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Term Paper on Biomembranes

Term Paper Contents:

  1. Term Paper on the Introduction to Biomembranes
  2. Term Paper on the Cell Membranes
  3. Term Paper on the Membrane Lipids
  4. Term Paper on Cholesterol
  5. Term Paper on the Membrane Proteins
  6. Term Paper on the Membrane Lipid Bilayer
  7. Term Paper on the Membrane Fluidity
  8. Term Paper on the Membrane Asymmetry

1. Term Paper on the Introduction to Biomembranes:

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An animal cell is composed of many subcellular compartments. Each of these compartments, as well as the whole cell, is surrounded by a membrane. The outer cellular membrane, called the plasma membrane, is anchored to the cytoskeleton, which is a network of microfilaments and microtubules that interact extensively with each other and with the components of the plasma membrane. Among other functions, the cytoskeleton is responsible for the shape of the cell, for its mobility, and for the separation of chromosomes during cell division.

The proteins of the cytoskeleton networks, fall into three classes:

(1) Actin filaments, formed by the polymerization of 42,000- dalton G-actin subunits;

(2) Two types of tubulin, α and β, each 55,000 daltons; as α, β-dimers, they assemble into microtubules;

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(3) Intermediate filament proteins, so-called because the diameter of the filaments is between that for actin filaments (7 nm) and microtubules (11 nm).

Although different intermediate filament proteins are found in different cell types, they can be classified into five major groups:

1. Keratins are found in epithelial cells.

2. Neuronal filaments, composed of three proteins of 200,000, 150,000, and 68,000 daltons, occur in close association with axonal microtubules.

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3. Desmin filaments are found predominantly in muscle cells.

4. Glial fibrillary acidic protein (GFAP) is found exclusively in glial cells.

5. Vimentin-containing filaments are associated with mesenchymal cells.

Cytoskeletal Proteins

The intermediate filaments, which show a high degree of amino acid sequence homology, associate with the actin and tubulin filament systems in the cell to develop a cytoskeletal network that determines the morphology and mobility of the cell. Other proteins may also be involved, such as myosin in muscle to give it a contractile property and dynein with microtubules to give them mechanical properties. The cytoskeleton is also closely associated with the cell membranes, particularly the plasma membrane.

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2. Term Paper on the Cell Membranes:

Biologic membranes separate cells from their external environment and divide the interior of the cell into compartments. They are 75 to 90 A thick. The chemical composition of cell membranes varies widely. As a general estimate, a representative membrane is made up of about 50% protein, 45% lipid, and 5% carbohydrate.

Approximately 10% of the membrane proteins are glycoproteins. A major exception to this general chemical composition is the myelin sheath of nerves, one of the most widely studied biologic membranes. Myelin is made up of 20% protein, 75% lipid, and 5% carbohydrate.

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A bilayer of lipid forms the central structure of the membrane. The bilayer is composed primarily of phospholipids and is held together by physical forces, not covalent bonds. Proteins are interspersed throughout the bilayer. Some of the proteins are attached to the surface, whereas others are embedded within the lipids or penetrate completely through the bilayer and are exposed on both surfaces. Many of the membrane proteins are enzymes. Others are recognition factors, ion channels, transporters, or receptors.

Composition of Cell Membranes

Cytoskeleton, Membrane Protein and Lipid Bilary

3. Term Paper on the Membrane Lipids:

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The lipid composition of the human erythrocyte membrane, which is representative of the plasma membrane of most human cells. A much higher percentage of glycosphingolipids exists in myelin and correspondingly, much less phospholipid. However, both the erythrocyte membrane and the myelin are plasma membranes; that is, they are located at the cell surface and separate the extracellular fluid from the cytoplasm.

Membranes of the Mammalian Cell

This emphasizes the diversity in the lipid composition of cell membranes. Some of the intracellular membranes also have specialized lipid composition. For example, inner mitochondrial membranes contain almost no cholesterol and are the only mammalian membranes that contain appreciable amounts of cardiolipin.

Membrane Lipid Composition

Phospholipids:

The main lipids in myelin and the erythrocyte membranes are phospholipids. This is true of all biologic membranes; the basic structure of membranes is a bilayer composed of phospholipids. In this arrangement the hydrocarbon chains of the phospholipid fatty acyl groups project into the center of the bilayer. The hydrophilic glyceryl-phosphorylbase components of the phospholipids are called head groups and are located on the outside of the bilayer, where they interact with water or other polar and charged molecules.

