Cell Membrane Structure and Composition PDF

Thi s Photo by Unknown author is licensed under CC BY-SA. Cell Membrane Structure and Composition  Hereditary spherocytosis Hereditary spherocytosis is a genetic disease and a type of hemolytic anemia. Anemia is characterized by symptoms such as fatigue, dizziness, hair...

Thi s Photo by Unknown author is licensed under CC BY-SA. Cell Membrane Structure and Composition  Hereditary spherocytosis Hereditary spherocytosis is a genetic disease and a type of hemolytic anemia. Anemia is characterized by symptoms such as fatigue, dizziness, hair loss and eye yellowing. In hereditary spherocytosis, red blood cells lose their biconcave, flexible shape (necessary to pass through narrow capillaries) and become spherical. In this disease, proteins attaching the cell membrane to the cytoskeleton are dysfunctional (ex: spectrin, ankyrin)  Red blood cell membrane: an Illustration of cell membrane structure Plasma membranes is supported Spectrin (part of the by a meshwork of proteins called the cell cortex in RBCs) Cell Cortex (actin, myosin, and actin- binding proteins such as spectrin and ankyrin in red blood cells) Plasma membrane is a thin fatty film studded with proteins and coated with carbohydrates. The carbohydrates attached to the plasma membrane proteins and Ank:ankyrin lipids on the outside of the plasma Sp: Spectrin membrane form a sugar coating Act: actin Plasma called glycocalyx. membrane Glycocalyx  All plasma membranes are composed of lipids and proteins All plasma membranes (either surrounding the cell or the organelles) consist of lipid and proteins. Membrane lipids are arranged in two sheets called lipid bilayer.  Plasma membrane lipids The lipids in the plasma membrane are phospholipids (major lipid components), cholesterol and glycolipids. All plasma membrane lipids are amphipathic (they have a hydrophilic head and a hydrophobic tail).  Phosphatidylcholine: an example of phospholipid Phosphatidylcholine is the most common phospholipid in biological membranes. The hydrophilic head of phosphatidylcholine is composed of a choline molecule and a phosphate group. Glycerol links the hydrophilic head to the hydrophobic tails. The hydrophobic tails are composed of two fatty acids (14 and 24 carbon atoms). The hydrophobic tails could be saturated (no double bonds between the carbon atoms) or unsaturated (with double bond(s) between the carbon atoms). Plasma membrane phospholipids form a lipid bilayer The hydrophilic heads face water on both surfaces of the bilayer The hydrophobic tails are all shielded from the water and lie next to one another in the interior (like the filling in a sandwich).  Phospholipid bilayers form sealed compartments Phospholipid bilayers spontaneously close in on themselves to form sealed compartments. The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water. The layer of the lipid bilayer facing the cytosol is called the cytosolic monolayer (or face) and the layer facing the exterior of the cell or the lumen of an organelle (ex:Golgi, ER) is called the non-cytosolic monolayer (or face).  The Lipid bilayer is a flexible two-dimensional fluid The plasma membrane is flexible and its components (lipids and proteins) can move freely within the layer. Phospholipids can rotate, move laterally within the same layer and flip to the other layer (rarely, with the aid of flippase enzymes). Therefore, the plasma membrane is described as fluid.  Several factors affect the fluidity of the plasma membrane The temperature ❖ fluidity increases as the temperature increases. Lipid composition ❖ cholesterol tends to stiffen plasma membranes. Phospholipid tail saturation degree: ❖ lipid bilayers with unsaturated hydrocarbon tails are more fluid. Phospholipid tail length: ❖ lipid bilayers with shorter fatty acid chains are more fluid.  New membrane is synthesized on the SER Phospholipids are made and incorporated into the cytosolic face of the smooth endoplasmic reticulum plasma membrane. The phospholipids are randomly distributed to the non-cytosolic face of the smooth endoplasmic reticulum’s membrane.  