Plasma Membrane: Meaning, Structure and Functions

In this article we will discuss about:- 1. Meaning of Plasma Membrane 2. Structure of Plasma Membrane 3. Functions.

Meaning of Plasma Membrane:

The membrane enclosing a cell is called cell membrane or plasma membrane (animal cells) and plasma lemma (plant cells). It contains proteins and lipids in the ratio of 80 : 20 in bacteria on one extreme and on the other extreme 20 : 80 in some nerve cells. The over all composition of most of the cell membranes is 40-50% protein and 50-60% lipids; both the components vary in their composition.

The lipids found in membranes are of three kinds:

(1) Phospholipids,

(2) Glycolipids, and

(3) Steroids.

They have been further classified into different types (Fig. 2.6). The proportion of these lipids varies in different membranes. For example, plasma membrane is composed of 55% phospholipids. 5% glycolipids, 20% steroids and 20% other lipids.

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

Bacterial membrane constrains a high proportion of cholesterol (70%) and a lesser proportion of phospholipids (30%). The different types of phospholipids found in the biological membranes are summarised in Table 2.3.

Structure of Plasma Membrane:

Electron microscopic studies have revealed that the 7-8 nm thick plasma membrane has two electron dense regions separated by an electron light central region (Fig. 2.7). These three layers together are called “trilaminar”. Robertson termed them as “unit membrane”; he proposed the “unit membrane hypothesis” according to which all the biological membranes have “trilaminar” organisation.

The most widely accepted model of plasma membrane is the “fluid mosaic model” which was proposed by Singer and Nicholson in 1972.

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) The protein molecules are embedded within the lipid bilayer. The lipid bilayer is a fluid and the lipid molecules are in a state of “liquid crystal” i.e., they are not fixed on a position and at the same time, they are not free to move. The membrane protein and lipids, both can have lateral movement within the lipid bilayer.

1. Lipid Bilayer:

The lipid bilayer is made up of two lipid layers, each layer being one molecule thick. This organisation is common to all biological membranes, but there are notable differences in the particular kinds of lipids present. Each lipid molecule has a ‘hydrophilic’ head and one or two ‘hydrophobic’ tails, making them “amphipathic” molecules.

The hydrophilic ends of the lipid molecules are oriented toward the outside of the membrane of the cell, while their hydrophobic tails are oriented inward, the latter constitute the interior hydrophobic region of the membrane (Fig. 2.7). The tails of the lipid molecules are made up of fatty acids (Fig. 2.8), both saturated and unsaturated fatty acids may be present.

In myelin membrane, unsaturated fatty acids constitute less than 10%, while in the mitochondrial and chloroplast membranes, unsaturated fatty acids make up more than 50% of the fatly acids. Tails of saturated fatty acids extend 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 (about 50% each) in the biological membranes but the organellar membranes contain a high proportion (75-80%) of proteins. Integral proteins are embedded within the lipid bilayer, and they can move laterally within the bilayer.

The region (domain) of the protein molecule lying within the lipid bilayer is “hydrophobic” while that lying out side the bilayer is “hydrophilic”.

The protein molecules that pass through the lipid bilayer and are exposed on both the sides of the lipid bilayer are called trans-membrane (Fig. 2.9.). Trans­-membrane proteins have one or more regions containing 21-26 hydrophobic amino acids which are coiled into anα-helix.

The membrane proteins are of different kinds regarding their organisation within the membrane:

(i) Proteins with single membrane-spanning region (hydrophobic region), and

(ii) Proteins with multiple membrane-spanning regions.

Proteins with single membrane-spanning region: These are of two types:

(a) Group I Proteins:

Group I proteins are those whose N-terminal end is exposed to the exterior of the cell, while the C-terminal end is exposed in the cytoplasm (Fig. 2.9).

(b) Group II Proteins:

Such proteins have their C-terminal end exposed to the exterior of the cell, while their N-terminal end is exposed into the cytoplasm (Fig. 2.9). Such proteins are less common.

Proteins with multiple membrane-spanning regions (hydrophobic regions): These are of two types:

(a) Proteins with ‘odd’ number of hydrophobic regions: In such proteins, the N-terminal and C-terminal regions lie on different sides of the membrane (Fig. 2.9).

(b) Proteins with ‘even’ number of hydrophobic regions: These proteins have both, their N-terminal and C-terminal ends on the same side of the lipid bilayer (Fig. 2.9)

Lipids are synthesized in the ER, and are transported to the cytoplasmic surface of the membrane, from where they are transported 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). The inner surface (cytoplasmic surface), on the other hand, is charged negatively (-) due to the predominance of unsaturated fatty acid chains in the lipid molecules forming the inner monolayer.

Thus there is an asymmetry in the organisation of the lipid bilayer of the plasma membrane. One important property of the plasma membrane is that it can produce “vesicles” by a process of budding. The vesicles can fuse with the membrane by the reverse process.

Functions of Plasma Membrane:

Besides enclosing the cell and protecting it from the external environment, the plasma membrane has several important functions, such as, regulating the movement of materials inside and outside the cell, metabolic functions, communication between different cells and adhesion between cells.

1. Movement of Materials:

Movement (import and export) of materials occur by different mechanisms, e.g:

(a) Simple diffusion,

(b) Facilitated diffusion,

(c) Active transport, and

(d) Endocytosis and exocytosis.

