Cell Biology Course (Bio-1101) Lecture 3: Cell Membrane PDF

Cell Biology Course (Bio-1101) Lect. 3: Cell membrane Assoc. Prof. / Reda M. Mansour Fluid mosaic model of plasma membrane 1- Fluid because elastic fluid lipid molecules 2- Mosaic because it consisted of different components (lipid + Protein + Carbohydrate). E-Phase P-Phase Outside cell =...

Cell Biology Course (Bio-1101) Lect. 3: Cell membrane Assoc. Prof. / Reda M. Mansour Fluid mosaic model of plasma membrane 1- Fluid because elastic fluid lipid molecules 2- Mosaic because it consisted of different components (lipid + Protein + Carbohydrate). E-Phase P-Phase Outside cell = extracellular Plasma membrane = E-Face Inside cell; intracellular = P- face Plasma membrane  Cell membrane (= Plasma lemma = Plasma membrane) A. Structure: - It is about 7.5 nm thick. - It consists of an inner and an outer leaflet called the phospholipid bilayer with integraI (intrinsic) and peripheral (extrinsic) proteins. Lipid and protein molecules are sometimes attached to carbohydrates. - The inner leaflet faces protoplasmic face (P-face) while the outer leaflet faces the extracellular matrix (E-phase). - Under transmission electron microscopy (TEM), it is a trilaminar structure called unit membrane. B. Function: 1. Structural and functional integrity of cell. 2. Semipermeable; allow some molecules to enter cell & prevent others to enter. 3. Molecules recognition and transport inside and out side the cell. 4. Signal transduction. 5. Cell to cell contact and communication. 5. Potential difference maintenance in nervous tissue.  Cell Membrane (Fluid mosaic model) A. Lipid molecules: - It is composed of 1- Phospholipids (Lipid + Phosphate group) 2- Glycolipids (Lipid + Carbohydrate) 3- Cholesterol. - It is freely permeable to small, lipid-soluble, nonpolar molecules but is impermeable to charged ions. - Composition: 1. Phospholipids (major content of membrane): - It is amphipathic and consisted of: a- Polar (hydrophilic) head: Glycerol + Phosphate group b- 2 nonpolar (hydrophobic) fatty acid tails (16-18 carbon chain): one saturated and one unsaturated. 2. Glycolipids (Lipid + Carbohydrate): on outer leaflet (toward extracellular). 3. Cholesterol, only 2% of plasmalemma lipids, in both leaflets. Note: Fluidity: - It is because elastic fluid lipid molecules a. Fluidity increases with increased temperature and decreased with saturation of the fatty acyl tails. b. Fluidity decreases with an increase in the membrane's cholesterol content. A phospholipid particle  B. Membrane proteins * They are of two types: 1- Peripheral proteins (extrinsic = on external or internal surfaces): Proteins interacts with other membrane proteins or with the hydrophilic head part of the lipid bilayer. 2- Integral proteins (intrinsic = within lipid bilayer): - Proteins are embedded in phospholipid bilayer. - Proteins interacts with the hydrophobic part of lipid bilayer. * Protein to lipid ratio in is about 1:1 in cell membranes of most cells while 1:4 in myelin cells. * Proteins are immobile (held in place by cytoskeleton) or diffuse laterally in the phospholipid bilayer.  Integral proteins According to structure, these proteins are divided into: 1- Non-transmembrane proteins not span the lipid bilayer. 2- Transmembrane proteins span the lipid bilayer& divided into: a- Single pass transmembrane - proteins pass only once through membrane. b- Multipass transmembrane proteins - proteins loops and crosses the membrane several times. Note: 1- Siglecs (sialic acid-binding immunoglobulin-type lectins) are transmembrane proteins present on normal cells and signal immune cells that normal cells are self and should not be killed. Certain cancer cells express sialic acid residues on their surface to evade immune system and not killed. 2- Channels, carrier, and pumps are transmembrane protein. Types of membrane proteins according to structure Transmembrane protein Non-transmembrane protein C. Glycocalyx (cell coat): It is located on the outer surface of plasmalemma. 1. Composition. It consists of polar oligosaccharide linked covalently to most proteins (glycoproteins) and some lipids (glycolipids) of the plasma lemma. 2. Function: Glycocalyx helps in: a. Attachment to extracellular matrix. b. Binding of antigens and enzymes to the cell surface. c. Cell–cell recognition and Communication. d. Protection. e. Activation of immune cells.  Major Classes of Membrane Proteins Class Functions Receptor Binds to signaling molecules (Signal transduction). Enzyme e.g., energy utilization , digestion,... etc. Channel Transports ions and small molecules according to conc. gradient without the expenditure of energy (passive diffusion). Pump Transports ions and small molecules against conc. gradient with expenditure of energy (active transport). Carrier Bind small molecules and transport them via reversible conformational changes (act with passive diffusion & active transport). Linker Attach cell to the extracellular matrix protein Structural Attach neighboring cells to each other protein  Some functions of membrane proteins 1- Signal transduction. 2- Enzymatic activity. 3- Transport. 4- Cell-cell recognition. 5- Attachment to the cytoskeleton & extracellular matrix. Cell membrane transport processes  Cell membrane transport processes These processes include transport of a single molecule (uniport) or cotransport of two different molecules in the same (symport) or opposite (antiport) direction. A. Passive transport: - It requires no energy. - Molecules move across the plasma membrane down concentration or electrochemical gradient. 