Plasma Membrane, Structure & Function (BIS 3023) PDF

BIS 3023 MOLECULAR & CELL BIOLOGY MEMBRANE STRUCTURE & FUNCTION Prof. Dr. Wan Iryani Wan Ismail Faculty of Science & Marine Environment Universiti Malaysia Terengganu 21030 Kuala Nerus Terengganu [email protected]  Outline Plas...

BIS 3023 MOLECULAR & CELL BIOLOGY MEMBRANE STRUCTURE & FUNCTION Prof. Dr. Wan Iryani Wan Ismail Faculty of Science & Marine Environment Universiti Malaysia Terengganu 21030 Kuala Nerus Terengganu [email protected]  Outline Plasma Membrane Structure Fluidity Membrane Proteins Membrane Functions Carbohydrates in Cell-Cell Recognition Membrane Proteins & Lipids Selective Permeability of the Lipid Bilayer Membrane Gradient Transport Proteins Direction of Transports Summary  Plasma Membrane The plasma membrane (also known as the cell membrane or cytoplasmic membrane) is a biological membrane that separates the interior of a cell from its outside environment. Functions: 1. Integrity of the cell: Size and shape 2. Controls transport: “selective permeable” 3. Excludes unwanted materials from entering the cell 4. Maintains the ionic concentration of the cell & osmotic pressure of the cytosol 5. Forms contacts with the neighboring cells – tissue 6. Sensitivity – first part of the cell that gets affected by the changes in the extracellular environment  Structure Comprised of a phospholipid bilayer Phospholipids are the most abundant lipids present in the plasma membrane – about 75% Phospholipid – Glycerol + 2 fatty acids – Addition of a phosphate group Phospholipids are amphipathic molecules – Contains both hydrophobic and hydrophilic regions – Hydrophobic fatty acids “tails” – Hydrophilic phosphate “head group” – Makes the phospholipid both polar and non-polar  Structure Plasma membrane also contains embedded proteins Many lipids and proteins are also modified through the addition of carbohydrates – Glycoproteins – Glycolipids Membrane proteins must be able to change shape to function Some of them even laterally diffuse through the membrane So the membrane must be a fluid-friendly  The Davson–Danielli Model (or paucimolecular model) In 1935, Hugh Davson and James Danielli proposed the plasma membrane model The model describes a phospholipid bilayer that lies between two layers of globular proteins and it is trilaminar and lipoproteinous It was the first model that attempted to describe the position of proteins within the lipid bilayer found in membranes. The model was also described as a ‘lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers.  The Davson–Danielli Model (or paucimolecular model) The Danielli-Davson model got support from electron microscopy. In high magnification electron micrographs, membranes appeared as two dark parallel lines with a lighter colored region in between. Proteins appear dark in electron micrographs and phospholipids appear light – possibly indicating proteins layers either side of a phospholipid core. The total thickness of the membranes too turned out to be about 7.5 nm.  Falsification Evidence for the Davson– Danielli Model Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size. Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane. Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place. Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively. When the two cells were fused, the markers became mixed throughout the membrane of the fused cell. This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli). Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane. These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localized to the outside of the membrane structure. Fluorescent antibody tagging of membrane proteins Freeze fracture  Fluid Mosaic Model (1972) Due to these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972. This model, known as the fluid-mosaic model, remains the model preferred by scientists today According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers.  Fluidity Fluid mosaic model states that a membrane is a fluid structure with a mosaic of proteins embedded in it Phopholipids in the plasma membrane can move within the bilayer As temperature cools, membranes switch from a fluid to a solid state Membranes must be fluid to work properly, they are usually about as fluid as salad oil  Fluidity Membrane fluidity is due to several factors 1. Temperature: lipids move around more with increase temperature 2. Lipid packing: Lipids with shorter fatty acids are less stiff 3. Saturation of fatty acids: More C=C bonds (more unsaturated) increase fluidity 4. Cholesterol: decreases fluidity at warmer temperature; but at low temperature hinders solidification  Membrane Proteins Membrane proteins determine most of the membrane’s specific functions Proteins links on the extracellular side to an extracellular matrix of proteins – supports the cells with an tissue Proteins links on the cytoplasmic side to the cytoskeleton via adaptor proteins Three kinds of membrane proteins 1. Extrinsic or Peripheral Proteins - bound to the surface of the membrane - function mainly as enzymes 2. Intrinsic or Integral Proteins - penetrate the hydrophobic core - Transmembrane proteins 3. Lipid Anchored Proteins/Lipid-Linked Proteins  Membrane Functions Major functions of membrane proteins 1. Transport 2. Enzyme activity 3. Signal Transduction 4. Cell-Cell recognition 5. Intercellular joining 6. Attachment to the cytoskeleton and extracellular matrix (ECM)  Membrane Functions Transport: (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane. Enzyme Activity: A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. Signal Transduction: A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell.  