These polar groups are represented schematically by the circles to which the wavy lines (the fatty acyl chains) are attached. The lipid bilayer is composed of two leaflets, the outer phospholipid leaflet that faces the extracellular fluid and the inner phospholipid leaflet that faces the cytoplasm. Each leaflet is 25 Å thick, with the head group occupying 10 Å and the fatty acyl chains 15 Å. The total thickness of the bilayer is 50 Å, 30 Å of which is comprised of the hydrocarbon core containing the fatty acyl chains of both leaflets.

Phospholipid Composition:

The membrane lipid bilayer contains a mixture of phospholipids, most of which are glycerol derivatives called phosphoglycerides. Sphingomyelin is the one exception; it is a phospholipid derivative of sphingosine. In each fraction the choline phosphoglycerides are the most prevalent phospholipid.

The ethanolamine phosphoglycerides are the second most abundant phospholipid, and the serine and inositol phosphoglycerides comprise about 15% of the total. Sphingomyelin, which contains a phosphorylcholine head group, is rich in the plasma membrane. Cardiolipin, a bis(phosphatidyl)glycerol, is present in substantial amounts only in the inner mitochondrial membrane.

Phospholipid Fatty Acid Composition:

The fatty acid composition of the different phospholipids varies considerably. For example, the choline phosphoglycerides are rich in palmitic (16:0) and linoleic (18:2) acids, whereas the ethanolamine and serine phosphoglycerides are rich in arachidonic acid (20:4) and the 22-carbon polyunsaturated fatty acids. By contrast, sphingomyelin is rich in saturated fatty acids and 24- carbon fatty acids.

The fatty acids also are not evenly distributed between the sn-1 (sn, stereospecific numbering) and sn-2 positions of the glycerophospholipids. Saturated fatty acids are more prevalent in the sn-1 position and polyunsaturated fatty acids in the sn-2 position. Monounsaturated fatty acids tend to be more evenly distributed among both positions. Ether-linked hydrocarbon groups, when present, always are in the sn-1 position. They occur in the alkyl ether phosphoglycerides and plasmalogens.

4. Term Paper on Cholesterol:

Cholesterol is inserted into the lipid bilayer between phospholipid molecules, in both leaflets of the lipid bilayer. Its hydroxyl group is oriented toward the aqueous environment and interacts with the polar head groups of the phospholipids. The nonpolar rings and hydrocarbon tail of cholesterol are positioned So that they interact with the hydrocarbon chains of the phospholipid fatty acyl groups.

Phosphoglycerides of Cell Membranes

The planar ring structure of the steroid nucleus penetrates to a depth of about the first 10 carbons of the phospholipid fatty acyl chains. The hydrocarbon chain of cholesterol occupies the region between carbon 11 and the methylterminus of the fatty acid. The amount of cholesterol contained in the various cell membranes differs considerably.

Fatty Acid Composition

Insertion of Cholesterol into the Membrane Lipid Bilayer

For example, cholesterol comprises about 25% of the lipids by weight in the plasma membrane, where the molar ratio of cholesterol to phospholipid is about 0.5 to 0.8, but it is not present in the inner mitochondrial membrane. Likewise, all the cholesterol in the plasma membrane is in the free or non-esterified form, whereas both cholesterol and cholesterol esters are contained in the endoplasmic reticulum.

Cholesterol Exchange and Surface Transfer:

Cholesterol is held in the lipid bilayer by physical interactions, primarily between the planar steroid nucleus and the adjacent phospholipid fatty acid hydrocarbon chains. Because it is not held in the bilayer through covalent bonds, cholesterol can move in and out of the plasma membrane. In some cases, cholesterol in the membrane exchanges with cholesterol in the surface coats of plasma lipoproteins, and no overall change occurs in membrane cholesterol content.

It is possible, however, to transfer cholesterol in or out of a cell by such a process provided that the cholesterol that moves is immediately channeled into another pathway, such as conversion to a cholesterol ester. If an accumulation or release of cholesterol occurs, the process is called surface transfer. The factor that determines whether a net transfer of cholesterol occurs, and, if so, in which direction, is the molar ratio of unesterified cholesterol to phospholipid in the two structures.

