Cell Membrane & Vesicular Transport PDF

1 Cell membrane& vesicular Transport ILOs By the end of this lecture, students will be able to 1. Relate the molecular structure of membrane lipids to its different membrane functions. 2. Correlate the variable peripheral & integral membrane molecules to membrane function. 3. Int...

1 Cell membrane& vesicular Transport ILOs By the end of this lecture, students will be able to 1. Relate the molecular structure of membrane lipids to its different membrane functions. 2. Correlate the variable peripheral & integral membrane molecules to membrane function. 3. Interpret the diagrammatic presentation of the fluid mosaic model 4. Interpret endocytosis & exocytosis as vital processes for transport across the cell membrane. 5. Differentiate between types of endocytosis in normal and pathological conditions. Cell Components The cell is composed of two basic parts: cytoplasm (Gr. kytos, cell, + plasma, thing formed) and nucleus. Individual cytoplasmic components are usually not clearly distinguishable in common hematoxylin and eosin-stained preparations; the nucleus, however, appears intensely stained dark blue or black. (Why?) Cytoplasm The outermost component of the cell, separating the cytoplasm from its extracellular environment, is the plasma membrane (plasmalemma). However, even if the plasma membrane is the external limit of the cell, there is a continuum between the interior of the cell and extracellular macromolecules. The cytoplasm is composed of a matrix, or cytosol, in which are embedded the organelles, the cytoskeleton, and deposits of carbohydrates, lipids, and pigments. Higher levels of organization Plasma Membrane All eukaryotic cells are enveloped by a limiting membrane composed of phospholipids, cholesterol, proteins, and chains of oligosaccharides covalently linked to phospholipids and protein molecules. The cell, or plasma, membrane functions as a selective barrier that Page 1 of 5 regulates the passage of certain materials into and out of the cell and facilitates the transport of specific molecules. One important role of the cell membrane is to keep constant the intracellular milieu, which is different from the extracellular fluid. Membranes also carry out a number of specific recognition and regulatory functions (to be discussed later), playing an important role in the interactions of the cell with its environment. Membranes range from 7.5 to 10 nm in thickness and consequently are visible only in the electron microscope (so, what is visible on the light microscope?) Molecular structure of the cell membrane (Figure 2) 1- Membrane phospholipids. Structure: two long, nonpolar (hydrophobic) hydrocarbon chains linked to a charged (hydrophilic) head group. (Why it appears black in electron micrograph? Is it a single line?) o Organization: the hydrophobic (nonpolar) chains directed toward the centre of the membrane and the hydrophilic (charged) heads directed outward. o Cholesterol is also a constituent of cell membranes. Role of cholesterol; (It will be discussed later with fluidity of the membrane). Note: ✔ This represents the fluid mosaic model of the cell membrane. ✔ The lipid composition of each half of the bilayer could be different according to its functional role. ✔ The trilaminar appearance are characteristic of all the internal cellular membranes (eg. Nuclear, mitochondrial, and endoplasmic reticulum) as well as the plasma membrane. ✔ Some of the lipids, known as glycolipids, possess oligosaccharide chains that extend outward from the surface of the cell membrane and thus contribute to lipid asymmetry. (Figure 2) 2- Proteins, About 50% of the plasma membrane components Types: Page 2 of 5  o Integral proteins are directly incorporated within the lipid bilayer. Some integral proteins span the membrane one or more times, from one side to the other. Other proteins are large enough to extend across the two lipid layers and protrude from both membrane surfaces (transmembrane proteins). Integrins are transmembrane proteins that are linked to cytoplasmic cytoskeletal filaments and to extracellular molecules. Through these linkages there is a constant exchange of influence, in both ways, between the extracellular matrix and the cytoplasm. o Peripheral proteins exhibit a looser association with membrane surfaces. They may protrude from either the outer or inner surface. o Organelle-specific membranes proteins confer unique functions on certain organelles. Correlate the functional forms of integral membrane protein in Fig. 2 with its type. 3- The carbohydrate moieties of glycoproteins and glycolipids