1. Cell Size Limitations
Why are cells so small?

2. Most cells are between 2µm and 200µm
A micrometer is 1 millionth of a meter!
Too small to be seen with naked eye

3. Why can’t organisms be one big giant cell?

4. Limits Diffusion limits cell size
Movement from higher concentration to lower concentration
Larger the distance, slower the diffusion rate
A cell 20 cm would require months for nutrients to get to the center
Smaller cell size allows the cell to be more efficient

5. DNA limits cell size
larger cells need more DNA. Needs more of everything
If the cell was very large, there would not be enough DNA to support the cell.
Most cells have just one nucleus

6. Surface Area to Volume Ratio Limits Size of Cell

7. Surface Area to Volume Ratio Limits Size of Cell
Volume increase more rapidly than surface area.
Cell size doubles, 8x as much volume, but only 4x as much surface area
It would need 8x more nutrients and will have 8x more waste to remove
If E.coli were left unreglated, it could engulf the Earth in one day because it doubles in volume every 30 minutes!!

8. Surface Area to Volume Ratio Limits Size of Cell
So, if too big is a problem, what’s the solution? Cells divide before they become too big
Process of cell division is called mitosis

9. The Volume of Cell is Important
Volume determines the amount metabolism in the cytoplasm
Metabolism will require import of precursors
Metabolism will result in the export of secretions
And the export of excretory products

10. The Surface Area is also important….
Surface area will determine the exchange of materials between the cell and its environment
Bigger cells will metabolise more
But they will need more surface to support that metabolism
And the sites of metabolism in bigger cells will be further from the surface of the cell

11. This is a problem then….
Bigger cells have a greater metabolism than smaller cells
BUT bigger cells have a proportionally less surface for exchange
How to increase surface area with increasing size?
Increase the surface area of the cell through different shapes

13. Villi in the Small Intestines

14. Tissues and Diffusion
Tissues are a collection of cells that function together
Tissues need a way for diffusion to occur to all cells