Surface Area to Volume ratio

1. Surface Area to Volume ratio

2. Why can't cells continue to grow larger and larger to become giant cells, like a blob? • Why are most cells, whether from an elephant or an earthworm, microscopic in size? • What happens when a cell grows larger and what causes it to divide into two smaller cells rather than growing infinitely larger? • This investigation provides you with a 'hands-on' activity that simulates the changing relationship of Surface Areas -to- Volume for a growing cell.

3. Background information • Cells are roughly cubic or spherical in shape and are limited in size by the ratio between surface area and volume (SA/V); cells divide when they reach a certain surface area. • The volume of a cell is related to the amount of surface area-to-volume ratio it has. • The chemical activity within the cell and the surface area of the cell limits the exchange of nutrients, gases and waste products with its environment.

4. As a cell grows larger, its rate of producing wastes and requiring nutrients increases faster than the surface area through which molecules must exit and enter. • Volume increases by the cube, while surface area increases by the square. • As a cell grows larger, its surface area becomes too small to maintain life functions. • Cells remain small and therefore maintain a large surface-area-to volume ratio

5. As the cell increases in size,the SA/V ratio decreases (the V ↑ faster than the SA, i.e. cubed function vs. squared function). • There is less surface (CM) to meet the needs of the volume (chemical activity within the cytoplasm & organelles). • If chemical reactions within the cytoplasm cannot be supplied with nutrients or remove wastes, the cell will die OR it can divide into two cells and thus obtain a favorable, i.e. large SA/V ratio.

6. The important point is that the surface area to volume ratio gets smaller as the cell gets larger. • In summary, increasing cell size results in a decrease in the SA/V ratio for the cell. This reduction in the SA/V ratio combined with the increased distance to the cell center makes diffusion of materials into & out of the cell less efficient.

7. Some cells have unique shapes& structures to overcome SA/V ratio. E.g. intestinal epithelial cells on villi contain microvilli (a “brush border”) to ↑ SA for diffusion of nutrients; RBC’s are biconcave shape to ↑ SA/V ratio for more efficient gas diffusion. • What other shape might cells have to ↑ SA/V ratio?

8. Practical Activity • You will be given copies of the Cubic Cell Models on heavy, colored paper. The four cell models are then cut out, folded, and taped together. The models represent one cube-shaped cell at increasing stages of growth. The smallest stage represented is 1cm long on a side; the largest stage is 4cm on a side.