Cell membrane composition Lecture NO:1, 1st MBBS Dr Muhammad Ramzan

Cell membrane – the definition Is a biological membrane that separates the interior of all cells from the outside environment Also called as plasma membrane Protects and maintains the integrity of a cell www.biology - on line.org

Cell membrane – the background Is selectively permeable to ions and organic molecules Controls the movement of substances in and out of the cells. Source of Communication B/W with the cells Fundamental structure consists of Phospholipids Bilayer with embedded proteins It is thin, Pliable and 7-10nms thick and envelops the cell www.Encyclopedia britannica.com

Plasma membrane – electron micrograph Rail road track The plasma membrane is referred to as rail road track that appears as Dense lines on : Electron micrographs that run parallel and are separated by an intertwining space (space between the 2 lines appears divided)

Cell membrane - Rail road track

Cell membrane composition Proteins, Lipids and CHOs Cell membrane consists of: Proteins (55%) Phospholipids (25%), Cholesterol (13%) Other lipids (4%) And Carbohydrates (3%).

Membrane proteins – the major component Proteins are the major component of the plasma membrane and consist of 50-55% by weight Proteins are much larger molecules than lipids 1/one mole of protein = 50 -100 moles of lipids

Synthesis of Membranous proteins The Ribosome Membranous proteins are synthesized in the RER of all the cells Are transported to the Golgi and further to the plasma membrane for their maintenance CHOs groups are added to the polypeptide chains both in RER and Golgi apparatus Most of the Trans membrane proteins are Glycoproteins with their oligosaccharides exposed on their cellular surface

Membrane proteins – Types Two (2) Major membranous proteins are of 2 types based on the Nature of the membrane and protein interactions Integral/ Tran membrane/intrinsic proteins 1 Peripheral membrane/Extrinsic proteins 2

Integral /Trans membrane proteins Exposed to aqueous environment on both sides These proteins cross the entire thickness of the membrane and are its structural/functional component 1 Are firmly attached to membrane and lipid Bilayer Cannot be removed without disrupting the cell 2 Are mostly Glycoproteins to span the lipid Bilayer 3 Include: Ion channels; Transport proteins, GPCR, LDL –receptors and enzymes www.biology - on line.org

Trans membrane/Integral Proteins

Trans membrane Proteins - GPCR

Peripheral proteins - Characteristics Do not pass through the lipid Bilayer Are Indirectly associated with the membrane through protein - protein interactions or Ionic bonds or Directly to the surface of integral proteins, either on the Cytoplasmic or extracellular side 1 They do not cross through the lipid Bilayer 2 They can be removed from the membrane through polar agents without disrupting phospholipids Bilayer 3

Integral and Peripheral proteins

Peripheral proteins – the role Some of the peripheral proteins are water soluble enzymes that act on the water soluble heads of the : membranous Phospholipids These Phospholipases hydrolyze various bonds in the heads of phospholipids and : Degrade the damaged or aged cell membranes

RBC membrane composition – A model special characteristics Plasma membrane of the Human Erythrocyte is a model for membrane studies as it has : No nuclei and internal membranes 1 Can easily be isolated for biochemical analysis 2 Studies of the RBC plasma membrane provided the first evidence of the existence of Phospho lipid Bilayer 3

Tran membrane proteins of RBC the types Major Trans membrane proteins are : Glycoproteins, Glycophorins and Band 3 Aquaporin RH(Rhesus) proteins The red cell glucose transporters (GLUT-1)

Tran membrane proteins of RBC Glycophorins are proteins that contribute to the Glycosylation Aquaporins is a water channel protein in renal tubules RH (Rhesus) protein is one of the most immunogenic human blood group system The Red cell Glucose transporter(GLUT1) is passive one and is present in other tissues also. Band 3 is an anion transporter for HCO3 and Cl͞

Trans membrane Proteins of RBC

Integral protein of RBC- Role of Band 3 anion transporters

Membranous lipids – 3 types The cell membrane consists of 3 classes of amphepathic lipids: Phospholipids, Glycolipids and Cholesterol The amount of each depends upon the type of cell In majority of the cases phospholipids are the most abundant In RBC studies 30% of the plasma membrane is lipid

Membranous lipids

Cell membrane – Fluid mosaic model pieces of elements are fitted togother It describes the structure of plasma membrane as a mosaic of its components which: Includes Proteins; phospholipids, Cholesterol and CHO It gives fluid character to the cell membrane in which proteins move freely and laterally in lipid Bilayer Fluidity is due to the Unsaturated FAs that are unable to pack www.dictionary.com

Lipid Bilayer and its composition Lipids and proteins It is primarily a thin layer of Phospholipids, both on the Cytoplasmic and extracellular surfaces : Resulting in Bilayer with embedded proteins The head is Hydrophilic with Phospholipids and tail is Hydrophobic having CH and FAs in the middle It is maintained by the : 1. hydrophobic, non covalent interactions and H2 bonds www.wikipedia.org

Phospholipids Bilayer

Lipid Bilayer and Membrane fluidity the basis The FAs are usually unsaturated; have kinks, unable to pack and move freely within the membrane as Fluid 1 The membrane is soft and flexible as a result. 2 It allows the movements of the moles especially of proteins laterally, and prevents vertical movements 3 Lipid Bilayer go through self assembly process for membrane formation in SER

lipid Bilayer – the significance Lipid Bilayer is critical for membrane function: It isolates the 2 aqueous compartments : Cytoplasmic and extra cellular from each 1 Ensures membrane Semi permeability 2 Maintains Fluidity of the cell membrane 3 Embedded proteins give Fluid Mosaic appearance to the membrane 4

Membranous Carbohydrates (CHO) Carbohydrates form a layer on the extracellular surface of the cell membrane – Glycocalyx It protects the cell from physical and chemical injuries and act as adhesions B/W the cells Prevents the invasion of pathological agents like Bacteria and Virus

Significance of carbohydrates Carbohydrates are attached to the Lipids as Glycolipids on the outer cell surface as : blood group antigens – important in blood transfusions Glycoproteins are Proteins with CHOs These serve as cell surface receptors GPCR and hormones like . All ant. Pituitary hormones)