exoplasmic domain; they are important components of specific molecules called receptors that participate in important interactions such as cell adhesion, recognition, and response to protein hormones. As with lipids, the distribution of membrane proteins is different in the two surfaces of the cell membranes. Therefore, all membranes in the cell are asymmetric. Glycocalyx (cell coat) In the electron microscope the external surface of the cell shows a fuzzy carbohydrate-rich region called the glycocalyx. This layer is composed of carbohydrate chains linked to membrane proteins and lipids and of cell-secreted glycoproteins and proteoglycans. Functional significance of glycocalyx The glycocalyx has a role in cell recognition and attachment to other cells and to extracellular molecules. Transport across the cell membrane Movement of molecules and ions across membranes occurs by different mechanisms including: a. Passive transport (simple and facilitated diffusion). b. Active transport (pump and cotransport carrier). (Fig 3) c. Bulk movement of materials into and out of the cells (endocytosis & exocytosis) which will be discussed in this section. Mass transfer of material also occurs through the plasma membrane. This bulk uptake of material is known as endocytosis. The corresponding name for release of material in bulk is exocytosis. However, at the molecular level, exocytosis and endocytosis are different processes that utilize different protein molecules. Page 3 of 5 Endocytosis: (Gr. endon, within, + kytos, cell) The process whereby a cell ingests macromolecules, particulate matter, and other substances from the extracellular space. Mechanisms of endocytosis: (Figure 3) 1. Phagocytosis (cell eating) It is a nonselective process of engulfing larger particulate matter, such as microorganisms, cell fragments, and degenerated cells (e.g., defunct red blood cells), by specialized cells known as phagocytes. The engulfed material is enclosed in a vesicle called the phagosome. 2. Pinocytosis Nonselective process where small invaginations of the cell membrane form and entrap extracellular fluid forming a pinocytotic vesicle that pinch off from the cell surface and may fuse with lysosomes (see the section on Lysosomes later in this chapter). Receptor-Mediated Endocytosis It is a selective process that involves engulfment of macromolecules. This process depends on interaction between two structures: Receptor proteins (cargo receptors) in the cell membrane. Cargo receptors are transmembrane proteins. Macromolecules (ligands), (What is ligand?) such as low-density lipoproteins and protein hormones that bind with its receptor on the cell membrane. The receptors are either originally widely dispersed over the surface or aggregated in special regions called coated pits. Binding of the ligand to its receptor causes widely dispersed receptors to accumulate in coated pits. The coated pit invaginates and pinches off from the cell membrane, forming a coated vesicle that carries both the ligand and its receptor into the cell. Page 4 of 5 Note: Caveolae is a special form of endocytosis where the coating protein is caveolin (will be discussed later) Fate of the endocytotic vesicle (Figure 4) The coated vesicles soon lose their clathrin coat and fuse with early endosomes near the periphery of the cell, a system of vesicles and tubules located in the cytosol near the cell surface. The clathrin molecules separated from the coated vesicles are moved back to the cell membrane to participate in the formation of new coated pits. If some contents of the early endosome require degradation, they are transferred deeper in the cytoplasm into the late endosome. This similar set of tubules and vesicles, located deeper in the cytoplasm near the Golgi apparatus, helps to prepare its contents for eventual destruction by lysosomes. The membranes of all endosomes contain ATP-linked H+ pumps that acidify the interior of the endosomes by actively pumping H+ ions into the interior of the endosome. Fate of the Endosome contents Receptors that are separated from their ligand by the acidic pH of the early endosomes may return to the cell membrane to be reused. For example, low-density lipoprotein receptors are recycled several times. The ligands typically are transferred to late endosomes. However, some ligands are returned to the extracellular milieu to be used again. An example of this activity is the iron-transporting protein transferrin. Occasionally, both the receptor and the ligand (e.g., epidermal growth factor and its receptor) are transferred to the late endosome, and then to a