Plasma Membrane: What is it? Composition, Function, Models, Polarity, Structure and Permeability

It is a biological membrane that separates the interior of all cells from the external environment.

Also known as the cell membrane or cytoplasmic membrane, and historically called the plasmalemma , it consists of a lipid bilayer with integrated proteins.

Cell membranes are involved in a variety of cellular processes such as cell adhesion, ionic conductivity, and cell signaling and serve as the binding surface for various extracellular structures, including the cell wall, the carbohydrate layer called glycocalyx, and the intracellular network of fibers. protein called the cytoskeleton.

In the field of synthetic biology , cell membranes can be artificially reassembled.

Composition

Cell membranes contain a variety of biological molecules, especially lipids and proteins.

The composition is not established, but is constantly changing due to fluidity and changes in the environment, even fluctuating during different stages of cell development.

Specifically, the amount of cholesterol in the cell membrane of the human primary neuron changes, and this change in composition affects fluidity throughout the developmental stages.

Material is incorporated into, or removed from, the membrane through a variety of mechanisms:

  • The fusion of intracellular vesicles with the membrane (exocytosis) not only excretes the contents of the vesicle but also incorporates the components of the vesicle membrane into the cell membrane. The membrane can blister around the extracellular material that is pinched to become vesicles.
  • If a membrane is continuous with a tubular structure made of membrane material, then the tube material can be fed into the membrane continuously.
  • Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), there is an exchange of molecules between the lipid and aqueous phases.

Lipids

The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and sterols.

The amount of each depends on the cell type, but in most cases phospholipids are the most abundant, often contributing to more than 50% of all lipids in plasma membranes.

Glycolipids only make up a minimal amount of about 2% and sterols make up the rest. In GR studies, 30% of the plasma membrane is lipid.

However, for most eukaryotic cells, the composition of plasma membranes is approximately half that of lipids and half that of proteins by weight.

The fatty chains in phospholipids and glycolipids generally contain an even number of carbon atoms, typically between 16 and 20.

The 16- and 18-carbon fatty acids are the most common. Fatty acids can be saturated or unsaturated, with the double bond configuration almost always’ cis.

The length and degree of unsaturation of the fatty acid chains have a profound effect on the fluidity of the membrane as the unsaturated lipids create kinks, preventing the fatty acids from packing together, thus lowering the melting temperature (increasing the fluidity) of the membrane.

The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation.

The entire membrane is held together by the non-covalent interaction of the hydrophobic tails, however the structure is quite fluid and does not lock rigidly in place.

Under physiological conditions, the phospholipid molecules in the cell membrane are in a liquid crystalline state. It means that lipid molecules diffuse freely and exhibit rapid lateral diffusion along the layer in which they are present.

However, the exchange of phospholipid molecules between the intracellular and extracellular leaflets of the bilayer is a very slow process. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.

Furthermore, a fraction of the lipids in direct contact with the integral membrane proteins, which is tightly bound to the surface of the protein is called the ring lipid layer; behaves as part of the protein complex.

Phospholipids forming lipid vesicles

Lipid vesicles or liposomes are circular bags that are enclosed by a lipid bilayer.

These structures are used in laboratories to study the effects of chemicals on cells by delivering these chemicals directly to the cell, as well as to gain more information on the permeability of the cell membrane.

Lipid vesicles and liposomes are formed by first suspending a lipid in an aqueous solution and then shaking the mixture by sonication, resulting in a vesicle.

By measuring the efflux rate from inside the vesicle to the ambient solution, it allows the researcher to better understand the permeability of the membrane.

Vesicles can be formed with molecules and ions within the vesicle by forming the vesicle with the desired molecule or ion present in solution.

Proteins can also be embedded in the membrane by solubilizing the desired proteins in the presence of detergents and binding them to the phospholipids in which the liposome is formed.

These provide researchers with a tool to examine various functions of membrane proteins.

Carbohydrates

Plasma membranes also contain carbohydrates, predominantly glycoproteins, but with some glycolipids (cerebrosides and gangliosides).

Carbohydrates are important in the role of cell recognition in eukaryotes; are located on the cell surface where they recognize host cells and share information, viruses that bind to cells using these receptors cause an infection.

For the most part, glycosylation does not occur on the membranes within the cell; in general, glycosylation occurs on the extracellular surface of the plasma membrane.

Glycocalyx is an important feature in all cells, especially epithelia with microvilli.

Protein

The cell membrane has a high protein content, typically around 50% of the membrane volume.

These proteins are important for cells because they are responsible for various biological activities. About a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms.

Membrane proteins consist of three main types: integral proteins, peripheral proteins, and lipid-anchored proteins.

Function

The basic function of the cell membrane is to protect the cell from its environment.

The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment.

The cell membrane controls the movement of substances in and out of cells and organelles. In this way, it is selectively permeable to ions and organic molecules.

