multicellular

• if organisms are multicellular it can be harder to supply all the cells with the same nutrients - they are relatively big and have a low volume to surface area ratio
• multicellular organisms have a higher metabolic rate
• multicellular organisms are very active a large number of cells are respiring quickly so need a rapid and quick supply of glucose and oxygen
• to ensure every cell has a good enough supply, multicellular organisms need a transport system
• mammals - circulatory system which is used to transport glucose and oxygen around the body using blood

  also carries hormones, antibodies, and waste like CO2