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16.2: Plasma Membrane Structure - Biology

16.2: Plasma Membrane Structure - Biology


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In eukaryotic cells, the plasma membrane surrounds a cytoplasm filled with ribosomes and organelles. Some organelles (nuclei, mitochondria, chloroplasts) are even surrounded by double membranes. All cellular membranes are composed of two layers of phospholipids embedded with proteins. All are selectively permeable (semi-permeable), allowing only certain substances to cross the membrane. The unique functions of cellular membranes are due to their different phospholipid and protein compositions. Decades of research have revealed these functions (see earlier discussions of mitochondrial and chloroplast function for instance). Here we’ll describe general features of membranes, using the plasma membrane as our example.

A. The Phospholipid Bilayer

Gorter and Grendel predicted the bilayer membrane structure as early as 1925. They knew that red blood cells (erythrocytes) have no nucleus or other organelles, and thus have only a plasma membrane. They also knew that the major chemical component of these membranes were phospholipids. The space-filling molecular model below shows the basic structure of phospholipids, highlighting their hydrophilic (polar) heads and hydrophobic tails.

Molecules with hydrophilic and hydrophobic domains are amphipathic molecules. Gorter and Grendel had measured the surface area of red blood cells. They then did a ‘blood count’ and then disrupted a known number of red blood cells. They then measured the amount of phospholipids in the membrane extracts. From this, they calculated that there were enough lipid molecules per cell to wrap around each cell twice. From these observations, they predicted the phospholipid bilayer with fatty acids interacting within the bilayer. Curiously, Gorter and Grendel had made two calculation errors in determining the amount of phospholipid per cells. Nevertheless, their errors compensated each other so that, while not strictly speaking correct, their conclusion remained prophetic! Common membrane phospholipids are shown below.

Amphipathic molecules mixed with water spontaneously aggregate to ‘hide’ their hydrophobic regions from the water. In water, these formed actual structures called liposomes that sediment when centrifuged!

Liposome membrane structure is consistent with the predicted phospholipid bilayer, with the hydrophobic tails interacting with each other and the polar heads facing away from each other, forming a phospholipid bilayer. This led to a picture of membrane architecture based on phospholipid interactions. An iconic illustration of the phospholipid bilayer, with its hydrophobic fatty acid interior and hydrophilic external surfaces is drawn below.

Liposome membrane structure is consistent with the predicted phospholipid bilayer, with the hydrophobic tails interacting with each other and the polar heads facing away from each other, forming a phospholipid bilayer. An iconic illustration of the phospholipid bilayer, with its hydrophobic fatty acid interior and hydrophilic external surfaces is drawn below.

B. Models of Membrane Structure

In 1935, Davson and Danielli suggested that proteins might be bound to the polar heads of the phospholipids in the plasma membrane, creating a protein/lipid/protein sandwich. Decades later, J.D. Robertson observed membranes in the transmission electron microscope at high power, revealing that all cellular membranes had a trilamellar structure. The classic trilamellar appearance of a cellular membrane in the electron microscope is illustrated below

The trilamellar structure is consistent with the protein-coated hydrophilic surfaces of a phospholipid bilayer in Davson and Danielli’s protein-lipid-protein sandwich. Observing that all cellular membranes had this trilamellar structure, Robertson he further proposed his Unit Membrane model: all membranes consist of a clear phospholipid bilayer coated with electron-dense proteins.

The static view of the trilamellar models of membrane structure implied by the Davson-Danielli or Robertson models was replaced in 1972 by Singer and Nicolson’s Fluid Mosaic model (see The fluid mosaic model of membranes. Science 175:720- 731). They suggested that in addition to peripheral proteins that do bind to the surfaces of membranes, many integral membrane proteins actually span the membrane. Integral membrane proteins were imagined as a mosaic of protein ‘tiles’ embedded in a phospholipid medium. But unlike a mosaic of glazed tiles set in a firm, cement-like structure, the protein ‘tiles’ were predicted to be mobile (fluid) in a phospholipid sea. In this model, membrane proteins are anchored in membranes by one or more hydrophobic domains; their hydrophilic domains would face aqueous external and cytosolic environments. Thus, like phospholipids themselves, membrane proteins are amphipathic. We know that cells expose different surface structural (and functional) features to the aqueous environment on opposite sides of a membrane. Therefore, we also say that cellular membranes are asymmetric. A typical model of the plasma membrane of a cell is illustrated below.