The cell membrane also plays a role in anchoring the cytoskeleton to give the cell shape and in binding to the extracellular matrix and other cells to bind them together and form tissues.

Fungi, bacteria, most archaea, and plants also have a cell wall that provides mechanical support to the cell and prevents the passage of larger molecules.

The cell membrane is selectively permeable and capable of regulating what enters and leaves the cell, facilitating the transport of materials necessary for survival.

The movement of substances through the membrane can be “passive”, which occurs without the input of cellular energy, or “active”, which requires the cell to expend energy in its transport.

The membrane also maintains cell potential. The cell membrane works as a selective filter that allows only certain things to enter or leave the cell.

The cell employs several transport mechanisms that involve biological membranes:

1. Osmosis and passive diffusion: some substances (small molecules, ions) such as carbon dioxide (CO 2) and oxygen (O2) can move through the plasma membrane by diffusion, which is a passive transport process.

Because the membrane acts as a barrier to certain molecules and ions, they can occur in different concentrations on the two sides of the membrane.

Diffusion occurs when small molecules and ions move freely from a high concentration to a low concentration to balance the membrane.

It is considered a passive transport process because it does not require energy and is driven by the concentration gradient created by each side of the membrane.

Such a concentration gradient across a semipermeable membrane establishes an osmotic flux for water.

Osmosis in biological systems involves a solvent that moves through a semi-permeable membrane in a similar way to passive diffusion since the solvent still moves with the concentration gradient and does not require energy.

While water is the most common solvent in cells, it can also be other liquids and supercritical liquids and gases.

2. Transmembrane protein channels and transporters: transmembrane proteins extend through the lipid bilayer of membranes; They work on both sides of the membrane to transport molecules through it.

Nutrients, such as sugars or amino acids, must enter the cell and certain products of metabolism must leave the cell.

Such molecules can passively diffuse through protein channels such as aquaporins in facilitated diffusion or are pumped across the membrane by transmembrane transporters.

Protein channel proteins, also called permeases, are generally quite specific, and only recognize and transport a limited variety of chemicals, often limited to a single substance.

Another example of a transmembrane protein is a cell surface receptor that allows cell signaling molecules to communicate between cells.

3. Endocytosis: endocytosis is the process by which cells absorb molecules by enveloping them.

The plasma membrane creates a small inward deformation, called invagination, in which the substance to be transported is captured.

This intussusception is caused by proteins on the outside of the cell membrane, which act as receptors and cluster in depressions that eventually promote the accumulation of more proteins and lipids on the cytosolic side of the membrane.

The deformation is detached from the membrane inside the cell, creating a vesicle that contains the captured substance. Endocytosis is a pathway to internalize solid particles (“cell consumption” or phagocytosis), small molecules and ions (“cell consumption” or pinocytosis), and macromolecules.

Endocytosis requires energy and is therefore a form of active transport.

4. Exocytosis: just as the material can enter the cell by invagination and formation of a vesicle, the membrane of a vesicle can fuse with the plasma membrane, extruding its contents to the surrounding medium.

Exocytosis occurs in various cells to remove undigested residues of substances introduced by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cell barrier.

In the process of exocytosis, the undigested food vacuole containing debris or the secretory vesicle sprouted from the Golgi apparatus first moves through the cytoskeleton from inside the cell to the surface.

The membrane of the vesicle comes into contact with the plasma membrane. The lipid molecules of the two bilayers rearrange and the two membranes fuse.

A passageway forms in the fused membrane and the vesicles discharge their contents outside the cell.

Prokaryotes

Prokaryotes are divided into two different groups, Archaea and Bacteria, with bacteria that are further divided into gram-positive and gram-negative.

Gram-negative bacteria have a plasma membrane and an outer membrane separated by periplasm, however, other prokaryotes have only one plasma membrane.

These two membranes differ in many ways. The outer membrane of gram-negative bacteria differs from other prokaryotes because of the phospholipids that form the exterior of the bilayer, and the lipoproteins and phospholipids that form the interior.

The outer membrane typically has a porous quality due to its presence of membrane proteins, such as gram-negative porins, which are pore-forming proteins.

The inner plasma membrane is also generally symmetric while the outer membrane is asymmetric due to proteins such as those mentioned above.

Also, for prokaryotic membranes, there are several things that can affect fluidity. One of the main factors that can affect flowability is the fatty acid composition.

For example, when Staphylococcus aureus bacteria were cultured at 37 ◦C for 24 h, the membrane exhibited a more fluid state rather than a gel-like state. This supports the concept that at higher temperatures the membrane is more fluid than at colder temperatures.

When the membrane becomes more fluid and needs to stabilize more, it will produce longer fatty acid chains or saturated fatty acid chains to help stabilize the membrane.

Bacteria are also surrounded by a cell wall made up of peptidoglycan (amino acids and sugars).

