Unicellular organisms consist of a single cell that carries out all the necessary functions of life, while multicellular organisms are made up of multiple cells that work together to perform complex functions.
Unicellular organisms, such as bacteria and yeast, are often simpler in structure and function. They reproduce quickly and can adapt to a variety of environments. Their single-cell nature means that they must accomplish all life processes, like metabolism and reproduction, within that one cell.
On the other hand, multicellular organisms, like humans, plants, and animals, have specialized cells that form tissues and organs with specific functions. This specialization allows for greater complexity. For instance, in humans, some cells are designed for muscle contraction, while others are geared toward transporting oxygen.
In summary, the key difference lies in the number of cells: unicellular organisms operate as a single entity, while multicellular organisms rely on a collective of cells that work in harmony, each contributing to the overall function and survival of the organism.