What limits cell size ?
Different types of cells reach different sizes. In general the reasons for cell size limits are due to the mechanisms needed for cell survival and how cells' requirements are met by the structures that form and are contained within cells. (Click on the diagrams on the right for details about the structures of different 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 membrane
- Mechanical structures necessary to hold the cell together (and the contents of the cell in place)
The above limitations affect different types of cells to different extents.
Notes about each of the main limitations of cell size follow.
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, while
- the surface area of the cell increases only to the square of the linear increase.
Examples of simple formulae:
Volume of a Cube:
Surface Area of a Cube:
Volume = r3
Surface Area = 6 r2
Volume of a Sphere:
Surface Area of a Sphere:
Using the above formulae, it is easy to express the ratios of surface area to volume for these very simple shapes:
Surface Area / Volume
ratio for a Cube:
Surface Area / Volume
ratio for a Sphere:
= 3/r= 6/d
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 to other 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 nutrients needed to sustain the cell enter via the surface of the cell (supply) and are needed in quantities related to the cell volume (requirement). Put another way, more cytoplasm results in higher demands for supplies via the cell membrane.
Surface-area : Volume ratio particularly limits the size of bacterial cells, i.e. prokaryotic cells.
This is because, prokaryotic cells are incapable of endocytosis (the process by which small patches of the cell membrane enclose nutrients in the external environment, breaking-away from the structure of the cell membrane itself to form membrane-bound vesicles that carry the enclosed nutrients into the cell.) Endocytosis and exocytosis enable eukaryotic cells to have larger surface-area : volume ratios than prokaryotic cells because prokaryotic cells rely on simple diffusion to move materials such as nutrients into the cell - and waste products out of the cell.
2. Nucleo-cytoplasmic ratio
Not all cells have a membrane-bound nucleus. Eukaryotic cells (including plant cells and animal cells) have nuclei and membrane-bound organelles, while prokaryotic cells (i.e. bacteria) do not. Nuclei contain information needed for protein synthesis and so control the activities of the whole cell.
Each nucleus can only control a certain volume of cytoplasm.
This is one of the limitations of the size of certain biological cells.
Some cells overcome this particular limitation by having more than one nucleus, i.e. some special types of cells have multiple nuclei. Cells that contain multiple nuclei are called multinucleate cells and are 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 labelled a "plasma membrane") even if the cell also has a cell wall. The structure of cell membranes consist of phospholipids, cholesterol and various proteins. It must be flexible in order to enable important 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 of the plasma membrane that enables it to perform its many functions also results in its fragility to environmental 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 reduced or complete loss of protein function.
- Water potential: Even small reductions in the water potential of the cytoplasm can result in too much water entering the cytoplasm, causing a fragile animal cell to burst due the outward pressure from the fluid inside the cell membrane.
As the size of cells increase, the risk of damage to the cell membrane also increases.
This limits the maximum size of cells - especially of animal cells because they do not have cell walls.
See below for more about the effects on cell size of the structures that hold cells together.
4. Structures that hold the cell together
As indicated on the pages about animal cells, plant cells and bacteria cells, the contents and internal structures of cells vary according to the general type of cell and its specific function within the organism. Some cells are complex structures that contain 100s or 1000s of structures (including different types of organelles) within the cell membrane. For example, 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 mechanical structures 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:
- Support from outside the cell membrane:
Most cells have some form of "extracellular" support.
Plant cells and bacteria cells have cell walls - although they are different types of cell walls. The structure of plant cell walls consists of cellulose microfibrils forming a mesh (imagine a fine net) around the outer surface of the cell membrane and a matrix of polysaccharides including pectins and hemicelluloses occupying the regions defined by the mesh of microfibrils. The overall effect is formation of a strong composite structure that supports and protects its contents e.g. against damage to the cell membrane due to expansion of the cytoplasm due to endocytosis.
Cell walls enable plant cells to be larger than animal cells - plant cells are usually bigger than animal cells.So what form of extracellular support do animal cells have ?
Glycocalyx: External to the cell membrane, animal cells have a fine outer-layer of extracellular polymeric material (glycoprotein) which is called the glycocalyx. It consists of short-chain polysaccharides and provides some mechanical support - but much less support than that provided by a cell wall.
Glycocalyx isn't limited to animal cells. It also forms the capsule, or "slime layer" of some bacteria (prokaryotes).
- Support from within the cell membrane:
i.e. "intracellular" support.
The cell membrane and the cytoplasm and organelles within it are inter-connected by many protein structures that, together, form the cytoskeleton of the cell. The functions of the cytoskeleton include protecting and supporting the structure of the cell as well as helping to maintain the shape of the cell.