Passive transport is the cellular process of moving molecules and other substances across membranes.

Passive transport differs from active transport in that it does not involve any chemical energy. Rather, passive transport relies on the innate permeability of the cell membrane and its component proteins and lipids.

There are four main types of passive transport:

Diffusion is the overall movement of material from an area of higher concentration to an area of lower concentration.

The difference in concentration between the two areas is called the concentration gradient; diffusion tends to continue until the concentration gradient is gone.

Once this happens, material still tends to move between the areas but with no net gain for one side or the other.

In the human body, waste products are removed from the bloodstream in the kidneys via dialysis, an example of diffusion. (Learn more about Diffusion Through A Membrane.)

Facilitated diffusion is the carrier-mediated transport of large molecules through the cell membrane using transport proteins embedded within the cell membrane.These molecules would otherwise not be able to breach the cell membrane, but the transport proteins effectively "transport" them through.

This process is still diffusion, however, so the concept of the molecules moving from a higher concentration (outside the cell) to a lower concentration (inside the cell) without utilizing any chemical energy still applies.

The exchange of oxygen and carbon dioxide in the alveoli of the lungs is an example of facilitated diffusion. (Learn more about Membrane Proteins.)

Filtration is the movement of solute molecules and water across a membrane by normal cardiovascular pressure. The size of the membrane pores dictate the molecules that may pass.

Some functions of the liver and kidneys are based upon filtration.

Check out these resources for more information about cell membranes:

• Active Transport
• Passive Transport
• Carrier Proteins
• Membrane Proteins
• Lipid Bilayer
• Endocytosis
• Exocytosis
• Diffusion Through a Membrane
• Osmosis in a Plant Cell
• Osmosis in an Animal Cell
• Plasma Membrane - Plant Cell
• Plasma Membrane - Animal Cell
• My Cell Membranes flashcards
• Biology Quiz: Cell Membranes