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Intro to Anatomy 4: Cell Structure and Function
Intro to Anatomy 4: Cell Structure and Function
The Lone Ranger
Published by The Lone Ranger
01-07-2007
Default Physical Constraints on Cell Size


Physical Constraints on Cell Size:
The relationship between an objectís surface area and its volume determines how large a cell can grow. Because surface area is a square function, if you double an objectís size, its surface area increases by a factor of 22 or four times. Volume, however, is a cubic function, so if you double an objectís size, its volume increases by 23 or eight times. This means that as objects grow larger, their volumes increase at a much faster rate than do their surface areas.


If its proportions remain the same, as an object grows larger,
its volume increases much faster than does its surface area.


What does this have to do with cells? Well, itís important to remember that everything that comes into a cell Ė oxygen, food, water, etc. Ė must come in across the plasma membrane. Similarly everything that is expelled from a cell Ė poisonous metabolic waste products, for example Ė must leave across the plasma membrane. This means that the size of the plasma membrane sets a limit to how quickly a cell can absorb and excrete materials.

So, the size of a cellís plasma membrane determines how quickly it can absorb necessary substances like oxygen and nutrients, and how quickly it can expel dangerous substances like CO2. The problem is that the size of the cellís plasma membrane is a function of the cellís surface area.

On the other hand, the amount of material inside the cell that requires oxygen and nutrients Ė and that generates dangerous waste products like CO2 Ė is a function of the cellís volume.


So as a cell increases in size, it quickly reaches a point where it simply isnít possible for it to grow any larger and survive. If it were to grow any larger, the cellís relatively small surface area relative to its relatively large volume would mean that it couldnít bring in oxygen and nutrients (and expel poisonous metabolic wastes) fast-enough to keep itself alive.

So if an organism is to grow large, it must be made of many small cells, each of which has sufficient surface area relative to its volume to keep itself supplied with nutrients and oxygen, and to avoid poisoning by its own metabolic wastes. Movies such as The Blob that feature enormous single-celled organisms wandering about the countryside and devouring unwary teenagers are pure fantasy. No single-celled organism could get even close to that size. In fact, the largest single-celled organisms are just barely visible to the naked eye Ė if you have good eyesight, that is.



Not that big a threat, actually.





An organism can increase its surface area while keeping its volume
constant by dividing into lots of smaller subunits (i.e.
cells).


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