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MCAT Biology Question — Cell Size

The size of a cell is least dependent on which of the following?


  1. Intracellular solute concentration
  2. Surface area to volume ratio
  3. Resting membrane potential
  4. Extracellular solute concentration




This question asks the examinee to identify the factor that has the least influence on the size of a cell. The osmotic gradient between the intracellular and extracellular environment is critical in determining the flow of water into or out of the cell. When a cell is placed in a hypotonic environment, water flows into the cell causing the cell in swell up and increase in size. Conversely, when a cell is placed in a hypertonic environment, water flows out of the cell causing the cell to shrivel up and decrease in size. Thus, choices A and D can be eliminated.


The surface area to volume ratio is another important factor that determines the upper limit that a cell can reach. As a cell increases in size, the metabolic requirements of the cell also increase. In response to the increase in metabolic requirements, the exchange of nutrients and waste products between the cell and its environment also increases. Since many of these exchanges are limited to transmembrane protein channels, a favorable ratio of a high surface area to volume is required. The surface area is proportional to the square of the radius while the volume is proportional to the radius cubed. Thus, if a cell becomes too large, the volume becomes to great in comparison to the surface area and the metabolic demands of the cell cannot be met. Therefore, choice B is also vital in determining the size of the cell. Lastly, the polarity of the cell, which is established by the sodium/potassium ATPase pumps determines the resting membrane potential involved in action potentials. This electrical gradient has no effect on cellular size.


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