Membranes: Structure and Function PDF

Membranes Structure and function, by Siu Wai (Phyllis) TSANG, PhD TUNG WAH COLLEGE email: [email protected] Office#: 3190-6713 Prepared by SWT 2024 email: [email protected] 1 What defines a cell? The plasma membrane (cell membrane) defines the boundary...

Membranes Structure and function, by Siu Wai (Phyllis) TSANG, PhD TUNG WAH COLLEGE email: [email protected] Office#: 3190-6713 Prepared by SWT 2024 email: [email protected] 1 What defines a cell? The plasma membrane (cell membrane) defines the boundary of the cell and any of its internal compartments The plasma membrane is a semi-permeable barrier Some internal compartments (e.g., organelles like mitochondria, nucleus and chloroplasts of eukaryotic cells) are surrounded by membranes https://qph.cf2.quoracdn.net/main-qimg-deec9fd62a82cda8e57762f837e0303c-pjlq https://images.nagwa.com/figures/explainers/903150780879/1.svg Prepared by SWT 2024 email: [email protected] 2 Principles of biological membranes The biological membrane is a lipid bilayer with proteins of various functions (enzymes, transporters) embedded in or associated with the bilayer and it’s stable in water All of the internal membranes of cells are part of an interconnected, functionally specialized, and dynamic endomembrane system Although the lipid bilayer is impermeable to charged or polar solutes, cells of all kinds have many membrane transporters and ion channels that catalyze transmembrane movement of specific solutes Prepared by SWT 2024 email: [email protected] 3 Membrane components outside (environment) inside (cytoplasm) Component Location Phospholipids Main fabric of the membrane Integral proteins Embedded in the phospholipid bilayer; may or may not extend through both layers Attached to the phospholipid bilayer (either the layer facing the outside environment of Peripheral proteins the cytoplasm), but not embedded in its hydrophobic core Attached to proteins or lipids on the extracellular side of the membrane (forming Carbohydrates glycoproteins and glycolipids) Cholesterol Tucked between the hydrophobic tails of the membrane phospholipids Prepared by SWT 2024 email: [email protected] 4 Membrane components (cont’d) Textbook Ch.7 p.181 Because each type of living cell has its own functions, the types of lipid, protein and carbohydrates found in each type of membrane vary The two layers of the phospholipid bilayer membrane are NOT identical in composition (e.g., bulkier molecules occur more often in the inner side of the membrane) Prepared by SWT 2024 email: [email protected] 5 Main classes of plasma membrane proteins Integral membrane proteins / transmembrane proteins : amphipathic; embedded in and/or extend through a membrane; ≥1 hydrophobic regions at membrane interior; ≥1 hydrophilic regions extended outward Peripheral proteins: bound to membrane primarily through interactions with integral membrane protein or interact directly with the lipid bilayer Textbook Ch.7 p.194 Prepared by SWT 2024 email: [email protected] 6  Integral membrane / transmembrane proteins https://slideplayer.com/slide/15376044/93/images/3/Integral+Proteins+Peripheral+Proteins+Transmembrane+Protein.jpg https://www.sciencefacts.net/wp-content/uploads/2022/10/Peripheral-Membrane-Proteins.jpg Prepared by SWT 2024 email: [email protected] 7 Peripheral proteins Lipid-anchored membrane proteins: located on one of the surfaces of the lipid bilayer Fatty acid / isoprenyl anchor: attached by covalent linkage either to a fatty acid or isoprenyl GPI anchor: attached by covalent linkage to a glycosylphosphatidylinositol Textbook Ch.7 p.194 Prepared by SWT 2024 email: [email protected] 8 Major classes of membrane lipids 1. Phospholipids Possess a small polar head group (such as choline) attached via a phosphate to a fatty acid- Textbook Ch.7 p.184 containing glycerol or sphingosine backbone Glycerol-based  phosphoglycerolipids (phosphoglycerides) Sphingosine-based  phosphosphingolipids The amphipathic detergent sodium dodecyl sulfate (SDS) is commonly used in labs to disrupt membranes and solubilize membranes and membrane proteins for biochemical analysis Prepared by SWT 2024 email: [email protected] 9 Major classes of membrane lipids (cont’d) 2. Glycolipids Formed by adding carbohydrate groups to lipids Like phospholipids, some glycolipids are glycerol Textbook Ch.7 p.184 based and are called glycoglycerolipids Others are sphingosine-based and are called glycosphingolipids The human ABO blood groups, e.g., involve glycosphingolipids known as A antigen and B antigen that serve as specific cell surface markers of the different groups of red blood cells Prepared by SWT 2024 email: [email protected] 10 