Biological Control of Cell Membrane Structural Properties

You learned about DNA and proteins in your high school biology class, but you may not remember much about the cell’s membrane which is based on a dynamic, fluctuating sandwich structure. This cellular envelope controls what chemicals enter and exit the cell, partly due to molecular machines such as channels and pumps in the membrane, and partly due to the sandwich structure itself. This sandwich structure is a barrier to certain types of chemicals. But the membrane permeability and the operation of the molecular machines depend on the details of the sandwich structure. And as recent research has been finding, contra evolutionary expectations, organisms actively maintain and fine-tune the sandwich structure in response to environmental challenges.

The cell membrane’s sandwich structure consists of two layers facing away from each other. Each layer is made up of an array of phospholipid molecules lined up next to each other. Phospholipid molecules consist of a water-loving (or hydrophilic) head, a phosphate group, and two oily (or hydrophobic) hydrocarbon tails.

The two layers face away from each other in a tail-to-tail arrangement. This creates a very oily, low dielectric, interior of the sandwich. While small oily molecules are able to pass through this membrane structure, it is difficult for any water-loving molecule, including water itself, to pass through the barrier. So when this sandwich structure forms a closed sphere surrounding the cell, the inside compartment is separated and protected from the outer aqueous environment. But the sandwich structure is not a simple, rigid, structure. It fluctuates, and this influences the membrane performance.

At lower temperatures the sandwich structure has a more rigid, gel, phase and at higher temperatures it has a less rigid, fluid phase. As with the freezing and melting of water there is a distinct phase change, between the gel and fluid phases of the sandwich structure, which occurs over a rather narrow temperature range. This melting point temperature is strongly influenced by the degree of attraction between the adjacent phospholipid tails. Depending on the temperature and this degree of attraction, the sandwich structure may be like a gel or like a fluid, and this is important because the phase influences the membrane permeability and the membrane’s molecular machines.

Within the membrane sandwich structure, the phospholipid tails are attracted to each other via the weakest chemical force, van der Waals interactions. Unlike the stronger chemical bonds, van der Waals interactions do not arise from the trading or sharing of electrons. So how do these interactions work?

Consider two neighboring atoms. As the electrons quickly move about, uneven charge distributions can occur across the atom. One side of an atom may temporarily be positively charged, and the other side negatively charged. Such charges influence the neighboring atom. For instance, a negative charge will tend to repel the electrons of the neighboring atom causing an attractive, uneven charge distribution in that atom. The two atoms can then continue with synchronized, fluctuating charge distributions. But all of this depends greatly on the distance between the two neighboring atoms. If they are too far apart (or too close together), the entire interaction, weak as it is, becomes insignificant.

The distance between adjacent phospholipid tails depends on their shape. If the tails have two hydrogen atoms for each carbon atom, then there are no double bonds. Such saturated chains have a consistent, linear, shape and they pack tightly together at the van der Waals preferred distance.

Unsaturated chains, on the other hand, have double bonds which cause structural kinks, loose packing and therefore weaker van der Waals interactions. And the particular location of the double bond is important, as some locations disrupt the van der Waals interactions more than others.

So all of this means that the number and location of hydrogen atoms in the phospholipid tails is an important tuning parameter (there are other tuning strategies as well), determining the phase of the sandwich structure and, in turn, the cell’s membrane performance. This is particularly important for organisms that are subject to greater temperature variations, such as poikilotherms. Such temperature variations can cause unwelcome phase changes in the membrane’s sandwich structure.

Physiological response to temperature change

Years ago it was thought that the various protein machines in the cell’s membrane were more or less randomly distributed. It is yet another example of the influence of evolutionary thinking on biology. If the biological world is a fluke, then aren’t biological designs, such as the cell’s membrane architecture, random?

Now we know better. The cell membrane architecture is anything but random. In fact, the attention to detail is enormous. This includes the phase of the sandwich structure and its tuning mechanisms, such as the degree to which the phospholipid tails are saturated. Here are quotes from representative research papers discussing how organisms monitor and control their membrane fluidity, particularly in response to temperature variations:

E. coli incorporates increasing proportions of saturated and long-chain fatty acids into phospholipids as growth temperature is increased. It was found that this compositional variation results in the biosynthesis of phospholipids that have identical viscosities at the temperature of growth of the cells. [link to paper]

Numerous studies have shown that fluidity is an important factor in the function of biological membranes. Changes in fluidity affect the activity of membrane-bound enzymes and the activity of transporters, as well as the permeability of membranes to nonelectrolytes, water, and cations. Given that temperature has profound effects on membrane fluidity, it is not surprising that poikilotherms adjust the composition of their membranes in ways that defend fluidity in the face of changes in body temperature. …

Although the ways in which membrane composition is altered in response to temperature are not always consistent among species, tissues, cells, or even organelles, a few important trends have emerged. One prominent response to a decrease in the body temperature of poikilotherms is an increase in the percentage of unsaturated fatty acids that make up the phospholipids. … Phospholipids with saturated fatty acids pack readily into bilayers, whereas phospholipids with unsaturated (and therefore, kinked) acyl chains tend to disrupt hydrophobic interactions among acyl chains of adjacent phospholipids. An increase in the proportion of unsaturated fatty acids thus results in an increase in membrane disorder and fluidity, which tends to oppose the ordering effect of a drop in temperature. [link to paper]

The phospholipid composition of plasma membranes from the kidney of rainbow trout, Salmo gairdneri, was determined over a period of 21 days as fish were acclimating between temperatures of 5 and 20 degrees C. Proportions of phosphatidylethanolamine (PE) were significantly higher (29.03 vs. 23.26%) in membranes of 5 degrees C- than 20 degrees C-acclimated trout [link to paper]

Our observations suggest that a physical parallel to the changes of lipid composition is the maintenance of an optimal lipid order in the hydrophobic core of the cytoplasmic membranes. It can be interpreted as a tendency of Bacillus subtilis to keep the lateral pressure in its membranes at an optimal value, independent of the temperature of cultivation. [link to paper]

Not only is the cell membrane intricate and complex (and certainly not random), but it has tuning parameters such as the degree to which the phospholipid tails are saturated. It is another example of a sophisticated biological design about which evolutionists can only speculate. Random mutations must have luckily assembled molecular mechanisms which sense environmental challenges and respond to them by altering the phospholipid population in the membrane in just the right way. Such designs are tremendously helpful so of course they would have been preserved by natural selection. It is yet another example of how silly evolutionary theory is in light of scientific facts.