Exchange between organisms and their environment

All organisms need to exchange substances such as

Food waster gases and heat with their surroundings

These substances must diffuse between the organisms and their surroundings

Ficks law

Rate at which a substance can diffuse

Rate of diffusion = surface area x concentration gradient/ distance

As organisms get bigger

Their surface area/ volume ratio gets smaller

Also means as organisms become bigger it becomes more difficult for them to exchange materials with their surroundings

Gases cross the respiratory surface by diffusion so they must have

A large surface area

A thin permeable surface

A mechanism to maximise the diffusion gradient

Cuboids

Volume = length x width x height

surface area = (4 x length x height) + (2 x height x width)

Cylinders

area of a circle πr2 circumference = 2πr

volume of a cylinder = πr2 X height

Surface area = (2πr X height) + 2πr

Sphere

Volume = 4/3 πr3

Surface area = 4πr2