Major classes of membrane lipids (cont’d) 3. Sterols The main sterol in animal cell membranes is cholesterol, a four-ringed molecule that is necessary for maintaining Textbook Ch.7 p.184 and stabilizing membranes in our bodies by acting as a fluidity buffer The membranes of plant cells contain small amounts of cholesterol and larger amounts of phytosterols, i.e., campesterol, sitosterol and stigmasterol Fungal membranes contain ergosterol that is similar in structure to cholesterol Prepared by SWT 2024 email: [email protected] 11 Membrane lipids – fatty acids Fatty acids are components of all membrane lipids except the sterols Most fatty acids in membranes are between 12 and 20 carbon atoms in length, with 16- and 18-carbon fatty acids especially common This size range appears to be optimal for bilayer formation because chains with fewer than 12 or more than 20 carbons are less able to form a stable bilayer Thus, the thickness of membranes is dictated primarily by the chain length of the fatty acids required for bilayer stability The fatty acids found in membrane lipids vary considerably in the presence and number of double bonds Prepared by SWT 2024 email: [email protected] 12 Membrane lipids – fatty acids (cont’d) Saturated fatty acids contain no double bonds because every C atom in their tails is Textbook Ch.7 p.186 bound to the maximum number of H atoms Unsaturated fatty acids contain one and two double bonds Prepared by SWT 2024 email: [email protected] 13 Carbohydrates in membranes Most membranes contain small but significant amounts of carbohydrates Glycoproteins: membrane proteins with carbohydrate chains covalently linked to amino acid side chains Via N-linked and O-linked glycosylation of membrane proteins Glycolipids: including glycosphingolipids, glycoglycerolipids and glycophosphatidylinositols These carbohydrate chains help in cell-to-cell recognition, e.g., A & B antigens Textbook Ch.7 p.201 Prepared by SWT 2024 email: [email protected] 14 Membrane asymmetry The lipid bilayer is fluid Most lipids in a variety of cell types are unequally distributed between the two monolayers that constitute the lipid bilayer This membrane asymmetry includes differences in both the kinds of lipids present and the degree of unsaturation of the fatty acids in the phospholipid molecules https://open.oregonstate.education/app/uploads/sites/178/2023/11/Figure-02-06.png Once established, asymmetry tends to be maintained Prepared by SWT 2024 email: [email protected] 15 Membrane fluidity Rather than being fixed in place within the membrane, the membrane lipids form a fluid bilayer that permits lateral diffusion of membrane lipids as well as proteins Lipid molecules move especially fast because they are much smaller than proteins Proteins move much more slowly, partly because they are much larger molecules and partly due to their interactions with cytoskeletal proteins inside the cell The fluidity of biological membranes is determined by the proportions of saturated and unsaturated fatty acid groups in the phospholipids and the composition and distribution of cholesterols Membranes function properly only in the fluid state Prepared by SWT 2024 email: [email protected] 16  Membrane fluidity (cont’d) A phospholipid molecule is capable of 3 kinds of movement in a membrane Rotation about its long axis Lateral diffusion by exchanging places with neighboring molecules in the same monolayer Transverse diffusion, or “flip-flop,” from one monolayer to the other Textbook Ch.7 p.187 Prepared by SWT 2024 email: [email protected] 17  Membrane fluidity (cont’d) https://open.oregonstate.education/app/uploads/sites/178/2023/11/Figure-02-07.png https://i1.wp.com/cms.jackwestin.com/wp-content/uploads/2020/02/mrmbane.png?resize=565%2C394&ssl=1 https://open.oregonstate.education/app/uploads/sites/178/2023/11/Figure-02-05-e1700178915664.png Prepared by SWT 2024 email: [email protected] 18 Membrane fluidity (cont’d) The membrane’s fluidity depends primarily on the types of lipids it contains, the length of the fatty acid chains and their degree of unsaturation Long-chain fatty acids have higher transition temperatures (i.e., less fluid) than short-chain fatty acids Completely saturated fatty acids have higher transition temp. The kinks caused by double bonds prevent hydrocarbon chains from fitting together snugly Textbook Ch.7 p.189 19 Prepared by SWT 2024 email: [email protected] Membrane fluidity (cont’d) Prepared by SWT 2024 email: [email protected] 20 Membrane fluidity − cholesterol Textbook Ch.7 p.190 For eukaryotic cells, membrane fluidity is also affected by the presence of sterols (i.e., mainly cholesterol in animal cell membranes) A typical animal