Question

bacteria use a homeostatic system to regulate the concentration of hydrogen ions (\text{h}^+) in intracellular fluid. as the concentration of \text{h}^+ increases, a bacterial cell produces more atp. the additional atp increases the activity of proteins called \text{h}^+ atpases. these proteins actively transport \text{h}^+ out of the intracellular fluid of the cell.

the top figure shows a set of linear relationships among three variables involved in regulating the concentration of \text{h}^+.

the bottom figure shows four path models (a through d) that represent possible homeostatic systems that regulate the concentration of hydrogen ions (\text{h}^+) in intracellular fluid. the regulated variable is represented by a dashed box. other variables are represented by a solid black box. an arrow connecting one box to another indicates a relationship between two variables. the sign above an arrow (+ or -) indicates whether the variables are positively or negatively related.

which path model accurately describes the relationships among the three variables?

a. model a
b. model b

Explanation:

Analyze the biological relationships

Using the Homeostasis, Homeostatic Regulation, and Homeostatic Control Loop knowledge points

• As the concentration of \(H^+\) increases, ATP production increases: positive relationship (\(+\)) from \(H^+\) concentration to ATP production.
• Additional ATP increases the activity of \(H^+\) ATPases: positive relationship (\(+\)) from ATP production to \(H^+\) ATPase activity.
• \(H^+\) ATPases actively transport \(H^+\) out of the cell, decreasing the concentration of \(H^+\): negative relationship (\(-\)) from \(H^+\) ATPase activity to \(H^+\) concentration.

Evaluate the path models

Using the Path Models, Linear Relationships, and Causal Diagrams knowledge points

• Model A:
• \(H^+\) concentration \(\xrightarrow{+}\) ATP production
• ATP production \(\xrightarrow{+}\) \(H^+\) ATPase activity
• \(H^+\) ATPase activity \(\xrightarrow{-}\) \(H^+\) concentration
• Model B:
• \(H^+\) concentration \(\xrightarrow{-}\) ATP production
• ATP production \(\xrightarrow{+}\) \(H^+\) ATPase activity
• \(H^+\) ATPase activity \(\xrightarrow{+}\) \(H^+\) concentration
• Model C:
• \(H^+\) concentration \(\xrightarrow{+}\) ATP production
• ATP production \(\xrightarrow{-}\) \(H^+\) ATPase activity
• \(H^+\) ATPase activity \(\xrightarrow{+}\) \(H^+\) concentration
• Model D:
• \(H^+\) concentration \(\xrightarrow{-}\) ATP production
• ATP production \(\xrightarrow{+}\) \(H^+\) ATPase activity
• \(H^+\) ATPase activity \(\xrightarrow{-}\) \(H^+\) concentration

Identify the correct model

Using the Causal Diagrams knowledge point

• Model A correctly matches all three signs: \(+\) from \(H^+\) to ATP, \(+\) from ATP to ATPase, and \(-\) from ATPase to \(H^+\).

Answer:

• a. Model A (Correct answer)
• b. Model B
• c. Model C
• d. Model D