1. Surface Area to Volume Ratio - video
Cells are very small, no matter what the size of the organism that they are part of.
Cells do not and cannot grow to be very large and this is important in the way living organisms are built and function.

2. Why are cells so small?
Nutrients and wastes move across a cell surface by diffusion.
The chemical reactions that take place is known as the cell’s metabolism
TOPIC 2: CELLS

3. Substances must be absorbed by the cell and waste products must be removed.
The rate at which this occurs is determined by the surface area of the cell.
As size of an object increases the ratio between the surface area and volume decreases.
Rate materials enter and leave the cell depends on surface area
Rate materials are used or produced depends on the volume

4. Why is this important?
The surface area relates to how much material can enter a cell at a time.
The bigger the area is the more material can enter in an amount of time

5. The volume relates to how much material is needed at a time.
The bigger the volume is the more material is
needed to maintain it for an amount of time.

6. Lets look at Surface areas and volumes………
Fill in the table below: a
Size of side (a)
Surface area of the box
(a x a x 6)
Volume of the box
(a x a x a)
Surface area to volume ratio 1 6
6 : 1 2 24 8 3 54 27 4 96 64 5
150
125
216 7
294
343
Size of side (a)
Surface area of the box
(a x a x 6)
Volume of the box
(a x a x a)
Surface area to volume ratio 1 6
6 : 1 2 3 4 5 7
Size of side (a)
Surface area of the box
(a x a x 6)
Volume of the box
(a x a x a)
Surface area to volume ratio 1 6
6 : 1 2 24 8
3 : 1 3 54 27
2 : 1 4 96 64
1.5 : 1 5
150
125
1.2 : 1
216
1 : 1 7
294
343
0.9 : 1 a a

7. What trend did you notice?
What happened to the area as side size increased?
The area increased much faster than the side size.
What happened to the volume?
The volume increased much faster than the side size
Which one (area or volume) increased the fastest?

8. a a a Look at cell size where a=1. Is it able to supply its needs?
Assume that a volume of 1 needs 1 lot of material to sustain it….And an area of 1 can provide 1 lot of material: a a a
Look at cell size where a=1. Is it able to supply its needs?
Look at cell size where a=6. Is it able to supply its needs?
What about a=7?
Area = 6 Volume =1 ….It can provide 6x as much material than it needs
Area = 216 Volume =216 ….It can just provide as much material than it needs

9. If the cell gets too large and the ratio of Surface Area to Volume decreases it can’t take in the essential materials or excrete the waste fast enough….. So it must divide.

10. The same idea is relevant for heat and waste needing to be given out by an organism.

11. Can you think of some organisms that have to deal with this problem?
Elephant – a very big organism produces lots of heat. How does it get rid of all the heat with a relatively low SA : Vol ratio?
Find out about elephant ears.
The Shrew – A very small organism with a relatively large SA : Vol ratio. How does it manage to stay warm in winter?
Find out about changes in metabolism.

12. 2.1.6: Surface area:volume
As a cell grows it has less surface area to obtain materials and to dispose of waste.
The rate of exchange of materials across the membrane becomes limiting and cannot keep up with the cell’s requirements.

13. Becoming multicellular has enormous advantages.
An organism can grow in size and its cells can differentiate, meaning they can take on specific functions, so the organism can grow in complexity as well as size.

14. How Many Cells Are in the Human Body???
About 100 Trillion

15. How Surface Area to Volume Ratio Limits Cell Size
A cell is a metabolic compartment where a multitude of chemical reactions occur.
The number of reactions increase as the volume of metabolic volume within a cell increases. (The larger the volume the larger the number of reactions)
All raw materials necessary for metabolism can enter the cell only through its cell membrane.
The greater the surface area the larger the amount of raw materials that can enter at only one time.
Each unit of volume requires a specific amount of surface area to supply its metabolism with raw materials. The amount of surface area available to each unit of volume varies with the size of a cell.
As a cell grows its SA/V decreases.
At some point in its growth its SA/V becomes so small that its surface area is too small to supply its raw materials to its volume. At this point the cell cannot get larger.