In this model, peripheral proteins have a hydrophobic domain that does not span the membrane, but that anchors it to one side of the membrane. Other peripheral (or socalled “surface”) proteins are bound to the membrane by interactions with the polar phosphate groups of phospholipids, or with the polar domains of integral membrane proteins.

Because of their own aqueous hydrophilic domains, membrane proteins are a natural barrier to the free passage of charged molecules across the membrane. On the other hand, membrane proteins are responsible for the selective permeability of membranes, facilitating the movement of specific molecules in and out of cells. Membrane proteins also account for specific and selective interactions with their extracellular environment. These interactions include the adhesion of cells to each other, their attachment to surfaces, communication between cells (both direct and via hormones and neurons), etc. The ‘sugar coating’ of the extracellular surfaces of plasma membranes comes from oligosaccharides covalently linked to membrane proteins (as glycoproteins) or to phospholipids (as glycolipids). Carbohydrate components of glycosylated membrane proteins inform their function. Thus, glycoproteins enable specific interactions of cells with each other to form tissues. They also allow interaction with extracellular surfaces to which they must adhere. In addition, they figure prominently as part of receptors for many hormones and other chemical communication biomolecules. Protein domains exposed to the cytoplasm, while not glycosylated, often articulate to components of the cytoskeleton, giving cells their shape and allowing cells to change shape when necessary. Many membrane proteins have essential enzymatic features, as we will see. Given the crucial role of proteins and glycoproteins in membrane function, it should come as no surprise that proteins constitute an average of 40-50% of the mass of a membrane. In some cases, proteins are as much as 70% of membrane mass (think cristal membranes in mitochondria!).

C. Evidence for Membrane Structure

Membrane asymmetry refers to the different membrane features facing opposite sides of the membrane. This was directly demonstrated by the scanning electron microscope technique of freeze-fracture. The technique involves freezing of isolated membranes in water and then chipping the ice. When the ice cracks, the encased membranes split along a line of least resistance… that turns out to be between the hydrophobic fatty acid opposing tails in the interior of the membrane. Scanning electron microscopy then reveals features of the interior and exterior membrane surfaces. Among the prominent features in a scanning micrograph of freeze-fractured plasma membranes are the pits and opposing mounds facing each other on opposite flaps of the membrane, as illustrated below.

Other features shown here are consistent with phospholipid membrane structure.

Cytochemistry confirmed the asymmetry of the plasma membrane, showing that only the external surfaces of plasma membranes are sugar coated, Check the link below for more detailed descriptions of the experiments.

Finally, the asymmetry of membranes was also demonstrated biochemically. In one experiment, whole cells treated with proteolytic enzymes, followed by extraction of the membranes and then isolation of membrane proteins. In a second experiment, plasma membranes were isolated from untreated cells first, and then treated with the enzymes. In a third experiment, proteins were extracted from plasma membranes isolated from untreated cells. Electrophoretic separation of the three protein extracts by size demonstrated that different components of integral membrane proteins were present in the two digest experiments, confirming the asymmetry of the plasma membrane. Again, for more details, check the link below.