lysosome, for eventual degradation. (Fig 4) Fig. 4 Formation of early & late endosomes Page 5 of 5 2 Lipid Structure & functions in bio-membranes: ILOs By the end of this lecture, students will be able to 1. Describe structure-function relationship of phospholipids 2. Describe the structure-function relationship of cholesterol 3. Interpret their importance in controlling cell membrane function Membranes can be regarded as a “fluid mosaic model” the fluid nature of the membranes allows greater flexibility to the cell than it would if the membranes were rigid. It also allows the motion of membrane components, required for some types of membrane transport as membranes are dynamic structures. The cell membrane is an asymmetric structure with the two sides of membrane being structurally and functionally different. This difference is due to the difference in composition and orientation of lipids, proteins and carbohydrates. ❖ How can this membranes’ structure serve its function? o Phospholipids: Phospholipids are ionic compounds. Like fatty acids (FA), phospholipids are amphipathic in nature. That is, each has a hydrophilic head, which is the phosphate group plus whatever alcohol is attached to it and a long, hydrophobic tail containing FA. There are two classes of phospholipids, those that have glycerol as a backbone (GLYCEROPHOSPHLIPIDS or PHOSPHOGLYCERIDES ) and those that have sphingosine as a backbone (SPHINGOPHOSPHOLIPIDS). ✓ Glycerophosphlipids (Phosphoglycerides): Of the two major phospholipid classes present in membranes, phosphoglycerides are more common and consist of a glycerol backbone to which are attached two fatty acids in ester linkage and a phosphorylated alcohol. Phosphoglycerides structurally similar to Figure (10-1): Triglycerides Vs Phospholipids triglycerides except that on the third carbon of glycerol instead of a fatty acid there is a phosphate attached to a polar head group. The fatty acid constituents are usually even-numbered carbon molecules, most commonly containing 16 or 18 carbons. They can be saturated or unsaturated with one or more cis double bonds. Page 1 of 5  The simplest phosphoglyceride is Phosphatidic acid, which is 1,2- diacylglycerol 3-phosphate, a key intermediate in the formation of other phosphoglycerides In most phosphoglycerides present in membranes, the phosphate is esterified to an alcohol such as ethanolamine and choline or glycerol. Lecithin (phosphatidyl choline): - It is the most abundant phosphoglyceride. The phosphorylated alcohol here is choline. - It represents a large proportion of the body’s store of choline. - Choline is important in nervous transmission. - Lecithin is also a major constituent of the surfactant preventing adherence, due to surface tension of the inner surfaces of the lungs. - Its absence from the lungs of premature infants causes respiratory distress syndrome. - There is a certain enzyme called lecithinase enzyme present in the venum of cobra. It splits the unsaturated FA from lecithin in cell membrane giving rise to lysolecithin. This substance can produce lysis of red cell membrane and haemolysis. Cephalin (phosphatidyl ethanolamine): It is another abundant phosphoglycerols (phosphoglycerides) which is also found in cell membranes. The base is ethanolamine. Cephalin is one of the important blood clotting factors. ✓ Sphingophospholipids: The second major class of phospholipids is composed of Sphingomyelin, which contains a sphingosine backbone rather than glycerol. A fatty acid (usually longer and less saturated than that of phophoglycerides) is attached by an amide linkage to the amino group of sphingosine, forming ceramide. The primary hydroxyl group of sphingosine is esterified to phosphorylcholine, forming sphingomyelin. As the name implies, sphingomyelin is prominent in myelin sheaths. They play a major role in signal transmission and cell recognition. Figure(10-2): Structure of Sphingomyein Page 2 of 5  ❖ What is the Structure-Function Relationship of phospholipids? Phospholipids are the predominant lipids of cell membranes. In membranes, the hydrophilic (polar) head of the phospholipid extends outward, interacting with the intracellular or extracellular aqueous environment. While, the hydrophobic portion of a phospholipid molecule is associated with the nonpolar portions of other membrane constituents, such as glycolipids, proteins, and cholesterol. The hydrophobic tail containing the fatty acids plays a major role in maintaining the fluidity of the Figure (10-3): Structure of membrane membrane, unsaturated fatty acids add to membrane fluidity. These unsaturated