Figure 1.7: Organized Systems Exist at Many Levels (A) This cellular-level system synthesizes and breaks down a cell protein called Protein T. (B) This organismal-level system determines the amount (and thus the concentration) of sodium (Na+) in the blood plasma and other extracellular body fluids of a human. (C) This community-level system helps determine the number of meadow voles (Microtus pennsylvanicus) in a field in the spring.