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Biology LibreTexts

15.1: Membranes

Membranes

Plasma membranes enclose and define the borders between the inside and the outside of cells. They are typically composed of dynamic bilayers of phospholipids into which various other lipid soluble molecules and proteins have also been embedded. These bilayers are asymmetric—the outer leaf being different than the inner leaf in lipid composition and in the proteins and carbohydrates that are displayed to either the inside or outside of the cell. Various factors influence the fluidity, permeability, and various other physical properties of the membrane. These include the temperature, the configuration of the fatty acid tails (some kinked by double bonds), the presence of sterols (i.e., cholesterol) embedded in the membrane, and the mosaic nature of the proteins embedded within it. The cell membrane has selectivity; it allows only some substances through while excluding others. In addition, the plasma membrane must, in some cases, be flexible enough to allow certain cells, such as amoebae, to change shape and direction as they move through the environment, hunting smaller, single-celled organisms. 

Amoebae Hunting Video

Cellular membranes

A subgoal in our "build-a-cell" design challenge is to create a boundary that separates the "inside" of the cell from the environment "outside". This boundary needs to serve multiple functions that include:

  1. Act as a barrier by blocking some compounds from moving in and out of the cell. 
  2. Be selectively permeable in order to transport specific compounds into and out of the cell. 
  3. Receive, sense, and transmit signals from the environment to inside of the cell. 
  4. Project "self" to others by communicating identity to other nearby cells.

Figure 1. The diameter of a typical balloon is 25cm and the thickness of the plastic of the balloon of around 0.25mm. This is a 1000X difference. A typical eukaryotic cell will have a cell diameter of about 50µm and a cell membrane thickness of 5nm. This is a 10,000X difference.

Note: possible discussion

The ratio of membrane thickness compared to the size of an average eukaryotic cell is much greater compared to that of a balloon stretched with air. To think that the boundary between life and nonlife is so small, and seemingly fragile, more so than a balloon, suggests that structurally the membrane must be relatively stable. Discuss why cellular membranes are stable. You will need to pull from information we have already covered in this class.

Fluid mosaic model

The existence of the plasma membrane was identified in the 1890s, and its chemical components were identified in 1915. The principal components identified at that time were lipids and proteins. The first widely accepted model of the plasma membrane’s structure was proposed in 1935 by Hugh Davson and James Danielli; it was based on the “railroad track” appearance of the plasma membrane in early electron micrographs. They theorized that the structure of the plasma membrane resembles a sandwich, with protein being analogous to the bread, and lipids being analogous to the filling. In the 1950s, advances in microscopy, notably transmission electron microscopy (TEM), allowed researchers to see that the core of the plasma membrane consisted of a double, rather than a single, layer. A new model that better explains both the microscopic observations and the function of that plasma membrane was proposed by S.J. Singer and Garth L. Nicolson in 1972.

The explanation proposed by Singer and Nicolson is called the fluid mosaic model. The model has evolved somewhat over time, but it still best accounts for the structure and functions of the plasma membrane as we now understand them. The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character. Plasma membranes range from 5 to 10 nm in thickness. For comparison, human red blood cells, visible via light microscopy, are approximately 8 µm wide, or approximately 1,000 times wider than a plasma membrane.

Figure 2. The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins. Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane.

 

The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates. The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with organism and cell type, but for a typical human cell, proteins account for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, and carbohydrates account for the remaining 10 percent of the composition by mass. However, the concentration of proteins and lipids varies with different cell membranes. For example, myelin, an outgrowth of the membrane of specialized cells, insulates the axons of the peripheral nerves, contains only 18 percent protein and 76 percent lipid. The mitochondrial inner membrane contains 76 percent protein and only 24 percent lipid. The plasma membrane of human red blood cells is 30 percent lipid. Carbohydrates are present only on the exterior surface of the plasma membrane and are attached to proteins, forming glycoproteins, or to lipids, forming glycolipids.

