Lecture 2: The Cell Membrane (Molecular Biology 2024/2025) PDF

Lecture 2: The cell membrane Lecturer: Dr. Michelle Kuzma Adapted from: Dept. Head, Dr. Danuta Mielżyńska-Švach Molecular biology 2024/2025 Housekeeping ⚫ Slides will be shared ⚫ Please no recording o Video should be released online ⚫ Email contact regarding lectures: o...

Lecture 2: The cell membrane Lecturer: Dr. Michelle Kuzma Adapted from: Dept. Head, Dr. Danuta Mielżyńska-Švach Molecular biology 2024/2025 Housekeeping ⚫ Slides will be shared ⚫ Please no recording o Video should be released online ⚫ Email contact regarding lectures: o [email protected] ⚫ Textbook: Essential Cell Biology, 6th ed. ▪ Bruce Alberts Cell membrane functions Membranes in the cell  Membrane structure All membranes in cells are built according to the same scheme. They always consist of the following compounds: ❑ lipids ❑ proteins ❑ sugars (carbohydrates) bound to: o lipids (glycolipids) o proteins (glycoproteins)  Membrane lipids Membrane lipids are divided into three groups based on their chemical structure: ❑ phospholipids ❑ sphingolipids ❑ sterols  Phospholipids Phospholipids are lipids composed of: ❑ two fatty acids, ❑ glycerol (an alcohol), ❑ phosphoric acid, ❑ a functional group attached to the phosphate that confers characteristic properties.  Phospholipids Common functional groups that are linked to the phosphate residue are: ❑ ethanolamine, ❑ choline, ❑ inositol, ❑ serine.  Sphingolipids Sphingolipids are lipids composed of: ❑ sphingosine (a long-chain amino alcohol), ❑ a fatty acid, ❑ phosphoric acid (optional), ❑ a functional group (i.e., ethanolamine, choline, serine, etc.). Sphingolipids are divided into two subgroups: ❑ sphingomyelins ❑ glycolipids  Sphingomyelin Sphingomyelin is made up of: ❑ sphingosine, ❑ a fatty acid, ❑ phosphoric acid, ❑ a functional group (i.e., serine, ethanolamine or choline). Critical in: ❑ brain matter ❑ neural tissue ❑ myelin sheath of nerve endings Sphingomyelin structure  Glycolipids Glycolipids are made up of: ❑ sphingosine, ❑ a fatty acid, ❑ one or more sugar molecules. The simplest glycolipids are cerebrosides that contain glucose or galactose. More complex glycolipids are gangliosides that contain up to seven sugar residues.  Glycolipid structure Ceramide  Sterols Sterols are alcohols belonging to the family of steroids. The most important representative of animal sterols is cholesterol. Cholesterol is a cyclic compound containing a branched side chain. sterol skeleton Cholesterol  Structure of membrane lipids Membrane lipids contain one or two fatty acid residues ❑ the fatty acid residues contain an even number of carbon atoms (usually 16 to 18) ❑ at least one bond in the fatty acid residue could be unsaturated The presence of unsaturated bonds causes the rest of the fatty acid to effectively take up more space. Structure of fatty acids  Structure of membrane lipids Membrane lipid molecules are amphiphilic meaning that they simultaneously exhibit: ❑ a hydrophilic (“water-loving”), polar end, ❑ a hydrophobic (“water-fearing”), nonpolar end. The hydrophilic part of the membrane phospholipid molecule, depending on its chemical structure, can: ❑ be electrically charged, ❑ have the polar character of an electric dipole.  Structure of membrane lipids Despite differences in structure, each of the various types of membrane lipids have a hydrophilic head and one to two hydrophobic tails. Structure of membrane lipids  Lipid bilayer The cell membrane is made up of a phospholipid bilayer, which is formed by two layers of phospholipids. They are arranged so that the: ❑ hydrophilic parts (polar heads) are on the surface of the bilayer, ❑ hydrophobic parts (hydrocarbon chains) face the interior of the bilayer. The escape of membrane lipids from the bilayer is prevented by the aqueous environment outside and inside of the cell. Lipid bilayer Lipid bilayer  Membrane proteins Membrane proteins are categorized by the degree of binding to the lipid bilayer: ❑ integral ❑ peripheral ❑ surface  Integral membrane proteins Integral membrane proteins are embedded within the plasma membrane and are divided into: ❑ monotopic membrane proteins that are attached to one side of the plasma membrane, ❑transmembrane proteins, which span across the entire thickness of the lipid bilayer, ❑ polytopic membrane proteins that span across the plasma membrane multiple times.  