the heart (1.1-1.4)

• explain why larger organisms need a mass transport system but smaller organisms do not

  -larger organisms have smaller SA:V ratio

  -some cells are far from the surface

  -so diffusion alone cannot supply oxygen/glucose/nutrients to the cell
• why do some organisms rely on a mass transport system to move substances around their bodies?

  because they are too large for diffusion to move substances around their bodies quickly enough

  they rely on a mass transport system to move substances efficiently over a long distance by mass flow
• why do some organisms not need a mass transport system to move substances around their bodies?

  because diffusion is usually fast enough to meet the organism's requirements and they have a large SA:V ratio