The molecule that can MOST easily pass through the plasma membrane without assistance is carbon dioxide (CO2). Diffusion is a type of passive transport that occurs when molecules move in favor of a concentration gradient.
During diffusion, molecules move high concentration to a region of low concentration.
Oxygen (O2) and carbon dioxide (CO2) are non-polar gasses that move across cell membranes by a process called simple diffusion.
Simple diffusion is a type of passive transport that does not require energy to move substances across cell membranes.
