What is the Cell Membrane?

The cell membrane is known by different names like plasma membrane or cytoplasmic membrane, or biological membrane. The term "cell membrane" was first introduced by C. Nageli and C. Cramer in the year 1855. Later on, in 1931, the term "plasmalemma" for cell membrane was given by J. Plowe. The cell membrane separates the cell's internal environment from the extracellular space. This separation allows the protection of cells from their environment. 

The cell membrane is covered by an external layer known as the "cell wall" in some cells like prokaryotic cells, plant cells, and fungal cells. The cell membrane is present in all cells starting from the old prokaryotic cells to the new eukaryotic cells. The plasma membrane is primarily made up of lipids, proteins, and carbohydrates. It is 7.5-10 nm in thickness. 

Properties of the Cell Membrane

• The major constituents of the cell membrane are lipids, carbohydrates, and proteins.
• The cell membrane is semi-permeable.
• It is alive and is metabolically active.
• It is present in all cells.
• It is very thin and is visible only through an electron microscope.

The Function of the Cell Membrane

The cell membrane performs numerous functions in the cells:

• It maintains the integrity of the cell.
• It keeps the cell safe from the external environment.
• It allows entry of only selected substances through it and thus helps in cellular transport. 
• It plays an instrumental role in cell signaling and cell communication.
• It maintains the shape and structure of the cell.
• It also plays an important role in cell motility.
• The sexual as well as asexual reproductions take place with the help of the cell membrane.

Structure of the Plasma Membrane 

Many scientists and researchers worldwide gave many theories and models to describe the structure of the plasma membrane. The three main models are:

1. Sandwich model
2. Unit membrane model
3. Fluid mosaic model

Sandwich Model

Hugh Davson and James Danielli gave this model of the plasma membrane in the year 1935. This model is known as the sandwich or paucimolecular model. 

According to this model:

• The membrane surrounding the cytoplasm is composed of a lipid bilayer. One layer of bilayer faces the cell's interior, and another layer would face the cell's external environment. 
• The lipid bilayer is coated with globular proteins on both sides.
• The lipid layer is associated with globular proteins by the electrostatic interactions between charged amino acids of a globular protein and the polar group present in the lipid.
• Sometimes Van der Waals interaction is also responsible for the association of globular protein and lipids.
• This model is also known as the lipid-protein sandwich.

Limitations of the sandwich model

• According to this model, the plasma membrane is symmetrical in shape. It is not bifacial. 
• This model does not account for the permeability of the plasma membrane.
• The solidification of the plasma membrane was also not supported by this model.

Unit Membrane Model

Robertson gave this model of plasma membrane structure in the year 1953. Robertson shows that when the plasma membrane fixed with osmium tetroxide is observed under the electron microscope, it appears to be tri-laminar in structure. The tri-laminar structure looks like two parallel outer darker layers and one central layer that is light in appearance. The characteristics of the unit membrane model are as follows:

• The unit membrane model states that the plasma membrane has a bimolecular lipid leaflet sandwiched between the outer and inner layers of protein that ars structured in the pleated sheet configuration.
• This type of membrane structure is present in many cellular organelles.
• The organelle is a structure that performs one or more specific functions, much like an organ does in the human body.

Drawbacks of the unit membrane model

This model was not able to explain the uniformity in the feature of all biological membranes. The membrane covering the mitochondria and endoplasmic reticulum was different in structure from the plasma membrane, as shown by F.S. Sjöstrand in the year 1963.

Fluid Mosaic Model 

This model is the most widely accepted model that describes the structure of the plasma membrane. S.J. Singer and G. Nicolson gave it in the year 1972. The fluid mosaic model has the following characteristics:

• According to this model, the cell membrane comprises four components: phospholipids, proteins, carbohydrates, and cholesterol.
• These components form a mobile mosaic in the fluid-like environment created by a sea of phospholipids.
• This model suggests that proteins, carbohydrates, cholesterol are incorporated into the plasma membrane rather than present as sheets, as stated by the previous model of the cell membrane.
• The fluid property of the biological membrane was determined through experiments like X-ray and calorimetry. 
• The fluidity of the cell membrane was attributed to the lipid bilayer, as explained by this model.
• The phospholipid layer has a hydrophobic or water-fearing tail, whereas its head is water-loving. 
• As the plasma membrane is mostly lipid, lipid-soluble substances could easily pass through the membrane compared to water-loving substances. 

Proteins Present in the Cell Membrane

The proteins found in the cell membrane are classified into three main types:

1. Integral proteins 

These types of proteins are embedded in the lipid bilayer. They are found to have hydrophobic interactions with the hydrophobic tails of lipids that make the cell membrane structure stable. The integral proteins are also referred to as intrinsic proteins. Band 3 protein is an example of an intrinsic protein. It consists of 14 transmembrane segments and is present in the plasma membrane of the red blood cells. 

2. Peripheral proteins 

These proteins are found in the exterior of the cell membrane. They make hydrophilic interactions with the polar heads of the plasma membrane. 

3. Lipid anchored proteins

These proteins are also present at the plasma membrane surface but are involved in the covalent interaction with the lipid bilayer.

These membrane-spanning proteins are present thought the membrane and assist in the transport of ions and molecules across the biological membrane. The membrane-bound proteins are those proteins that are attached to the plasma membrane. 

The Function of Cholesterol in the Membrane

The cholesterol is mostly found in the cell membrane of animals but is absent in the cell membrane of plants and prokaryotes, in which the cell walls protect the cell from the external environment. It is responsible for giving strength and flexibility and makes the membrane selectively permeable so that only limited molecules and ions could enter the cell. Cholesterol also accounts for the optimum fluidity of the cell membrane. 

Role of the Plasma Membrane in Cellular Transport 

The plasma membrane is semi-permeable and allows an easy passage to lipid-soluble substances. The movement of substances across the membrane takes place from high concentration to low concentration. The red blood cell membrane plays an important role in the uptake of nutrients and oxygen. Similarly, the plant cell membrane serves as an anchor point for the cytoskeleton. 

Context and Application

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors of Science in Biology
• Bachelors of Science in Zoology
• Master of Science in Zoology