1. Simple diffusion: - It occurs through phospholipid bilayer and exhibits little specificity for transported molecules. - It needs no ion channels or carrier proteins. - transport small nonpolar molecules ( e.g., O2 and N2) and small, uncharged polar molecules ( e.g., H2O, CO2, glycerol). 2. Facilitated diffusion - It occurs via ion channels and/or carrier proteins. - It exhibits specificity for the transported molecules. - It is faster than simple diffusion. - Types: a. Ion channel proteins permit small water-soluble molecules and ions such as Cl-, K+, Na+ b. Aquaporins are channels for the rapid transport of water. c. Carrier proteins undergo reversible conformational changes to transport specific molecules. These proteins function in both passive and active transport. Notes: 1- Cystinuria is a hereditary condition caused by abnormal carrier proteins that are unable to remove cystine from the urine, resulting in the formation of kidney stones. 2- Cystic fibrosis is a hereditary disease with malformed chloride channel proteins. So, NaCl increases in cell causing water entry and secretion of mucin (mucus) causing obstruction of bronchiolar pass way of lung. Examples of ions channel: 1- K+ leak ungated channels: - They are always open are the most common ion channels. 2- Voltage-gated channels :open due to the potential difference across the membrane changes. 3- Mechanically gated channels: - Open in response to a mechanical stimulus. 4- Ligand-gated channels: - Open in response to signaling molecule. Ligand-gated ion channels are probably the location where anesthetic agents act to block the spread of action potentials. Note: - lonophores are lipid-miscible molecules that bind with ions and insert them into the phospholipid bilayer of cell membrane into the cell. lonophores are frequently fed to cattle and poultry as antibiotic agents and growth-enhancing substances.  B. Active transport: - It requires energy (ATP) and transport molecules against concentration & electrochemical gradient via carrier proteins or pump. 1. Primary active transport e.g. Na+-K+ pump that transport 3 Na+ molecules out of cell and 2 K+ into the cell ( = in opposite directions or anti port) to maintain constant cell volume and mediated by Na+-K+ adenosine triphosphatase (ATPase). 2. Secondary active transport e.g. glucose transport involves the symport (in same direction) of glucose and Na+ molecule across enterocytes. Notes: ATP-binding cassette transporters (ABC transporters): In eukaryotes, ABC transporters function to export toxins and drugs from the cytoplasm into the extracellular space using ATP. Multidrug-resistant (MDR) proteins are ABC transporters that are present in certain cancer (malignant) cells that are able to transport the cytotoxic drugs outside cells. So, drugs become less effective.  Osmosis It is the movement of water from hypotonic solution (with low salt conc.) to hypertonic solution (with high conc. of salt). (a) Animal cell: e.g. red blood cell ruptured (lysed) when placed in hypotonic soln. & shrinkage (shriveled) when placed in hypertonic soln. (b) Plant cell: Plant cells become turgid in hypotonic soln. and plasmolysed when placed in hypertonic soln. Uptake (Endocytosis) and release (Exocytosis) of large materials by animal cells via vesicles.  I- Endocytosis There are 3 types of endocytosis: 1- Phagocytosis (cellular eating) 2- Pinocytosis (cellular drinking) 3- Receptor-mediated endocytosis. 1- Phagocytosis: - It is the engulfment of solid particles (e.g. bacteria) by pseudopodia forming food vacuole which then mixed with hydrolytic enzymes of lysosomes. e.g. Macrophages and neutrophils engulf bacteria. 2- Pinocytosis: - It is the uptake of liquid material forming pinocytic vesicle. - The plasma membrane coated pits & coated vesicle. 3- Receptor-mediated endocytosis: - It is the engulfment of liquid material & particles with the help of receptor.  II. Exocytosis: - It is the release of material from the cell by fusion of a secretory granule membrane and the plasma membrane. Types: a. Regulated secretion: - It is the release of stored materials (usually in vesicles) in response to extracellular signal - e.g. release of enzymes & neurotransmitter. b. Constitutive secretion: - It is continuous release of material (e.g., collagen and plasma proteins) without any storage or signal step. Venoms, such as those of some poisonous snakes, inactivate acetylcholine receptors of skeletal muscle sarcolemma at neuromuscular junctions. Graves' disease (hyperthyroidism) is an autoimmune disease in which antibodies specifically bind specifically to plasma membrane receptors leading to become overactive. 1. A herpetologist is bitten by a poisonous snake and is taken to the emergency department with progressive muscle paralysis. The venom is probably incapacitating his (A) Na+ channels. (B) Ca2+ channels. (C) phospholipids. (D) acetylcholine receptors. (E) spectrin. 2. Cholesterol functions in the plasmalemma to (A) increase fluidity of the lipid bilayer. (B) decrease fluidity of the lipid bilayer. (C) facilitate the diffusion of ions through the lipid bilayer. (D) assist in the transport of hormones across the lipid bilayer. (E) bind extracellular matrix molecules