Net transfer will occur from the structure containing the higher to the one containing the lower cholesterol/phospholipid molar ratio. Surface transfer may be an important mechanism for cholesterol movement, particularly for the efflux of cholesterol from cells to prevent excessive accumulation in tissues such as the walls of arteries. High-density lipoproteins (HDLs) are the main acceptors in the extracellular fluid for cholesterol released from cells by the surface transfer mechanism.

Glycosphingolipids:

The carbohydrate is not free; it is a component of either glycolipids or glycoproteins. All the glycolipids in animal cells are glycosphingolipids. They are derivatives of ceramide and therefore contain sphingosine and a long-chain fatty acid in amide linkage. This is the same structure contained in sphingomyelin. With the glycosphingolipids, however, the phosphorylcholine group of sphingomyelin is replaced by one or more carbohydrate residues.

Insertion of Orientation of Glycosphingolipids

Glycosphingolipids are inserted into the membrane lipid bilayer in the same manner as cholesterol. This also indicates the structure of several carbohydrate chains in the commonly occurring glycosphingolipids. The ceramide group is contained within the lipid bilayer, with the sphingosine and fatty acid hydrocarbon chain parallel to and interacting with the fatty acyl chains of the phospholipids. By contrast, the carbohydrate group projects out from the surface of the bilayer, interacting with the phospholipid head groups and the surrounding water.

5. Term Paper on the Membrane Proteins:

As an initial approximation, membrane proteins can be divided into two general types, peripheral and integral. Peripheral proteins are bound loosely to the membrane and can be removed by mild treatment with solutions of high ionic strength, chelating agents such as ethylenediamine tetraacetate, or treatment with enzymes such as phospholipase C. They are soluble in aqueous solution and do not contain tightly adherent lipid. Peripheral proteins comprise about 30% of the membrane proteins.

The remainder of the membrane proteins, integral proteins, are tightly bound and removed only by such drastic treatments as extraction with detergents. Lipid adheres to the integral proteins when they are removed from the membrane, and these proteins usually are insoluble when they are introduced into aqueous media unless detergent is present. Many different individual proteins make up each of these two classes of membrane proteins, and their molecular weights vary widely.

Integral Membrane Proteins:

Two general types of proteins are embedded in the lipid bilayer. One type spans the membrane lipid bilayer only once. Such proteins include the low-density lipoprotein (LDL) receptor and glycophorin, the main membrane glycoprotein of the erythrocytes. These proteins contain a single α-helical membrane- spanning segment composed of 18 to 22 nonpolar amino acid residues that interact with the phospholipid fatty acid chains in the lipid bilayer. Most of the structure of these proteins is contained outside the lipid bilayer, both in the extracellular fluid and in the cytoplasm.

The second type of integral protein is of erythrocytes, which is an anion transporter that exchanges bicarbonate for chloride ions. Each of the two subunits of this transporter crosses the lipid bilayer 12 times, and the membrane-spanning segments are connected by hairpin loops. In this case most of the protein structure is contained within the lipid bilayer.

Other membrane proteins that span the bilayer several times include rhodopsin, cytochrome P450, Ca+ +-ATPase, and the β-adrenergic receptor. Many of the integral proteins are glycoproteins that contain a number of carbohydrate chains. The carbohydrate chains are attached to the extracellular domain and project into the surrounding fluid.

Structural Differences in Two Intrinsic Membrane Proteins

Glycoproteins:

Many biologically active proteins are glycoproteins; some are secreted from the cell, such as antibodies, and others become a part of a membrane. The initial steps in the biosynthesis of the carbohydrate prosthetic groups occur in the lumen of the endoplasmic reticulum (ER). The growing peptide chain emerges from the ribosome on the ER and inserts through the membrane of the ER into the lumen under the influence of a signal recognition protein (SRP), which is bound to an SRP receptor in the ER membrane.

The proteins being synthesized are targeted for a particular location in the cell by the first approximately 10 to 40 amino acid residues emerging from the ribosome. The sequence is recognized by the SRP and the ribosome and thus becomes attached to the ER. The sequence continues to translate the messenger ribonucleic acid (mRNA) message into a polypeptide chain, which is inserted through the ER membrane into the lumen.