Integral membrane proteins The structure of integral membrane proteins is reinforced by the highly hydrophobic nature of the lipid component of the membrane. The hydrophobic amino acid side chains of integral membrane proteins interact with the hydrophobic hydrocarbon tails of the membrane lipids. The hydrophilic parts of integral membrane proteins face internally allowing passage of some polar molecules and water. Integral membrane proteins Non-penetrating integral membrane proteins Integral proteins that do not penetrate the plasma membrane are divided into: ❑ outer monolayer proteins (2), ❑ inner monolayer proteins (3), ❑ internal membrane proteins that are located between the two monolayers (i.e., in the hydrophobic part) (4).  Peripheral membrane proteins Peripheral membrane proteins are found on both the inner and outer surfaces of the cell membrane. Peripheral proteins can be bound to the cell membrane by: ❑ electrostatic/ionic bonding, ❑ hydrogen bonding, ❑ van der Waals forces.  Cell surface proteins Cell surface proteins occur only on the outer surface of the cell membrane. Surface proteins are connected to the cell membrane by an anchored element (anchor motif) (e.g., a protein loop or lipid). Types of membrane proteins  Functions of membrane proteins Transport membrane proteins enable the transport of substances across the membrane. Structural membrane proteins link cells together or to the extracellular matrix. Receptor membrane proteins are part of the body's signaling system. Enzymatic membrane proteins catalyze chemical reactions that occur on the surface or inside cells. Functions of membrane proteins  Glycolipids and glycoproteins Some proteins and lipids in the outer layer of the cell membrane covalently attach to sugars. Most membrane proteins attach to short sugar chains such as oligosaccharides to form glycoproteins. Some membrane proteins attach to long polysaccharide chain(s) to form proteoglycans. A single protein molecule can attach to multiple sugar chains. A single lipid molecule can attach to only one sugar chain to form a glycolipid.  Glycolipids and glycoproteins All the sugars (carbohydrates) that make up glycoproteins, proteoglycans and glycolipids are found only on the outer side of the cell membrane. These sugars form a sugar coating called the carbohydrate layer or glycocalyx. The glycocalyx is involved in: ❑ protecting the cell surface, ❑ recognizing other cells, ❑ forming contacts between cells, ❑ merging cells into larger groups. Structure of glycocalyx Cell membrane structure  Cell membrane properties Characteristic features of the cell membrane are: ❑ selective permeability, ❑ fluidity, ❑ asymmetry, ❑ heterogeneity.  Membrane permeability The primary function of the membrane is to create a barrier that controls the passage of molecules across the itself. Small nonpolar molecules (oxygen, carbon dioxide) diffuse passively through the lipid bilayer. Small uncharged polar molecules (water, ethanol) diffuse passively through the lipid bilayer. Larger uncharged molecules (amino acids, glucose) do not diffuse through the lipid bilayer. Ions and electrically charged molecules do not diffuse through the lipid bilayer. Membrane permeability  Membrane fluidity Fluidity is how well all components of the cell membrane can move. The cell membrane is an elastic, two-dimensional fluid. The fluidity of the membrane is influenced by its composition (i.e. the content of: ❑ cholesterol (-) ❑ unsaturated fatty acids (+)) The lipid bilayer is elastic meaning it can bend and return to its original conformation. Membrane fluidity  Membrane fluidity Membrane lipid molecules can perform various types of movements, such as: ❑ segmental movement (flexion) - changing the position of fatty acid chains in relation to the axis of the molecule ❑ rotational movement - around the axis of the molecule (frequent) ❑ translational movement: ❑ lateral movement - in the plane of the membrane (frequent), ❑ transverse movement ("flip-flop") - between membrane layers (rare).  Membrane fluidity Segmental movement is caused by the movement of the hydrocarbon chains of phospholipids. The more mobile the hydrocarbon chains are, the larger the effective volume the chains occupy (i.e., looser packing). Factors influencing the fluidity of the cell membrane: ❑ length of the hydrocarbon chains (14 - 24 C) ❑ number of unsaturated bonds ❑ amount of cholesterol ❑ temperature Lateral and rotational movements take place within the same layer, respectively. Membrane fluidity  Membrane fluidity Transverse movement of the "flip-flop" type occurs because of the passage of lipids from the outer layer to the inner layer of the membrane and vice versa. Transverse movement is classified as: ❑ uncatalyzed ❑ catalyzed by enzymes (flippases) Membrane fluidity  Membrane fluidity Membrane integral proteins can undergo: ❑ rotational movements - rotate around the axis of the molecule ❑ lateral movements - move in the plane of the membrane Due to size of the proteins, the rotational and lateral movements are slower than those of lipids. Membrane proteins do not exhibit transverse movement (i.e., the "flip-flop" type).  