Different types of cells reach different sizes. In general the reasons for cell size limits are due tothe mechanisms needed for cell survival and how cells" requirements are met by the structures that formand are contained within cells. (Click on the diagrams on the right for details about the structures ofdifferent types of cells.)

The factors limiting the size of cells include:

Surface area to volume ratio (surface area / volume)Nucleo-cytoplasmic ratio Fragility of cell membraneMechanical structures necessary to hold the cell together (and the contents of thecell in place)

The above limitations affect different types of cells to different extents.

Notes about each of the main limitations of cell size follow.

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1. Surface area to volume ratio

When the size of a cell (having a simple *shape)increases:

the cell volume increases to the cube of the linear increase, whilethe surface area of the cell increases only to the square of the linearincrease.

Examples of simple formulae:


Volumeof a Cube:

Surface Area of a Cube:

Volume = r3

Surface Area = 6r2

where r is the length of eachside of the cube.

Volumeof a Sphere:

Surface Area of a Sphere:

*
*

where r is the radius of thesphere.

The diameter (d) of thesphere is twice the radius so the above could be re-written in terms ofdiameter using the relationship d=2r

*As shown on the right, cells have various and often irregular shapes so it is a simplification to consider the formulae for cubes and spheres. They are convenient shapes for easy calculations and comparison. A sphere is the 3-dimensional shape that has the minimum possible surface area/volume ratio.


Using the above formulae, it is easy to express the ratios of surface area to volume for these verysimple shapes:


Surface Area / Volumeratio for a Cube:

=6/r

where r is the length of eachside of the cube.

Surface Area / Volumeratio for a Sphere:

=3/r= 6/d

where r is the radius of thesphere.

The diameter (d) of thesphere is twice the radius so the above could be re-written in terms ofdiameter using the relationship d=2r


So, in the cases of very simple shapes such as cubes and spheres,the larger the size of the object (r), the smaller it"s surface area to volume ratio. Expressed toother way, the smaller the size of the object (e.g. a cell), the larger its (surface area) /volume ratio.

A large (surface area) / volume ratio is helpful because nutrientsneeded to sustain the cell enter via the surface of the cell (supply) and areneeded in quantities related to the cell volume (requirement).Put another way, more cytoplasm results in higher demands for supplies via the cell membrane.


This is because, prokaryotic cells are incapable of endocytosis (the process by which smallpatches of the cell membrane enclose nutrients in the external environment, breaking-away from thestructure of the cell membrane itself to form membrane-bound vesicles that carry the enclosednutrients into the cell.) Endocytosis and exocytosis enable eukaryotic cells to have larger surface-area: volume ratios than prokaryotic cells because prokaryotic cells rely onsimple diffusion to move materials such as nutrients into the cell - and wasteproducts out of the cell.

Note that some animal cells increase theirsurface area by forming many tiny projections called microvilli.

2. Nucleo-cytoplasmic ratio

Not all cells have a membrane-bound nucleus. Eukaryotic cells (including plant cells and animal cells) have nuclei and membrane-boundorganelles, while prokaryotic cells (i.e. bacteria) donot. Nuclei contain information needed for protein synthesis and so control the activities of thewhole cell.


Each nucleus can only control a certain volume of cytoplasm.

This is one of the limitations of the size of certain biologicalcells.


Some cells overcome this particular limitation by having more than one nucleus, i.e. some specialtypes of cells have multiple nuclei.Cells that contain multiple nuclei are called multinucleate cells andare also known as multinucleated cells and as polynuclear cells.A multinucleate cell is also called a coenocyte.Examples of multinucleate cells include muscle cells in animals and the hyphae (long,branching filamentous structures - often the main mode of growth) of fungi.

3. Fragility of the cell membrane

All cells have and need a cell membrane (sometimes labelleda "plasma membrane") even if the cell also has a cell wall. The structure of cell membranesconsist of phospholipids, cholesterol and various proteins. It must be flexible in order to enableimportant functions of cell membranes such as exocytosis(movement of the content of secretory vesicles out of the cell), endocytosis(movement of the content of secretory vesicles into of the cell) etc.. However the structure ofthe plasma membrane that enables it to perform its many functions also results in its fragility toenvironmental variation e.g. in temperature and water potential.

Temperature: Even small increases in temperature can reduce the(hydrophobic) interactions between the hydrocarbon tails of the phospholipids - leading to reducedor complete loss of protein function.Water potential: Even small reductions in the water potential of thecytoplasm can result in too much water entering the cytoplasm, causing a fragile animal cell toburst due the outward pressure from the fluid inside the cell membrane.

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As thesize of cells increase, the risk of damage to the cell membrane also increases.

This limits the maximum size of cells - especially of animal cellsbecause they do not have cell walls.


4. Structures that hold the cell together

As indicated on the pages about animal cells,plant cells and bacteriacells, the contents and internal structures of cells vary according to the general type ofcell and its specific function within the organism. Some cells are complex structures that contain100s or 1000s of structures (including different types of organelles) within the cell membrane. Forexample, in a typical animal cell specialized organelles occupy around 50% of the total cell volume.In order for cells to survive they must remain intact so sufficient mechanicalstructures must hold the cell contents together.

The cell membrane (mentioned above) has many important functions including enclosing the contents of the cell -but it is not solely responsible for providing enough structure to hold the cell together.

Cells need sufficient structural support, which is provided by:

See also cell functions (in general), the functions of the cell membrane and table to compare plant, animal and bacterial cells.