World Builders™                                                                  Session Four  --  Microbiology

Limits to Cell Size

How big can cells get?  Is there a limit?  What sets the limit?

Could This Happen?

Well, no.

(Fortunately for Dr Viau)

There are limits to the sizes that cells can reach.

These limits are governed by the relationship of what is inside the cell to the area of the membrane that encloses the cell.

The incredible giant cell would have so much chemistry going on inside itself that it could not get food, minerals, and water to come in through the cell wall fast enough to keep it alive.

However, tiny cells that cause us to get the flu have enough area on the outside to keep the chemistry inside working very well!

Let's think about this for a minute. The cell is a little parcel of just the right chemicals. To live, it must keep its chemical contents balanced.

This is accomplished by

• letting necessary chemicals into the cell -- such as oxygen, water, and minerals
• letting waste products get out through the cell membrane.

A process called diffusion allows chemicals to pass through the cell membrane.  The cell has no mouth, no lungs, no digestive system. Everything has to happen through the membrane. The relationship between the surface area of the membrane and the volume of the contents of the cell becomes the limiting factor in cell size.

Here is something interesting. Look at how the area (size of the surface) of the outside changes with the shape even though the volume (inside) stays the same.

 Compute the Area Total Surface Area (the outside) Volume  (the inside) 2 inches by 2 inches      by 2 inches ( 2 * 2) * 6 sides =  24 square inches Area = 24 square inches 2 * 2 * 2 = 8 cubic inches 1 inch by 1 inch by  8 inches (1 * 1) *2 sides = 2 plus ( 1 * 8) * 4 sides = 32 ( 2 + 32 ) = 34 square inches Area = 34 square inches 1 * 1 * 8 = 8 cubic inches 2 inches by 1 inch  by 4 inches ( 2 * 1 ) * 2 sides = 4 plus (4 * 1 ) * 2 sides = 8  plus (4 * 2) * 2 sides = 16  ( 4 + 8 + 16 ) = 28 Area = 28 square inches 2 * 1 * 4 =   8 cubic inches

When a cell gets too big it will starve from lack of incoming chemicals, or poison itself with wastes that cannot be excreted rapidly enough, so it will try to maximize the surface area to volume ratio. (This works in water. On land increased surface area also means faster dehydration and heat loss (or gain).

However -- the cell becomes more fragile as it elongates and thins itself. This creates practical limits to how large the cell can grow.

A strategy that increases the area of the cell surface and also strengthens it is to grow little pleats and ridges. Here are some examples of cells that have done this.

 Design Microbiology Page

Photos from archives at Biology Department, University of Bowling Green, Ohio
Header shows strands of algae growing  in Yellowstone National Park
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .