Cellular Biology - Topic 3: The Plasma Membrane PDF

1.Maila CELLULAR BIOLOGY Group Nº 5 BLOCK II: Plasma membrane and transport TOPIC 3: THE PLASMA MEMBRANE INDEX 1. Membranes in the eukaryotic cell 2. Membrane lipids...

1.Maila CELLULAR BIOLOGY Group Nº 5 BLOCK II: Plasma membrane and transport TOPIC 3: THE PLASMA MEMBRANE INDEX 1. Membranes in the eukaryotic cell 2. Membrane lipids 2.1. Lipid bilayers and membrane compartments 2.2. Plasma membrane structure 2.3. The fluid mosaic model of the PM 2.4. Trilaminar structure of the PM 2.5. Asymmetrical distribution of lipids 2.6. Plasma membrane fluidity 2.7. Membrane microdomains: Lipid rafts 2.8. Restrictions of lateral of lateral mobility mobility for PM proteins 3. Membrane proteins 3.1. Types of membrane proteins 3.2. Diverse functions of membrane proteins 4. Membrane carbohydrates 4.1. Glycocalyx 4.2. Blood groups 5. Sugar binding proteins: Lectins 6. Plasma membrane proteins can also be solubilized 7. Knowledge test & single choice test 1  1.Maila CELLULAR BIOLOGY Group Nº 5 1. Membranes in eukaryotic cells: Membranes are essential because they separate the cell cytoplasm from the outside medium (extracellular plasma membrane) and cell organelles from each other (intracellular). These membranes can be slightly different from each other as there are some small differences in composition between intracellular and extracellular membranes. However, they all have the same basic structure: lipid bilayers. 2. Membrane lipids 2.1. Lipid bilayers and membrane compartments Lipid bilayers make a hydrophobic barrier between the intracellular and extracellular media. Both media are polar as they are aqueous. What membrane lipids have in common is that all of them are amphipathic molecules, that is to say, they have polar (hydrophilic heads that vary from one lipid to another) and nonpolar regions (hydrocarbon chains of fatty acids that form the hydrophobic tails). That is of great importance since, as we said, those lipids have to interact not only with the internal hydrophobic segment, but also with the external and internal environment. The main components are phospholipids and sphingolipids. When these molecules are found in a water-rich environment, they tend to organize in three different ways or structures: - Bilayer: polar heads face contact with the water and nonpolar tails face each other (avoiding interaction of hydrophobic parts with water-rich environment) - Micelle: it forms a hydrophobic court where tails face each other and heads are distributed pointing towards the outside media. - Liposome: it encloses a tiny aqueous environment. Polar heads are distributed so they face either the external part or the internal part of the liposome. This way, tails will be protected by facing each other. Lipid bilayers will spontaneously self-assemble to create large compartments. It is impossible that a 2  1.Maila CELLULAR BIOLOGY Group Nº 5 bilayer lasts indefinitely with its edges exposed to water, so they eventually have to compact free-edges, so they fuse to form a spherical membrane. As so, a planar phospholipid bilayer with edges exposed to water would be energetically unfavorable, and a sealed compartment formed by the fusing of phospholipid bilayer would be energetically favorable. 2.2 Plasma membrane structure: The plasma membrane is a fluid lipid bilayer with proteins inserted. All lipids are amphipathic: - Phospholipids: More than half of the total lipid membrane. They have a phosphate group which enables them to bind to any polar group (for example, serine). - Sphingolipids: One quarter of the lipids of the membrane. They have a sphingosine backbone. Most of them are GLYCOLIPIDS (cerebrosides or gangliosides), meaning that they have sugars covalently attached to the hydrophilic head. - Cholesterol (only in animal cells): About 20%. It has a big hydrophobic part and a small polar part. Actually, it is particularly rich in the plasma membrane, because the animal cells plasma membrane is very exposed to the external environment, and therefore it is more subjective to changing temperature. So cholesterol maintains membrane fluidity levels within physiological meanings. Proteins are also essential components of the plasma membrane, and there are two kinds: - Integral/Intrinsic proteins span the entire membrane, either fully or partially, and are embedded/inserted within the lipid bilayer. They often function as channels, transporters, or receptors, allowing molecules to pass through or signaling between the inside and outside of the cell. - Peripheral/Extrinsic proteins are not embedded in the membrane, but instead are loosely attached to the surface of the membrane (not directly bound) by non-covalent interactions, either on the inner or outer side. They play roles in signaling, maintaining the cell’s shape, and interacting with the cytoskeleton or extracellular matrix. Lipids and proteins in the outer side of the membrane are glycosylated since they are attached to 3 1.Maila CELLULAR BIOLOGY Group Nº 5 carbohydrates groups. The carbohydrates are always in the extracellular part of the membrane, creating the glycocalyx. The reason why there’s no carbohydrates on the inner part is that when vesicles are created in the ER (endoplasmatic reticulum), sugars are added in the internal part of these compartments; later, when the vesicles fuse with the PM (plasma membrane), those sugars will be facing just the outside part of the membrane. Because of this, the plasma membrane is asymmetric. 2.3. The fluid mosaic model of the PM The term fluid comes from the fact that membrane lipids are moving constantly. According to this model, the plasma membrane will constitute a fluid sea of lipids with proteins that would be the pieces of the mosaic moving constantly (proteins would be the “ships” navigating that “lipid sea”). The membrane can interact with components of the extracellular matrix and the cytoskeleton (intracellular), interacting as an interphase between them. There are many proteins which can bind and physically attach to additional proteins or collagen fibers in the ECM, and to cytoskeleton fibers in the internal part. This provides a mechanism of transmission of mechanical tension between the extracellular matrix and the intracellular part. 2.4. Trilaminar structure of the PM This can be seen with the TEM (transmision electron) microscopy under high magnification. We see two parallel electron-dense lines that represent the phospholipid heads stained by heavy metals when the sample is processed in TEM microscopy. These two lines are separated by an electro lucid strip. Metals accumulate preferentially at the side of phospholipid heads, so it generates electron-dense lines. There are two lines representing that membranes are lipid bilayers. Heads would be stained and the electro-lucid space in between would correspond to the parts occupied by hydrocarbon tails. This cannot be seen under low magnification since it would show a single line instead of two. 4 1.Maila CELLULAR BIOLOGY Group Nº 5 2.5. Asymmetrical distribution of lipids They are distributed asymmetrically: - Only in the outer layer: glycolipids are always facing the extracellular space. - Evenly distributed: they can be found in both perimeters. For example, phosphatidylcholine, sphingomyelin and cholesterol. - Only in the inner layer: phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol. They participate in cell signaling processes. The phosphatidylserine is usually found only in the inner layer, but when the cell goes through apoptosis (cell death) it gets translocated to the outer layer. 2.6. Plasma membrane fluidity Fluidity of the membrane will condition the mobility and function of inserted membrane proteins and lipid second messengers. There are several factors that affect the fluidity of the membrane. - Fatty acid unsaturations (double C-C bonds) are very important in terms of membrane fluidity. The more insaturations the membrane has, the more fluid it will be. Phosphatidylcholine is the most abundant lipid in cell membranes and it has an insaturation that affects the spatial arrangement of the molecule. Because of this distortion, the tail can’t be packed tightly with the rest of the molecules and this increases membrane fluidity. - Temperature: The lower the temperature is, the more rigid the membrane is, and the 5  1.Maila CELLULAR BIOLOGY Group Nº 5 higher the temperature is, the more fluid the membrane is. - Cholesterol: it can either increase or decrease fluidity in animal cells. Animal cells aren’t protected by a cell wall, and temperature of the extracellular media can sometimes change, so they are highly exposed to external environment. Therefore, cholesterol plays a role as a buffer of changes in fluidity. ○ In the case of high temperatures, cholesterol decreases fluidity, because thanks to its rigidity, it doesn’t allow phospholipids and sphingolipids to move as fast as they could. ○ In the case of low temperatures, cholesterol increases fluidity and prevents excessive packaging by letting some space in between the lipid molecules. Lipids in the membrane act like a fluid and they are constantly moving and diffusing. The most common movement is the lateral movement. However, it is also possible that lipids move from one part of the bilayer to the other. This movement is called flip-flop (catalyzed by flippase enzymes) and it is less common. This lipid mobility allows different physiological events, which wouldn’t be possible if the plasma membrane was completely rigid. Resealing: When an amoeba is cut, the membrane can reseal so the amoeba will survive. Besides, if we make a small hole in a frog egg cell, the membrane can reseal itself to close the puncture. Membrane fusion: if we have two different cells, with different labelled proteins, and we fuse 6 1.Maila CELLULAR BIOLOGY Group Nº 5 them up, after a while (40 min) they are completely mixed up. This proves that plasma membrane proteins are moving around the membrane. 2.7. Membrane microdomains: Lipid rafts Although membranes are fluid as a whole, there are some microdomains that have limited movement of lipids. This is the case of lipid rafts, domains particularly enriched in cholesterol and glycosphingolipids, which go together as a single unit. The lipids don’t move individually, but the raft does move, keeping its structure. Due to their particular lipid composition, lipid rafts tend to accumulate better certain types of membrane proteins, receptors, etc. It is believed they play a role as cell signalling centers. 