Cell Membrane - Biochemistry II PDF

Course Title : Biochemistry II Course Code : FM111 Department : Biochemistry Lecture Title: Cell Membrane Lecturer Name : Ass. Prof. Dr. Omnia Safwat El-Deeb Lecture Objectives: Each student at this lecture should be able to: Define cell membrane...

Course Title : Biochemistry II Course Code : FM111 Department : Biochemistry Lecture Title: Cell Membrane Lecturer Name : Ass. Prof. Dr. Omnia Safwat El-Deeb Lecture Objectives: Each student at this lecture should be able to: Define cell membrane Determine cell membrane structure Know the different components of cell membrane Identify asymmetry of cell membrane Content: Cell membrane definition Cell membrane structure Different components of cell membrane Asymmetry of cell membrane  Prepared By: Ass. Prof. Dr. Omnia Safwat El-Deeb Ass. Prof. Dr. Rowaida Raafat Ibrahim Medical Biochemistry Department Faculty of Medicine. Tanta University  Biological membranes These are membranes that: - Separate the cell from the surrounding environment. - Subdivide the cell into compartments  Biochemical composition of biological membranes Biological membranes are formed of three main components: 1. Amphipathic lipids 2. Proteins 3. Carbohydrates The major lipids of the membranes are phospholipids (glycerol- phospho-lipids and sphingomyelin), cholesterol and glycolipids. Amphipathic lipids are self assembled into bilayer sheath with polar heads facing outwards and the non-polar tails facing each other (inside). 1. Peripheral (extrinsic) proteins: that are attached to the polar heads of the lipid bilayer by hydrophilic interactions. 2. Integral (intrinsic) proteins which have three forms: a. Completely hidden in the hydrophobic part (hydrophobic protein) b.Partly hidden in the hydrophobic part and partly attached to the hydrophilic heads c. Traversing proteins: attached to the lipid bilayer by hydrophobic and hydrophilic interactions. Functions of membrane proteins 1. Enzymes activity. 2. Acting as antigens, or receptors for hormones. 3. Acting as transport proteins (channels) for transport of small molecules. 4.Playing a special role in certain cell-surface activities (via glycoproteins and glycolipids) which: a. Keep cell individuality (by acting as identity markers). b. Develop cell adhesion capacity (where cells can adhere to other neighboring cells by means of interaction between their surface glycoproteins). Enzymatic protein Receptor protein Channel protein Recognition protein 1. Are either covalently attached to proteins (glycoproteins) and to a lesser amount to lipids (glycolipids). 2. Being located on the exterior side of the plasma membrane or the luminal side of the endoplasmic reticulum. 3. The sugar moiety includes: glucose, galactose, mannose, fucose, N-acetyl-galactosamine, N-acetyl-glucosamine, and sialic acid. 4- Functioning in : cell-cell recognition, adhesion, and receptor action  Described by: O (1) Fluid mosaic model & O (2) Asymmetry of biological membranes  Fluid Mosaic Model a.Are formed of two-dimensional layers (of amphipathic phospholipids and sterols with the non polar regions of the lipids facing each other and their polar heads facing outwards). Their thickness is 60-100 A. b. Are having globular proteins embedded at irregular intervals held by hydrophobic interactions (between the hydrocarbon chains of lipids and the hydrophobic amino acids of the protein). Hydrophobic interactions O Hydrophobic interactions occur between 2 or more nonpolar molecules when they're in polar environments (most commonly Water). O Their 'dislike' to water causes the molecules to stick together or fold in a certain way, in order to interact with the polar environment as little as possible c. Protrusion of some proteins from one or both sides , - Some traverse the entire width of the membrane, -Some proteins are loosely attached to the polar sides of the membrane Factors Affecting Membrane Fluidity 1-Fatty acid composition & sterol content affect fluidity: The more the fluidity of the membrane……. i. The more unsaturated FA & the more the short chain FA. ii.Increased membrane stiffness and decreased fluidity is provided by hydrophobic saturated FA (aligned in ordered manner). 2-Tempereture: The higher temperature the higher fluidity. Transition temperature at which the structure undergoes the transition from the ordered to disordered one. The higher saturated FA (SFA) content of membranes the higher the transition temperature. 3. Cholesterol (acts as modulator): in the membrane by producing intermediate state of fluidity via controlling the formation of crystalline form & prevention of excessive solidification or fluidity. 4. Integral proteins  Asymmetry of biological membranes: O 1) Regional asymmetry e.g. membranes of the villous border of mucosal cells are macroscopically visible, but those at gap and tight junctions and at synapses occupy a smaller region. O 2) Inside outside asymmetry: a) Lipid asymmetry: O Choline containing phospholipids e.g. phosphatidylcholine and sphingomyelin are more distributed on the outer layer. O Phosphatidyl-serine and phosphatidyl ethanolamine are more distributed on the inner layer. O Cholesterol is greatly present on the outer layer b) Protein asymmetry: O Proteins are irregularly distributed within the membranes. c) Carbohydrate asymmetry: O Carbohydrates are exclusively present on the outer surface of membranes. d)Specific enzymes: are present on the inside and outside of the biological membranes e.g. O In plasma membrane, there is 5'- nucleotidase, Na+/K+ Atpase and adenylate cyclase. O In mitochondrial membrane, there is ATP synthetase. Questions 1. Enumerate Functions of cell membrane 2. Classify inside outside asymmetry References: - Harper's Illustrated Biochemistry - Lippincott's illustrated Reviews, Biochemistry Thank You SUE