16.1: Components and structure

A cellʼs plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment. Cells exclude some substances, take in others, and excrete still others, all in controlled quantities. The plasma membrane has many significant functions:

• It must be very flexible to allow certain cells, such as red and white blood cells, to change shape as they pass through narrow capillaries.
• It carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the immune responseʼs “self” versus “nonself” distinction.
• It functions as a medium for complex, integral proteins, and receptors to transmit signals. These proteins act both as extracellular input receivers and as intracellular processing activators.
• It imparts selective permeability essential for maintaining a resting membrane potential and osmoregulation.

Fluid mosaic model

The fluid mosaic model describes the plasma membrane structure 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 (figure 16.1). The protein, lipid, and carbohydrate proportions in the plasma membrane vary with cell type.

Figure 16.1: Schematic of the cell membrane. Plasma membranes range from 5 to 10 nm in thickness.

Lipids

The membraneʼs main fabric comprises amphiphilic, phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules (figure 16.2) are in contact with the aqueous fluid both inside and outside the cell. Hydrophobic or "water-hating" molecules tend to be nonpolar. They interact with other nonpolar molecules in chemical reactions, but generally do not interact with polar molecules. A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached. This arrangement gives the overall molecule a head area (the phosphatecontaining group), which has a polar character or negative charge, and a tail area (the fatty acids), which has no charge. The head can form hydrogen bonds, but the tail cannot (figure 16.2).

Figure 16.2: Structure of a phospholipid.

Cholesterol, another lipid comprised of four fused carbon rings, is situated alongside the phospholipids in the membraneʼs core (figure 16.2).

Specific phospholipids play key roles in the membrane; phosphatidylcholine, serine, inositol, and ethanolamine (figure 16.3) play various roles in the membrane.

Figure 16.3: Important membrane lipids.

• Phoshatidylcholine is primarily found on the outer leaflet, while phosphatidylserine, inositol, and ethanolamine are all located predominately on the inner leaflet.
• Both phosphatidylserine and inositol are negatively charged at physiological pH allowing for binding of positively charged molecules.
• Sphingolipids and glycolipids are less abundant and play key roles in nervous tissue membranes. Disorders of sphingolipid or glycolipid metabolism are often severe (e.g., Tay-Sachs or Neimann-Pick disease).

Proteins

Proteins comprise the plasma membrane's second major component. Integral proteins, or integrins, as their name suggests, integrate completely into the membrane structure, and their hydrophobic membranespanning regions interact with the phospholipid bilayerʼs hydrophobic region.

Peripheral proteins are on the membrane's exterior and interior surfaces, attached either to integral proteins or to phospholipids. Peripheral proteins, along with integral proteins, may serve as enzymes, as structural attachments for the cytoskeletonʼs fibers, or as part of the cellʼs recognition sites. Scientists sometimes refer to these as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens. Additional proteins can be lipid anchored on the exterior of the membrane.

Carbohydrates

Carbohydrates are the third major plasma membrane component. They are always on the cell's exterior surface and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of two to sixty 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. We collectively refer to these carbohydrates on the cellʼs exterior surface — the carbohydrate components of both glycoproteins and glycolipids — as the glycocalyx (meaning “sugar coating”). The glycocalyx is highly hydrophilic and attracts large amounts of water to the cellʼs surface. This aids in the cellʼs interaction with its watery environment and in the cellʼs ability to obtain substances dissolved in the water.

Membrane fluidity

The integral proteins and lipids exist in the membrane as separate but loosely attached molecules. The membraneʼs mosaic characteristics 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.

Temperature can also influence membrane rigidity. Decreasing temperatures compress saturated fatty acids with their straight tails, and they press in on each other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the “kinks” in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This “elbow room” helps maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phospholipids would “freeze” or solidify.

References and resources

Text

Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 4: Cell Structure, Chapter 5: Structure and Function of the Plasma Membranes.

Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 4: The Structure and Function of the Plasma Membrane.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 49.

Figures

Grey, Kindred, Figure 16.1 Schematic of the cell membrane. Plasma membranes range from 5 to 10 nm in thickness. 2021. https://archive.org/details/16.1_20210926. CC BY 4.0. Added Cell membrane detailed diagram blank by LadyofHats. Public domain. From Wikimedia Commons.

Grey, Kindred, Figure 16.2 Structure of a phospholipid. 2021. Chemical structure by Henry Jakubowski. https://archive.org/details/16.2_20210926. CC BY-SA 4.0. Added Cell membrane detailed diagram 4 vi by P.T.Đ. CC BY-SA 4.0. From Wikimedia Commons.

Grey, Kindred, Figure 16.3 Important membrane lipids. 2021. Chemical structure by Henry Jakubowski. https://archive.org/details/16.3_20210926. CC BY 4.0.