New membrane is then matured in the Golgi membrane The new plasma membrane is then delivered to the Golgi plasma membrane. In the Golgi membrane, specific phospholipids are transferred back to the cytosolic monolayer (ex: phosphatidylserine). Therefore, the plasma membrane is described as asymmetrical. Phosphatidylserine (light green) Phosphatidylethanolamine (yellow) Phosphatidylcholine (red) Sphingomyelin (brown)  Phospholipids and glycolipids are distributed asymmetrically Glycolipids are in the non-cytosolic monolayer of the lipid bilayer and face the exterior of the cell. phosphatidylethanolamine (yellow) Phosphatidylcholine (red) phosphatidylinositols (dark green) Sphingomyelin (brown) Glycolipids (hexagonal blue) Phosphatidylserine (light green) cholesterol (green)  Plasma membrane asymmetry is preserved during membrane transfer Plasma membrane is transported by a process of vesicle budding (from the Golgi apparatus) and fusion (with the cell membrane or the plasma membrane of other organelles). Membranes retain their orientation during transfer between cell compartments (lipids in the cytosolic monolayer always face the cytosol). Most plasma membrane functions are carried out by membrane proteins Plasma membrane proteins have a variety of functions: ❖ Transporters ❖ Ion channels ❖ Anchors ❖ Receptors ❖ Enzymes  Membrane proteins associate with the lipid bilayer in different ways Multipass Single pass β barrel There are two general types of membrane proteins: ❖ Integral transmembrane proteins, monolayer-associated, lipid-linked (anchored) ❖ Peripheral proteins-attached or lipid-attached. The distribution of membrane proteins is also asymmetrical.  Transmembrane proteins usually cross the lipid bilayer as an α Helix The backbone of a polypeptide chain is hydrophilic. Single pass transmembrane proteins have a hydrophobic region.  Transmembrane proteins usually crosse the lipid bilayer as an α Helix Multi pass transmembrane proteins have amphipathic alpha helices (plural of helix). Plasma membrane proteins can move laterally in the lipid bilayer Staining membrane proteins allows us to visualize membrane fluidity.  The movement of membrane proteins can be restricted Cells can confine particular proteins to localized areas: ❖ by binding the cell cortex. ❖ by binding extracellular matrix molecules. ❖ by binding proteins on the surface of another cell. ❖ be restricted by barriers. Example: epithelial cells in the gut tight junctions prevent proteins from the apical plasma membrane to mix with proteins in the lateral and basal plasma membrane  The cell surface is coated with carbohydrates All carbohydrates added to proteins and lipids face outside of the cell. This sugar coating is called the carbohydrate layer or glycocalyx. The carbohydrates of glycoproteins and proteoglycans often function in cell recognition and adhesion. Glycocalyx allows cell-cell adhesion  The functions of the cell membrane All these properties allow the cell membrane to get involved in various essential functions such as: ❖ Cell signaling ❖ Transport ❖ cell growth and motility ❖ Cell-cell recognition ❖ intercellular adhesion  Learning outcomes Describe the structure, location and function of the cell cortex. Explain how lipid bilayer is formed and why is it considered fluid and asymmetrical. List the main membrane components, explain how they move within the lipid bilayer and how their movement (membrane fluidity) is regulated and restricted. Illustrate in a diagram a cell membrane with its phospholipids and different membrane proteins. Include examples of lipid bilayer asymmetry. Predict how removal (or inhibition) of any components of the cell membrane affects its properties and function. Predict the results of cell fusion experiments. Contrast different types of integral proteins (single pass, multipass, β-barrel transmembrane protein, anchored protein, etc.) and integral and peripheral proteins in terms of structure and function.  Learning outcomes Explain how the polypeptide chain of a transmembrane protein, with its hydrophilic backbone, can span the hydrophobic interior of the lipid bilayer. Explain where and how new membrane is synthesized and explain how phospholipids are asymmetrically distributed to produce even lipid bilayers. Describe how membranes retain their orientation during transfer between cell compartments. Describe the structure and function of the glycocalyx and with an example explain its role in tissue formation.  Keywords you need to know before studying this lecture ❖ Amino acid ❖ Amino acid side chains ❖ Hydrophobic side chaines ❖ Hydrophilic side chaines ❖ Charged/uncharged molecules ❖ Hydrogen bond ❖ Lipid ❖ Polar /unpolar molecules ❖ Partial negative charge ❖ Partial positive charge ❖ Primary structure of a protein ❖ solute/solvent/solution ❖ Single/double covalent bond ❖ Secondary structure of a protein ❖ Protein N and C terminal