1. GIANT AMOEBA EATS
NEW YORK CITY
Surface-to-Volume Ratios and the
Benefits of Being Multicellular

2. PREDICTIONS
do you think cells are so small?
Why
 _____________________________
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 _____________________________
___
 _____________________________

3. GIANT AMOEBA EATS
NEW YORK CITY
will likely never see that headline
You
cells get larger, they need more
As
food and produce more waste
more materials must be
Therefore,
able to move in and out of the cell
through the cell membrane

4. SURFACE-TO-VOLUME
RATIO
growing cell needs larger surface
A
area through which to exchange
materials
cell’s volume increases, its outer
As
surface grows too. But the volume
increases at a faster rate than the
surface area. So if a cell gets too
large, it won’t have enough openings
to allow materials in and out

5. WHY AREN’T THE RATES THE
SAME?
Surface-to-Volume is actually a
Because
ratio
Surface-to-Volume ratio decreases as
The
cell size increases
the number of cells but not the
Increasing
size keep a high surface-to-volume ratio

6. SURFACE-TO-VOLUME RATIO

7. BENEFITS OF BEING
MULTICELLULAR
A single cell as big as your are would have an
incredibly small surface-to-volume ratio. The
cell would not survive because its outer surface
would be too small to allow in the materials it
would need
 Remember, multicellular organisms grow by?
Producing more cells
 Why is an elephant larger than a human being?
It has more cells, not larger cells

8. MANY KINDS OF CELLS
organisms are able to do
Multicellular
lots of other things besides grow
is because different cells are
This
specialized to do different jobs
for example have specialized
Humans
cells like muscle, eye, and brain cells
which let us walk, run, watch a movie,
think, and so on.

9. MATH BREAK
The shape of a cell can affect its surface-to-volume ratio.

Examine the cells below and answer the questions that follow
4
A
2 B
2
2
1
2
What is the surface area of Cell A? Cell B?

28 Cell B? 24
What is the volume of Cell A?

8 8
Which of the two cells has the greater surface-to-volume

ratio?
Cell A 28:8

10. YOUR PET PARAMECIUM
 Imagine you have a pet Paramecium, a type
of unicellular organism. The dimensions of
your Paramecium are 125 m x 50 x 20. If 7
food molecules can enter through each
square micrometer of surface every minute,
how many molecules can it eat in 1 minute?
 Ifyour pet needs 1 food molecule per cubic
micrometer of volume every minute to
survive, how much would you have to feed it
every minute?