environments on Earth, and elsewhere, such as lakes, rivers, and oceans, have a tendency to dilute molecules. A cell enclosure, or membrane, provides a way to keep molecules together at relatively high concentrations. A membrane will also retain water, allowing for chemical reactions to proceed even when outside conditions are desiccating.
In the previous chapter, we discussed the lipids, some of which contain a hydrophilic head (which is attracted to water) and a hydrophobic tail made from a fatty acid (which is repelled from water). One important class of lipids that are involved in membrane formation is the phospholipids in which the charged polar end is a phosphate group (Figure 5.4). It is worth examining these in more detail to see how they form a cellular membrane. The principles are the same for all membrane lipids.
Figure 5.4 Amphiphilic molecules such as phospholipids that make up some cell membranes. They have a hydrophilic end (attracted to water) and a hydrophobic end (repelled from water).
A remarkable characteristic of these so-called amphiphilic phospholipid molecules is that when added to water, they have a tendency to assemble spontaneously in such a way that the hydrophilic head is oriented into the water, and the hydrophobic tails, which would like to escape the water surrounding the molecules, are attracted toward each other to expel water. The result is a lipid bilayer membrane (Figure 5.5). These bilayers themselves tend to assemble into vesicles, small spherical structures with fluid in the inside. This shape, like a water droplet, is a minimal energy shape.
Figure 5.5 A simplified diagram showing the structure of a lipid bilayer that makes up cell membranes.
The phospholipids can also assemble in a single layer whereby a ball of lipids is formed, the tails pointing toward the center with a layer of hydrophilic heads on the outside of the ball. These are called micelles. They are less interesting than bilayer vesicles because vesicles have a hollow center in which cellular components can collect.
This property, whereby amphiphilic molecules can form a membranous layer, is by no means rare. Even fatty acids on their own, such as the long-chained carboxylic acid, decanoic acid, extracted from meteorites (Chapter 12) tend to form membranous vesicles, suggesting that this is a fundamental property of this class of molecules and that the assembly of the first cell membranes on Earth may not have been a difficult or extraordinary event.
Within the lipid membrane other proteins are incorporated, resulting in a complex system that regulates the interaction of the internal cell environment with the outside environment. Many of these proteins are trans-membrane proteins acting, for example, as thin channels both for ions and molecules to move into the cell and for wastes to be expelled. Some of these proteins act as a line of communication, allowing the cell to sense changes in physical and chemical conditions in the outside environment and respond to them.
5.5.1 Gram-Negative and -Positive Prokaryotic Membranes
The lipid bilayers found in life are assembled into more complex structures. It was recognized early in the study of cellular membranes that the bilayer is filled with different proteins that play roles in attachment to surfaces, transport of nutrients into the cell, and movement of waste outward. This led to the concept of the fluid-mosaic model of the membrane, where the word “mosaic” is a reference to its similar appearance to a mosaic or tiled art work. The complexity of membranes reflects the fact that although crudely we can think of it as a “bag” to hold cell constituents in, it is also a gateway in and out of the cell during the cell's many interactions with the environment. It is instructive to explore the complexity of the membrane a little more to understand these ideas.
In bacteria, membranes can be broadly separated into Gram-positive and Gram-negative membrane structures, a separation defined by how organisms stain in the Gram stain, a method of dyeing bacteria developed by microbiologist Hans Christian Gram (1853–1938) in the nineteenth century. In the Gram stain, microorganisms are first stained with crystal violet stain, which gives them a dark blue/violet color. After washing with ethanol and adding a paler dye (safranin), Gram-positive organisms retain the dye and show up as dark blue, whereas Gram-negative organisms lose it, becoming a red color. These differences are understood to be due to the quite different membrane structures in the two groups of organisms.
Gram-negative membranes (found, for example, in the ubiquitous gut bacterium E. coli) have an inner and outer membrane. The inner membrane is the plasma membrane. The space between the inner and outer membrane is termed the periplasm. The liquid in this space is somewhat more viscous than the cytoplasm and contains a wide variety of proteins involved in sugar and amino acid transport.
Sandwiched between the inner and outer membrane is a peptidoglycan mesh (Figure 5.6) containing just a few layers of peptidoglycan. Peptidoglycan is a complex matrix made up of sugars cross-linked to amino acids. It is one of the few structures in life that contains D-amino acids (Figure 5.7). The peptidoglycan helps the cell maintain its shape, and it is involved in cell division.
Figure 5.6 Gram-negative and Gram-positive cell membranes.
Figure 5.7 A typical structure of peptidoglycan. The pentaglycine cross-link is made from five glycine amino acids. N-acetylglucosamine and N-acetylmuramic acid are sugars and are derivatives of the sugar glucose.
Other structures in the membrane include porins, which are one class of outer-membrane proteins. They control the movement of ions and other small compounds across the cell membrane. There are other trans-membrane proteins involved in communication and interactions between the cell and the outside environment. Lipopolysaccharides (LPS), long sugar chains attached to the outer membrane with lipids, are involved in the attachment of cells to surfaces. A variety of lipoproteins within the inner envelope of the outer cell membrane take part in mediating biochemical reactions. They also play a role in linking the outer membrane to the peptidoglycan layer.
In Gram-positive cell walls (Figure 5.6) there is one cell membrane, with a periplasmic space above it, and the cell is surrounded by a thick peptidoglycan mesh made up of around 40 peptidoglycan layers (between 30 and 100 nm thick, compared to the Gram-negative peptidoglycan layer, which is just a few nanometers thick). Teichoic acids and lipoteichoic acids are long-chain sugars present on the surface and threaded through the peptidoglycan layers. They are involved in the integrity of the peptidoglycan layers, but some also play a role in cell attachment, like the LPS in the Gram-negative cell wall. An example of a Gram-positive organism is the very versatile bacterium Bacillus subtilis, found in diverse places from soils to spacecraft assembly rooms.
The structures outside of the plasma membrane in both Gram-positive and negative bacteria are generally referred to as the cell wall.
Discussion Point: Are Prokaryotes
