Plasma Membrane: Structure and Function

Introduction

All cells are separated from its surrounding by a thin, flexible and fragile structure called a plasma membrane. It is also known as the cell membrane. Ernst Overton first studied its lipid nature in the 1890s. E. Gorter and F. Grendel, 1925 first proposed that the cellular membranes might contain a lipid bilayer [1]. In the 1920s and 1930s, cell physiologists obtained evidence that it is more than simply a lipid bilayer. In 1935 Danielle and Davson proposed a model in which proteins are attached to the membrane lipid at the lipid-aqueous interface. They described the plasma membrane as “lipoids” [2]. Detailed studies have been done since the invention of the electron microscope. A Series of studies on plasma membrane finally led to the most widely accepted model proposed by Singer and Nicolson in 1972. This model is known as the fluid mosaic model [3].

Membrane Structure and Composition

A plasma membrane is a lipid-protein assembly where the components are arranged in a thin sheet, held together by non-covalent interactions. The lipids are amphipathic; it consists of a hydrophilic part (water-soluble), and the other part is hydrophobic (water-insoluble). In the plasma membrane, the lipid molecules aggregate to minimize the water contact by the – hydrophobic end. The hydrophilic end is arranged towards the interior and exterior of the cell. Thus, the lipids form a bi-layered structure where each face (outer and inner) is called the leaflet. Besides lipids, proteins and carbohydrates are also present.

Membrane Lipids

Three main kinds of lipids (glycerides) are present in the membrane. These are:

  1. Phosphoglycerides
  2. Sphingolipids
  3. Cholesterol  

Phosphoglycerides

The presence of a phosphate group in the lipid molecule make it phospholipid. Because most lipids have a glycerol backbone, it is also called phosphoglycerides. These are diglycerides—only two of the hydroxyl groups of the glycerol are esterified to fatty acids, and the third is esterified to a hydrophilic phosphate group. The phosphoglycerides include phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI). The head group of these lipids is highly hydrophilic (water-soluble).

On the other hand, the fatty acyl chains are hydrophobic, unbranched hydrocarbons approximately 16 to 22 carbons in length. The fatty acyl chain may be saturated or unsaturated. Sometimes, phosphoglycerides may contain a saturated fatty acyl chain and another unsaturated fatty acyl chain.

Sphingolipids

It is sphingosine derived and a relatively less abundant class of lipid. Sphingosine is amino alcohol containing a long hydrocarbon chain. The sphingolipids consist of sphingosine linked to a fatty acid by its amino group. This molecule is a ceramide. Various sphingosine-based lipids have – additional groups esterified to the terminal alcohol of the sphingosine moiety. If the substitution is a carbohydrate, the molecule is a glycolipid which may be a cerebroside (simple sugar substitution) or a ganglioside (substitution of a cluster of sugars like sialic acid). Sphingolipids are similar to phosphoglycerides. However, its fatty acyl chain is longer and more saturated.

Cholesterol

It is yet another lipid of the membrane. It may constitute up to 50 percent of all lipids in some animals. Its hydrophilic hydroxyl group is arranged towards the membrane surface, and the remaining portion is embedded in the lipid bilayer.

Significance of Lipid Bilayer

The fatty acyl chains of both leaflets of the lipid bilayer span a width of about 30˚A, and each row of head groups is 15˚A thick. Thus, the entire lipid bilayer is only about 60˚A (6 nm) thick. The hydrocarbon chains are never exposed to the surrounding aqueous solution. As the lipid bilayer is flexible, the overall shape of the membrane can change during locomotion and cell division. Fusion and fission are other attributes of membrane flexibility. The lipid bilayer maintains the internal composition of the cell/organelle. The lipids present in the membrane are distributed asymmetrically. Further, the membrane of different cells has slightly different lipid compositions.

Membrane Sugar

The eukaryotic plasma membrane contains 2-10 percent (w/w) carbohydrates. Ninety percent of these carbohydrates are attached to the membrane protein, forming glycoprotein. The rest are attached to the lipids forming glycolipid. All of the sugar molecules of the membrane face the extracellular side or the luminal side of the organelle. The carbohydrates of glycoprotein are present as branched oligosaccharides. The sugar molecule is attached to the protein through an N-glycosidic or O-glycosidic bond.

Significance of Membrane Sugar

The carbohydrate projections from the cell membrane mediate the interaction of the cell with the surrounding or with the other cell. It helps short the newly synthesized protein to a particular cell location. The glycolipids on the RBC membrane determine a person’s blood type (A, B, AB, or O).

Membrane Protein

Different types of cells contain hundreds of proteins in/on their membrane. Each of these proteins has a defined orientation to the cytoplasm. The asymmetrical distribution of proteins is called “sidedness.” Membrane proteins are grouped into three categories. These are:

  1. Integral proteins 
  2. Peripheral proteins 
  3. Lipid-anchored proteins

Integral Proteins

These proteins pass through the membrane from the cytoplasmic to the extracellular side. They have a hydrophobic domain spanning the lipid bilayer once or several times and hydrophilic domains protruding from the lipid bilayer on both sides. The integral proteins can function as a receptor, transporter, or electron carrier.

Peripheral Proteins

The peripheral proteins are associated with the membrane through weak electrostatic bonds. They are entirely located on the lipid bilayer on either side. Depending on the prevalent condition, these proteins form a dynamic relationship with the membrane, recruited to the membrane or released from it. The cytoskeletal proteins form a fibrillar network in the cytoplasm and are attached to the cytoplasmic face of the membrane. The peripheral proteins on the cytoplasmic side of the lipid bilayer function as a signal transducer in the cell or may act as an enzyme.

Lipid-anchored proteins

The lipid anchored proteins are located on either side of the lipid bilayer and are covalently attached to a lipid. The lipid can be phosphatidylinositol, a fatty acid, or a phenyl group to which protein is linked (ex: GPI-anchored protein).

Functions of the Plasma Membrane

  • The plasma membrane forms a boundary that separates the cell contents from the surrounding.
  • It also functions as an insulating layer that conserves heat produced in the cell.
  • Plasma membrane gives dynamic shape to the cell; otherwise, movement is impossible.
  • It forms different compartments (organelle) inside the cell. These organelles provide an additional – chemical environment within a cell for the occurrence of diverse reactions.
  • The plasma membrane also facilitates cell fusion and fission.
  • The plasma membrane also hosts many chemical reactions which occur within the membrane, or the reaction is facilitated by an enzyme attached to the membrane.
  • It provides a barrier to the free passage of ions/molecules across it.

Refrences

[1]. Cooper, G. M. “The Cell: A Molecular Approach 2nd edition Boston University.” Sunderland (MA): Sinauer Associates. (2000).

[2]. Danielli, James Frederic, and Hugh Davson. “A contribution to the theory of permeability of thin films.” Journal of cellular and comparative physiology 5.4 (1935): 495-508.

[3]. Singer, S. Jonathan, and Garth L. Nicolson. “The Fluid Mosaic Model of the Structure of Cell Membranes: Cell membranes are viewed as two-dimensional solutions of oriented globular proteins and lipids.” Science 175.4023 (1972): 720-731.