The idea that membranes are fluid was also tested. In yet another elegant experiment, antibodies were made to mouse and human cell membrane proteins. Membranes were isolated and injected into a third animal (a rabbit most likely). The rabbit saw the membranes and their associated proteins as foreign and responded by making specific anti-membrane antibody molecules. The antibodies against each membrane source were isolated and separately tagged with different colored fluorescent labels so that they would glow a different color when subjected to ultraviolet light. After mouse and human cells were mixed under conditions that caused them to fuse, making human-mouse hybrid cells. When added to fused cells, the tagged antibodies bound to the cell surface proteins. After a short time, the different fluorescent antibodies were seen to mix under a fluorescence microscope under UV light. The fluorescent tags seemed to moving from their original location in the fused membranes. Clearly, proteins embedded in the membrane are not static, but are able to move laterally in the membrane, in effect floating and diffusing in a “sea of phospholipids”. The mouse antibodies as seen in the hybrid cell right after fusion are cartooned below.

D. Membrane Fluidity is Regulated

1. Chemical Factors Affecting Membrane Fluidity

As you might imagine, the fluidity of a membrane depends on its chemical composition and physical conditions surrounding the cell, for example the outside temperature. Factors that affect membrane fluidity are summarized below.

Just as heating a solution causes dissolved molecules and particulates to move faster, phospholipid and protein components of membranes are also more fluid at higher temperatures. If the fatty acids of the phospholipids have more unsaturated (C=C) carbon bonds, these hydrophobic tails will have more kinks, or bends. The kinks tend to push apart the phospholipid tails. With more space between the fatty acid tails, membrane components can move more freely. Thus, more polyunsaturated fatty acids in a membrane make it more fluid. On the other hand, cholesterol molecules tend to fill the space between fatty acids in the hydrophobic interior of the membrane. This reduces the lateral mobility of phospholipid and protein components in the membrane. By reducing fluidity, cholesterol reduces membrane permeability to some ions.

2. Functional Factors Affecting Membrane Fluidity

Evolution has adapted cell membranes to different and changing environments to maintain the fluidity necessary for proper cell function. Poikilothermic, or coldblooded organisms, from prokaryotes to fish and reptiles, do not regulate their body temperatures. Thus, when exposed to lower temperatures, poikilotherms respond by increasing the unsaturated fatty acid content of their cell membranes. At higher temperatures, they increase membrane saturated fatty acid content. Thus, the cell membranes of fish living under the arctic ice maintain fluidity by having high levels of both monounsaturated and polyunsaturated fatty acids. What about fish species that range across warmer and colder environments (or that live in climates with changing seasons). For these fish, membrane composition can change to adjust fluidity to environment.

The warm-blooded (homeothermic) mammals and birds maintain a more or less constant body temperature. As a result, their membrane composition is also relatively constant. But there is a paradox! Their cell membranes are very fluid, with a higher ratio of polyunsaturated fat to monounsaturated fats than say, reptiles. The apparent paradox is resolved however, when we understand that this greater fluidity supports the higher metabolic rate of the warm-blooded species compared to poikilotherms. Just compare the life styles of almost any mammal to a lazy floating alligator, or a snake basking in the shade of a rock!

E. Making and Experimenting with Artificial Membranes

Membrane-like structures can form spontaneously. When phospholipids interact in an aqueous environment, they aggregate to exclude their hydrophobic fatty tails from water, forming micelles. Micelles are spherical phospholipid monolayer vesicles that self-assemble, a natural aggregation of the hydrophobic fatty acid domains of these amphipathic molecules.

A micelle is drawn below.

Micelles can further self-assemble into spherical phospholipid bilayers called liposomes (below).

When formed in the laboratory, these structures behave somewhat like cells, for example, forming a pellet at the bottom of a tube when centrifuged. Liposomes can be custom designed from different kinds of phospholipids and amphipathic proteins that become integral to the liposome membranes. When liposomes can be prepared in the presence of specific proteins or other molecules that can’t cross the membrane. The trapped molecules cannot get out of this synthetic ‘organelle’. Such were the studies that allowed the identification of the mitochondrial respiratory chain complexes. The ability to manipulate liposome content and membrane composition also make them candidates for the drug delivery to specific cells and tissues (google liposome for more information).

