One evolutionary trend is that as you move from simpler animals to more complex animals, Systems become more specialized.
They use carbo dioxide from the atmosphere during the process of photosynthesis. Also, after the producers die, their organic compounds are carried on to the decomposers.
The answer is most likely: common ancestor.
The cell membrane is a lipid bilayer that prevents that passage of water and ions. This allows cells to maintain a higher concentration of sodium ions out the outside of the cell.
