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.
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
•
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
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
• 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.
Lets look at Surface areas and
volumes………
• Fill in the table below:
a
Size of
side (a)
a
a
Surface
area of
the box
(a x a x 6)
Volume of Surface
the box
area to
(a x a x a) volume
ratio
1
6
1
6:1
2
24
8
3:1
3
54
27
2:1
4
96
64
1.5 : 1
5
150
125
1.2 : 1
6
216
216
1:1
7
294
343
0.9 : 1
• 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?
• 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
= 6 Volume =1 ….It can provide 6x as much
its needs? Area
material than it needs
• Look at cell size where a=6. Is it able to supply
= 216 Volume =216 ….It can just provide as
its needs? Area
much material than it needs
• What about a=7?
• 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.
• The same idea is relevant for heat and waste
needing to be given out by an organism.
Can you think of some organisms that
have to deal with this problem?
1. 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.
2. 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.
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.
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.
How Many Cells Are in the
Human Body???
About 100 Trillion
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.