Restriction of lateral movement of proteins The lateral movement of proteins can be restricted due to attachment to: ❑ the cell cortex inside of the cell, ❑ extracellular matrix molecules outside of the cell, ❑ proteins on the surface of another cell.  Membrane asymmetry Membrane asymmetry means that respective layers (leaflets) of the cell membrane have a different composition of lipids and proteins. The outer layer of the cell membrane contains: ❑ mainly phosphatidylcholines and sphingomyelin, ❑ surface proteins, ❑ a large amount of glycolipids and glycoproteins. The inner layer of the cell membrane contains mainly: ❑ lipids with electrically charged polar heads like phosphatidylserine, ❑ lipids that easily form hydrogen bonds, like phosphatidylethanolamine. Membrane asymmetry Choline Serine Ethanolamine  Membrane heterogeneity The cell membrane is non-uniform or heterogeneous. Most of the cell membrane is made of a lipid bilayer. The main components of which are phospholipids, cholesterol, glycolipids and proteins. Additionly, there are independent structures: ❑ lipid rafts (rafts) ❑ caveolae  Membrane heterogeneity Lipid rafts are: ❑ flat and dynamic areas of the cell membrane, ❑ rich in cholesterol and sphingolipids, ❑ involved in signalling and transport. Caveolae are: ❑ bottle-shaped invaginations of the cell membrane, ❑ rich in cholesterol, sphingolipids and caveolin, ❑ involved in signalling, endocytosis and transcytosis. Rafts and caveolae are not found in the membranes of lymphocytes, erythrocytes and nerve cells.  Lipid raft Intercellular space 1.Non-raft membrane 2. Lipid raft 3. Raft-associated transmembrane protein 4. Nonraft-associated transmembrane protein 5. Glycosylation modifications (on glycoproteins and glycolipids) 6. GPI-anchored protein 7. Cholesterol 8. Glycolipid Caveola  Membrane transport small molecules large molecules passive active bulk transport transport transport osmosis ATPases simple diffusion Co-transporters endocytosis exocytosis facilitated diffusion phagocytosis pinocytosis receptor-mediated endocytosis  Passive transport Osmosis H2O molecules can diffuse directly through a lipid bilayer. The passage of water molecules from an area of h ​ igh H2O concentration to an area of ​low H2O concentration is called osmosis. The process of osmosis is relatively slow. That is why some cells contain specialized channels in their cell membrane called aquaporins to facilitate the transport of water molecules. Osmosis Aquaporin structure  Passive transport Does not require input of external energy Passive transport of a substance with an electrical charge depends on the: ❑ concentration gradient, ❑ membrane potential. The net force of ion movement is created by the electrochemical gradient, which determines the direction of passive transport. Always occurs in the direction of the concentration gradient (i.e., high to low) Passive transport  Passive transport Simple diffusion is the process by which solutes pass through a cell membrane along the concentration gradient of the solution. The rate of diffusion depends on: ❑ the difference in concentrations (directly proportional), ❑ the electric field across the membrane (charge equalization), ❑ the gradient of hydrostatic pressure across the membrane, ❑ the permeability coefficient of the given substance, ❑ the temperature.  Passive transport Facilitated diffusion (passive-mediated) does not require external energy input. Facilitated diffusion in cell membranes can occur by two means: ❑ a protein channel ❑ a protein transporter The entity facilitating transport is a membrane protein. Passive transport  Ion channels Facilitated diffusion can occur through ion channels. Protein ion channels: ❑ connect intracellular and extracellular spaces, ❑ are filled with water. Protein ion channels are selective depending on: ❑ the diameter and shape of the ion channel, ❑ the arrangement of charged amino acids lining the channel, ❑ the type of ion (i.e., anion, cation).  