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Transport of small molecules across the plasma membrane

Passive transport

It is passive because the cell performs it without energy consumption+ç. It is spontaneous, in favor of a concentration gradient. The molecules pass because the membrane is permeable to water and polar substances. The transport will be faster the smaller the molecule. It can be by several mechanisms:

Simple diffusion

  • If the molecules are soluble in the membrane (for example, small lipid molecules, oxygennitrogen or CO2), they can pass through it directly.
  • In the case of polar molecules or other ions, they can pass through channel proteins or membranous channels, which allow their passage at a speed greater than their diffusion through the membrane.

Osmosis. It is a particular case of simple diffusion, which consists of the water passing through the membrane according to the concentration gradient. It will move from the medium with the lowest concentration to the medium with the highest concentration until the concentrations of the solutions equilibrate.

Difusión a través de la membrana celular

By BruceBlaus. When using this image in external sources it can be cited staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. (Own work) [CC BY 3.0], via Wikimedia Commons

Facilitated difussion

It also occurs in favor of gradient. Membrane proteins called permeases or transporter proteins or "carriers" intervene that bind to the molecule, change its conformation, and transport it to the other side of the membrane, where they recover their original conformationThe carbohydrates, amino acids and nucleosides would have this type of transport.

Active transport

With energy inputagainst the concentration, osmotic pressure, or electrical gradient, it requires transporter proteins, also called pumps.

It can only be made by some types of specialized proteins. The necessary energy is obtained from the hydrolysis of ATP, which is why these proteins are said to be ATPase.

The Na+/K+ pump

It is the most important ionic bomb. Animal cells have a higher concentration of K+ ions in their internal environment than outside, and a higher concentration of Na+ in the extracellular environment than inside.

The differences in concentration are due to the fact that the pump, for each hydrolyzed ATP, expels three Na+ ions towards the outside and introduces two K+ ions towards the inside, against the concentration gradient.

Fundamental insights on the transport of small molecules across the membrane

Transport of small molecules across the plasma membrane


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