F. The Plasma Membrane is Segragated into Regions with Different Properties of Fluidity and Selective Permeability

As we will see shortly, fluidity does not result in an equal diffusion of all membrane components around the cell membrane surface. Instead, extracellular connections between cells as well as intracellular connections of the membrane to differentiated regions of the cytoskeleton, effectively compartmentalize the membrane into subregions. To understand this, imagine a sheet of epithelial like those in the cartoon below.

The sheet of cells exposes one surface with unique functions to the inside of the organ they line. It exposes the opposite surface, one with a quite different function, to the other side of the sheet. The lateral surfaces of the cells are yet another membrane compartment, one that functions to connect and communicate between the cells in the sheet. Components, i.e., membrane proteins illustrated with different symbolic shapes and colors, may remain fluid within a compartment. Of course, this macrodifferentiation of cell membranes to permit cell-cell and cell-environmental interactions makes intuitive sense.

The recent observation that cellular membranes are even more compartmentalized was perhaps less anticipated. In fact, membranes are further divided into microcompartments. Within these compartments, components are fluid but seldom move between compartments. Studies indicate that cytoskeletal elements create and maintain these micro-discontinuities. For example, integral membrane proteins are immobilized in membranes if they are attached to cytoskeletal fibers (e.g., actin) in the cytoplasm. Furthermore, when aggregates of these proteins line up due to similar interactions, they form kind of fence, inhibiting other membrane components from crossing. By analogy, this mechanism of micro-compartmentalization is called the Fences and Pickets model; proteins attached to the cytoskeleton serve as the pickets. The movement across the fences (i.e., from one membrane compartment to another) is infrequent. Extra kinetic energy is presumably needed for a molecule to ‘jump’ a fence between compartments. Hence, this kind of motion, or hop diffusion distinguishes it from the Brownian motion implied by the original fluid mosaic model.


Structure of Plasma Membrane (With Diagram) | Botany

1. The membrane which bounds the protoplasm (Fig. 292) of the cell of all living organisms including plants and animals is known as plasma membrane or cell membrane or plasmalemma.

2. Plasma membranes range from 7.5 nm to 10 nm in thickness.

3. These are composed of approximately 60% protein and 40% phosphoglyceride.

4. According to Davson and Danielli (1935) plasma membranes are made up of a central region consisting of phosphoglycerides and an outer denser region composed of proteins.

5. The phosphoglyceride molecules are believed to be arranged in two rows with their hydrophilic polar heads towards the outer edges and their hydrophobic hydrocarbon tails in the centre.

6. Plasma membranes are selectively permeable controlling the passage of materials into and out of the cells.

7. The proteins of the membrane include enzymes and compounds of the active transport system.

8. The membrane is relatively impermeable to charged ions, which enter the cell by means of active transport system.


INTRODUCTION

Budding yeast Saccharomyces cerevisiae developed efficient stress response systems that allow adaptation of this unicellular organism to rapidly changing environmental conditions. Recent studies identified eisosomes as integral parts of major stress response pathways. Eisosomes are furrows in the plasma membrane of yeast and other fungi that represent stable membrane domains with unique lipid and protein compositions. The membrane of eisosomes is thicker than the surrounding membrane (Bharat et al., 2018) and it is enriched in sphingolipids and ergosterol (Grossmann et al., 2007 Stradalova et al., 2009). Therefore, eisosomes have the characteristics of lipid rafts and other related lipid domains such as caveolae. Several transmembrane and membrane-associated proteins have been identified that play key roles in the formation and function of eisosomes. Among them are Pil1 and Lsp1, two homologous BAR domain proteins that form a half-pipe-shaped polymer associated with the curved base of the eisosome, and tetraspan proteins including Nce102 that might cluster lipids and transmembrane proteins to form the membrane domain of the eisosome (for a review, see Douglas and Konopka, 2014).

