3.3.1 Surface Area to Volume Ratio

cube + sphere surface area and volume

cube: sa = length² x 6, v = length³

sphere: sa = 4 pi r², v = 4/3 pi r³

RELATIONSHIP between the size of an organism and its SA:V ratio

as the size of the organism increases, the surface area to volume ratio decreases, the metabolic rate decreases

single-celled organisms + multi-cellular organisms

-single = larger SA:V, exchange substances by diffusion directly across cell membrane

-multi = smaller SA:V, cannot exchange enough to survive, diffusion pathway is too long to reach the middle of organism, too slow rate of diffusion

ADAPTATIONS of multi-cellular organisms 1)

a flattened shape

e.g. flat worms - exchange across surface

-are flat = increases SA:V

-are thin = short diffusion pathway

ADAPTATIONS of multi-cellular organisms 2)

have specialised exchange surfaces

-features of these
-large surface area
-very thin
-selectively permeable
-mass transport system - maintains steep concentration gradient