What can happen through the cell membrane

Diffusion through a membrane

Living cells are enclosed by a membrane, the structure of which is dealt with in a separate section of my cytology pages. Not only are the cells surrounded by a membrane on the outside, but most of the cell organelles inside the cell are also surrounded by a membrane. Mitochondria, chloroplasts and cell nuclei are even enclosed by two membranes, which is related to the evolutionary biological origin of these organelles (keyword: endosymbiotic theory).

A biological membrane consists of a lipid bilayer in which various proteins are embedded. As you probably already know, a lipid molecule consists of a hydrophilic "head" and two hydrophobic "tails". The lipid bilayer therefore consists of three areas, as the following figure shows:

A lipid bilayer

A lipid bilayer is a barrier to most particles

If a molecule wants to diffuse from the external medium into the interior of the cell, it must first pass through a hydrophilic layer, which is formed by the outward-facing "heads". Once the molecule has penetrated this layer, it has to pass through a very thick hydrophobic layer, which is formed by the "tails" of the two lipid layers. This is followed by another hydrophilic layer, which is formed by the "heads" of the inner lipid layer.

Suppose such a molecule is hydrophilic (for example, glucose). Then it can pass through the first hydrophilic layer without any problems. But when it reaches the inner hydrophobic layer, the molecule is repelled and the path is over. No matter how high a concentration gradient from outside to inside helps, diffusion is not possible, at least not in a "reasonable" time.

Let us now assume the reverse case, that the molecule is hydrophobic. Then the passage should already fail because of the first hydrophilic layer of the lipid bilayer. However, there are a few exceptions. Fatty acids, for example, can pass through the lipid bilayer without problems, even though they are hydrophobic.

Only small molecules can pass through the lipid bilayer

Very small, electrically neutral molecules such as oxygen, nitrogen or carbon dioxide can usually penetrate the membrane unhindered. It is much more difficult for water to diffuse through the lipid bilayer, but it is still possible. Some hydrophobic substances such as steroid hormones or fatty acids can also pass through the lipid bilayer without any problems [1].

Factors that have an influence on the diffusion through a membrane

The rate of diffusion of a substance through a lipid bilayer depends on the following factors [2]:

  • Cell membrane thickness: the thicker the cell membrane, the slower the diffusion rate
  • Diameter of the molecules: the larger the molecules, the slower the diffusion rate
  • Concentration gradient: the greater the concentration gradient, the higher the diffusion rate
  • temperature: the higher the temperature, the higher the diffusion rate
  • Lipid solubility: the better the molecules dissolve in lipids, the higher the diffusion speed
  • Membrane surface and Composition of the membrane: Depending on the chemical composition of the lipids and depending on the protein content, the diffusion of substances through the membrane can be made easier or more difficult.

Facilitated diffusion through pore proteins

There is no consensus in the specialist literature as to whether diffusion through pore proteins can already be described as "facilitated diffusion". After all, a pore protein is nothing more than a more or less complexly structured "hole" in the membrane through which the particles can then enter the cell or flow out of it. The rate of diffusion through such a pore can therefore not be greater than without a membrane that hinders free diffusion.

A lipid bilayer with proteins

Here you can see such a pore protein as it pervades the lipid bilayer. If the pore inside the protein is hydrophilic, hydrophilic particles can diffuse through the protein. If, on the other hand, the pore is hydrophobic, the protein is suitable for hydrophobic particles. However, such pores always work according to the sieve principle: Only particles with a diameter smaller than that of the pore are allowed to pass through.

For experts: sodium and potassium channels

Sodium channels are pore proteins that only allow sodium ions to pass through. Sodium ions are positively charged and significantly smaller than potassium, calcium or magnesium ions, which are also positively charged. This is where the simple sieve principle comes into play. Neutral or electrically negatively charged particles, which are just as small as sodium ions or even smaller, cannot pass through a sodium channel because there are electrical charges inside the pore, which ensure that only positive ions can pass through.

Potassium channels are also pore proteins; they only allow potassium ions to pass through, but not sodium ions. At first one wonders here, since potassium ions are larger than sodium ions. The sieve principle does not seem to work here.

Don't worry, it still works. In principle, potassium channels only allow hydrated ions to pass through, i.e. ions that are surrounded by a shell of water. However, a hydrated sodium ion is larger than a hydrated potassium ion. This is due to the higher charge density of the sodium ions. Both ions are simply positively charged, but sodium ions are smaller than potassium ions, so the positive charge is concentrated in a smaller volume - the charge density is greater. Ions with a high charge density attract more water molecules than ions with a low charge density. Therefore, the sodium ions gather more water molecules than the potassium ions and are therefore larger in hydrated form than the potassium ions.

The following applies to pore proteins: They can only let through particles in the direction of the concentration gradient. Theoretically, the particles would at some point come through the membrane without any help, but with the help of the pore proteins this is much faster. Therefore one can speak of a facilitated diffusion.

Pore ​​proteins can be specific, i.e. only allow certain particles to pass through. We saw this using the example of the sodium and potassium channels (see expert box). But there are also pore proteins that are quite unspecific. The sieve principle always applies, however, particles that are larger than the pore cannot pass through.

Facilitated diffusion through carrier proteins

Carrier proteins are membrane proteins that not only represent simple "holes" in the membrane, but also sophisticated mechanisms for the specific transport of substances, possibly also against the concentration gradient (active transport). Carrier proteins are so important for the function of the cell that I have moved this topic to a separate page (which, of course, I have not yet written).

We'll continue with diffusion through a permeable membrane ...