Ion channels The function of ion channels is to temporarily increase the permeability of the membrane to selected inorganic ions. An ion channel can be: ❑ open, which allows ions to pass through freely, ❑ closed, which allows ions to pass through periodically. Opening and closing of the channel is in response to external stimuli (i.e., temperature, electrochemical gradient, mechanical stimuli, concentration gradient). The concentration of the opening agent affects the number of open channels. This is the most efficient type of transport. Ion channels  Transporters Transporters (carrier proteins) are responsible for the movement across cell membranes of mostly: ❑ small water-soluble, organic molecules, ❑ some inorganic ions. Each transporter is highly selective (i.e., each often transports only one type of solute.) Transporters open only on one side of the cell membrane, but never on both sides at the same time. Transporters  Facilitated diffusion A carrier protein for facilitated diffusion undergoes different confirmational states. For the transport of glucose there is an: ❑ outward open state - binding sites for the solute are exposed to the outside of the cell membrane, ❑ closed state - binding sites are inaccessible from both sides of the cell membrane, ❑ inward open state - binding sites for the solute are exposed to the inside of the cell membrane. Facilitated diffusion  Facilitated diffusion Facilitated diffusion depends on: the concentration gradient around the transporter, the rate of interactions between the carrier protein and the transported substance, the rate of conformational changes of the protein, hormones. Insulin increases the transport of: glucose in adipose tissue and muscle cells, amino acids in the liver. Glucocorticoids increase the transport of amino acids into liver cells  Coupled transport Coupled transport is a type of carrier transport under facilitated diffusion. The characteristic feature is that the transporter has binding sites for two substances. Depending on the direction of transport of the substance in relation to the direction of flow of the accompanying substance, we distinguish the two types of transport: ❑ symport: when both substances flow in the same direction ❑ antiport: when the flow of the substances are in opposite directions Coupled transport  Active transport Active transport: ❑ occurs against the concentration gradient of the substance being transported, ❑ requires energy input, ❑ supplies the cell with substances, such as amino acids, sugars, sodium and potassium ions, etc., ❑ ensures an appropriate osmotic pressure. We distinguish two types of active transport: ❑ primary ❑ secondary  Active transport Primary (direct) active transport There is a direct relationship of transport with the process of energy release (e.g., through ATP hydrolysis). Secondary active transport A transported substance (e.g., Na+) is transported down its an electrochemical gradient, which determines the co-transport with a second substance (e.g., sugar, amino acid) against its gradient. The electrochemical gradient is the source of energy for secondary active transport.  Primary active transport Sodium-potassium pump The pump uses the energy released during ATP hydrolysis to move Na+ ions out of the cell and K+ ions into the cell. During this process, a phosphate group released from ATP is attached to the transporter. The Na+/K+ pump helps maintain a low concentration of Na+ and a high concentration of K+ inside the cell. The ATP-driven Na+/K+ pump occupies a central position in the energy management of animal cells. Sodium-potassium pump  Secondary active transport Glucose transport The transport protein simultaneously: ❑ allows sodium ions to move along their concentration gradient, ❑ transports a glucose molecule into the cell against its concentration gradient, ❑ uses the electrochemical gradient of Na+ to drive active glucose import. Glucose transport  Bulk transport Bulk transport is used to transport large molecules (i.e., amino acids, proteins and others) that cannot pass directly through the cell membrane barrier. Transport of large molecules requires disruption of the cell membrane. This process is completed via vesicles. Types of bulk transport: ❑ endocytosis ❑ exocytosis  Bulk transport Endocytosis is the uptake of substances into the cell including viruses, bacteria and other cells (or cell fragments) by enclosing them in a membrane-bound vesicle formed by the outer cell membrane. Exocytosis is the removal of undigested waste or secretion of compounds (e.g., hormones) from the cell exported via a membrane-bound vesicle, which then fuses with the outer cell membrane. Transmembrane transport  Transmembrane transport Types of endocytosis: ❑ phagocytosis ❑ pinocytosis ❑ receptor-mediated endocytosis Stages of phagocytosis: ❑ uptake of macromolecules or bacteria ❑ formation of a phagosome ❑ transport of substances enclosed within the phagosome to a primary lysosomes ❑ formation of a secondary lysosomes as a result of the fusion between a phagosome with a primary lysosome  Transmembrane transport Stages of pinocytosis: ❑ uptake of fluids and substances dissolved within them ❑ formation of a pinosome ❑ transport of substances