Are there limits to cell size? Why or why not?

Answer 1

Ultimately a size limit is imposed by the surface area: volume ration.

Imagine if you placed a gallon of ice cubes in the sun and a gallon block of ice. The individual ice cubes would melt far faster even though the volume is identical because the amount of ice in contact with the external world is much higher and so it reaches equilibrium far faster.

Cells have the same problem. A cell can only absorb nutrients, oxygen and so forth across the cell membrane, and it can only get rid of wastes by pumping them out through the cell membrane. However as a cell gets bigger the amount of internal material producing waste and demanding nutrients increases relative to the amount of cell membrane capable of providing nutrients and eliminating wastes. This is the SA:volume dilemma faced by all organisms. As an organism gets larger it has less relatively contact with the external environment.

That SA:vol problem imposes an absolute maximum size limit on a cell. Once a cell gets too big it simply can not rely on diffusion to get waste products from the centre of the cell out into the environment or to get food and oxygen form the environment into the centre of the cell. Once that happens the centre of the cell dies. There are methods of getting around that problem via the use of various pumps and chemical scavengers and so forth but those solutions are expensive for the cell. Any cell that gets too large and starts relying in such solutions will be unable to compete with smaller cells and will be doomed to starvation.

The largest cells are arguably the yolk cells of some birds such as cassowaries. These cells can be several centimetres across but they manage that because the ACTIVE part of the cell is just a tiny point on the cell surface. The rest of the cell is nothing but a big ball of fat which doesn’t require oxygen or nutrients and doesn’t produce wastes. In that way cells can get infinitely large, but it’s cheating solution to the problem since the cell becomes little more than a bag.

For a productive active cell the absolute theoretical size limit is just a few centimetres and the absolute real-world limit just a few millimetres in diameter due to the problems imposed by decreasing surface area.

Answer 2

No, there are no limits to cell size. Prokaryotes and unicellular eukaryotes are really small cells and this is good enough 'coz all the functions needed for their survival and reproduction can be carried out efficiently in this small-sized cell.
Multicelled Eukaryotes have the additional sophistication of compartmentalization in their cells to acheive the really complex functions and to accomodate the multitude of biomolecules required to perform these tasks. So, their cells are larger and the size depends on the species. Even then, some cells can be really large, depending on the associated function. For example, the Egg. Ova in humans are really big compared to other human cells and an entire ovum is a single cell. In birds, the single celled egg can be as small as the humming bird egg or as large as that of an Ostrich. Here the large cell accomodates the yolk required for the nourishment of the embryo. So, u can't really say cell sizes are restricted.

Answer 3

Yes and no. In theory, there are no limits to cell size - however, the cell needs certain requirements in order to live. The problems faced by large cells are: obtaining energy, removing waste, and rapid intracellular transport needed for the maintenance of cell functions.

Take one of the largest cells known to man - the Ostrich Egg. The rest of the ostriches body (especially the liver) must pump energy into this cell for its extended survival on its own. Not only that, but the waste removal systems have to be fairly complex for a cell this size. An example of intracellular transport complexities are the largest cells in our bodies -- the neuron. The length of these cells in the spinal column (the axon) requires complex systems of microtubule production that shuttle proteins and other materials through the very long cytosol. Transport in these cells can take days, even months!

Answer 4

Surface:volume ratio is not the only factor.

Not all factors are known but another one is the ratio of the quantity of a cytoplasmic component (we don't know exactly what) against the quantity of DNA. For example there are salamanders which are haploid (one copy of all the DNA), diploid (2 copies) and pentaploid (5 copies). The salamanders are more or less equal in size. However the cells of the diploid are double in size of the haploid and of the pentaploid 5 times bigger. In order for the organisms to have the same size this means that the organs have respectively half and five time less cells.

Eggs are special cases and the larger size is due to the storage of nutrients

Answer 5

leviter_atin is correct :-)

BTW, the largest bacterium know so far is Thiomargarita namibiensis. It is far larger than researchers ever expected bacteria to be; they thought that these considerations of SA/ volume would make such a large bacterium impossible!

eg see links in this Google:

http://www.google.com/search?sourceid=navclient&ie=UTF-8&rls=RNWE,RNWE:2004-20,RNWE:en&q=%22Thiomargarita+namibiensis%22

Answer 6

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