Two functions have been attributed to eisosomes: regulation of amino acid-polyamine-organocation (APC) transporters (Bianchi et al., 2018 Gournas et al., 2018 Moharir et al., 2018) and sensing of membrane stress (Athanasopoulos et al., 2015 Kabeche et al., 2015). APC transporters are evolutionary conserved nutrient transporters that import small molecules such as amino acids and nucleobases. Based on detailed studies of a few of these transporters, it has been predicted that the 26 yeast APC transporters use the proton gradient across the plasma membrane to drive import of the nutrients. Activity of APC transporters and thus the import of the nutrients they pump is mainly regulated by endocytosis and degradation of these proteins. The rate-determining and key regulatory step of this down-regulation is the ubiquitination of the transporters by the ubiquitin ligase Rsp5, which tags the APC transporters for endocytosis and lysosomal degradation via the multivesicular body pathway. Ubiquitination efficiency is affected by both the activity of transporter (pumping transporters expose degron-like sequences Keener and Babst, 2013) and the regulation of Rsp5 adaptor proteins, called ARTs (Lin et al., 2008 Nikko et al., 2008). In addition, eisosomes function as storage compartments of APC transporters that protect inactive transporters from ubiquitination (Grossmann et al., 2008 Spira et al., 2012 Gournas et al., 2018 Moharir et al., 2018). This is accomplished possibly by the special lipid environment of the eisosomes that is predicted to stabilize the ground state of the transporters and/or by blocking access of the Rsp5 ubiquitin ligase to the eisosome-localized transporter.

The membrane stress sensing function of eisosomes is linked to the two homologue proteins Slm1 and Slm2 that localize to eisosomes (Kamble et al., 2011 Olivera-Couto et al., 2011). These proteins contain BAR domains and PIP2 (phosphoinositide-4,5 bisphosphate)-binding PH domains. Membrane stress or tension causes the release of Slm1/2 from eisosomes and the association of these proteins with the Tor complex 2 (TORC2) signaling complex, which activates TORC2 kinase and triggers a membrane stress response (Berchtold et al., 2012 Niles et al., 2012). One consequence of this stress response is the phosphorylation and inactivation of Orm1 and Orm2, two ER proteins that inhibit sphingolipid synthesis. Orm1/2 are phosphorylated by Ypk1/2, the major effector kinases that operate directly downstream of TORC2 (Roelants et al., 2011). TORC2–Ypk1/2-mediated inactivation of Orm1/2 causes up-regulation of sphingolipid synthesis, which ultimately helps to relieve membrane tension. Furthermore, it has been suggested that during acute membrane tension eisosomes flatten, thereby providing additional membrane to the cell surface (Kabeche et al., 2015). However, this model was based on data obtained from osmotically stressed spheroplasts (yeast without cell wall), resulting in nonphysiological swelling of the cells.

In our study we revisited the response of eisosomes to different stress conditions and found a link between membrane tension and APC transporter regulation, explaining why these two types of regulation localize to the same structure. Furthermore, we found an unexpected delay in the derepression of sphingolipid synthesis on membrane stress, suggesting other sources contribute membrane to the cell surface during acute cell expansion.