2.8 Restrictions of lateral mobility for PM proteins: The lateral mobility of the plasma membrane can be restricted in several ways: - When the PM proteins are bound to the cytoskeleton, they will stay fixed in that location. When they are bound to the extracellular matrix (ECM) or when they are binding to another cell, proteins will stay fixed as well. - Diffusion barriers: Enterocytes (cells of the intestinal lining) usually have tight junctions, which separate the apical and the basolateral domain. These junctions will prevent the passage of plasma membrane components from one domain to another. Proteins belonging to the basolateral domain can move along it but they can not cross it to reach the apical domain, and vice versa. 7 1.Maila CELLULAR BIOLOGY Group Nº 5 3. Membrane proteins 3.1. Types of membrane proteins There is a great variety of proteins on the PM. They can associate to the PM in three ways: Transmembrane proteins: These proteins either partly or fully cross the membrane. Those that completely cross the membrane are called single-pass or multi-pass transmembrane proteins, depending on how many times they pass through it. To interact with the lipid bilayer, transmembrane proteins have hydrophobic domains, which allow them to stay within the membrane’s hydrophobic core. Many transmembrane proteins are also multimeric, meaning they are made up of multiple subunits, which can be either homomers (same subunits) or heteromers (different subunits). The part that crosses the membrane is usually formed by α-helices, helping to hold them in place. Lipid-linked or glycolipid-linked proteins: These proteins are attached to a lipid or glycolipid tail, which anchors them into the membrane. Even though they don’t span the membrane like transmembrane proteins, they are still considered integral proteins because they are firmly attached to the membrane through the lipid. Peripheral (protein-attached) proteins: These proteins don’t directly interact with the lipid bilayer but are instead non-covalently bound to other membrane proteins. They are attached through weaker interactions like electrostatic forces or hydrogen bonds. These proteins are found on either the inside or outside of the membrane and help with cell signaling, maintaining cell shape, and connecting to the cytoskeleton. 8 1.Maila CELLULAR BIOLOGY Group Nº 5 3.2. Diverse functions of membrane proteins 1. Some proteins participate in solute transport: This is the case of sodium ATPases, which are the most active proteins in the human body, because they generate the membrane potential. 2. Some others have enzymatic activity, for example in the electron transport within mitochondrial membranes, and they catalyze redox reactions. 3. Another function is the signal transduction, which is the case of growth factor receptors. 4. Others play a role in cellular recognition: the Human Leukocyte Antigen (HLA), also called Major Histocompatibility Complex (MHC). They have a role in our immune system, they get recognised by our immune cells. They are usually glycosylated: glycoproteins. 5. Some proteins from the PM are also involved in the adhesion between cells (cadherins) as well as in extracellular matrix adhesion (integrins). 4. Membrane carbohydrates 4.1. Glycocalyx Many of the lipids and proteins have glucidic residuals bound to them. Those sugars will be always facing the outer part of the membrane. This happens because sugars are added to 9  1.Maila CELLULAR BIOLOGY Group Nº 5 the internal part in the ER and packed into vesicles. When those vesicles fuse with the PM, the carbohydrates will only be facing the outer surface, never the inner part. After that, they will be linked to lipids and proteins, creating a structure made of glycolipids and glycoproteins called glycocalyx. The glycocalyx has many functions: - Protection of the membrane. The presence of carbohydrates protects the membrane against external stimuli. It is the case of enterocytes, which have a very thick glycocalyx in order to be protected from enzymatic activity. - Facilitate solution and transport (Attracting water). - Cell adhesion and recognition. In order to label sugars, there are some stains which can be used, such as PAS stains (sugars turn pink) and alcian blue (it gives them a blue color). Ex: goblet cells. 4.2. Blood groups As mentioned before, there are different sugar residues in the outer surface of the RBC membrane. The type of residues we have in our membrane results in different blood groups: - People having O type blood have a single carbohydrate core containing, fucose, N-acetyl-glucosamine and galactose. - A and B type blood differ in only one single residue: apart from the core, A have N-acetyl-galactosamine and B have galactose. - AB people have both A and B oligo chains in the outer surface of the RBC. The addition of one single residue to the carbohydrate oligo chain has a huge impact towards the recognition of the immune system: if the carbohydrates do not correspond (if they are recognised as foreign cells), there is going to be an immune attack and the blood will be rejected. The compatibility is the following: - A rejects B and AB - B rejects A and AB - AB accepts all kinds of blood (universal acceptor) - O only accepts O (universal donor). They can give blood to anybody but they can only receive blood belonging to the same group. 