cell contains large amounts of cholesterol—up to 50% of the total membrane lipid on a molar basis This intercalation of cholesterol molecules into the membrane of an animal cell affects membrane fluidity Cholesterol acts as a fluidity buffer 21 Prepared by SWT 2024 email: [email protected] Role of cholesterol in membrane fluidity Without cholesterol, the mobility of hydrocarbon lipid chains is disrupted, and fluidity increases at HIGH temperatures Without cholesterol, the ordered packing of the extended hydrocarbon lipid chains is restricted and thereby decreases fluidity at LOW temperatures Cholesterol helps maintain fairly constant fluidity in animal cell membranes despite fluctuations in temperature or degree of fatty acid saturation Prepared by SWT 2024 email: [email protected] 22 Fluid Mosaic Model Membranes are vital structures found in all cells A membrane consists of lipids molecules and many embedded proteins and carbohydrates, organized in a way described by fluid mosaic model These components give a fluid character to the membranes https://biologynotesonline.com/wp-content/uploads/2024/03/image-962.png Proposed by S. Jonathan Singer and Garth Nicolson in 1972 Prepared by SWT 2024 email: [email protected] 23 The functions of membranes Boundaries of the cell and its organelles (act as permeable barriers) Serve as sites for specific biochemical functions Possess transport proteins that regulate substances moving across the cell Contain protein molecules that act as receptors to detect extracellular signals Provide mechanisms for cell-to-cell contact, adhesion and communication Textbook Ch.7 p.177 Prepared by SWT 2024 email: [email protected] 24 Boundary and permeability barrier Defines the boundary of the cell and its compartments – organelles The interior of the cell (organelles) must be physically separated from the surrounding environment Desirable substances IN & undesirable substances OUT Phospholipid bilayer blocks the passage of polar molecules and ions An effective permeability barrier for these substances The intracellular membranes serve to compartmentalize functions within eukaryotic cells Prepared by SWT 2024 email: [email protected] 25 Specific functions Membranes have specific proteins either embedded in or localized on One of the most useful ways to characterize a specific membrane is to describe the particular enzymes, transport proteins, receptors and other molecules associated with it that can give the membrane a specific function Examples: The cell membrane contains enzymes that secrete the materials that make up the extracellular matrix The cell membranes of nerve cells contain ion transporters essential for proper signal transmission to muscles Prepared by SWT 2024 email: [email protected] 26 Transport across the membrane Most of the substances that move across membranes are NOT macromolecules Membrane proteins: carry out and regulate the transport of substances into and out of cells and their organelles nutrients, ions, gases, water and other substances Although substances, such as gases and very small or lipophilic molecules, can typically diffuse directly across cellular membranes, most substances needed by the cell are hydrophilic require transport proteins cells may have specific transporters to import glucose, amino acids or other nutrients Prepared by SWT 2024 email: [email protected] 27 Transport mechanisms Passive transport Simple diffusion and facilitated diffusion Direction of move may be dictated by concentration gradient or electrochemical potentials Requires NO energy Active transport Requires energy Passive transport Active transport Prepared by SWT 2024 email: [email protected] 28 Simple diffusion Movement of solutes / molecules from a region of higher concentration to a region of lower concentration Molecules move down a concentration gradient Net movement of molecules stops when equilibrium is reached Passive – requires no energy Prepared by SWT 2024 email: [email protected] 29 Osmosis Osmosis is the diffusion of water molecules from one side of a selectively permeable (semi-permeable) membrane to the other Diffusion: solutes + water molecules Osmosis: water molecules [permeable membrane] Cell membrane is naturally permeable to water Water flow across membranes is always passive Under normal conditions, most cells in the body neither swell nor shrink because there is no net movement of water across membranes Prepared by SWT 2024 email: [email protected] 30 Tonicity The effect of an extracellular solution on the volume of a cell Determined by the relative concentration of the non-penetrating solute molecules If a red blood cell is placed into a hypertonic solution, as water leaves