12. Why do cells divide?
1. DNA Overload
2. Efficiency
3. Growth
4. Repair and Replace
13. DNA Overload
• Cell becomes too large for the DNA to control. It
has to divide!
14. To be more EFFICIENT!
• Cell becomes too large the volume increases faster
than the surface area.
• The cell membrane is no longer efficient at allowing
nutrients and waste to pass through it.
15. Growth
• We start as a single cell and become a person
because cells divide!
16. Repair and Replace damaged or worn out
cells!
• Cell becomes too large for the DNA to control. It
has to divide!
19. Cell Sizes
There is a nerve
cell in the leg of
the giraffe that is
nearly two meters
long.
20. What Limits Cell Size?
All needed cell materials are
absorbed through the surface
of the cell.
These materials are used by the entire
volume of the cells “living stuff”.
22. 1 cm
3cm
Lets compare surface area to
volume ratios
23. 1 cm
SA = 6 cm2 3cm
Volume = 1cm3
SA = 54 cm2
6:1 Ratio Volume = 27 cm 3
2:1 Ratio
24. What Limits Cell Size?
The Surface Area of the Cell
Becomes to Small to Support the
Volume
25. That is why cells
divide ! !
How do cells divide ? ? ?
26. Surface Area & Volume
When cells get too • Volume increases
large it is more more rapidly than
difficult for them to surface area when
transfer sufficient cells grow, causing
amounts of O2 and the ratio of surface
nutrients in & waste area to volume to
product out decrease
28. Practice Problems
• Find surface area & volume of a cube that is
4cm on all sides:
• Surface area= ____cm x ____cm x ___ = ______cm
• Volume = ____cm x _____cm x _____cm = _______cm
• Ratio of surface area to volume is = surface area =
• volume
Ratio =
1.5:1
Surface area to volume ratio is another reason why a cell divides. In cell growth, cell volume increases much more rapidly than does the surface area. Decreased surface area to volume causes problem for the cell to get sufficient amounts of oxygen and nutrients in and waste products out.
G1 is the normal cell growth phase S=chromosomes are replicated and the synthesis of DNA molecules take place G1 normal cell growth phase “S” = chromosomes are repliicated and the synthesis of DNA molecules take place. G2= usually the shortest of the three phases of interphase. Here is when many organelles and molecules required for cell division are produces. M Phase= the actual cell division (mitosis)
