Final answer:
Single-celled organisms use diffusion to transport nutrients and remove waste, as their small size allows for direct exchange with the environment.
Step-by-step explanation:
Unicellular organisms rely on the process of diffusion to transport substances throughout the body. Unlike complex multicellular organisms that have evolved a circulatory system, single-celled organisms depend entirely on diffusion for the uptake of nutrients and the removal of waste products. This is because their small size allows for efficient exchange of materials directly with their environment, without the need for a transport system.