Cells are the smallest functional units of life and form all the other organisms, including plants, animals, human beings, etc. These life forms are capable of carrying out a number of life processes such as metabolism, growth, reproduction, etc. These activities require a number of chemical molecules and energy. Cells may be capable of making some of these molecules as part of their metabolism. However, they cannot synthesize every chemical they need and hence are dependent on their immediate environment for such chemicals. This is where the cell membrane comes into picture. Cell's outer layering is very special as it allows the entry of selective molecules only. Thus, only the chemicals that are needed are allowed access to the cytoplasm, while the rest are not allowed entry.
Thus, cell's external environment fulfills the requirement of chemicals that cannot be produced by the cell itself.
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