Plasma membrane: concept, structure and functions (2023)

In this article we will talk about:- 1.Importance of plasma membrane 2.Structure of the plasma membrane 3. Functions.

Concept of plasma membrane:

The membrane that encloses a cell is called the cell membrane or plasma membrane (animal cells) and plasmalemma (plant cells). It contains proteins and lipids in a ratio of 80 : 20 in bacteria at one end and 20 : 80 in some nerve cells at the other end. The overall composition of most cell membranes is 40-50% protein and 50-60% lipid. both components vary in composition.

There are three types of lipids in membranes:

(1) phospholipids;

(2) glycolipids, and

(3) Steroids.

They are further classified into different types (fig. 2.6). The proportion of these lipids varies in different membranes. For example, the plasma membrane consists of 55% phospholipids. 5% glycolipids, 20% steroids and 20% other lipids.

But the endoplasmic reticulum contains 65% phospholipids, 30% glycolipids and 5% steroids. The percentage of these types of lipids in mitochondrial membranes is 75% (phospholipids), 20% (glycolipids) and 5% (steroids).

The bacterial membrane limits the high content of cholesterol (70%) and a smaller content of phospholipids (30%). The different types of phospholipids found in biological membranes are summarized in Table 2.3.

Structure of the plasma membrane:

Electron microscopic studies have shown that the 7-8 nm thick plasma membrane has two regions of electron density separated by a central region of electron light (Fig. 2.7). These three layers are called together"trilaminair".Robertson called them "membrane of unity". define it"membrane unit case"according to which all biological membranes have a "three-layer" organization.

The most widely accepted model of the plasma membrane is the"liquid mosaic model"proposed in 1972 by Singer and Nicholson.

According to this model, the membrane is composed of lipids and proteins organized as follows (Fig. 2.7):

(a) Two monolayers of lipid molecules form a lipid bilayer.

(b) Protein molecules are embedded in the lipid bilayer. The lipid bilayer is liquid and the lipid molecules are in a "liquid crystal" state, i.e. they are not fixed in place and at the same time they are not free to move. Membrane protein and lipids can both move laterally within the lipid bilayer.

1. Lipid bilayer:

The lipid bilayer consists of two lipid layers, each layer is one molecule thick. This organization is common to all biological membranes, but there are notable differences in the specific types of lipids present. Each lipid molecule has a "hydrophilic" head and one or two "hydrophobic" tails, which form"ambipathetic"molecules.

The hydrophilic ends of the lipid molecules are oriented toward the outside of the cell membrane, while their hydrophobic tails are oriented inward, with the latter forming the inner hydrophobic region of the membrane (Fig. 2.7). The tails of lipid molecules are composed of fatty acids (fig. 2.8), both saturated and unsaturated fatty acids can be present.

In the myelin membrane, unsaturated fatty acids make up less than 10%, while in the membranes of mitochondria and chloroplasts, unsaturated fatty acids make up more than 50% of the fatty acids. The tails of saturated fatty acids stretch freely, but those of the unsaturated chain bend at the double bond.

2. Membrane proteins:

In general, the ratio of lipids and proteins is equal (each about 50%) in biological membranes, but organic membranes contain a high content (75-80%) of proteins. Incorporated proteins are embedded in the lipid bilayer and can move laterally within the bilayer.

The region (domain) of the protein molecule that is inside the lipid bilayer is "hydrophobic," while that outside the bilayer is "hydrophilic."

Protein molecules that pass through the lipid bilayer and are exposed on both sides of the lipid bilayer are called transmembranes (Fig. 2.9.). Transmembrane proteins have one or more domains containing 21-26 hydrophobic amino acids coiled into an α-helix.

Membrane proteins are of different types in terms of their organization within the membrane:

(i) Proteins with a single membrane spanning domain (hydrophobic domain) and

(ii) Proteins with domains spanning multiple membranes.

Proteins with a single membrane domain: there are two types:

(ONE)Group I Proteins:

Group I proteins are proteins whose N-terminus is exposed to the outside of the cell, while the C-terminus is exposed to the cytoplasm (Fig. 2.9).

