Plasma Membrane Structure

The plasma membrane is a dynamic, fluid double-membraned structure that forms the external boundary around the cytosol of cells. The plasma membrane, also known as the plasmalemma or the cell membrane, is selectively permeable, regulates the molecular traffic across the boundary of the cells. In plant cells and bacterial cells, a cell wall is present on the exterior of the plasma membrane. It also participates in cell signalling and adhesion, maintains a stable internal environment, thus providing overall integrity of the cell.

Composition of Plasma Membrane

Plasma membranes are composed of proteins and lipids, but the protein-to-lipid ratio varies among cell types. In eukaryotic cells, carbohydrates are abundant in the plasma membrane, but either remain bound to proteins (as glycoproteins) or lipids (as glycolipids).

Lipids

Three classes of lipid molecules are present in the plasma membrane

Phospholipids: Form the main fabric of the plasma membrane. Each of these phospholipid molecules has a phosphorus head group attached to it.

Glycolipids: Present in the outer leaflet of the plasma membrane, with the carbohydrate portions exposed on the cell surface.

Sterol: Cholesterol (in animal cells), Stigmasterol and sitosterol (in plants, bacteria cells) provide rigidity to the plasma membrane.

Proteins

Membrane proteins take part in various dynamic processes and are classified into two groups.

Peripheral Protein: The proteins associated with the membrane that can be released by gentle extraction methods, keeping the lipid bilayer intact. Example: Ankyrin, etc.

Integral Protein: These are mostly transmembrane proteins that are held very tightly in the lipid bilayer and cannot be released by gentle extraction methods. Example: Glycophorin, etc.

Carbohydrates

Glycolipids and glycoproteins are mostly involved in cell-to-cell recognition.

Structure of Plasma Membrane

In 1935, scientists Davson and Danielli proposed a model to explain the structure of the plasma membrane. This early ‘Sandwich Model’ suggested that the phospholipids form a lipid bilayer with the hydrophilic heads facing outward and the hydrophobic tails inward. Two protein layers cover the hydrophilic ends of the lipids, resulting in a P-L-L-P form (Lipid bilayer sandwiched between two protein layers). This model is obliterated and is replaced by the Fluid Mosaic Model.

Fluid Mosaic Model

In 1972, scientists S.J. Singer and G.L. Nicolson proposed the ‘Fluid Mosaic model’ of the plasma membrane, which emphasises the quasifluid nature of the membrane and presents it as a mosaic of various components. The model depicts the plasma membrane mainly as a structure composed mainly of the phospholipid bilayer, where proteins, sterols, and carbohydrates are embedded.

Diagrammatic representation of the Fluid Mosaic Model

Lipid Bilayer

Two leaflets of amphiphilic phospholipid molecules are the major components.
One phospholipid molecule comprises a glycerol backbone along with 2 fatty acid molecules associated (comprising the tail), and one phosphate-containing group (part of the head),
Polar head groups are hydrophilic and remain in contact with the intracellular or extracellular aqueous phase. The presence of phosphate imparts a negative charge.
Non-polar tails constitute the hydrophobic interior of the plasma membrane and are neutral.
Phospholipids form a lipid bilayer and separate the extracellular and intracellular fluids.
In animal cells, certain phospholipids like phosphatidylcholine, phosphatidylserine, and sphingomyelin comprise the framework of membranes and are stabilised by cholesterol.
These phospholipids are asymmetrically distributed between the two leaflets of the bilayer- the inner leaflet consists of phosphatidylserine and phosphatidylethanolamine predominantly, while the outer leaflet contains sphingomyelin and phosphatidylcholine.
Lipid composition and temperature determine the fluidity of the bilayer; an increase in temperature tends to increase membrane fluidity, and a lower temperature results in the gel-like organisation.

Membrane Proteins

The second major component of the plasma membrane.
The peripheral proteins, also known as the extrinsic proteins, remain bound to the hydrophilic head of the lipid bilayer or the integral proteins by electrostatic and hydrogen bonds. They are of the lipid bilayer. They are mostly soluble in water.
The integral proteins, also known as intrinsic proteins, integrate completely into the lipid bilayer; few of these are linked with one layer and pass through only a portion of the membrane, whereas other integral proteins span from one side of the membrane to another side (transmembrane proteins).
All transmembrane proteins are amphipathic. They might be

Single-pass proteins: They typically have a hydrophobic transmembrane portion consisting of 20-25 amino acid residues.

Multi-pass proteins: They contain two or more transmembrane domains that span the lipid bilayer. The coiling of these proteins into α- helix facilitates the spanning through the bilayer.

On the basis of functions, membrane proteins can also be classified as structural proteins, catalytic proteins, transport proteins, etc. Transport proteins are discussed in brief.

Channel proteins always mediate passive transport (down the concentration or electrochemical gradient of the solute), but carrier protein-mediated transport might be active or passive; thus, transport through channel proteins occurs at a faster rate.

Co- transporter

Carrier proteins that transfer two solutes simultaneously through the cell membrane.

Symporter: The co-transporter proteins involve the simultaneous transfer of two solutes in the same direction.

Antiporter: The co-transporter proteins involve the simultaneous transfer of two solutes in the opposite direction.

Uniporter

Carrier proteins that carry a single solute from one side of the membrane to the other.

Diagrammatic representation of carrier proteins

Leaky Channels

Non-gated channel proteins that remain perpetually open, regardless of any signal or stimuli, enable ions to move across the membrane down their gradients, ensuring a steady flow of ions.

Gated Channels

Channel proteins that only open in response to some specific signal.

Representation of a gated channel

Ligand-Gated Channels: Binding of a ligand opens the protein.

Mechanical Gated Channels: Mechanical stress opens the protein.

Voltage-Gated Channels: A Change in voltage across the membrane opens the protein.

Carbohydrates

Carbohydrates are mostly present towards the exterior of the membrane and form glycolipids or glycoproteins, remaining attached to lipids and proteins, respectively. When present on the exterior surface of the cells, glycolipids and glycoproteins together are known as glycocalyx, which is extremely hydrophilic and attracts huge quantities of water towards the cell surface. Glycocalyx helps the cell to acquire any substance dissolved in water and to be in constant interaction with its fluid-like environment.

Frequently Asked Questions (FAQs)

Q1. Is the plasma membrane permeable?

The plasma membrane is selectively permeable. The lipid layer allows the hydrophobic molecules and small polar molecules to diffuse, but large polar molecules are not allowed to diffuse through the membrane.

Q2. How can a plasma membrane get damaged?

Pathogens (bacteria, viruses, and fungi), specific protein aggregates or chemicals can sometimes provoke damage to the plasma membrane.