CELL SIZE AND SURFACE AREA TO VOLUME RATIO
Cells vary greatly in size. The smallest cells are bacteria called mycoplasmas, with diameter between 0.1 m to 1.0 m. The bulkiest cells are bird eggs, and the longest cells are some muscle cells and nerve cells. Most cells lie between these extremes .
Cell size and shape are related to cell function. Bird eggs are bulky because they contain a large amount of nutrient for the developing young. Long muscle cells are efficient in pulling different body parts together. Lengthy nerve cells can transmit messages between different parts of body. On the other hand, small cell size also has many benefits. For example human red blood cells are only 8 m in diameter and therefore can move through our tiniest blood vessels i.e. capillaries.
Most cells are small in size. In relation of their volumes, large cells have less surface area as compared to small cells. Figure shows this relationship using cube-shaped cells. The figure shows 1 large cell and 27 small cells. In both cases, the total volume is same:
Volume = 30 m X 30 m X 30 m = 27,000 m3
In contrast to the total volume, the total surface areas are very different. Because a cubical shape has 6 sides, its surface area is 6 times the area of 1 side. The surface areas of cubes are as follows:
- Surface area of 1 large cube = 6 X (30 m X 30 m) = 5400 m2
- Surface area of 1 small cube = 6 X (10 m X 10 m) = 600 m2 and
- Surface area of 27 small cubes = 27 X 600 m2 = 16, 200 m.