Glycolysis:

• Occurs in the cytoplasm.
• Does not consume oxygen.
• It is a common pathway for both aerobic and anaerobic respiration.
• It results in the production of  2 molecules of pyruvic acid, 2 ATP as net product and

2 NADH₂

N.B: Although glycolysis makes 4 ATP, the net ATP production by this step is 2 ATP (because 2 ATP were used to start glycolysis). This means only 2 net ATP are available for cell use.

The teacher is required to show a simplified diagram showing steps of the process of glycolysis.

Also, there is a need to explain the gross amount of ATP formed during glycolysis.

If oxygen is available to the cell, what will happen to pyruvate?

The pyruvate will enter into the mitochondria & thus, aerobic respiration will begin.
Aerobic Respiration

• Occurs in the mitochondria
• Includes the Krebs Cycle & the Electron Transport Chain
• Pyruvic acid from glycolysis diffuses into mitochondria & reacts with coenzyme A to form  acetyl-CoA (2-carbon compound) in the intermembrane space of mitochondria.

As a result, 2CO₂ and 2NADH₂ are produced from pyruvate in the intermembrane space of mitochondria.

Kreb's Cycle

There is no need to discuss each step of the reaction, but the outline of Kreb's Cycle can be shown using diagrams.

• Named after biochemist Hans Krebs.
• Kreb's Cycle is also known as the Citric acid Cycle
• It is a metabolic pathway that indirectly requires O₂ 
• Requires 2 cycles to metabolize glucose.  

The Acetyl Co-A (2C) enters the Kreb's Cycle & joins with Oxaloacetic Acid (4C) to make Citric Acid (6C).

Here the teacher is expected to put the summary of the Kreb's Cycle as follows.

Table 10.1: Summaryf the Kreb’s Cycle

Electron Transport Chain

N.B: As this concept might be difficult for students, the teacher is should show only the outline.

• Found in the inner mitochondrial membrane or cristae.
• Contains 4 protein-based complexes that work in sequence moving H⁺ from the matrix across the inner membrane (proton pumps)
• A concentration gradient of H⁺ between the inner & outer mitochondrial membrane occurs
• H⁺ concentration gradient causes the synthesis of ATP by chemiosmosis
• Energized e- & H⁺ from the 10 NADH₂ and 2 FADH₂ (produced during glycolysis & Krebs cycle) are transferred to O₂ to produce H₂O (redox reaction)

O₂ + 4e- + 4H⁺ ==> 2H₂O

Table 10.2: Summary of Aerobic respiration

N.B: Most cells produce 36- 38 molecules of ATP per glucose (40% efficient).

Group work

Focus Question:  How efficient is aerobic respiration in generating ATP?

• Complete oxidation of glucose releases 686 kcal/mol.
• Phosphorylation of ADP to form ATP requires at least 7.3 kcal/mol.

Aerobic respiration is remarkably efficient in the transfer of chemical energy from glucose to ATP.

• Estimated efficiency in eukaryotic cells is about 40%.
• Energy lost in the process is released as heat. Approximately 60% of the energy from glucose is lost as heat. Some of the heat is used to maintain our high body temperature (37°C).
•  Estimated efficiency of aerobic respiration can be calculated  as:- 

(Amount of energy in one ATP molecule X total amount of ATP per mole of glucose X 100) / Total amount of kilo calories per mole of glucose

(7.3 kcal/mol ATP x 38 mol ATP/mol glucose x 100) / 686 kcal/mol glucose

Assessment questions

• What are the products of glycolysis?
• Where does glycolysis take place?
• How many NADH₂    and FADH₂ are formed in glycolysis?
• How many ATP molecules are formed from one molecule of glucose at the end of aerobic respiration?
• What is the role of oxygen in aerobic oxidation of food?