Cell Size (with audio)
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Eukaryotic Cell Size
Why are most eukaryotic cells
between 10 and 100 m in diameter?
How big is that?

Remember 1 mm is the smallest mark on a
metric ruler.
1

2
Remember it takes 1000 micrometers (m)
to equal 1 millimeter (mm).
1
2
How big is that?

Let’s use our metric conversions and
figure out how big a typical cell is if
measured in millimeters.
10 to 100 m = _______ to ______ mm
Eukaryotic cells are microscopic

Answer:
10-100 m = 0.01 -

0.1
mm
In other words, one cell is smaller than
1 millimeter. The largest of eukaryotic cells
are 1/10 of a mm in diameter (0.1 mm)!
1
2
Certainly we have small and
large eukaryotic organisms
(Think of an amoeba, a mouse and a redwood tree).

Why don’t we find smaller and larger
eukaryotic cells to match?
What keeps cells from getting
smaller than 10 micrometers?

This is what we call the LOWER LIMIT on cell
size.

It is related to the minimum amount of space it
would take to hold all the essential cell
structures.

If you try to make a cell smaller than 10 m,
something essential would not fit and therefore
the cell would not survive.
Analogy


There is a "lower limit" on
suitcases.
If you try to take a
suitcase smaller than some
minimum size on a
vacation, you would not
be able to fit all the
essential things you’d
need to "survive" into
your bags.
But what about mammalian red
blood cells?


RBCs are only 5 m -smaller than the usual
lower limit.
How is this possible??
RBCs don’t really violate the rule ….



It is just than in their
maturation process, the
nucleus of RBCs is discarded.
Leaving out such a prominent
structure allows these cells to
be unusually small.
Thus mature RBCs are small
enough enough to fit through
the tiniest of blood vessels,
the capillaries.
What keeps cells from getting
larger than 100 micrometers?

This is what we call the UPPER LIMIT on
cell size.

This is related to the ability of the cell to
supply its metabolic needs.
Metabolic Needs
Volume

To survive a cell must
have sufficient nutrients
and gases for its size.

A cell’s metabolic needs
are defined by its
volume*. The larger the
cell the greater its
metabolic needs will be.
*
Volume refers to the internal space of a
cell. By analogy, a box’s volume refer
to how much “stuff” it could hold.
Needs
Supplying those Needs
Nutrients
Wastes
Gases

Everything that enters and
leaves a cell must come
through its cell membrane.
This would include nutrients,
gases and wastes.

To supply its needs a cell
must have enough surface
area* to get those needed
materials in and wastes out
quickly.
*
Surface area refers to the covering
of a cell. By analogy, a box’s
surface area could be measured by
the amount of wrapping paper it
would take to cover it completely.
Surface Area to Volume Ratio

To meet its metabolic needs, a cell must have
sufficient surface area for its volume. This
relationship is described as a "large surface area to
volume (SA/V) ratio."

In other words, a cell must have enough cell
membrane to be able to transport what it needs in
and out at a fast enough rate to survive. The ratio
of the two is critical.
What happens as a cell grows?
SA
V

As a cell grows, its surface area (SA) increases.

As a cell grows, its volume (V) increases.
So what’s the problem?


If both the SA (supply) and V (needs/demand) increase
with increasing cell size, why can’t a cell grow as large as
it wants?
The problem is that while both SA and V increase, they
don’t grow at the same rate. The volume (V = needs)
increases faster than the surface area (SA= supply).
SA
V
Therefore, the SA/V ratio actually
decreases with increasing cell size.
SA / V

Remember cells need a large SA/V ratio.

And, the larger the cell the smaller that ratio
will be.
The decreasing SA/V ratio
limits cells from growing
larger than 100 m.

Without enough surface area for its
size, such a large cell will not be able
to supply its own needs.
It is all about supply and demand

As cells grow larger, while both the supply
and demand increase, the "needs" quickly
outpace the "supply."

At that point, the cell must stop growing or
divide into two smaller cells or it will die.
Without the ability to supply its own needs,
the cell would either have insufficient
energy to live or poison itself with its own
wastes.
What about frog eggs?

Frog eggs are single
cells and are over
1 mm in size -- larger
than the usual upper
limit.

