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 membranes Protects and maintains the integrity of a cell

Cell membrane – the background Is selectively permeable to ions and organic molecules Controls the movement of substances in and out of the cells. The fundamental structure consists of Phospholipid Bilayer with embedded proteins It is thin, Pliable and 7-10nms thick and envelops the cell

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

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

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

Cell membrane - Rail road track

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 one mole of protein = 50 -100 moles of lipids

Membranous proteins – the synthesis the Ribosomes Membranous proteins are synthesized in the Rough Endoplasmic Reticulum ( RER) of all the cells Are transported to the Golgi apparatus and further to the plasma membrane CHOs groups are added to the polypeptide chains both in RER and Golgi apparatus Most of the Trans membrane proteins are Glycoprotein with their oligosaccharides exposed on cellular surface

Membrane proteins – Types Two (2) Major membranous proteins are two - 2 Based on the nature of the membrane + protein interactions Integral/ Tran membrane/intrinsic proteins Peripheral membrane/Extrinsic proteins

Membrane proteins- Integral proteins These proteins cross the entire thickness of the membrane and is its structural component Cannot be removed without disrupting the cell Integral proteins are the abundant type of Glycoproteins to span the lipid Bilayer and : May include Ion channel and Transport proteins, (GPCR) Protein Coupled Receptors (GPCR) and LDL receptors

Peripheral proteins – do not pass through 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 They do not cross through the hydrophobic lipid Bilayer they can be removed from the membrane through polar agents without disrupting the phospholipids Bilayer

Integral and Peripheral proteins

Tran membrane/Integral proteins of RBC the types Transmembrane proteins cross the lipid Bilayer and have lateral mobility in the membrane Major ones are Glycoproteins, Glycophorins and Band 3 Aquaporin RH(Rhesus) proteins The red cell glucose transporters (GLUT-1) Band 3 is an ion transporter and is a Dimmer/tetramer Band 3 Tetramer; crosses the lipid Bilayer. Its Cytoplasmic domain interacts with the cytoskeleton via : Ankyrin and Spectrin

Trans membrane Proteins of RBC

Tran membrane proteins of RBC Glycophorins, general proteins that contribute to Glycosalytion Glycophorin C crosses the Bilayer to interact with Protein 4.1 with in the junctional complex of Cytoskeleton Aquaporin is a membrane water channel protein RH (Rhesus) proteins are associated with Band 3 and 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.

Integral proteins of RBC – Two (2)

Integral proteins of RBC – Band 3 Band 3 is a large glycoprotein with 929 amino acids and 14 spanning α regions/sites It is an anion transporter of HCO3¯ (bicarbonates) and Cl¯ for red blood cell membrane. It has 2 globular structure containing internal channels : for anions to travel across the lipid Bilayer

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 cases phospholipids are the most abundant In RBC studies 30% of the plasma membrane is lipid

Plasma membrane composition - the lipids

Membranous lipids/lipid Bilayer - self assembled According to fluid mosaic model, the biological membranes can be considered as a 2 dimensional liquid : Where all lipids and proteins molecules diffuse more or less easily . Lipid Bilayer go through self assembly process in the formation of membranes in Endoplasmic Reticulum

Phospholipid Bilayer – the composition It primarily consists of a thin layer of amphepathic phospholipids on both the : Cytoplasmic and extracellular surfaces resulting in Bilayer The head is composed of Phospholipids and Hydro phobic tail is in the middle having CH and Fas It is maintained by the hydrophobic and non covalent inter- actions and H2 bonds

Lipid Bilayer and Membrane fluidity The FAs in lipid Bilayer are usually unsaturated; have kinks, unable to pack and therefore they : move freely in the interior of the membrane 1 So that the membrane is soft and flexible 2 It also allows the movements of the molecules laterally, and prevents vertical movements 3

Phospholipid Bilayer

lipid Bilayer – the significance 2 aspects Lipid Bilayer is critical for membrane function: Firstly, as barrier B/W the 2 aqueous compartments as interior is occupied by hydrophobic moles FAs 1 Phospholipid Bilayer is viscous fluid not solids 2

Membranous Carbohydrates Carbohydrates form a layer on the extracellular surface of the cell membrane – the Glycocalyx It protects the cell from physical and chemical injury and act as adhesions to attach the cells together Provides attachments to the agents like Bacteria/Virus

Significance of carbohydrates Carbohydrates are attached to the Lipids as Glycolipids on the outer cell surface as : blood group antigens- blood transfusions Glycoproteins are Proteins to which CHO are attached These serve as cell surface receptors and hormones (G- Protein Coupled receptors(GPRC) – All Pituitary hormones)