Final answer:
A single, flattened layer of epithelial cells is called simple squamous epithelium, which is efficient for rapid diffusion due to its thin layer.
Step-by-step explanation:
A single, flattened layer of epithelial cells represents what is known as simple squamous epithelium. These cells are shaped like thin scales and are particularly adapted for functions where rapid passage of chemical compounds is required, such as in the lungs' alveoli for gas exchange, in kidney tubules, or in the lining of capillaries.
The simple epithelium allows for efficient diffusion because of the minimal barrier it poses due to its single thin layer of cells.