the cell, it shrinks If a red blood cell is placed into a hypotonic solution, as water enters the cell, it swells, and the hemoglobin begins to leak out (hemolysis) If a red blood cell is placed into an isotonic solution, it will neither crenate nor hemolyze Textbook Ch.8 p.214 Prepared by SWT 2024 email: [email protected] 31 Facilitated diffusion The passive movement of solutes (e.g., molecules, ions…) across the biological membrane by means of a transport protein (i.e., protein channel or carrier protein) located in the membrane (e.g., integral membrane protein) Solutes move along the concentration gradient via a selective process Movement runs across the biological membrane at a significant rate Passive – requires no energy Prepared by SWT 2024 email: [email protected] 32 Facilitated diffusion – carrier protein Textbook Ch.8 p.219 Prepared by SWT 2024 email: [email protected] 33 Facilitated diffusion – protein channel Aquaporins, a.k.a. water channels, are protein channels that speed up osmosis without changing the direction of water movement 45ec-9a63-765cd08eadd18407356371333153109.png https://www.vedantu.com/question-sets/b3b9ce92-d71d- Textbook Ch.8 p.225 Prepared by SWT 2024 email: [email protected] 34 Transport proteins Uniport The transport protein allows passage of a single type of molecules across the cell membrane Symport The transport protein allows passage of two different types of molecules in the same direction across the membrane Antiport The transport protein allows passage of two different types of molecules in opposite directions Textbook Ch.8 p.218 Prepared by SWT 2024 email: [email protected] 35 Channel proteins (ion channels) Ion channels selectively allow the passage of only one kind of ions Ligand-gated ion channels Triggered by binding of specific substances e.g., nicotinic acetylcholine receptor for the passage of Na+ ions Mechanically-gated ion channels In response to mechanical forces e.g., sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound Voltage-gated ion channels In response to changes in the membrane potential e.g., calcium channels at the synapse of neurons Prepared by SWT 2024 email: [email protected] Passive transport – end result https://open.oregonstate.education/app/uploads/sites/157/2021/02/0305_Simple_Diffusion_Across_Plasma_Membrane-1.jpg Prepared by SWT 2024 email: [email protected] 37 Active transport A transport process that requires energy This process allows molecules to be pumped across the cell membrane against a concentration gradient, https://www.sciencefacts.net/wp-content/uploads/2020/03/Active-Transport.jpg i.e., from low [conc.] to high [conc.] Adenosine triphosphate (ATP) Prepared by SWT 2024 email: [email protected] 38 Active transport (cont’d) Mediates transport of small molecules or ions as well as larger molecules, such as starch, across a cell membrane via transport proteins based on cellular energy Major functions of active transport Make possible the uptake of essential nutrients from surrounding Allows various substances to be removed from the cells Enables the cell to maintain constant imbalance electro-potential Actively transported across the plasma membrane by processes known as exocytosis and endocytosis Textbook Ch.8 p.226 Prepared by SWT 2024 email: [email protected] 39 Exocytosis vs endocytosis 胞吐作用 內吞作用 Prepared by SWT 2024 email: [email protected] 40 Active transport mechanisms Coupled transporters couple uphill transport of one solute to the downhill transport of another ATP-driven pumps use hydrolysis of ATP to uphill transport Light-driven pumps couple transport to light absorption Textbook Ch.8 p.229 41 Prepared by SWT 2024 email: [email protected] Transport processes of the erythrocyte Textbook Ch.8 p.212 Erythrocyte = red blood cell Prepared by SWT 2024 email: [email protected] 42 Exocytosis of neurotransmitters at synapse Prepared by SWT 2024 email: [email protected] 43 Function – signal transduction Membrane proteins detect and transmit electrical and chemical signals  signal transduction Membrane proteins mediate cell adhesion and cell-to-cell communication Cells are in contact with other cells Gap junctions in animal cells Plasmodesmata in plant cells Prepared by SWT 2024 email: [email protected] 44 Membrane self-sealing When lipid bilayers are disrupted, they will reseal Small cell membrane breaks spontaneously seal via the lateral flow of lipid molecules Prepared by SWT 2024 email: [email protected] 45  Cell structure and function Kahoot! Games https://kahoot.it/solo/?quizId=26c7ecc3-7911-4372-ba0b- d88eb895fb80 Prepared by SWT 2024 email: [email protected] 46  ~ The end ~ Thank you! Prepared by SWT 2024 email: [email protected] 47