enclosed within the pinosome to a primary lysosome ❑ formation of a secondary lysosome as a result of the fusion between a pinosome with a primary lysosome  Transmembrane transport Stages of receptor-mediated endocytosis Membrane stage: ❑ receptor located on the surface of the cell membrane called the pit ❑ binding of a specific molecule (i.e., ligand to the respective receptor) Intracellular stage: ❑ formation of an endosome, which progresses from an early to a late stage ❑ movement of the endosome to specific cellular compartments or to a lysosome Types of endocytosis Bulk transport  Lysosomal degradation The degradation of macromolecules takes place in lysosomes, which contain numerous specific hydrolases.  Protein degradation In a healthy organism, 3 - 5% of proteins are degraded; in a sick organism much more are broken down. Therefore, protein metabolism must be under constant and strict control. In cells of a healthy organism, proteins are degraded if: ❑ the lifespan has ended, ❑ the structure is improper, ❑ the protein is damaged, ❑ there is an excessive amount of that kind of protein.  Protein degradation Protein degradation in eukaryotic cells occurs in two ways: Lysosomal proteolysis, referred to as non-selective degradation In lysosomes, exogenous proteins and old endogenous proteins (e.g., structural proteins) are degraded. Proteasomal proteolysis, referred to as selective degradation, which is associated with the ubiquination.  Ubiquination The ubiquitin system consists of the following elements: ❑ ubiquitin - made of 76 amino acid residues, ❑ ubiquitin-activating enzyme (E 1), ❑ ubiquitin-conjugating enzyme (E 2), ❑ ubiquitin ligase (E 3), ❑ proteasome, ❑ ubiquitin-detaching enzyme - deubiquitinase (DUB).  Ubiquitin Ubiquitin (a globular protein, Ub) attaches to the protein that is to be degraded in order for the proteasome enzyme complex to recognize it. Ubiquitin is composed of: ❑ alpha-helix segments (marked in blue), ❑ beta-sheets (marked in green).  Proteasomes Proteasomes are found in all eukaryotic cells and breakdown proteins bound to ubiquitin. Proteasomes are present in the cytoplasm and in the nucleus. The number of proteosomes varies and depends on the cell's need for protein breakdown. On average, there are about 30,000 proteasomes in a single eukaryotic cell.  Structure of the proteasome Proteasomes are large, high-molecular-weight enzyme complexes. Proteasome structure: ❑ cylindrical made of 28 proteases (central part) ❑ the active sites of the proteases are directed towards the interior of the proteosome ❑ the ends of the cylinder are closed by large protein complexes that resemble plugs. Structure of the proteasome  Proteasome functions Proteasome functions: ❑ bind to proteins to be degraded ❑ unfold a protein and bring it into the "cylinder" ❑ cut (lyse) proteins into short peptides ❑ release the peptides from either end of the cylinder Energy (from ATP hydrolysis) is required to carry out this process Degradation via proteosomes Organelle degradation (autophagy) Dying organelles, such as mitochondria, endoplasmic reticulum membranes, nuclei and peroxisomes send signals to form autophagosome membranes. Autophagosomes: ❑ enclose the damaged organelles, ❑ isolate the organelles from the cytosol, ❑ degrade the organelles after joining with a primary lysosomes within the secondary lysosomes (autolysosome).  Organelle degradation Degradation of the nucleus (nucleophagy): Fragments of the nucleus are subject to degradation in the event of damage to DNA and/or improper separation of chromosomes during cell division. Nucleophagy causes the formation of structures in the cell called micronuclei that contain: ❑ parts of chromosomes, ❑ whole chromosomes, ❑ fragments of the nuclear envelope.  Organelle degradation Mitochondrial degradation (mitophagy): One of the main signals for mitophagy is oxygen deprivation (hypoxia). Lysosome degradation (lysophagy): The signal for lysophagy is: ❑ increased permeability of lysosomal membranes, ❑ appearance of proteins typical of lysosomal membranes in the cytoplasm, ❑ ubiquitination of lysosomal surface proteins.  Organelle degradation Ribosome degradation (ribophagy): The signal for ribophagy is the demand for nitrogen, specifically, for amino acids, such as arginine (Arg) and leucine (Leu), and nucleotides. Proteasome degradation (proteaphagy): Proteaphagy occurs through the binding of appropriate receptors to autophagosome proteins.  Literature Essential Cell Biology, B. Alberts, D. Bray, K. Hopkin Volume 2: Chapter 11. Membrane Structure Chapter 12. Transport Across Membranes Chapter 15. Intracellular Compartments and Protein Transport (Endocytosis Pathways Only)