Much remains to be learned about these processes. For example, many protein chains that are part of a membrane have internal sequences of hydrophobic amino acid residues. These segments, as a-helices, remain embedded in the lipid bilayer of the membrane. Depending on the number of these hydrophobic segments, which can be calculated from the free energy transfer of each successive amino acid from a lipid environment to water, the protein loops back and forth from the cytoplasmic to the luminal side of the subcellular particle or from the cytoplasmic to the extracellular side of the plasma membrane.

Carbohydrate residues are attached to the polypeptide segments in the lumen of the ER. Following transport to the Golgi complex, the carbohydrate groups are further processed before delivery to predestined target sites, which may be membranes or extracellular secretions.

Peripheral Proteins:

The peripheral proteins are contained entirely in the aqueous environment and are attached to the surface of the lipid bilayer. Some of the attachment occurs through ionic interactions between charged amino acid residues and the head groups of the phospholipids. Ions such as Ca++ often form a bridge between an anionic phospholipid head group such as serine and an anionic amino acid group such as aspartate. In other cases the surface protein is attached to the membrane phospholipid head group through a covalent linkage.

Phosphatidylinositol Glycan Anchors:

Many peripheral proteins are attached covalently to the lipid bilayer through phosphatidylinositol. These include alkaline phosphatase, 5′-nucleotidase, acetylcholinesterase, and the Thy-1 antigen. The proteins are linked from their C-terminal amino acid residue to phosphoethanolamine, which is linked to a chain of carbohydrate residues called a glycan.

Components of the carbohydrate chain include mannose, glucosamine, galactose, and N-acetylgalactosamine. The glycan chain is attached covalently to the inositol residue of phosphatidylinositol, which is a part of the membrane lipid bilayer.

Proteins attached to the cell surface by phosphatidylinositol glycan anchors are synthesized with a leader sequence consisting of nonpolar amino acid residues at the C-terminal end. During processing, this leader sequence is removed, and the resulting C-terminal amino acid group (cysteine in the case of the Thy-1 antigen) becomes attached to the phosphorylethanolamine glycan chain.

Peripheral proteins attached through phosphatidylinositol glycan anchors are released from the cell in response to certain stimuli. These stimuli activate a phosphatidylinositol specific phospholipase C that hydrolyzes the phosphorylinositol glycan group from the diacylglycerol backbone. In some cases the released diacylglycerol or the glycan structure may act as a second messenger.

Attachment of Peripheral Protein to the Membrane Lipid Bilayer

Fatty Acylation of Membrane Proteins:

Many proteins attached to cell membranes contain covalently bound palmitic or myristic acid. The palmitic acid is in ester or thioester linkage to an internal amino acid residue, whereas the myristic acid is in amide linkage to an N-terminal glycine residue. The fatty acyl group probably facilitates the attachment of the protein to membranes by penetrating into the lipid bilayer. Membrane proteins that contain fatty acyl groups include the transferrin receptor, rhodopsin, the nicotinic acetylcholine receptor, and the Ca++-ATPase of the sarcoplasmic reticulum.

6. Term Paper on the Membrane Lipid Bilayer:

The phospholipid bilayer is composed of two rows of phosphor glycerides that have their fatty acyl groups pointed toward each other and their glyceryl-phosphoryl-base head groups oriented outward to the extracellular and cytoplasmic surfaces. Therefore the inside of the bilayer is composed of nonpolar fatty acyl hydrocarbon chains, whereas the outside surfaces that interact with the aqueous environment contain the polar phospholipid head groups.

At body temperature the lipid bilayer is in a fluid like physical state analogous to an oil droplet. This is called the liquid crystalline state. If the membrane is cooled, the lipids pass into a solid, or gel, state. The temperature at which the lipid bilayer changes from the liquid crystalline to the gel state is known as the phase-transition temperature.

Physical States of Membrane Lipids

Under physiologic conditions the membrane is above the phase-transition temperature, and the liquid crystalline state predominates. Some regions of the bilayer are in the gel state, however, and both states coexist in most membranes. In other words, domains of gel structure exist, probably rich in cholesterol, sphingomyelin, and phospholipids with saturated fatty acyl chains, interspersed among the liquid crystalline domains.