fatty acids form kinks that increase the space between the phospholipids increasing the membrane’s fluidity. In addition, the increased space allows certain small molecules to cross the membrane quickly and easily. In the same time, they form weak non-covalent bonds with each other, holding the bilayer together. Therefore, phospholipid molecules are responsible for the semi-permeability of the cell membrane. They prevent passage of water soluble substances and ions; however they allow passage of small non-polar and fat soluble substances. The amounts and fatty acid compositions of the various phospholipids vary among the different cellular membranes. Clinical Implications: There are many diseases associated with problems in the ability of the phospholipid bilayer to perform these functions. One of these is Alzheimer’s disease, characterized by brain shrinkage and memory loss. One idea explaining why Alzheimer’s disease occurs is the formation of plaque sticking to the phospholipid bilayer of the brain neurons. Interference with the membrane’s phospholipids block communication between the brain neurons, eventually leading to neuron death and in turn causing the symptoms of Alzheimer’s, such as poor short-term memory. One of the facts about the Fluid-Mosaic membrane model is that the components of the bilayers are free to move. Phospholipids have several types of movements, rotational where phospholipid rotates on its axis to interact with its immediate neighbors. lateral, where the phospholipid moves around in one leaflet. Finally, it is possible for phospholipids to Page 3 of 5  move between both leaflets of the bilayer in transverse movement, in a “flip-flop” manner, these movements are important for cell signalling. o Cholesterol: Compounds containing 27 carbon structures with four rings. A Cholesterol is a very important steroid to the body. When cholesterol binds to a fatty acid it forms a Cholesteryl ester. Cholesteryl esters have a lower solubility in water due to their increased hydrophobicity. In the memebrane cholesterol can be present on it’s own or combined B with proteins Figure (10-4): A: Cholesterol Cholesterol Helps control the Fluidity of Cell Membranes: B:Cholesteryl - Cholesterol is an amphipathic molecule, meaning, like Ester phospholipids, it contains a hydrophilic and a hydrophobic portion. - The cholesterol molecules are randomly distributed across the phospholipid bilayer. - Cholesterol's hydroxyl (OH) group aligns with the phosphate heads of the phospholipids. The remaining portion of it tucks into the fatty acid portion of the membrane. - Because of the way cholesterol is shaped, part of the steroid ring is closely attracted to part of the fatty acid chain on the nearest phospholipid. This helps slightly immobilize the membrane (decrease the fluidity) and make it less soluble to very small water- soluble molecules that could otherwise pass through more easily. Figure (10-5): Cholesterol in - Without cholesterol, cell membranes would membrane be too fluid, not firm enough, and too permeable to some molecules. - However, in cold temperature Cholesterol increases the fluidity as it helps separate the hydrophobic tails of phospholipids so that the fatty acid chains can't come together and cyrstallize. Cholesterol Helps Secure Important Proteins in the Membrane: - The plasma membrane contains many proteins that perform important functions, because certain proteins' size or shape requires a thicker phospholipid bed to sit in, and because certain proteins need to stick together to function properly, the fluidity of the cell membrane where the molecules are constantly moving randomly, could pose a problem. Page 4 of 5 - In these areas the plasma membrane contains high concentrations of cholesterol and glycosphingolipids which aggregate more making these parts of the membrane thicker, and making it ideal for accommodating certain proteins. - The previous association of cholesterol and glycosphingolipids together with proteins form “lipid rafts” which are microdomains of the plasma membrane that function to organize and regulate membrane signalling. - A special type of lipid rafts is the “Caveolae” which are flask like invaginations in the plasma membrane (while lipid rafts are generally flat regions) they are mainly present in fat cells and muscle cells. - Caveolae plays an important role in cell signalling, they can bud from the plasma membrane to form a vesicle for endocytosis (cellular process in which substances are brought into the cell) or can flatten into the membrane to help cells with stand mechanical stress. Page 5 of 5