Bis2A 6.0: Cellular Membranes vBis2Ateam
This module will outline the structure and function of the cellular membrane. Discussed in detail are the membrane components and the factors that affect membrane fluidity and permeability.

Section SummaryPlasma 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: 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 selectively, 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: Block some compounds from moving in and out of the cell.
  2. Be selectively permeable: 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: communicate identity to other nearby cells.
The diamerter of a typical balloon is 25cm, compared to the thickness of the plastic of the balloon, which is around 0.25mm. This is a 1000X difference. A typical eukaryotic cell will have a cell diameter of about 50um, and a cell membrane thickness of 5nm. This is a 10,000X difference.
This illustration shows a red balloon next to an electron micrograph of a eukaryotic cell.
Possible Discussion
The ratio of membrane thickness compared to the size of an average eukaryotic cell (figure 1), 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. ([link])

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.
This illustration shows a phospholipid bilayer with proteins and cholesterol embedded in it. Integral membrane proteins span the entire membrane. Protein channels are integral membrane proteins with a central pore through which molecules can pass. Peripheral proteins are associated with the phospholipid head groups on one side of the membrane only. A glycoprotein is shown with the protein portion of the molecule embedded in the membrane and the carbohydrate portion jutting out from the membrane. A glycolipid is also shown with the lipid portion embedded in the membrane and the carbohydrate portion jutting out 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, protein accounts for about 50 percent of the composition by mass, lipids (of all types) account for about 40 percent of the composition by mass, with the remaining 10 percent of the composition by mass being carbohydrates. 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 attached to lipids, forming glycolipids.

Phospholipids

The main fabric of the membrane is composed of amphiphilic, phospholipid molecules. Amphiphilic molecules such as phospholipids consist of hydrophilic or “water-loving” areas and hydrophobic, or "water-hating" areas. On a phospholipid, the three-carbon glycerol backbone attached to a phosphate-containing group (review module 3.1) composes the 'head' group of a phospholipid. The 'tail' group consists of fatty acid chains which can be either saturated or unsaturated.



The amphiphilic nature of the phospholipid is vital to the structure of a plasma membrane. In water, phospholipids spontaneously arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out. In this way, they form a lipid bilayer—a barrier composed of a double layer of phospholipids that separates the water and other materials on one side of the barrier from the water and other materials on the other side.

This phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails. The hydrophilic head group consists of a phosphate-containing group attached to a glycerol molecule. The hydrophobic tails, each containing either a saturated or an unsaturated fatty acid, are long hydrocarbon chains.
An illustration of a phospholipid shows a hydrophilic head group composed of phosphate connected to a three-carbon glycerol molecule, and two hydrophobic tails composed of long hydrocarbon chains.
Possible Discussion
In your own words, explain why phospholipids tend to spontaneously orient themselves into something resembling a membrane when placed in water.
Possible Discussion
The fact that phospholipids spontaneously form bilayers and other similar structures is a huge advantage for cells. Why? What would the effects be on the cell if this process was not spontaneous?
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 proteins (some specialized types are called integrins) 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 ([link]). 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. Some complex proteins are composed of up to 12 segments of a single protein, which are extensively folded and embedded in the membrane ([link]). 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 be attached (interact with) either to integral membrane proteins or simply interact weakly with the phospholipids within the membrane.

Integral membranes proteins may have one or more alpha-helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3). (credit: “Foobar”/Wikimedia Commons)
The left part of this illustration shows an integral membrane protein with a single alpha-helix that spans the membrane. The middle part shows a protein with several alpha-helices spanning the membrane. The right part shows a protein with two beta-sheets spanning the membrane.
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) ([link]). 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).

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, that can expand and contract; 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 bound hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. In contrast, unsaturated fatty acids do not contain a maximal number of hydrogen atoms, but they do contain some double bonds between adjacent carbon atoms; a double bond results in a bend in the string of carbons of approximately 30 degrees.

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.
This illustration shows two phospholipid bilayers side by side. On the left, there are multiple unsaturated lipids in the bilayer, on the right the bilayer is completely composed of saturated lipids.
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. A cold environment tends to compress membranes composed largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. 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.

Link to Learning

Visit this site to see animations of the fluidity and mosaic quality of membranes.

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 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.

Cholesterol fits between the phospholipid groups within the membrane.
This illustration cholesterol.
Review of the Components of the Membrane
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 (for example, 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

Why is it advantageous for the cell membrane to be fluid in nature?

The fluid characteristic of the cell membrane allows greater flexibility to the cell than it would if the membrane were rigid. It also allows the motion of membrane components, required for some types of membrane transport.

Selective Permeability

Selective permeability of the cell membrane refers to its ability to differentiate between different types of molecules, only allowing some molecules through while blocking others. Some of this selective property stems from the intrinsic diffusion rates for different molecules across a membrane. A second factor affecting the relative rates of movement of various substances across a biological membrane is activity of various protein-based membrane transporters, both passive and active, that will be discussed in more detail in subsequent sections. First we take on the notion of intrinsic rates of diffusion across the membrane.

