and yet it owes its enormity to more than 1,000 trillion microscopic cells,

and on the epically small end of things,

there are likely millions of unicellular species,

yet there are very few we can see with the naked eye.

Why is that?

Why don't we get unicellular elephants,

or blue whales,

or brown bears?

To find out, we have to peer into a cell's guts.

This is where most of the cell's functions occur,

enclosed by a cellular membrane

that acts as the doorway into and out of the cell.

Any resources the cell needs to consume,

or waste products it needs to expel,

first have to pass through this membrane.

But there's a biological quirk in this setup.

A cell's surface and volume increase at different rates.

Cells come in many shapes,

but imagining them as cubes will make the math easy to calculate.

A cube has six faces.

These represent the cell membrane, and make up its surface area.

A cube measuring one micrometer on each side,

that's one millionth of a meter,

would have a total surface area of six square micrometers.

And its volume would be one cubic micrometer.

This would give us six units of surface area for every single unit of volume,

a six to one ratio.

But things change dramatically if we make the cube ten times bigger,

measuring ten micrometers on each side.

This cell would have a surface area of 600 square micrometers

and a volume of one thousand cubic micrometers,

a ratio of only .6 to one.

That's less than one unit of surface area to service each unit of volume.

As the cube grows, its volume increases much faster than its surface area.

The interior would overtake the membrane,

leaving too little surface area for things to quickly move in and out of the cell.

A huge cell would back up with waste and eventually die and disintegrate.

There's another plus to having multitudes of smaller cells, too.

It's hardly a tragedy if one gets punctured, infected, or destroyed.

Now, there are some exceptionally large cells that have adapted to cheat the system,

like the body's longest cell,

a neuron that stretches from the base of the spine to the foot.

To compensate for its length, it's really thin,

just a few micrometers in diameter.

Another example can be found in your small intestine,

where structures called villi fold up into little fingers.

Each villus is made of cells with highly folded membranes

that have tiny bumps called microvilli to increase their surface area.

But what about single-celled organisms?

Caulerpa taxifolia, a green algae that can reach 30 centimeters long,

is believed to be the largest single-celled organism in the world

thanks to its unique biological hacks.

Its surface area is enhanced with a frond-like structure.

It uses photosynthesis to assemble its own food molecules

and it's coenocytic.

That means it's a single cell with multiple nuclei,

making it like a multicellular organism but without the divisions between cells.

Yet even the biggest unicellular organisms have limits,

and none grows nearly as large as the elephant, whale, or bear.

But within every big creature are trillions of minuscule cells

perfectly suited in all their tininess

to keeping the Earth's giants lumbering along.