Membrane Functions Cell-cell recognition: Some glyco-proteins serve as identification tags that are specifically recognized by other cells. Intercellular joining: Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions Attachment to the cytoskeleton and extracellular matrix (ECM): Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes Membrane Functions Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates on the plasma membrane. Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or more commonly to proteins (glycoproteins).  Membrane Proteins & Lipids Synthesized in the endoplamic reticulum and golgi apparatus  Selective Permeability of the Lipid Bilayer Cells must exchange material with its surroundings i.e., controlled by plasma membrane Plasma membranes are selective permeable which regulates the cells molecular traffic Permeability is the property that determines the effectiveness of the plasma membrane as barrier Hydrophobic molecules such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Ionic and polar molecules, don’t cross the membrane easily, they require transport mechanism provided by transport proteins  Membrane Gradient Selective permeability of the plasma membrane allows the cells to control the concentration of ions in and out side of the cell This results in distinct distribution of positive and negative ions inside and outside of the cell Typically inside of the cell is more negatively charged. The difference in electric charge between inside and outside is called electrical gradient Since it occurs across the plasma membrane its called as membrane potential It can be measured with tiny glass electrodes It varies from cell to cell Very important in the functions of neuron and muscle cells  Transport Proteins Allow passage of hydrophilic substances across the membrane It is specific for the substances it moves There are numerous types 1. Channel proteins: Have a hydrophilic channel where certain molecules (proteins) or ions can use as a tunnel to move across the membrane 2. Carrier proteins: Bind to molecules (proteins) and change shape to shuttle them across the membrane 3. Pumps: Carrier proteins that require the hydrolysis of ATP (or GTP) to move substances Transport Proteins Video: https://www.youtub e.com/watch?v=tSJ0 LnOHpTw  Video: https://www.youtube.com/watch?v=_bPFK DdWlCg  Direction of Transport Two basic things decide the direction of transport 1. Concentration of what is being moved (chemical force) 2. Available energy (electrical force) This combination of forces acting on an ion is called the electrochemical gradient. Two types of transport 1. Passive Transport – Diffusion, Osmosis, Facilitated 2. Active Transport – Primary active transport, Secondary active transport, Exocytosis, Endocytosis  Passive Transport (Doesn’t Require Energy) Simple Diffusion  Passive Transport (Doesn’t Require Energy) Osmosis Water moves across a semipermeable membrane from an area of high concentration to an area of low concentration Tonicity is the ability of a solution to cause a cell to gain or lose water  Passive Transport (Doesn’t Require Energy) Facilitated Diffusion Transport proteins speed the passive movement of molecules across the membrane (down gradient) Channel proteins include – Aquaporins: for facilitated diffusion of water – Ion channels that open or close in response to stimulus (gated channels)  Active Transport (Require energy) Transport requires the expenditure of energy (membrane potential) Usually provide through the hydrolysis of ATP → ADP + Pi Cell is moving a substance against its concentration gradient Cell is forming transport vesicles Cell is internalizing something  Types of Active Transport Primary active transport – molecules are moved against its concentration gradient i.e. from low concentration to high concentration (use energy from ATP hydrolysis) – Electronic pump = A transport protein that generates voltage across a membrane Secondary Active transport – molecules are moved against its concentration gradient i.e. from low concentration to high concentration – movement is dependent upon another ions concentration gradient Bulk transport - Exocytosis – Cell secretion - Endocytosis – Cell internalization - Phagocytosis (“cellular eating”) - Pinocytosis (“cellular drinking”) - Receptor mediated endocytosis (binding of ligands to the receptor triggers vesicle formation).  In exocytosis, transport vesicles migrate to the membrane fuse with it and release their contents  More information… https://www.technologynetworks.com/immu nology/articles/endocytosis-and-exocytosis- differences-and-similarities-334059 Label the cytosis? Questions? Thank you