Factors Limiting Cell Size

Although cells are microscopic, their average size varies from 5 to 20 micrometers (0.05 to 0.02 mm), according to Dr. Max Fogiel, author of "The Biology Problem Solver." Cell type, external nutrient concentrations and growth proteins, as well as the relation between cell volume and membrane area, are some factors that limit cell size.

Cell Type

• Cells differ in shape and size according to their function. Erythrocytes or red blood cells are 6 to 8 micrometers in diameter while the defense cells called monocytes can reach up to 30 micrometers in diameter. Platelets, which help in the healing process, are an average of 1 to 4 micrometers in diameter.

External Nutrient Concentrations and Growth Factors

• Cells are able to monitor nutrient concentration in the external environment, regulating their growth accordingly. The role of extracellular proteins called "growth factors" are also important in the determination of cell size in mammals and other animals. Unicellular creatures such as yeasts have their size regulated only by external nutrient concentrations.

Ratio of the Cell Volume to Membrane Area

• As a cell grows, its internal material called cytoplasm increases at a faster rate than its external membrane. A cell is extremely dependent of the membrane to exchange nutrients and waste products with the extracellular area. Therefore, the ratio of the cell volume to membrane extension is also a fundamental determinant of cell size and consequent division,