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  • The elephant is a creature of epic proportions,

  • 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.

The elephant is a creature of epic proportions,

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B2 US TED-Ed surface area surface area volume membrane

【TED-Ed】What is the biggest single-celled organism? - Murry Gans

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    Sh, Gang (Aaron) posted on 2016/08/20
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