1. Unicellular organisms carry out all the functions of life in a single cell, while multicellular organisms are composed of many cells working together. 2. In multicellular organisms, cells become specialized through differentiation to form tissues and carry out specific functions. This specialization allows the organism to operate more efficiently. 3. Cell differentiation occurs as certain genes are expressed and others are not, leading cells to develop specialized structures suited to their functions. Specialized cells like nerve and muscle cells cannot reproduce, while others like skin cells can.