Unicellular organisms are devoid of circulatory systems and will be found living in moist or watery environments to facilitate uptake of nutrients and gases and expulsion of waste by the process of diffusion.
Their cell is structured such that it can engage in direct exchange of substances with its environment. Water serves as the perfect medium or environment through which this exchange can occur.
Gases and nutrients simply diffuse into the cell or are moved into the cell by means of active transport or phagocytosis.