(a) Simple Diffusion:

Simple diffusion refers to the unaided movement of a substance from the region of its higher concentration to a region of its lower concentration till an equilibrium is achieved. Some solutes diffuse through the plasma membrane more readily than the others.

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

(b) Facilitated Diffusion:

It is similar to simple diffusion but the rate of the solute movement increases by interaction with specific membrane transporters. The transporters are “trans membrane proteins.”

(c) Active Transport:

It is the mechanism by which movement of solutes occurs in one direction (unidirectional), i.e., from lower to higher concentration. This is an energy requiring process. The energy is obtained from hydrolysis of ATP and from other sources.

(d) Endocytosis and Exocytosis:

Certain substances are imported within the cell or expelled out of the cell via membrane “vesicles”. The uptake of the external substances via vesicles is called endocytosis, while the excretion of substances via vesicles is called exocytosis.

Endocytosis is divided into two types. The uptake of large particles through 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 via endocytosis is called ligand. The ligand binds to specific receptors, i.e., trans membrane proteins, present in the membrane. This triggers the formation of endocyticvesicles, the process is called “internalization” of receptor.

Some specific proteins, called coat proteins, bind to the plasma membrane on the cytoplasmic side; subsequently, the membrane starts deformation and invagination (Fig. 2.10).

The coat proteins surround the invaginating membrane. Ultimately, a vesicle is formed that includes the extracellular substance. The vesicle is coated by two types of proteins: (i) adaptor, and (ii) calthrin. Such vesicles are called “calthrin-coated vesicles”. Adaptors bind to the integral membrane proteins of the vesicle and to the calthrin (Fig. 2.10).

Different types of adaptors exist in the vesicles of different origin. For example, endocytic vesicles formed from plasma membrane have HA2 adaptor, while the vesicles produced by Golgi complex have HA1 adaptor. These adaptors differ in their composition. HA1 adaptor consists of γ-adaptin, β’-adaptin P47 and P20. HA2 adaptor is composed of α-adaptin, β-adaptin, P50 and P17.

Calthrin forms the outher coat of the vesicles in the form of a polyhedral coat. Calthrin is a protein complex called “triskelion” which consists 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, its protein coat is removed. Subsequently, the vesicle fuses with the target membrane and releases its contents. The vesicle without the coat is called an endosome.

Exocytosis is the reverse process of endocytosis (Fig. 2.10). The substance to be excreted is enclosed within a vesicle at the Trans region of the Golgi complex. This vesicle is called “exocytic” or “secretory” vesicle; it is also coated with specific coal proteins (Fig. 2.10).

When the vesicle reaches the plasma membrane, it becomes uncoated and finally fuses with the membrane. The substance enclosed in the vesicle is, as a result, discharged outside the cell.

Thus there are three methods of physical transfer of materials (ranging from ions to small molecules and macromolecules) from outside into the cell:

(i) Through Channels:

The channels are made by trans membrane proteins. Ions are transferred by this process. Separate channels exist for K⁺, Na⁺, and Ca²⁺ etc.

(ii) By Receptor Itself:

The ligand, such as sugars, binds to the receptor and is transported from extracellular side to the cytoplasm side of the membrane.

(iii) Receptor Internalization:

The ligand binds to receptor which triggers the process of internalization. Vesicle is formed by endocytosis and the ligand is brought into the cell.

2. Metabolic Functions:

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

3. Communication Recognition and Adhesion:

Some important functions of the plasma membrane are the communication between cells, recognition and cell to cell adhesion. Such functions are carried out by “receptors” which are trans membrane proteins or integral proteins.

The extracellular substance, called “ligand” binds to the specific receptors. This binding triggers a change in the function of the membrane. It can transduce signal in the cytoplasm, the phenomenon is called “signal transduction”.

There are two types of signal transduction:

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

(ii) The ligand receptor binding may activate the G protein associated with the plasma membrane. G proteins are guanine nucleotide-binding trimeric proteins consisting of the subunits a (monomer) and βγ (dimer). G protein is inactive when the trimer (αβγ) is bound to GDP.

On activation, GDP (bound to the a subunit) is replaced by GTP and the G protein dissociates into the subunits α and βγ dimer. Then one of the active subunits (either α or βγ) acts upon the target proteins in the cytoplasm. It either activates or represses the target proteins.

Different cells may have different receptors, and therefore, they may respond to different signals. One type of receptor may respond to protein hormones, some other type of receptor respond to neuro transmitters (e.g., acetylcholine), while another type of receptors respond to antigens etc.

Formation of tissues and organs in multicellular organisms occurs when cells adhere to each other in specific ways. Glycoproteins arc known to be involved in cell-to-cell recognition and adhesion. Membrane junctions are formed in animal cells for different functions. “Tight junctions” prevent the movement of molecules through the spaces between adjacent cells.

Desmosomes (specialized areas of cell surface that serve to bind the surface to another structure) provide mechanical strength to hold the cells together in conditions when tissues are exposed to forces that lead to stretching.

“Gap junctions” occur in both, vertebrates and invertebrates, especially in tissues which require quick communication between cells, e.g., nerve cells, muscles etc. They enable small molecules to move from one cell to the other.