Surface Area to Volume Ratio Explained


some organisms are big and some are

small but they're all made up of cells

some tiny organisms like bacteria are

made up of a single cell and all the

materials they need must be supplied via

their cell membrane large organisms like

us humans are made up of millions of

cells to get the important stuff like

oxygen to all of the cells that make up

our bodies we have a transport system

that take things to where we need them

we rely on the blood to transport what

we need to where we need it but why are

larger organisms made of many cells

rather than a single giant cell and why

do we need such advanced transport

systems rather than simply allowing

materials like oxygen to diffuse from

the air through our skin to all of our

body cells the problem is surface area

consider two cubes of different sizes

the smallest cube is 1 centimeter by 1

centimeter by 1 centimeter and so has

faces each with an area of one

centimeter squared and there are six

faces so it has a total surface area of

six centimeters squared the larger cube

is 2 centimeters by 2 centimeters by 2

centimeters so each face is 4

centimeters squared with 6 faces it has

a total surface area of 24 centimeters

squared so the larger cube has a larger

surface area but the larger cube also

has a larger volume we need to compare

the surface area of each cube with its

own volume the small cube has a volume

of one by one by one which is one

centimeter cubed

it has 6 square centimeters of surface

area for every one cubic centimeter of

volume we can say that it has a surface

area to volume ratio of 6 to one if we

do the same for the larger cube we can

see that it has a volume of 2 times 2

times 2 which equals 8 centimeters cubed

this ratio of 24 square centimeters of

surface area to every 8 cubic

centimeters of volume can be simplified

to 3 to 1

six to one is bigger than three to one

it's obvious that the smaller cube has a

larger surface area to volume ratio

let's imagine that these cubes are

living cells that need oxygen to survive

for oxygen to spread throughout the

cells it must diffuse through the

surface as the small cell has more

surface area relative to its volume

every part of that volume can be

supplied with oxygen at a faster rate

compared with the larger cell it would

take too long for the center of the

larger cell to be supplied with what it


if a large organism were made of a

single cell the exchange of materials

simply wouldn't happen fast enough this

is what limits the maximum size of a

cell since our bodies have to be made of

many tiny cells it's important that the

materials those cells need are

transported directly to them this is the

reason why we have a circulatory system

with a really strong pump that is the

heart constantly shifting nutrients in

the blood throughout our body