Relative PermeabilitiesThe fact that different substances might cross a biological membrane at different rates should be relatively intuitive. There are differences in the mosaic composition of membranes in biology and differences in the sizes, flexibility and chemical properties of molecules so it stands to reason that the permeability rates vary. It is a complicated landscape. The permeability of a substance across a biological membrane can be measured experimentally and the rate of movement across a membrane reported in what are known as membrane permeability coefficients.

Membrane Permeablility CoefficientsBelow in Figure 7, a variety of compounds are plotted with respect to their membrane permeability coefficients (MPC) as measured against simple biochemical approximation of a real biological membrane. The reported permeability coefficient for this system is the rate at which simple diffusion through a membrane occurs and is reported in units of centimeters per second (cm/s). The permeability coefficient is proportional to the partition coefficient and is inversely proportional to the membrane thickness.



It is important that you are able to read and interpret the diagram below. The larger the coefficient, the more permeable the membrane is to the solute. For example, hexanoic acid is very permeable, a MPC of 0.9; acetic acid, water and ethanol have MPC between 0.01 and 0.001 (see the figure 5), they are less permeable than hexanoic acid. Where as ions, such as sodium (Na+) have an MPC of 10-12, and cross the membrane at a comparatively slow rate.

Membrane Permeability Coefficient diagram. The diagram was taken from BioWiki and can be found at http://biowiki.ucdavis.edu/Biochemistry/Lipids/Dynamics_of_Membrane_Lipids/Membrane_Permeability
MPC

While there are certain trends or chemical properties that can be roughly associated with different compound permeability (small thing go through "fast", big things "slowly", charged things not at all etc.) we caution against over-generalizing. The molecular determinants of membrane permeability are complicated and involve numerous factors including the specific composition of the membrane, temperature, ionic composition, hydration, the chemical properties of the solute, the potential chemical interactions between the solute in solution and in the membrane, the dielectric properties of materials, and the energy trade-offs associated with moving substances into and out of various environments. So, in this class, rather than try to apply "rules" we will strive to develop a general sense of some properties that can influence permeability and leave the assignment of absolute permeability to experimentally reported rates. In addition, we will also try to minimize the use of vocabulary that depends on a frame of reference. For instance, saying that compound A diffuses "quickly" or "slowly" across a bilayer only means something if the terms "quickly" or "slowly" are numerically defined or the biological context understood. More on that in the following section.

Possible Discussion
Make a list of compounds we have studied in this class thus far, sort them into "can" and "cannot" pass directly through the membrane.

Archaeal Membranes

One major difference between Archaea and either Eukaryotes or Bacteria is the 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 5 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 familier 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 phosphoglycerol, phosphoserine, phosphoethanolamine or phosphoinositol. The inherent stability and unique features of archaeal lipids makes them a useful biomarker for Archaea within environmental samples.

A second difference between bacterial and archaeal membranes that is associated with some archaea is the presence of monolayer membranes , as depicted in figure 8 and figure 9. Notice that the isoprenoid chain is attached to the glycerol backbones at both ends, forming a single molecule consisting of two polar head gropus attached via 2 isoprenoid chains.

The exterior surface of the plasma membrane is not identical to the interior surface of the same membrane.
This illustration shows general archaeal lipid.
Comparisons of different types of Archaeal lipids and bacterial/eukaryotic lipids
This illustration shows general archaeal lipid.
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?

Glossary

Peripheral membrane proteins
Proteins associated with but not embedded in the cell (plasma) membrane.
integral membrane proteins
Proteins that are a least partially embedded in the cell (plasma) membrane.
Glycolipid
A lipid molecule that also contains at least one sugar molecule attached.
Glycoprotein
A protein that has at least one sugar molecule attached.
Lipids
Nonpolar, hydrophobic molecules that include fats, oils, waxes, steroids, and the phospholipids that make up biological membranes.
Amphipathic
A molecule having both hydrophilic and hydrophobic regions.
Fatty Acids
A molecule made up of a long nonpolar hydrocarbon chain and a polar carboxyl group. Found in the lipids that comprise the membranes of bacteria and eukaryotes.
Unsaturated Fatty Acids
A fatty acid whose hydrocarbon chain contains at least one double bond between two adjacent carbon atoms.
Saturated Fatty Acids
A fatty acid whose hydrocarbon chain contains only single bonds between each carbon atom.
Triglycerides
A simple lipid in which three fatty acids are attached to one molecule of glycerol by ester linkages.
Phospholipids
A lipid containing a phosphate group. A major constituent of cellular membranes.
Steroids
A member of the family of lipids whose multiple rings share carbons; the basics structure by which steroid hormones are derived.
Cholesterol
A steroid derived compound found in all animal cell membranes.
Isoprenoids
Any molecule containing or derived from isoprene (2-methyl-1,3-butadiene) , a branched-chain unsaturated hydrocarbon. Isoprene has in fact two carbon-carbon double bonds. Isoprenoids contain from two to many thousands isoprene units. Primary unit of archaeal lipids and can be used as a biomarker for archaea.