(SI)Group II Proteins:

In such proteins, the C-terminus is exposed to the outside of the cell, while the N-terminus is exposed to the cytoplasm (Fig. 2.9). Such proteins are less common.

Proteins with domains that span multiple membranes (hydrophobic domains): There are two types:

(a) Proteins with an "odd" number of hydrophobic domains: in such proteins, the N-terminal and C-terminal domains are located on different sides of the membrane (Fig. 2.9).

(b) Proteins with an 'even' number of hydrophobic domains: these proteins have both their N-terminus and C-terminus on the same side of the lipid bilayer (Fig. 2.9)

Lipids are synthesized in the ER and transported to the cytoplasmic surface of the membrane, from where they are transferred to the outer monolayer of the lipid bilayer. The protein involved in this movement is called flippase.

The outer surface of the membrane is rich in carbohydrate groups such as glycoproteins or glycolipids (Fig. 2.9). In contrast, the inner surface (cytoplasmic surface) is negatively charged (-) due to the predominance of unsaturated fatty acid chains in the lipid molecules that make up the inner monolayer.

Thus, there is an asymmetry in the organization of the lipid bilayer of the plasma membrane. An important property of the plasma membrane is that it can produce "vesicles" through a budding process. Vesicles can fuse with the membrane through the reverse process.

Functions of the plasma membrane:

In addition to enclosing the cell and protecting it from the external environment, the plasma membrane has many important functions such as regulating the movement of materials in and out of the cell, metabolic functions, communication between different cells, and adhesion between cells.

1. Movement of materials:

The movement (import and export) of materials is done through different mechanisms, for example:

(a) Simple diffusion,

(b) facilitated diffusion;

(c) Active transportation and

(d) Endocytosis to exocytosis.

(ONE)Easy to spread:

Simple diffusion refers to the unassisted movement of a substance from an area of ​​higher concentration to an area of ​​lower concentration until equilibrium is reached. Some solutes diffuse across the plasma membrane more easily than others.

Therefore, the plasma membrane is referred to as a selectively permeable or differentially permeable membrane. When water molecules move through a differentially permeable membrane from a lower to a higher concentration of solutes, the process is called osmosis.

(SI)Facilitated diffusion:

It is similar to simple diffusion, but the rate of solute movement is increased by interaction with specific membrane transporters. Transporters are "transmembrane proteins".

(DO)Active transport:

It is the mechanism by which the movement of dissolved substances takes place in one direction (unidirectional), i.e. from lower to higher concentration. This is a process that requires energy. Energy is obtained from the hydrolysis of ATP and from other sources.

(HEY)Endocytosis to exocytosis:

Certain substances enter the cell or are removed from the cell via membrane 'vesicles'. Absorption of external substances through vesicles is called endocytosis while excretion of substances through vesicles is called exocytosis.

Endocytosis is divided into two types. The uptake of large particles by vesicles is called phagocytosis, while the uptake of small particles and water-soluble molecules, such as enzymes, hormones, antibiotics, etc., is called pinocytosis (Fig. 2.10).

The extracellular substance taken into the cell through endocytosis is called a ligand. The ligand binds to specific receptors, i.e. transmembrane proteins, present in the membrane. This causes the formation of intracellular vesicles, the process called "internalization" of the receptor.

Some special proteins called envelope proteins are attached to the plasma membrane on the cytoplasmic side. then the membrane begins to deform and collapse (Fig. 2.10).

Coat proteins surround the vaginal membrane. Finally, a vesicle is formed containing the extracellular substance. The vesicle is surrounded by two types of proteins: (i) adapter and (ii) calthrin. Such vesicles are called"calthrin-coated vesicles".Adapters bind to vesicle integral membrane proteins and calthrin (Fig. 2.10).

