Passive transport

Diffusion is the random movement of any particles from a region of high particle concentration to a region of low particle concentration. Examples of this are

the diffusion of oxygen from the air sacs of the lungs into the blood stream and the diffusion of carbon dioxide from the blood stream into the air sacs of the lungs. the diffusion of sugars from the small intestine into the bloodstream.

Osmosis is a special case of diffusion where water molecules randomly move from a region of low solute concentration (that means in fact high water concentration) through a partially permeable membrane to a region of high solute concentration (that means in fact low water concentration).

Biologists are especially interested in this special case as just about all living things involve water and cell plasma membranes. The water inside and outside of cells nearly always has substances dissolved in it. These create concentration differences and cause the movement of water molecules.

As we have seen the phospholipid part of a plasma membranes is waterproof. This is very important to control the movement of water in and out of cells. Water moves into and out of cells through the channel proteins of the plasma membranes. The size of the channels determine what size molecules can fit through. This is what makes the membrane partially permeable.

What is important to notice is that these processes are called passive because it is the particles that have the energy of movement and it is not the membrane that moves. The membrane just allow the movement or not.