10 1.Maila CELLULAR BIOLOGY Group Nº 5 5. Sugar binding proteins: Lectins Lectins are membrane proteins which bind to sugars. Many of these proteins are involved in cellular attachment and recognition. This is the case of neutrophils, white blood cells that can be recruited to infection sites. When a tissue gets injured or attacked somehow, blood vessels start expressing lectins in order to ligate to oligosaccharides from the membrane of the white cell. After that, they will be able to recruit the infected cell and to fight the threat that affects the stressed tissue. 6. Plasma Membrane proteins can be solubilized The plasma membrane can be solubilized by means of detergents, which are amphipathic molecules which make micelles. Micelles interact with hydrophobic parts of PM proteins and lipids and dissolve them. This process is useful to purify specific proteins and incorporate them into artificial membranes. This is why it was recommended to wash your hands with soap to eradicate coronavirus, because the plasma membrane of the virus is created with lipids and proteins and they can be solubilized with the amphipathic molecules of detergents. 11  1.Maila CELLULAR BIOLOGY Group Nº 5 7. Knowledge test & single choice questions: Topic 3 7.1 Knowledge test 1- What does it mean that the plasma membrane lipids are amphipathic? It means that they have a polar or hydrophilic part (the head) which is oriented towards polar media, and a non-polar or hydrophobic part (the tail) which gets away from it. While polar heads are in contact with polar media (intra & extracellular), hydrophobic tails are in close contact together. 2- Membrane lipids and proteins may have sugars attached to them (glycolipids, glycoproteins). What side of the membrane do sugars face, and why? Membrane sugars face the outer surface of the membrane, they are facing the extracellular space. Both glycolipids and glycoproteins are synthesized through enzymatic processes in the cell, primarily in the endoplasmic reticulum (ER) and Golgi apparatus, where sugars are added to lipids or proteins through a process called glycosylation. Then, the glycolipids, glycoproteins are packaged into vesicles and transported to the plasma membrane, as the sugars in those molecules are oriented towards the center of the vesicle, once the vesicle fuses together with the plasma membranes the sugars will be facing the extracellular space. 3- If all hydrocarbon tails in phospholipids and sphingolipids in the plasma membrane were completely saturated, what would be the consequences? If all hydrocarbon tails in phospholipids and sphingolipids were completely saturated, the fluidity of the membrane would decrease, which would hamper several processes, such as signaling, transport, and cell movement, and could result in severe cellular dysfunction. 4- Can you give some experimental evidence of membrane lipid mobility? 12  1.Maila CELLULAR BIOLOGY Group Nº 5 Lipids in the membrane act like a fluid, they can move or diffuse rapidly. These lipids can make either lateral movements, which are pretty common, or flip-flop movements, which are pretty rare. Some evidences are amoeba, frog egg and mammalian cells. (in the photograph they are explained.) 5- What are the differences between a transmembrane protein and an integral protein? The transmembrane proteins cross the plasma membrane completely. While the integral proteins might cross it entirely or they might just span one part of it. 6- Can you give some examples of physiological processes regulated by plasma membrane carbohydrates? 1-Protect membrane and facilitate solute dissolution and transport (attracting water) 2-Mediate cell adhesion and recognition. Moreover, it is important to select a blood type that is compatible with ours when making a blood transfusion, as erythrocytes show carbohydrates in their membranes (blood groups A,B) and cells of our immune system can recognize these different sugars and if the carbohydrates do not correspond to the person’s natural blood group Immune Attack & Transfusion rejection will happen. 7- The importance of washing your hands with soap was highly stressed to avoid SARS-coV2 infection.What is the rationale behind that recommendation? The thing is that the coronavirus itself has membranes around it. Therefore, using detergents, make micelles in water solution, such as soap, we would be able to dissolve the plasma membrane around the virus breaking it apart and effectively inactivating the virus. 13  1.Maila CELLULAR BIOLOGY Group Nº 5 7.2 Single choice questions 1.- Which type of protein spans the entire lipid bilayer of the plasma membrane? Transmembrane proteins 2.- Which lipid component of the animal cell plasma membrane helps maintain its fluidity within physiological ranges? Cholesterol. Why? Compared to other type cells, animal cells are naked, they do not have a cell wall as protection. Furthermore, they are exposed to several temperature changes which may affect plasma membrane fluidity. Therefore, thanks to cholesterol membrane can maintain its characteristic no matter the condition 3.- Which of the following statements best describes lipid distribution in the plasma membrane? Glycolipids are concentrated in the outer leaflet. 4.- Which property of soap makes it effective in precluding SARS-CoV2? Its ability to form micelles 5.- Which component of the ABO blood group antigen is responsible for their specificity? Sugars (Just a monosaccharide) 6.- If a person with type O blood receives a transfusion of type A blood, what will likely happen? Severe immune reaction 14