Diffusion and Osmosis
Lipid diffusion/simple diffusion
Diffusion is defined as the spreading of particles in a gas or any substance in a solution resulting in a net movement from an area of high concentration to an area of lo concentration. The particles move down a concentration gradient and so this process is passive, meaning it requires no energy.
The only substances that can diffuse through the phospholipid membrane are lipid soluble molecules such as steroids or very small molecules such as H2O O2 and CO2.