First of all, don't confuse atoms and cells, which
is a common mistake but which is kind of like confusing a grain of
sand with a giraffe - they are wildly different sizes (atoms are
tiny enough for there to be trillions in a cell) and cells are by
far more complicated.
There is a range of size in cells, but there
seem to be both a lower and upper limit on workable size.
Bacterial cells, which are complex and common but simple compared
to our cells, usually range in the 1-5 micrometer-diameter range.
Smaller is rare - one hypothesis is that even a simple cell needs
enough room for its important molecules and parts to move around and
interact in, and the
simplest of cells is still pretty complex. Viruses are much
smaller than bacteria, but they're not cellular - almost no chemistry occurs inside that
tiny transport package.
Relative sizes (use the bar under the picture).
Size restrictions at the upper limit can be
trickier to explain. One leading hypothesis, first discussed in
Section 2, has to do with the
relationship between surface area and volume. In the
case of a round cell, if you double the diameter of the cell, the
surface area increases fourfold (the square of 2) but the volume
increases eightfold (the cube of two); as a cell gets
bigger, the amount of "guts" it has, the parts that need to get
materials in from outside and which produce often-toxic wastes that must leave the
cell, increases at a much faster rate than the surface through
which both incoming and outgoing materials must move. Although there are "tricks"
that cells have evolved - for instance, circulation of materials
past the surface, or shape changes, like flattening, to increase
surface without greatly increasing volume - it still appears that
there are limits to how big they can get and still run
efficiently. Very few types of cells are big enough to be
seen without a microscope, and even then youd need really good
eyesight. A structure like a chicken egg might be considered
by some as a single cell, but it doesnt really fit the rules to be a legitimate cell. Typically, cells "top
out" at about 100 micrometers diameter.
Page on the
relationship between surface area and volume.
Various shapes give varied surface area for similar volumes.
And, because biology is full of exceptions to rules: giant
CELLS SHARE SOME FEATURES, beyond basic
life functions, no matter the type of cell:
They all use the same sort of
phosphate-and-lipid barrier, called the
or plasma membrane or
plasmalemma, to separate their
insides from their surroundings. The lipid (fatty / oily) nature of the
membrane blocks most water and solutes from moving freely through;
various proteins stuck into and through the membrane makes the
passage of certain materials possible and controllable. The
exact workings will be covered in more detail in a later section.
They all need an internal environment where
molecules and ions must be free to float around and react, so all
cells have a watery plasm, commonly called cytoplasm,
with enough solutes that
it is thick, almost jellyish.
Cytoplasm can have lots of "stuff" in it.
They share some basic functions that use the
same structures. Since all cells use
DNA as a code
carrier, it is reasonable that the DNA is similar. All cells
have DNA bound together with proteins in the form of at least one chromosome.
Chromosomes themselves can vary in form and number.
Getting lots of DNA into a small place.
Proteins are critical to cell function.
DNA carries codes for proteins in genes. Proteins need to be
made using the code.
acorn-shaped collection of proteins and RNA, are cell structures
where proteins are made, and are found in all cells.
Ribosomes are a type of organelle, internal
structures that have specialized functions within a cell.
Intro to ribosomes (video).