Some eukaryotic cells also have cell walls, but none are made of peptidoglycan.

The outer membrane of gram negative bacteria is rich in lipopolysaccharides, which are combined regions of lipids or oligosaccharides and carbohydrates that stimulate the natural immunity of the cell.

The outer membrane can blister in periplasmic protrusions under stress conditions or according to virulence requirements while encountering a host target cell, and therefore such blebs can function as virulence organelles.

Bacterial cells provide numerous examples of the various ways in which prokaryotic cell membranes adapt with structures that adapt to the organism’s niche.

For example, proteins on the surface of certain bacterial cells aid in their gliding movement.

Many gram-negative bacteria have cell membranes that contain ATP-driven protein export systems.

Models

Fluid mosaic pattern

Biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.

Although the lipid bilayers that form the base of membranes do form two-dimensional liquids on their own, the plasma membrane also contains a large number of proteins, which provide more structure.

Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton, and potentially lipid rafts.

Lipid bilayer

Lipid bilayers are formed through the process of self-assembly.

The cell membrane consists mainly of a thin layer of amphipathic phospholipids that spontaneously organize so that the hydrophobic ‘tail’ regions are isolated from the surrounding water while the hydrophilic ‘head’ regions interact with the intracellular (cytosolic) and extracellular faces of the resulting bilayer.

This forms a continuous spherical lipid bilayer. Hydrophobic interactions (also known as the hydrophobic effect) are the main driving forces in the formation of lipid bilayers.

An increase in the interactions between the hydrophobic molecules (causing the hydrophobic regions to clump together) allows the water molecules to bind more freely to each other, increasing the entropy of the system.

This complex interaction can include non-covalent interactions such as van der Waals, electrostatics, and hydrogen bonds.

Lipid bilayers are generally impermeable to polar molecules and ions.

Membrane polarity

The apical membrane of a polarized cell is the surface of the plasma membrane that faces the interior of the light.

This is particularly evident in epithelial and endothelial cells, but it also describes other polarized cells, such as neurons.

The basolateral membrane of a polarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces. Look outward, into the interstitium, and away from the lumen.

The basolateral membrane is a compound phrase that refers to the terms “basement (base) membrane” and “lateral (lateral) membrane,” which, especially in epithelial cells, are identical in composition and activity.

Membrane structures

The cell membrane can form different types of “supramembrane” structures such as caveola, postsynaptic density, podosome, invadopodium, focal adhesion, and different types of cell junctions.

These structures are generally responsible for cell adhesion, communication, endocytosis, and exocytosis.

They can be visualized by electron microscopy or fluorescence microscopy. They are made up of specific proteins, such as integrins and cadherins.

Cytoskeleton

The cytoskeleton lies below the cell membrane in the cytoplasm and provides a scaffold for the anchoring of membrane proteins, as well as to form organelles that extend from the cell.

In fact, elements of the cytoskeleton interact extensively and intimately with the cell membrane.

The cytoskeleton can form appendage-like organelles, such as cilia, which are microtubule-based extensions covered by the cell membrane, and filopodia, which are actin-based extensions.

These extensions are wrapped in a membrane and project from the cell surface to detect the external environment and / or make contact with the substrate or other cells.

Intracellular membranes

The cell’s contents, within the cell membrane, are made up of numerous membrane-bound organelles, which contribute to the overall function of the cell.

The origin, structure and function of each organelle leads to great variation in cell composition due to the individual uniqueness associated with each organelle .

  • Mitochondria and chloroplasts are considered to have evolved from bacteria, known as the endosymbiotic theory.
  • In eukaryotic cells, the nuclear membrane separates the contents of the nucleus from the cytoplasm of the cell.
  • The ER, which is part of the endomembrane system, which makes up a very large portion of the total cell membrane content.
  • The Golgi apparatus has two interconnected round Golgi cisterns.

Variations

The cell membrane has different lipid and protein compositions in different cell types and may therefore have specific names for certain cell types.

  • Sarcolemma in myocytes: “sarcolemma” is the name given to the cell membrane of myocytes (also known as muscle cells).
  • Oocyte oolemma : The oocyte oolemma, or egg cells, are not consistent with a lipid bilayer as they lack a bilayer and do not consist of lipids.
  • Axolemma: the plasma membrane specialized in nerve cell axons that is responsible for the generation of the action potential.

Permeability

The permeability of a membrane is the passive diffusion rate of molecules through the membrane. These molecules are known as permeable molecules.

Permeability depends mainly on the electrical charge and polarity of the molecule and, to a lesser extent, on the molar mass of the molecule.

Due to the hydrophobic nature of the cell membrane, small electrically neutral molecules pass through the membrane more easily than large, charged ones.

The inability of charged molecules to pass through the cell membrane results in the pH distribution of substances throughout the fluid compartments of the body.