Which process involves the movement of molecules or atoms down a concentration gradient across plasma membranes?

Diffusion is the process involves the movement of molecules or atoms down a concentration gradient across plasma membranes. It does not require any use of energy so it is a type of passive transport.

Diffusion can be divided into simple diffusion and facilitated diffusion. In simple diffusion, a molecule dissolve with the plasma membrane and transported inside the cell along the concentration gradient.

Facilitate diffusion also involved in transport of molecules along the concentration gradient but in this case, the molecules are transported by carrier protein present in the plasma membrane. Facilitated diffusion has a fast rate of transport than simple diffusion.

they deliver oxygen to the blood