This video lesson from TED Ed shows why a single celled organism can’t grow bigger in size compared to multi cellular organisms. When a single cell tries to increase its size, its volume increases faster than its surface area. Lesser surface area with increased volume leads to clogging of resources to inside and waste materials to outside. Eventually the cell dies. So the ratio of Surface area to volume is very important for a single celled organism. This is the basic idea behind the evolution of multi cellularity. If a single cell could grown large in size, multi cellular organisms would never have evolved.
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Blood consists of 3 types of cells, suspended in a liquid called plasma. These three types of cells, red blood cells (RBC), white blood (WBC) cells and platelets perform different functions throughout the body. RBC's transport oxygen to cells and is doughnut shaped. WBC's play major role in our immune system, and appears usually in spherical shape. Platelet are cell fragments and they help in blood clotting. This is a Scanning Electron Microscopic Image of Human RBC, WBC and Platelet.