In eukaryotic cells that contain organelles, where do the steps of the Krebs (citric acid) cycle and the electron transport system (ETS) occur?

Introduction / Context:
The compartmentalization of metabolism is a central theme in eukaryotic cell biology. Understanding which pathways live in which organelles helps make sense of energy flow. This question checks if you can locate the Krebs cycle and the electron transport system (ETS) within the cell.

Given Data / Assumptions:

• Eukaryotes have mitochondria, specialized for aerobic energy metabolism.
• Krebs cycle generates NADH and FADH2 from acetyl-CoA.
• ETS uses those reduced cofactors to produce ATP via oxidative phosphorylation.

Concept / Approach:
The Krebs cycle enzymes are in the mitochondrial matrix, while the electron transport chain complexes (I–IV) and ATP synthase are embedded in the inner mitochondrial membrane (with the matrix and intermembrane space forming the electrochemical gradient system). Thus, both processes occur in the mitochondrion, though in different subcompartments.

Step-by-Step Solution:

Verification / Alternative check:
Electron micrographs and biochemical fractionation show Krebs enzymes in matrix fractions and respiratory complexes in the inner membrane, validating mitochondrial localization.

Why Other Options Are Wrong:

• Plasma membrane/ER: not the site of eukaryotic oxidative phosphorylation.
• Chloroplast stroma: hosts Calvin cycle in plants, not the Krebs cycle/ETS.
• “None of these”: incorrect because mitochondria are correct.

Common Pitfalls:
Confusing bacterial localization (where respiration is at the plasma membrane) with eukaryotic localization (mitochondria).

Final Answer:
mitochondria.