Plasma Membrane NEET Notes | EduRev

  • Lignification: Lignin (coniferyl alcohol) is a cellulose derivative carbohydrate which deposits on walls of sclerenchyma,vessels and tracheids.
  • Excess of lignin decreases the economic importance of pulp.
  • PITS
    Pits are formed in lignified cell wall. Deposition of lignin occurs throughout the cell wall leaving some small thin walled areas called pits. Pits are generally formed in pairs on the wall of adjacent cells. Two pits of a pair are seperated by a thin membrane called pit membrane (completely permeable) (earliar composed of middle lamella and primary wall). But, after some time primary wall may be dissolved. There are two types of pit pairs
    (i) Simple pits - When diameter of a pit cavity is uniform throughout its length then such type of pits are called simple pits.
    (ii) Bordered pits - When diameter of pit cavity increases from inside to outside then such pits are called Bordered pits. In such pits, pit membrane have a thickening, composed of suberin called Torus. Torus functions like a valve to regulate the flow of materials.
  • Pits occur in sclerenchyma, vessels and tracheids. Tracheids in gymnosperms have maximum number of bordered pits.
  • Suberisation : Suberisation occurs on cork and casparian strips of endodermal cells. Suberin is a highly impermeable material. It is water tight and air tight material. So suberisation also leads to death of cell. Maximum suberisation occurs in middle lamella. It reduces the transpiration rate in plants.
  • Cutinisation: Cutin - is also hydrophobic, waxy substance. Cutinisation is the deposition of cutin on cell walls of leaf epidermis. It reduces the transpiration rate in plants.
  • Cuticularisation: Deposition of cutin on the surface of leaf. It leads to formation of cuticle.
  • Mucilage deposition: Mucilage deposits on the surface of hydrophytes.
  • Deposition of silica: Occurs on the Leaves of grasses, Equisetum, Atropa, Diatoms, rice.

The cell membrane is also known as the plasma membrane. It is the outermost covering of animal cells. It is a semi-permeable membrane composed of lipids and proteins . The main functions of the cell membrane include:

Protecting the integrity of the interior cell.

Providing support and maintaining the shape of the cell.

Helps in regulating cell growth through the balance of endocytosis and exocytosis.

The cell membrane also plays an important role in cell signalling and communication.

It acts as a selectively permeable membrane by allowing the entry of only selected substances into the cell.

What is Plasma Membrane?

An outermost envelope-like membrane or a structure, which surrounds the cell and its organelles is called the plasma membrane. It is a double membraned cell organelle, which is also called the phospholipid bilayer and is present both in prokaryotic and eukaryotic cells.
In all living cells, the plasma membrane functions as the boundary and is selectively permeable, by allowing the entry and exit of certain selective substances. Along with these, the plasma membrane also acts as a connecting system by providing a connection between the cell and its environment.

Structure of Plasma Membrane

A plasma membrane is mainly composed of carbohydrates, phospholipids, proteins, conjugated molecules, which is about 5 to 8 nm in thickness.
The plasma membrane is a flexible and lipid bilayer that surrounds and contains the cytoplasm of the cell. Based on their arrangement of molecules and the presence of certain specialized components, it is also described as the fluid mosaic model.
The fluid mosaic model was first proposed in the year 1972 by American biologist Garth L. Nicolson and Seymour Jonathan Singer. The fluid mosaic model describes in detail about the plasma membrane structure in the eukaryotic cells, how well it is arranged along with their components – phospholipids, proteins, carbohydrates and cholesterol. These components give a fluid appearance to the plasma membrane.

Functions of the Plasma Membrane

  • The plasma membrane functions as a physical barrier between the external environment and the inner cell organelles.
  • The plasma membrane is a selectively permeable membrane, which permits the movement of only certain molecules both in and out of the cell.
  • The plasma membranes play an important role in both the endocytosis and exocytosis process.
  • The plasma membrane also functions by facilitating the communication and signalling between the cells.
  • The plasma membrane plays a vital role in anchoring the cytoskeleton to provide shape to the cell and also maintains the cell potential.

Facts about Plasma Membrane

Both cell membrane and plasma membrane are often confused because of the similarity in words. But these two are the protective organelles of the cell and are very much different in their structure, composition and functions. The cell membrane is a type of plasma membrane and is not always the outermost layer of the cell.

Plasma Membrane Structure

Also referred to as the cell membrane, plasma membrane is the membrane found in all cells, which separate the inner part of the cell from the exterior. A cell wall is found to be attached to the plasma membrane to its exterior in plant and bacterial cells. Plasma membrane is composed of a lipid layer which is semipermeable. It is responsible to regulate the transportation of materials and the movement of substances in and out of the cell.
In addition to containing a lipid layer sitting between the phospholipids maintaining fluidity at a range of temperatures, the plasma membrane also has membrane proteins. This also includes integral proteins passing through the membrane which act as membrane transporters and peripheral proteins attaching to the sides of the cell membrane. It loosely serves as enzymes which shape the cell. Plasma membrane is selectively permeable to organic molecules and ions, it regulates the movement of particles in and out of organelles and cells.

