Cell membrane controls what can enter and leave as most of the molecular interaction in the cell occur in a water solution and require a fairly narrow range of temperature and acidity.

Protein molecules perform most of the basic functions like DNA replication, repair of cell structures, helping other molecules get in and out of the cell and regulation of molecular interactions. Proteins are a sequence of amino acids (there are about twenty different types of amino acids) and the function of each protein depends on the specific sequence of amino acids it contains and its shape.

Proteins also do various other functions like carrying oxygen, effect contraction and provide material to body structures. They also serve as hormones, antibodies and enzymes.

The single fertilized cell divides and multiplies to form millions of cells. The variation and formation of special cells such as gland cells and nerve cells happens by activating and deactivating different parts of the DNA depending on the differences in the immediate environment of their predecessors. Cell processes are controlled by interaction between different kinds of molecules within the cell and also by molecules from outside the cell like hormones and neurotransmitters. Exposure to chemicals or radiation may affect the functioning of cells.