1. WHY DO CELLS DIVIDE?
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2. WHY DO CELLS DIVIDE?
IT’S ALL ABOUT SIZE

3. WHY DO CELLS DIVIDE? (instead of just getting bigger)
Diffusion
DNA
Surface area-to-volume ratio

4. Diffusion Cells need: food and oxygen to eliminate wastes
to move all materials through cytoplasm

5. Diffusion Typical cell has a diameter of 20 µm
takes a fraction of a second for materials to move from cell membrane to the center of the cell
If it had 20 cm diameter
would take months to move materials same distance
The cell would die from starvation or poisoning from wastes before it got that big

6. DNA Cell needs: to respond to changing conditions rapidly
to produce large amounts of proteins and enzymes

7. DNA DNA directs the production of proteins and enzymes
DNA makes an RNA copy
RNA leaves the nucleus and travels to a ribosome
RNA directs protein production at the ribosome

8. DNA
If there is not enough DNA to control the cell the metabolism will slow and the cell will die

9. WHY DO CELLS DIVIDE? (instead of just getting bigger)
DNA
Larger cell = Greater metabolism

10. DNA Larger cell = Greater metabolism
Greater metabolism = More DNA needed to direct metabolism

11. Surface area-to-volume ratio
As size of a cell increases, volume increases at a much greater rate than does surface area

12. Surface area-to-volume ratio
Cell needs:
materials for processes
obtains materials through its cell membrane
larger organisms (or cells) need more “stuff”
more and more materials as its volume increases
surface area for absorbing required materials
(not enough surface area to supply the needs for larger cells)

13. Surface area-to-volume ratio
Example:
Surface area of a cube =
area of 1 side x 6
Area of 1 side = length x width
1 inch
= 1 in x 1 in
= 1 in2
Surface area = 1 in2 x 6
= 6 in2

14. Surface area-to-volume ratio
Example:
Volume of a cube =
length x width x height
= 1 in x 1 in x 1 in
1 inch
= 1 in3

15. Surface area-to-volume ratio
Example:
Surface area = 6 in 2
1 inch
Volume = 1 in3
Surface area : Volume ratio
= 6 : 1

16. Surface area-to-volume ratio
Example:
Surface area of a cube =
area of 1 side x 6
Area of 1 side = length x width
= 4 in x 4 in
4 inches
= 16 in2
Surface area = 16 in2 x 6
= 96 in2

17. Surface area-to-volume ratio
Example:
Volume of a cube =
length x width x height
= 4 in x 4 in x 4 in
4 inches
= 64 in3

18. Surface area-to-volume ratio
Example:
Surface area = 96 in 2
Volume = 64 in3
4 inches
Surface area : Volume ratio
= 96 : 64
= 3 : 2

19. Surface area-to-volume ratio
Example:
4 inches
1 inch
Surface area : Volume ratio
= 6 : 1
Surface area : Volume ratio
= 3 : 2