Plasma Membrane Function

This membrane is composed of a phospholipid bilayer implanted with proteins. It forms a stable barrier between two aqueous compartments, which are towards the outside and inside of a cell in plasma membrane. The embedded proteins perform specialized functions which include cell-cell recognition and selective transport of molecules. Plasma membrane renders protection to the cell along with providing a fixed environment within the cell. It is responsible for performing different functions. In order for it allow movement of substances such as white and red blood cells, it must be flexible such that they could alter shape and pass through blood capillaries.
In addition, it also anchors the cytoskeleton to render shape to a cell and in associating with extracellular matrix and other cells to assist the cells in forming a tissue. It also maintains the cell potential. Plasma membrane is responsible for interacting with other, adjacent cells which can be glycoprotein or lipid proteins. The membrane also assists the proteins to monitor and maintain the chemical climate of the cell, along with the assistance in the shifting of molecules across the membrane.

Plasma Membrane – Components

It is composed of the following constituents:

  • Phospholipids – forms the ultimate fabric of the membrane
  • Peripheral proteins – present on the outer or inner surface of phospholipid bilayer but are not implanted in the hydrophobic core
  • Cholesterol – folded between the hydrophobic tails of phospholipid membrane
  • Carbohydrates – found to be attached to the lipids or proteins on the extracellular side of the membrane, leading to the formation of glycolipids and glycoproteins
  • Integral proteins – found to be implanted in the phospholipid bilayer

Structure of Plasma Membrane

Plasma membrane is a fluid mosaic of proteins, lipids and carbohydrates. It is impermeable to ions and water soluble molecules crossing membranes only through carriers, transmembrane channels and pumps. The transmembrane proteins nourish the cell with nutrients, regulate the internal ion concentration and set up a transmembrane electrical potential. Change in a single amino acid in one Cl− channel and plasma membrane pump can lead to human disease cystic fibrosis. On the basis of location of the membrane in the body, lipids can make up anywhere from 20-80% of the membrane, the rest being proteins.
It is composed of a phospholipid bilayer, which is two layers of phospholipids back-to-back. Phospholipids are lipids with a phosphate group associated with them. The phospholipids have one head and two tails where the head is polar and water-loving or hydrophilic. Tails on the other hand are nonpolar and water fearing or hydrophobic.

