topic 1 - exchanging surfaces

• cells use diffusion to take in and out substances they need and don't need
• In humans, urea diffusion is into the blood plasma for removal from the body by the kidneys
• single cellular organisms can directly diffuse into and out the cell membrane, due to its large surface area compared to volume
• multicellular organisms have smaller surface areas compared to their volume so not enough substances can pass through
• animals have lots of blood vessels so that the substances can pass through quickly
• gas exchange surface in animals
• large surface areas so lot of substances can be diffused at once in multicellular organisms