Not All the Same

On a human level, it makes sense that differences are important and necessary: not everyone can be a clerk, a carpenter or a cardiologist. Differences is skills, abilities, and talents help make society function well. The need for differences to obtain function is a pattern we can see at every level of living things from ecosystems down to the molecules which make up cells. At the molecular level, the existence of difference is not just significant for utilitarian purposes, but has metaphysical significance as well.

In the typical high school biology course, students are briefly confronted with the details concerning the make up of cells. Within this, they are presented with a description of cell membranes which gives the appearance that this organelle is composed largely of a single type of molecule called a phospholipid – these phospholipids form a bilayer which is interspersed with various proteins. Diagrams of membranes will usually depict these phospholipids as being identical in form, but this is far from an accurate depiction of membrane structure. While this oversimplification has merits in helping students begin to understand how cells work, this unfortunately fosters the notion that things like cells could have easily come about by the outworking of random processes.

I outlined in another article (Phantoms and Phospholipids) how natural processes were unlikely to have formed phospholipids prior to the first cell. Even if we could account for a natural process that forms phospholipids, phospholipids alone would have been inadequate to function as a cell membrane – they would be too unstable, and could not adequately diffuse materials necessary for metabolism and replication. Other components are essential to cell membrane function, but lets first focus on the role of phospholipids.

A phospholipid is constructed by the attachment of particular molecules to the three binding sites of a molecule called glycerol: two fatty acid chains, and a phosphate bearing reagent. The fatty acids are hydrophobic while the phosphate reagent is hydrophilic. This difference in properties enables the molecules to naturally arrange into a bilayer in an aqueous environment with the fatty acid tales of each leaflet of the bilayer pointed inward – away from water.

Cell membranes are more correctly described as being asymmetric. That is, the outer leaflet of the bilayer has a different structure and function than the inner leaflet. This is owing to both the differences in the types of lipids and proteins found on each side of the bilayer. Many different types of phospholipids inhabit the membrane of any given cell and influence the function of the cell membrane at different regions of the cell.  

Phospholipids vary, first, according to the size and type of fatty acid chains which are attached to the molecule of glycerol. Fatty acid composition may differ in response to temperatures in the environment in order to maintain the fluidity of the membrane. The types of fatty acids in a phospholipid can also allow some regions of a cell membrane to be more rigid while others are more flexible influencing the overall function of the cell.

There are particular arrangements of phospholipids in association with surface and integral proteins which stabilizes their position in the membrane and enables their function. For instance, integral proteins (those which cross the full distance of the membrane) have one-way functions like transporting nutrients across the membrane. If these proteins are not properly oriented in the membrane, they would cease to function. The differences in fatty acids will alter the shape of the phospholipids which enables them to conform to the shape of proteins, and hold the proteins in place.

Phospholipids also vary according to the type of phosphate reagent present. In the interior leaflet, the arrangement of these different types of phospholipids will help direct the protein driven functions which occur near the cell membrane. In the exterior leaflet, the arrangement of phosphate reagents plays a role in cell signaling – the way cells “talk” to each other and coordinate their activity. Another type of lipid in the membrane known as sphingolipids produce bioactive signaling molecules which also modulates cell processes. These sphingolipids are organized into distinct domains referred to as “rafts” and are essential for cell function.[1]

Three types of phospholipids and a sphingolipid (far right). https://www.ncbi.nlm.nih.gov/books/NBK26871/

How does a cell come by all these different types of phospholipids? The controlled process within a cell to manufacture phospholipids is governed by over 60 different enzymes in multi-step biochemical pathways utilizing multiple organelles[2]. The production of these enzymes within the cell is dependent upon the import of amino acids through transport proteins in the cell membranes. Remember, though, that the proper orientation of the proteins is dependent upon the arrangement of phospholipids in the bilayer. This interdependence of proteins and phospholipids poses a “chicken-and-egg” problem in explaining the existence of cells and cell membranes.

Another “chicken-and-egg” problem relates to the organelles involved in the biochemical pathways mentioned above. These organelles include the endoplasmic reticulum, Golgi apparatus, peroxisome and the mitochondria. The function of each of these organelles is predicated on the existence of cell membranes – phospholipid production is dependent on the existence and function of phospholipids.

I hope this brief outline makes clear the utilitarian importance of having different types of phospholipids, but what are the metaphysical implications here? It is not just that there are different types of phospholipids, but that there is a purposeful arrangement of these molecular parts which yield particular functions. Further, there are the interdependent relationships between proteins and phospholipids. Together, these strongly imply the need for foresight and planning in order to accomplish these functions. This is not something one could expect to come about by the outworking of natural laws alone, but signals the involvement of intelligent agency in order to explain their existence.


[1] Mary L. Kraft. Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence it. Front. Cell Dev. Biol 2016; 4:154 https://dx.doi.org/10.3389%2Ffcell.2016.00154 accessed 2/6/21.

[2] Paolo Fagone and Suzanne Jackowski. Membrane phospholipid synthesis and endoplasmic reticulum function. Journal of Lipid Research. April (2009, suppl.):311-316. DOI 10.1194/jlr.R800049-JLR200 accessed online 2/6/21.

1 thought on “Not All the Same

  1. Carl, I really like your article that describes the complexity of cellular membrane diversity and then relates that to the metaphysical concept of “irreducible complexity”. Your inclusion of the “chicken and egg” conundrum was a crowning touch.

    Like

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