Organisms and environment (s.a. to v ratio)

Two types of exchange

• active - metabolic energy required e.g. active transport

• passive - no metabolic energy required e.g. diffusion and osmosis

Surface area to volume ratio

Surface area to volume ratio gets smaller as an object becomes larger

What features may an organism have?

• flattened shape so no cell is ever far from the surface

• specialised exchange surfaces with large areas to increase the surface area to volume ratio

Features of specialised exchange surfaces

• large surface area to volume ratio - increases rate of exchange

• very thin so diffusion distance is short

• selectively permeable to allow selected materials to cross

• movement of environmental medium to maintain a diffusion grad.

• transport system to ensure movement of internal medium e.g. blood in order to maintain a diffusion gradient

What is diffusion directly proportional to?

surface area x diff. in conc. / length of diff. path