Cells that utilise a lot of energy are characterized by the presence of a large number of

Step 1: Analyze the question The question asks which organelle is found in large numbers in cells that use a lot of energy, such as muscle cells, sperm cells, and liver cells.

Step 2: Explain the concept Cells get usable energy from cellular respiration, which produces ATP (adenosine triphosphate). Most ATP is made in the mitochondria, which is why they are called the "powerhouses of the cell." The more energy a cell needs, the more mitochondria it contains.

Step 3: Go through the options

• A. Vacuoles: These store water, food, and waste. They are not involved in energy production.
• B. Mitochondria: These are the site of aerobic respiration (the Krebs cycle and the electron transport chain), where most ATP is made. Their inner membrane is folded into cristae to increase the surface area for energy production.
• C. Endoplasmic reticulum: This transports materials within the cell and helps synthesise proteins (rough ER) and lipids (smooth ER). It is not the main energy producer.
• D. Ribosomes: These are the sites of protein synthesis, not energy release.

Step 4: Examples to remember

• Flight muscles of birds and insects have many mitochondria.
• Sperm cells have a mitochondria-packed midpiece that powers the tail for swimming.
• Active transport cells, such as kidney tubule cells and root hair cells, have many mitochondria.

Answer: B. Mitochondria

Exam tip: Whenever you see "energy," "ATP," "respiration," or "powerhouse," think mitochondria. Likewise, "protein synthesis" points to ribosomes, and "storage" points to vacuoles.