|Why are cells small?|
The main reason that cells are small has to do
with how the ratio of volume to surface area
increases as a cell gets bigger. One way to think
about this is imagine a cell as a water balloon.
If you cut the balloon open and lay it flat and
measure the area, that measurement would be the
surface area. The volume of water inside the
balloon would naturally be the volume. If you fill
the balloon with a little bit of water, the ratio
of volume to surface area will be low, but as you
add more water, the ratio of volume to surface
area will increase. This is because the balloon is
only stretching a little bit more to accommodate a
lot more water. The implications of this for cells
is that all nutrients have to pass through their
cell membrane which is only on the surface. These
nutrients then need to get to the center of the
cell. If the cell has too much volume, the
nutrients, such as oxygen, may be depleted before
they get to the center of the cell. Therefore,
cells generally don’t get too large because they
would have trouble transporting nutrients and
other molecules from the outside to the entirety
of the inside.
That being said, there are
exceptions and there are cells that are quite
large that employ different strategies to overcome
this difficulty. An example is egg cells which can
be fairly large (such as the ostrich egg) and
neurons that can be very long (especially in the
case of the giant squid).
First off all, not all cells are small! Sure, most of
the cells are small because they need to transport
tiny cargo in the form of proteins or lipids and,
most importantly, messages from the nucleus of the
cell, which is like the command center, to
everywhere inside the cell. Therefore it would
take a really long and inconvenient time if they
were really large.
But not all cells are small. Let's take a look :
red blood cells are about 5 microns across, cells
below our skin are about 10 microns across, fat
cells which are full of fat are 100 microns, and a
the largest neuron cell we have can be up to one
meter (40 inches) long ! It really depends on
Interesting question! In fact, cells come in
many shapes and sizes. For example, one of
the longest cells in the human body is a neuron
that extends the entire length of your leg! One
of the largest cells you will find in nature is
the oocyte; the female contribution to the embryo.
A chicken egg is a cell! So the largest cell in
nature is probably the ostrich egg.
Biologists are very interested in what
determines cell size. Is there a limit to the
size a cell can be? What constrains cell size? Do
bigger animals have larger cells or just more
cells? Keep asking questions!
The reason cells are so small is usually explained
in terms of surface-area-to-volume ratio. The
larger a cell becomes, the lower this ratio
becomes: surface area increases much slower than
volume. This becomes a problem when the cell can't
get enough food (based on its surface area) to
supply to all of its interior volume. So cell size
is the best compromise between having resources
and being able to supply resources.
It's because of the square-cube law.
Think about this: suppose you have a cube of
something, and you want to get chemicals in or out
of it. The only place to get in or out is on the
sides of the cube. Each of these sides is a square
of length and width equal to the length of the
sides, squared. Thus, the total surface area of
the cube is six times the square of the length. If
the cube's length is one micrometer, let's say,
then the cube's surface area is six square
micrometers. If the cube's length is two
micrometers, then the surface area is 24 square
However, the mass inside of the cube is not
proportional to its area, but to its volume. This
is equal to the cube of the length. Our
one-micrometer-long cube has a volume of one cubic
micrometer, and our two-micrometer-long cube has a
volume of eight cubic micrometers.
Notice that the ratio of area to volume is not the
same between these two hypothetical cubes! The
one-micrometer long cube has a surface area/volume
ratio of 6/1 = 6/micrometer. The
two-micrometer-long cube has a surface area/volume
ratio of 24/8 = 3/micrometer. This means that if
we need to get nutrients or something into the
larger cube, we can only do it half as fast!
As a consequence, cells have to be small, in
to maximize their surface area/volume ratios. If
they were bigger, they couldn't take in nutrients
fast enough, and they would starve to death.
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