Which of the following statements about diffusion is true?
Diffusion is the passive movement (meaning that no additional energy is needed to facilitate the movement) of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. This process continues until the concentration of substances is uniform (equivalent) throughout. Gas exchange for respiration is an example of diffusion — oxygen from blood to tissue cells, carbon dioxide in the opposite direction. Osmosis is also an example of diffusion— diffusion of water through a semipermeable membrane.