Biologic Membranes Lecture Notes 2024 PDF

Biologic Membranes September 2024 Dr. Sameh Sarray Ph.D Department of Medical Biochemistry  EC Functions of Biological membrane ICF F ▪Selectively permeable (chooses what may cross the...

Biologic Membranes September 2024 Dr. Sameh Sarray Ph.D Department of Medical Biochemistry  EC Functions of Biological membrane ICF F ▪Selectively permeable (chooses what may cross the membrane); ▪It defines the difference (gradient) between ECF/ ICF. ▪Mediate cell-cell communications ▪The type of membranes may be identified by the presence of specific protein markers, E.g. Na,K-ATPase is a marker of plasma membrane, while succinate dehydrogenase is a mitochondrial marker Location for some chemical reactions  Major components of cell Cell membranes mainly consists of lipids, proteins and carbohydrates linked to lipids or proteins carbohydrates never in a free form Different membranes have different protein/lipid ratio: The chemical composition of cell membranes varies widely ratio of 3 components vary between these Loading… different types of cells.  > Phospholipids 1. Phosphatidylcholine amphipatic. P 2. etahnolamine. P serine 3. alcohol + phosphate hydropholic : hydrophobic : fatty acid chain hydrophilic I hydrophobic I 4· cardiolip in & &. p inositol 5. head (outward) tail (inward) fatty acid chain 6. Sphingomyelin  Phospholipid molecules are not attached to their neighboring phospholipid molecule by any chemical bonds, thus to some Loading… extent they can move, laterally in one bilayer, or undergo transverse movement between bilayers; this is how a cell can alter its shape!!!! Lateral movement (lateral diffusion) Lipids mobility is about 2 mm per sec Protein mobility varies from 10-4 to 0.4 mm/sec Rotation across the bilayer (transverse diffusion, flip- flop) A phospholipid molecules flip-flop once in several hours Proteins do not rotate  Lipid Bilayers Structure: Phospholipds bilayer Lipid shape and properties determine structures: micelles, vesicles (liposomes) Bilayer structure is asymmetric : The lipids composition is different between the two leaflets;  Cholesterol & 3 - main OH group component hydropholi - 27 carbon - steroid rings hydrophobic talls hydrocarbon hydrophobic - · amphipatic 5-25% of membrane content; cholesterol is dispersed throughout cell membranes between phospholipids: OH group is near the polar head group of phospholipids and the steroid rings and hydrocarbon tails are oriented parallel to those of phospholipids.. Cholesterol content reduce the side movement of phospholipids, and this affect the membrane fluidity  Glycolipids Carb + llpid The least abundant(< 5%). Exp cerebroside (Gal as sugar) Carbohydrates are attached to phospholipid molecules by covalent interactions The carbohydrate portion is always oriented toward the outside of the cell The carbohydrate facilitate cellular recognition and in the connections that allow cells to connect to one another to form tissues.  Lipid Rafts Lipid rafts are functional membrane microdomains Thicker than the remainder of the bilayer Enrich in cholesterol and sphingolipids Cholesterol acts as dynamic “glue” which holds the raft together. Lipid raft are involved in: Signal transduction Protein trafficking Membrane fluidity  Membrane Proteins Membrane proteins are classified into 2 major categories: inside -Integral proteins: are either partially or totally immersed in the lipid bilayer compeletly inside (transmembrane) (1-3) Loading… -Peripheral proteins or extrinsic proteins: Proteins that adhere only temporarily to the membrane to an integral membrane protein (4) or to the the polar head groups of the phospholipids (5).  Carbohydrates of Cell Membranes Minor component 5-8% Never as free form: They are attached to protein or lipid as glycoprotein or glycolipid The carbohydrate portion of the glycolipids or glycoproteins protrude to the outside of the cell. Carbohydrates in the outer face of the membrane form the glycocalyx.  FLUID MOSAIC MODEL fluidity comes Mosaic effect because now from the movement Protein is dispersed The plasma membrane is composed of different kinds of proteins dispersed in the lipid bilayer producing a mosaic effect The bilayer is fluid: the phospholipids can move with their hydrophobic tails unsaturated saturated (appropriate mixture of SFA and USFA that is fluid at normal temperature of the cell).  Membrane the hotter the more fluid the shorter chain the more fluid the more double bond the more fluid fluidity & cholesterol gives opposite effect Influenced by two major factors: temperature and lipid composition of the membrane: At low Temperature- fluidity is less As the temperature increases the hydrophobic side chains undergo a transition from the ordered state to the disordered state -increase fluidity Fluidity of membrane is maintained by: Length of hydrocarbon chain Degree of unsaturation Short chain FA increase the fluidity Long chain FA decrease the fluidity More the number of double bond- greater is the fluidity Unsaturated FA that exist in cis configuration increase the fluidity; Trans FA decrease the fluidity of membrane ↑ fluidity unsaturated transfa b fluidity Cholesterol effects: At high temperature, it interferes with the movement of the hydrocarbon chains and tends to reduce the membrane’s fluidity. In contrast, at low temperatures, cholesterol decreases the rigidity of the hydrocarbon chains. Cholesterol acts as buffer maintain , membrane fluidity  Membrane Proteins Proteins in membranes can be classified as: Receptors Transporters Ion channels Structural components Members of all of these classes are often glycosylated. The type of membranes may be identified by the presence of specific proteins markers. END!