Non-Bilayer Structures:

There also may be regions of the membrane that are not in bilayer structure. Areas rich in lysophospholipids can form a hexagonal (HI) structure, whereas those rich in phosphatidyl-ethanolamine containing highly polyunsaturated chains can form in inverted hexagonal (HII) phase. Regions of non-bilayer structure would tend to occur at the interface between gel and liquid crystalline domains. They also are likely to occur at points of membrane fusion.

Lipid Motion:

The fatty acyl chains flex rapidly back and forth. In addition, the phospholipid can rapidly rotate around its long axis. Phospholipids also can move very rapidly laterally within each of the leaflets of the lipid bilayer. Thus a phospholipid molecule can exchange places with the one on either side of it within fractions of a second.

In this process the phosphoglyceride remains within the same leaflet of the lipid bilayer, and it does not cross from the extracellular fluid half to the cytoplasmic half of the bilayer structure. The opposite process, movement of a phospholipid molecule between the extracellular and cytoplasmic leaflets of the bilayer, known as flip-flop, occurs slowly compared to lateral movement within the bilayer leaflet.

Mobility of Phospholipids in the Membrane Lipid Bilayer

It is unfavourable energetically to move the polar phospholipid head group through the central nonpolar hydrocarbon region, a process that would be required for the phospholipid to cross to the opposite side of the bilayer. When flip-flop does occur, it may be facilitated by certain membrane proteins that penetrate into the lipid bilayer.

7. Term Paper on the Membrane Fluidity:

The degree of motion of the hydrocarbon chains within the lipid bilayer is called fluidity. As motion increases, fluidity increases. At body temperature the lipid bilayer exists in a fluid state. The more fluid the bilayer, the more permeable is the membrane. Unsaturated fatty acids present in the membrane phospholipids increase the fluidity of the membrane and make it more permeable.

By contrast, saturated fatty acids decrease the fluidity and permeability of the membrane. Integral proteins that penetrate through the membrane modulate the fluidity of the lipid bilayer. Cholesterol also modulates the fluidity, decreasing it in regions of the membrane that contain many unsaturated fatty acids and increasing fluidity in the regions composed primarily of saturated fatty acids. Therefore cholesterol can be considered as a modulator of membrane fluidity and permeability.

Cholesterol forms clustered regions within the membrane lipid bilayer; some areas contain 1 mole of cholesterol per mole of phospholipid, whereas others contain almost no cholesterol. This gives the membrane a patchy effect, with solid regions coexisting with adjacent fluid domains. In this way, areas within a membrane can have very different physical and permeability properties.

8. Term Paper on the Membrane Asymmetry:

Recent studies using cross-linking reagents and enzymes that degrade membrane constituents have indicated that biologic membranes are asymmetric. This is true for the protein, carbohydrate, and lipid components of the membranes. Different peripheral proteins are present on the two surfaces of the lipid bilayer.

Likewise, the trans-membrane proteins such as the large subunit of the Na+, K+-adenosine triphosphates (ATPase) are asymmetric, with the Na+ and ATP-binding sites located on the surface exposed on the cytoplasmic side and the K+ – and obtain (an inhibitor) binding sites located on the region exposed at the extracellular fluid surface. The carbohydrate chains of the glycoproteins also are asymmetrically distributed; they are oriented so that they project out into the extracellular fluid.

Furthermore, the lipid bilayer itself is asymmetric. Phos­phatidylcholine and sphingomyelin are concentrated to a greater extent in the leaflet of the bilayer that faces the extracellular fluid. Conversely, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are concentrated in the leaflet that faces the cell cytoplasm.

Asymmetry of Phospholipids in the Membrane Lipid Bilayer

Phospholipid Exchange Proteins:

The cell cytoplasm contains proteins that catalyze the transfer of phospholipids between different membranes. They are called phospholipid exchange proteins. These cytoplasmic proteins have molecular weights between 16,000 and 30,000, and most have isoelectric points about pH 5.0. Each phospholipid exchange protein is fairly specific for a given phospholipid class.

Although they are known as exchange proteins, they have been shown to catalyze the net transfer of phospholipid from one membrane to another, for example, from microsomes to mitochondria. Therefore one of their main functions probably is to move phospholipids from the ER, where they are synthesized, to sites where new membrane is being formed.

When phospholipid exchange proteins interact with a membrane, they remove or add phospholipids only in the half of the lipid bilayer that they face. Because of this, they probably contribute to the asymmetric distribution of phospholipids across the membrane lipid bilayer.