Using Models to Understand Cellular Respiration1

A scientific model is a simplified representation of reality that highlights certain key features of a process such as cellular respiration. Scientific models can help us understand how cellular respiration produces ATP.

One model of cellular respiration shows chemical equations that summarize the inputs and outputs of cellular respiration. The curved arrows represent coupled chemical reactions; the top reaction provides the energy needed for the bottom reaction.

1. Describe in words what these equations tell us about cellular respiration.

2a. Add another curved arrow to the above chemical equations to show hydrolysis of ATP.

2b. Explain why hydrolysis of ATP is useful.

This figure shows another model of cellular respiration. It focuses on the three main stages of cellular respiration.

3. To describe each stage of cellular respiration, choose the best matches (one per blank). (Use one of the matches twice.)

Glycolysis ___Krebs cycle ___Electron transport chain + ATP synthase ___ ___

a. Makes most of the ATP produced by cellular respirationb. Occurs in mitochondria (plural of mitochondrion)c. Uses glucose as an input

4. The figure does not show most of the inputs and outputs of cellular respiration. Add the missing inputs and outputs to the figure. (The electron transport chain needs O2 as input and produces H2O. The Krebs cycle produces CO2.)

5. Explain why mitochondria are often called the powerhouse of the cell.

1 By Dr. Ingrid Waldron, Dept Biology, Univ Pennsylvania, © 2019. This Student Handout (and a shorter simpler version) and Teacher Notes (with background information and instructional suggestions) are available at https://serendipstudio.org/exchange/bioactivities/modelCR. This activity is intended to follow "How do organisms use energy?" (https://serendipstudio.org/exchange/bioactivities/energy).

Since most of the ATP is made in the mitochondria, we will explore the structure and function of mitochondria. Mitochondria have an outer membrane and an extensive inner membrane, with many folds.

Structure is related to function, so we wonder, “How does the extensive, folded inner membrane of mitochondria contribute to the production of ATP?” To answer this question, we will analyze the figure below.

6. One of these figures shows a magnified view of a small part of the other figure. Which figure shows the more magnified view? the figure above ___ the figure below ___

The electron transport chain proteins are embedded in the inner membrane in the correct arrangement to pass electrons along and pump H + from the matrix to the intermembrane space.

As a result, the concentration of H + is higher in the intermembrane space and lower in the matrix. Because of this concentration gradient, H+ tends to diffuse from the intermembrane space to the matrix.

The only place that H+ can cross the inner membrane is through channels in the ATP synthase molecules. The movement of H + through ATP synthase provides the energy to make ATP . This is similar to how the flow of water or air through a turbine provides the energy to generate electricity.

7. Imagine a mitochondrion that has electron transport chain proteins and ATP synthase, but no inner membrane. Explain why the ATP synthase would not be able to make ATP.

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8. How do the many folds of the inner membrane contribute to greater ATP production?

9. List each type of input molecule that is required for cellular respiration, and state whether the molecule comes mainly from outside the cell or inside the cell. For each type of input molecule, explain how the body gets this molecule or explain how this molecule is regenerated inside each cell.

Input Molecule Source: Outside or Inside the Cell

How the Body Gets this Molecule orHow this Molecule is Regenerated Inside Each Cell

10. Use what you have learned to construct a model of cellular respiration in this drawing of a cell with a mitochondrion. (In an actual cell there are many mitochondria.) Write the three stages of cellular respiration in the appropriate blanks. Label each arrow with an appropriate input or output. Add other information about cellular respiration to make a more complete model.

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