Which of the following cell types has the least number of mitochondria?

Analyze the question: We need the cell type with the fewest mitochondria. Mitochondria produce ATP through aerobic respiration, so cells with a high energy demand have many, and cells with a low energy demand (or dead cells) have few or none.

Concept: The number of mitochondria in a cell reflects its metabolic activity.

Going through the options:

• A. Cardiac cells of the heart: The heart contracts continuously without rest, so these cells have a very large number of mitochondria (up to about 40% of the cell volume).
• B. Cells of the cornified layer: This is the outermost layer of the epidermis (the stratum corneum). Its cells are dead, flattened and filled with keratin, and they have lost their nucleus and organelles, including mitochondria. They have essentially none.
• C. Muscle cells of the bladder: These are smooth muscle cells. They contract slowly and have a moderate energy demand, so they have a moderate number of mitochondria.
• D. Muscle cells of the diaphragm: The diaphragm contracts rhythmically all the time for breathing, so these cells have many mitochondria.

Conclusion: The cornified layer cells are dead and keratinised, so they have the fewest mitochondria (practically none).

Tip: Link mitochondria count to activity. Active cells (heart, flight muscles, sperm tail, liver) have many mitochondria, while dead or inactive cells (cornified skin cells, mature red blood cells) have few or none. If an option says "cornified," "keratinised" or "dead," think "no organelles."