Phospholipids

Phospholipids are major constituents of the cell membrane, the outermost layer of cells. Like fats, they are composed of fatty acid chains attached to a polar head group. Specifically, there are two fatty acid tails and a phosphate group as the polar head group. The phospholipid is an amphipathic molecule, meaning it has a hydrophobic part and a hydrophilic part. The fatty acid chains are hydrophobic and cannot interact with water, whereas the phosphate-containing head group is hydrophilic and interacts with water.

Note

Make sure to note in Figure 3 that the phosphate group has an R group linked to one of the oxygen atoms. R is a variable commonly used in these types of diagrams to indicate that some other atom or molecule is bound at that position. That part of the molecule can be different in different phospholipids—and will impart some different chemistry to the whole molecule. At the moment, however, you are responsible for being able to recognize this type of molecule (no matter what the R group is) because of the common core elements—the glycerol backbone, the phosphate group, and the two hydrocarbon tails.

Figure 3. A phospholipid is a molecule with two fatty acids and a modified phosphate group attached to a glycerol backbone. The phosphate may be modified by the addition of charged or polar chemical groups. Several chemical R groups may modify the phosphate. Choline, serine, and ethanolamine are shown here. These attach to the phosphate group at the position labeled R via their hydroxyl groups.
Attribution: Marc T. Facciotti (own work)

A phospholipid bilayer forms as the basic structure of the cell membrane. The fatty acid tails of phospholipids face inside, away from water, whereas the phosphate group faces outside, hydrogen bonding with water. Phospholipids are responsible for the dynamic nature of the plasma membrane.

 

Figure 4. In the presence of water, some phospholipids will spontaneously arrange themselves into a micelle. The lipids will be arranged such that their polar groups will be on the outside of the micelle, and the nonpolar tails will be on the inside. A lipid bilayer can also form, a two layered sheet only a few nanometers thick. The lipid bilayer consists of two layers of phospholipids organized in a way that all the hydrophobic tails align side by side in the center of the bilayer and are surrounded by the hydrophilic head groups.  
Source: Created by Erin Easlon (own work)

Note: possible discussion

Above it says that if you were to take some pure phospholipids and drop them into water that some if it would spontaneously (on its own) form into micelles. This sounds a lot like something that could be described by an energy story. Go back to the energy story rubric and try to start creating an energy story for this process—I expect that the steps involving the description of energy might be difficult at this point (we'll come back to that later) but you should be able to do at least the first three steps. You can constructively critique (politely) each other's work to create an optimized story.

Note: possible discussion

Note that the phospholipid depicted above has an R group linked to the phosphate group. Recall that this designation is generic—these can be different than the R groups on amino acids. What might be a benefit/purpose of "functionalizing" or "decorating" different lipids with different R groups? Think of the functional requirements for membranes stipulated above.

Membrane proteins

Proteins make up the second major component of plasma membranes. Integral membrane proteins are, as their name suggests, integrated completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the hydrophobic region of the the phospholipid bilayer. Single-pass integral membrane proteins usually have a hydrophobic transmembrane segment that consists of 20–25 amino acids. Some span only part of the membrane—associating with a single layer—while others stretch from one side of the membrane to the other, and are exposed on either side.  This type of protein has a hydrophilic region or regions, and one or several mildly hydrophobic regions. This arrangement of regions of the protein tends to orient the protein alongside the phospholipids, with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid. 

Peripheral proteins are found on either the exterior or interior surfaces of membranes; and weakly or temporarily associated with the membranes. They can  interact with either integral membrane proteins or simply interact weakly with the phospholipids within the membrane.

Figure 5. Integral membranes proteins may have one or more α-helices (pink cylinders) that span the membrane (examples 1 and 2), or they may have β-sheets (blue rectangles) that span the membrane (example 3). (credit: “Foobar”/Wikimedia Commons)

Carbohydrates

Carbohydrates are the third major component of plasma membranes. They are always found on the exterior surface of cells and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2–60 monosaccharide units and can be either straight or branched. Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allow cells to recognize each other (one of the core functional requirements noted above in "cellular membranes"). 

