In the context of mitochondria, why do some cells (e.g., skeletal muscle cells) contain more mitochondria than others (e.g., some resting connective tissue cells)?

• A. Cells that secrete large amounts of protein contain more mitochondria because mitochondria are the main site of ribosome assembly and protein translation.
• B. Cells that synthesize more DNA require more mitochondria because mitochondria are the primary site of DNA replication in eukaryotes.
• C. Cells exposed to more sunlight contain more mitochondria because mitochondria perform photosynthesis to generate ATP.
• D. Cells with higher ATP demand for sustained activity require greater oxidative phosphorylation capacity, so they contain more mitochondria.

Answer: (D)