What is a primary reason cells remain small?

• A. Exchange with the environment occurs at the cell surface and is limited by surface area.
• B. The nucleus controls organismal growth.
• C. Metabolic rate does not depend on cell size.
• D. Cells purposefully stay small to preserve DNA integrity.

Answer: (A)

The key idea is that exchange with the environment happens across the cell membrane and is limited by surface area. Substances like nutrients in and wastes out cross the membrane, and the rate of this exchange roughly follows the area of the cell’s surface. Meanwhile, the cell’s needs—the amount of nutrients required and waste produced—scale with the cell’s volume. As a cell grows, its volume increases faster than its surface area, so the surface area-to-volume ratio drops. That makes diffusion and transport across the membrane less capable of meeting the cell’s demands, leading to inefficient exchange. Keeping cells small (or adopting shapes that maximize surface area) helps ensure that nutrients can enter and wastes can leave quickly enough to sustain metabolism.

The other ideas aren’t the primary reason: growth is regulated by signals and the cell cycle, not simply by the nucleus; metabolic rate does depend on size in the sense that larger cells require more resources; and staying small to protect DNA isn’t the main constraint driving cell size.