Membrane fluidity

The mosaic characteristic of the membrane, described in the fluid mosaic model, helps to illustrate its nature. The integral proteins and lipids exist in the membrane as separate molecules and they "float" in the membrane, moving somewhat with respect to one another. The membrane is not like a balloon, however, in that can expand and contract dramatically; rather, it is fairly rigid and can burst if penetrated or if a cell takes in too much water. However, because of its mosaic nature, a very fine needle can easily penetrate a plasma membrane without causing it to burst, and the membrane will flow and self-seal when the needle is extracted.

The mosaic characteristics of the membrane explain some but not all of its fluidity. There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. By contrast, unsaturated fatty acids do not have a full complement of hydrogen atoms on their fatty acid tails, and therefore contain some double bonds between adjacent carbon atoms; a double bond results in a bend in the string of carbons of approximately 30 degrees.

Figure 6. Any given cell membrane will be composed of a combination of saturated and unsaturated phospholipids. The ratio of the two will influence the permeability and fluidity of the membrane. A membrane composed of completely saturated lipids will be dense and less fluid, and a membrane composed of completely unsaturated lipids will be very loose and very fluid.

Note: possible discussion

Organisms can be found living in extreme temperature conditions. Both in extreme cold or extreme heat. What types of differences would you expect to see in the lipid composition of organisms that live at these extremes?

 

Saturated fatty acids, with straight tails, are compressed by decreasing temperatures, and they will press in on each other, making a dense and fairly rigid membrane. When unsaturated fatty acids are compressed, the “kinked” tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This “elbow room” helps to maintain fluidity in the membrane at temperatures at which membranes with high concentrations of saturated fatty acid tails would “freeze” or solidify. The relative fluidity of the membrane is particularly important in a cold environment.  Many organisms (fish are one example) are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to the lowering of the temperature.

Cholesterol

Animals have an additional membrane constituent that assists in maintaining fluidity. Cholesterol, which lies alongside the phospholipids in the membrane, tends to dampen the effects of temperature on the membrane. Thus, this lipid functions as a "fluidity buffer", preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Thus, cholesterol extends, in both directions, the range of temperature in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.

 

Figure 7. Cholesterol fits between the phospholipid groups within the membrane.

Review of the components of the membrane

Archaeal membranes

One major difference between archaea and either eukaryotes or bacteria is the lipid composition of the archaeal membranes. Unlike eukaryotes and bacteria, archaeal membranes are not made up of fatty acids attached to a glycerol backbone. Instead, the polar lipids consist of isoprenoid (molecules derived from the five carbon lipid isoprene) chains of 20–40 carbons in length. These chains, which are usually saturated, are attached by ether bonds to the glycerol carbons at the 2 and 3 positions on the glycerol backbone, instead of the more familiar ester linkage found in bacteria and eukaryotes. The polar head groups differ based on the genus or species of the Archaea and consist of mixtures of glyco groups (mainly disaccharides), and/or phospho groups primarily of phosphoglycerol, phosphoserine, phosphoethanolamine or phosphoinositol. The inherent stability and unique features of archaeal lipids have made them a useful biomarker for archaea within environmental samples, though approaches based on genetic markers are now more commonly used. 

A second difference between bacterial and archaeal membranes that is associated with some archaea is the presence of monolayer membranes, as depicted below. Notice that the isoprenoid chain is attached to the glycerol backbones at both ends, forming a single molecule consisting of two polar head groups attached via two isoprenoid chains.

 

Figure 8. The exterior surface of the archaeal plasma membrane is not identical to the interior surface of the same membrane.

Figure 9. Comparisons of different types of archaeal lipids and bacterial/eukaryotic lipids

Note: possible discussion

In many cases—though not all—the archaea are relatively abundant in environments that represent extremes for life (e.g., high temperature, high salt). What possible advantage could monolayered membranes provide?

 

The components and functions of the plasma membrane
Component Location
Phospholipid Main fabric of the membrane
Cholesterol Between phospholipids and between the two phospholipid layers of animal cells
Integral proteins (e.g., integrins) Embedded within the phospholipid layer(s); may or may not penetrate through both layers
Peripheral proteins On the inner or outer surface of the phospholipid bilayer; not embedded within the phospholipids
Carbohydrates (components of glycoproteins and glycolipids) Generally attached to proteins on the outside membrane layer