1. LIMITS TO SIZE: Why are Cells Small Lab?

2. Why are cells so small?
The answer to this question has as much to do with mathematics as biology. Imagine a cell that is shaped like a cube. As the cell increases, its surface area to volume ratio changes.

3. Surface area
Surface area is the total area of the outside surface. For cubes it is expressed in squared units (such as cm2 or m2).
The S.A. of a cell will determine the amount of materials that can be exchanged between the cell and its environment.
Equation: SA = length x width x # of sides
© 2010 Paul Billiet ODWS

4. Volume
Volume is the amount of space something occupies. It is expressed in cubic units (such as cm3 or m3).
The volume of a cell will determine the amount of chemical reactions or the metabolism of the cell.
Equation: V= length x width x height
© 2010 Paul Billiet ODWS

5. Surface Area to Volume Ratio
Surface Area to Volume Ratio is the comparison of the amount of surface area (what is on the outside) to the volume (what is on the inside)
Cells need to have a large surface area to its volume (hence a high S.A. to V ratio) because wastes need to be able to exit, and nutrients need to be able to enter the cell quickly.

6. Growing cells
1cm
2cm
3cm
4cm
5cm
6cm
© 2010 Paul Billiet ODWS

7. How are surface area and volume affected by growth?
SA/V
Volume
(cm3)
Surface area
(cm2)
Length of side
(cm) 1 2 3 4 5 6 6 24 54 96
150
216 1 8 27 64
125
216
6:1
3:1
2:1
1.5:1
1.2:1
1:1
© 2010 Paul Billiet ODWS

8. Comparing Surface Area and Volume changes
(cm)
© 2010 Paul Billiet ODWS

9. Surface Area:Volume Ratio
(cm)
© 2010 Paul Billiet ODWS

10. As cells get larger, the S.A.:V gets __________________.
smaller

11. Since transport of materials in and out of the cell can only happen at the cell’s surface, what problem does this pose?
Bigger cells have a greater volume (meaning a greater metabolism) than smaller cells
BUT bigger cells have a proportionally less surface for exchange.
Diffusion can’t take place as quickly!
© 2010 Paul Billiet ODWS

12. 1. Throw out extensions Microvilli of small intestine epithelium
What are ways to increase surface area while increasing size.
1. Throw out extensions
Microvilli of small intestine epithelium
Image Credit

13. 2. Flatten into a thin film
Image Credits
Chick blastodisc

14. 3. Or just divide the cytoplasm into smaller volumes
8-cell Embryo
Early human embryos
Zygote
Image Credit
Image Credit

15. Do larger organisms have larger cells or more cells than smaller organisms?

16. Multicellular organisms show the same adaptations
Flowering plants have an extensive, branched rooting system to absorb water and minerals
Image Credit

17. Multicellular organisms show the same adaptations
Mammals have a long small intestine with internal folding to absorb digested food
Image Credit C. Candalh, INSERM