1) The document discusses how surface area to volume ratio limits cell size. As cells increase in size, their volume increases much faster than their surface area. 2) A smaller surface area to volume ratio makes it more difficult for cells to exchange materials by diffusion with their surroundings. 3) Organisms with smaller sizes, such as microbes, have much higher surface area to volume ratios that allow for more efficient exchange of materials compared to larger organisms.

1. Drawings from a microscope
http://www.gtac.edu.au/site/gcasts/HotPotato_CellStructure_Quiz/CellStructure_Quiz.htm

2. 1. If a red blood cell has a diameter of 8 mm and a
student shows it with a diameter of 40 mm in a
drawing, what is the magnification of the
drawing?
A. × 0.0002
B. × 0.2
C. ×5
D. × 5000

3. 2. A student observes and draws an Amoeba, using the high
power lens of a microscope. The diameter of the drawing is
100 mm. The actual diameter of the Amoeba is 100 µm.
What is the magnification of the drawing?
A. 0.001
B. 100
C. 400
D. 1000

4. Surface area to volume ratio
Carissa Fletcher
Inter Community School

5. 2.1.6 Explain the importance of the surface area to volume ratio as a factor
limiting cell size.
OR
http://peppysdevelopments.wordpress.com/2010/11/04/of-mice-and-men-
%E2%80%93-a-guide-to-getting-rid-of-mice-in-your-house/
http://goafrica.about.com
Which loses heat fastest?
All organisms need to exchange substances such as food, waste, gases and heat with
their surroundings.

6. The relationship between radius,
surface area and volume.
Note that as the radius of a cell increases from 1x to 3x (left), the surface area increases from 1x to 9x,
and the volume increases from 1x to 27x.Nature Education 2010 All rights reserved.

7. Exchange of substances
• The rate of exchange of substances depends on the organism's
surface area that is in contact with the surroundings.
• The requirements for materials depends on the volume of the
organism.
• Ability to meet the requirements depends on the surface area:
volume ratio.
• As organisms get bigger their volume and surface area both get
bigger.
• The volume increases much more than surface area.

8. Challenge – design a cell
Using a plasticine make a model cell that you
think:
1. Has the highest surface/volume ratio.
2. That is optimally shaped for diffusion of
substances.

9. Comparison of different animals
Organism Length Surface Area Volume SA/V Ratio
(m2) (m3) (m-1)
1 mm (10-6 m) 6 x 10-12 10-18 6,000,000
100 mm (10-4 m) 6 x 10-8 10-12 60,000
10 mm (10-2 m) 6 x 10-4 10-6 600
1m (100 m) 6 x 100 100 6
100 m (102 m) 6 x 104 106 0.06
This means that as organisms become bigger it becomes more difficult for them to
exchange materials with their surroundings.