There are different types of adapters in vesicles of different origin. For example, intracellular vesicles formed from plasma membrane have an HA2 adapter, whereas vesicles produced from the Golgi complex have an HA1 adapter. These adapters differ in their composition. The HA1 adapter consists of γ-adapter, β-adapter P47 and P20. The HA2 adapter consists of α-adaptor, β-adaptin, P50 and P17.

Calthrin forms the outer layer of vesicles in the form of a polyhedral layer. Calthrin is a protein complex called a "triskelion" consisting of 3 light and 3 heavy chains. Each light chain has a molecular weight ranging from 30,000 to 40,000 daltons. The molecular weight of each heavy chain is 180,000 daltons.

When the coated vesicle reaches the target membrane, the protein layer is removed. The vesicle then fuses with the target membrane and releases the contents. The vesicle without the fur is called an endosome.

Exocytosis is the reverse process of endocytosis (Fig. 2.10). The substance to be secreted is enclosed in a vesicle in the Trans region of the Golgi complex. This vesicle is called"extracellular"of"secretary"vesicle; it is also covered with specific carbohydrate proteins (Fig. 2.10).

When the vesicle reaches the plasma membrane, it becomes uncoated and eventually fuses with the membrane. As a result, the substance contained in the vesicle is expelled out of the cell.

Thus, there are three methods of physical transport of materials (ranging from ions to small molecules and macromolecules) out of the cell:

(I)Through channels:

Channels are made of transmembrane proteins. Ions are transported through this process. There are separate channels for K⁺, al⁺, in Ca²⁺enz.

(ii)From the receiver itself:

The ligand, like sugars, binds to the receptor and is transferred from the extracellular side to the cytoplasmic side of the membrane.

(iii)Receptor Internalization:

The ligand binds to the receptor triggering the internalization process. The vesicle is formed by endocytosis and the ligand is taken into the cell.

2. Metabolic functions:

The plasma membrane plays an important role in metabolism. Several enzymes are found on the cell surface, such as those involved in the extracellular breakdown of nutrients and those involved in cell wall biosynthesis. In prokaryotes, respiratory enzymes are located in the plasma membrane.

3. Communication recognition and attachment:

Some important functions of the plasma membrane are cell-to-cell communication, cell-to-cell recognition, and adhesion. Such functions are performed by "receptors" which are transmembrane or integral proteins.

The extracellular substance, called"connector"binds to specific receptors. This binding causes a change in membrane function. It can transmit signal to the cytoplasm, the phenomenon is called "signal transduction".

There are two types of signal switching:

(i) When a ligand binds to the receptor (a transmembrane protein), it activates the kinase activity of the receptor's cytoplasmic domain, leading to its phosphorylation. The phosphorylated receptor binds to the target protein in the cytoplasm.

(ii) Ligand receptor binding can activate the plasma membrane-associated G protein. G proteins are tripartite proteins that bind guanine nucleotides and are composed of α (monomer) and βγ (dimer) subunits. The G protein is inactive when the trimer (αβγ) binds to GDP.

Upon activation, GDP (bound to the α subunit) is replaced by GTP and the G protein is cleaved into the α and βγ dimer subunits. Then one of the active subunits (either α or βγ) acts on target proteins in the cytoplasm. Activates or suppresses target proteins.

Different cells may have different receptors and therefore may respond to different signals. One type of receptor may respond to protein hormones, another type of receptor responds to neurotransmitters (eg, acetylcholine), while another type of receptor responds to antigens, etc.

The formation of tissues and organs in multicellular organisms occurs when cells connect to each other in specific ways. Glycoproteins are known to be involved in cell-to-cell recognition and adhesion. Membrane junctions are formed in animal cells for various functions. "Tight junctions" prevent the movement of molecules through the spaces between neighboring cells.

Desmosomes (specialized parts of the cell surface that serve to connect the surface to another structure) provide mechanical strength to hold cells together in conditions where tissues are subjected to forces that lead to stretching.

"Gap-crossings"they occur in both vertebrates and invertebrates, especially in tissues that require rapid communication between cells, e.g. nerve cells, muscles, etc. They allow small molecules to move from one cell to another.