Fluid Mosaic Model

The description of the structure of plasma membrane can be carried out through the fluid mosaic model as a mosaic cholesterol, carbohydrates, proteins and phospholipids.
First proposed in 1972 by Garth L. Nicolson and S.J. Singer, the model explained the structure of plasma membranes. The model evolved with time however, it still accounts for the functions and structure of plasma membranes the best way. The model describes plasma membrane structure as a mosaic of components which includes proteins, cholesterol, phospholipids, and carbohydrates it imparts a fluid character on the membrane.
Thickness of the membrane is in the range 5-10nm. The proportion of constituency of plasma membrane i.e., the carbohydrates, lipids and proteins vary from cell to cell. For instance, the inner membrane of the mitochondria comprises 24% lipid and 76% protein, in myelin, 76% lipid is found and 18% protein.
Chief fabric of this membrane comprises phospholipid molecules that are amphiphilic. The hydrophilic regions of such molecules are in touch with the aqueous fluid outside and inside the cell. The hydrophobic or the water hating molecules on the other hand are non-polar in nature. One phospholipid molecule comprises a three-carbon glycerol backbone along with 2 fatty acid molecules associated to carbons 1 and 2, and one phosphate-containing group connected to the third carbon.
This organisation provides a region known as head to the molecule on the whole. The head, which is a phosphate-containing group possesses a polar character or a negative charge while the tail, another region containing fatty acids, does not have any charge. They tend to interact with the non-polar molecules in a chemical reaction however, do not typically interact with the polar molecules.
The hydrophobic molecules when introduced to water, have the tendency to form a cluster. On the other hand, hydrophilic areas of the phospholipids have the tendency to form hydrogen bonds with water along with other polar molecules within and outside the cell. Therefore, the membrane surface interacting with the exterior and interior of cells are said to be hydrophilic. On the contrary, the middle of the cell membrane is hydrophobic and does not have any interaction with water. Hence, phospholipids go on to form a great lipid bilayer cell membrane separating fluid inside the cell from the fluid to the exterior of the cell.
The second major component is formed by the proteins of the plasma membrane. Integrins or integral proteins integrate fully into the structure of the membrane, along with their hydrophobic membrane, ranging from regions interacting with hydrophobic regions of phospholipid bilayer. Typically, single pass integral membrane proteins possess a hydrophobic transmembrane segment consisting of 20-25 amino acids. Few of these traverse only a portion of the membrane linking with one layer whereas others span from one to another side of the membrane, thereby exposing to the flip side.
Few complex proteins consist of 12 segments of a one protein, highly convoluted to be implanted in the membrane. Such a type of protein has a hydrophilic region/s along with one or more mildly hydrophobic areas. This organisation of areas of the proteins has the tendency to align the protein along with phospholipids where the hydrophobic area of the protein next to the tails of the phospholipids and hydrophilic areas of protein protrudes through the membrane is in touch with the extracellular fluid or cytosol. The third most important component of the plasma membrane are carbohydrates.
They are generally found on the outside of the cells and linked either to lipids to form glycolipids or proteins to form glycoproteins. The chain of this carbohydrate can comprise two to sixty monosaccharide units which could be branched or straight. Carbohydrates alongside peripheral proteins lead to the formation of concentrated sites on the surface of the cell which identify each other.
This identification is crucial to cells as they permit the immune system to distinguish between the cells of the body and the foreign cells/tissues. Such glycoproteins and glycoproteins are also observed on the surface of viruses, which can vary thereby preventing the immune cells to identify them and attract them.
On the exterior surface of cells, these carbohydrates, their components of both glycolipids and glycoproteins are together known as glycocalyx, which is extremely hydrophilic in nature attracting huge quantities of water on the cell surface. This helps the cell to interact with its fluid like environment and also in the ability of the cell to acquire substance dissolved in water.

Difference between Cell Membrane and Plasma Membrane

Plasma membrane and cell membrane are often confused to be similar terms. However, they are quite different from each other. The plasma membrane encloses the organelles of the cell, whereas, the cell membrane encloses the entire cell components.
Read on to know what is cell membrane and plasma membrane, cell membrane and plasma membrane function and the difference between the cell membrane and the plasma membrane.

Difference Between Cell Membrane and Plasma Membrane

Following are the important difference between cell membrane and plasma membrane:


DISEASE | Bactericides and Fungicides

M. Kilian , U. Steiner , in Encyclopedia of Rose Science , 2003

Compounds Interfering with Cell Membrane Structure

Cell membranes are bilayers containing phospholipids, sterols and globular proteins. The structure is an essential requirement for stability. The guanidines have long-chain alkyl groups and act as nonspecific detergents. They belong to the multisite-acting fungicides. The alteration of permeability of cell membranes and interaction with mitochondrial membranes are considered to be primary actions. The lipophilic alkyl chain attaches to the lipid fraction of membranes whilst the polar guanidino portion remains in the aqueous phase. Dodine is used as a protectant fungicide in roses, although curative and eradicant actions are described. Among the side-effects of dodine, the bactericidal action of the fungicide may be mentioned.


CELL (PLASMA) MEMBRANE - PowerPoint PPT Presentation

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