How is this possible??
Unfertilized eggs are
metabolically suppressed
Therefore their needs are very low.
 And the limited surface area
available is adequate to supply
those needs.

However as soon as they are fertilized,
eggs become metabolically active

If nothing changed, the
SA/V ratio would be too
low for survival.

But within minutes after
fertilization, the single
large frog egg cell begins
to divide and divide and
divide. Soon the original
volume of the egg has
been divided into
hundreds of cells no larger
than 100 m.
In summary --Eukaryotic cells are typically larger than
10 m because you need at least 10 m to
hold the minimum structures for survival.
 Eukaryotic cells are usually smaller than
100 m because a decreasing SA/V ratio
limits a growing cell’s ability to supply its
own needs.

Testing your understanding

How is it possible for
most prokaryotic
cells to be smaller
than 10 m?
This is related to their lower limit

Prokaryotic cells lack
a true nucleus and any
membrane bound
organelles. Thus less
space is required to
hold their essential
components than is
true for the more
complex eukaryote.
References
Section 4.2 (page 54) of text including
Figure 4.2B
 Figure in study guide showing the
effect of increasing cell size on surface
area, volume and SA/V ratio

It would be unusual to find a eukaryotic cell
larger than 100 µm because:
a.
b.
c.
d.
e.
a cell that size could not hold everything it needs
to survive
it would have too much surface area for its
volume
it wouldn’t be able to supply its metabolic needs
its volume would be too small for its surface area
it would exceed the lower limit for eukaryotic
cell size
It would be unusual to find a eukaryotic cell
larger than 100 µm because:
b.
a cell that size could not hold everything it needs to survive
it would have too much surface area for its volume
c.
it wouldn’t be able to supply its metabolic needs
a.
d.
e.
its volume would be too small for its surface area
it would exceed the lower limit for eukaryotic cell size
Remember that how large a cell can grow is restricted by
the surface area to volume ratio (balance of “supply and
demand”). As cell grows, the surface area increases
more slowly than the volume so that at some point
(around 100 µm), the cell doesn’t have enough surface
area to supply its growing metabolic needs.
It would be unusual to find a eukaryotic cell
smaller than 10 µm because:
a.
b.
c.
d.
e.
a cell that size could not hold everything it needs
to survive
it would have too much surface area for its
volume
it would have too little surface area for its
volume
its volume would be too small for its surface area
it would exceed the upper limit for eukaryotic
cell size
It would be unusual to find a eukaryotic cell
smaller than 10 µm because:
a.
b.
c.
d.
e.
a cell that size could not hold everything it needs
to survive
it would have too much surface area for its volume
it would have too little surface area for its volume
its volume would be too small for its surface area
it would exceed the upper limit for eukaryotic cell size
Remember that all the structures a cell needs to survive
must be contained within its cell boundaries. As cells
decrease in size there is less and less space inside to
“pack” the minimum number of organelles. At some
point (around 10 µm in diameter), cells simply run out of
room.
A
B
Cell ____ has the greatest volume.
Cell ____ has the greatest surface area
Cell ____ has the greatest surface area to
volume ratio.
C
A
B
C
Cell C has the greatest volume.
Cell C has the greatest surface area
Cell A has the greatest surface area to
volume ratio.
Remember that as a cell increases in size, its surface area
and volume both increase, but the ratio of the two
decreases. In this case, cell A would be able to supply its
needs more easily than cell C.
Fill in the blanks with either UPPER or LOWER.

Surface area to volume ratio limits a cell’s _______ limit.

Cells are not usually smaller than 10 µm because of the
_____ limit on eukaryotic cell size.

Finding a cell larger than 100 µm would be unusual, as this
would exceed the _______ limit on eukaryotic cell size

A eukaryotic cell 3 µm in diameter would be outside the
typical _______ limit for cell size.
(How might this be possible?)
Fill in the blanks with either UPPER or LOWER.

Surface area to volume ratio limits a cell’s UPPER limit.

Cells are not usually smaller than 10 µm because of the
LOWER limit on eukaryotic cell size.

Finding a cell larger than 100 µm would be unusual, as this
would exceed the UPPER limit on eukaryotic cell size

A eukaryotic cell 3 µm in diameter would be outside the
typical LOWER limit for cell size.
(One possible explanation is that the cell has found a way to survive
with fewer cell structures than usual).