Cytology - Part II


Cell Volume Verses Surface Area

Except for special reproductive cells such as egg cells or some neurons, most cells are very small. The average size of cells range between 1 and 100 µm (micrometers)



Living cells are full of a very active, changing, shifting fluid called cytoplasm or cytosol. Today we know that the cytoplasm is a complex and dynamic solution of microfilaments, microtubules, and other molecules which enable cells to supplement the diffusion of needed raw materials into their interior.

Single cells are rarely ever able to be large and round because of the fundamental limitation of diffusion. This difficulty of maintaining close contact with their surroundings is clearly demonstrated in the diagram below.

If cubes made of porous gelatin are exposed to a blue dye, then sliced open after a few minutes, only the outer millimeter or so will be stained. Thus as the cell increases in size the middle is less likely to receive nutrients from the surroundings.

Volume will always increase faster than surface area because volume is based on a cube (a * a * a) while surface area increases as the square (a * a * 6). As soon as "a" is greater than 6 the numerical value for volume will exceed the surface area.

This physical explanation for the limitation in cell size is called the Surface-Volume Hypothesis.

Nevertheless, cells have developed ways of overcoming the limitations of simple diffusion. As the cell grows and the surface-volume ratio becomes more and more difficult for its membrane to cope with the cell can either divide or curtail its metabolism.

Cells can also overcome the surface-volume obstacle by:

  • cytoplasmic streaming(*), the circular movement of the cytosol, usually around the central vacuole within plant cells. This bulk movement is much more efficient in distributing needed materials.

     

  • having a large central portion of the cell containing a water filled vacuole, the metabolically active cytoplasm constrained to a thin periferal band between the cell wall and the tonoplast.

     

  • increasing the surface area by numerous projections called microvilli as found in the epithelial cells of the intestines.

     

  • reducing the metabolic requirements such as in egg cells where most of the volume is reserved for food storage so most of the cell is essentially in suspended animation.

 

Be sure to visit the Cytoskeleton Tutorial at The Biology Project for more information about cytoplasmic streaming and biology in general.


Modified July 8, 2005