Biomembranes PDF

Biomembranes Dr. Margaret Doherty  1 2 4 3 Know the Describe in Learning Know the components...

Biomembranes Dr. Margaret Doherty  1 2 4 3 Know the Describe in Learning Know the components of a detail membrane functions of a biomembrane Become transport objectives biomembrane familiar with the fluid mosaic model, the phospholipid bilayer and the extracellular matrix Functions of biological membrane Boundaries around cells (Plasma Membrane) Boundaries around distinct subcellular compartments (Nucleus, Mitochondria, Lysosomes, Golgi bodies, etc.) Compartmentalize and Segregate intracellular events and separate cells from one another Membranes mediate regulation of cellular functions by: Acting as selective barriers Allowing inside environment of cells or organelles to differ from outside Basic components in biological membrane Two basic components: Lipids Proteins Some membranes also contain carbohydrate Composition of lipid, protein and carbohydrate varies from one membrane to another Ratio of Lipid to Protein is not fixed Varies widely depending on the specific functions of the membrane Major lipids in the biological membrane Phospholipids Glycolipids Cholesterol Lipids in membranes are Amphipathic Lipids orientated to prevent Hydrophobic region interacting with water molecules Amphipathic molecule  Cross section of a micelle Orientation of molecule is such that the Hydrophobic Fatty Acid chains are hidden inside the Micelle and the Hydrophilic Head groups interact with the surrounding water molecules Cell membranes The most broadly accepted model of membrane structure is the fluid mosaic model This structure was first postulated by Singer and Nicholson in 1972. Phospholipids of membranes are arranged in a bilayer Form a fluid crystalline matrix The lipid molecules can move laterally - fluidity and flexibility Relative impermeable to highly polar molecules The components are free to diffuse laterally in two dimensions i.e. the mosaic is not fixed or static (fluid) How is the lipid bilayer formed? Phospholipid is made up of: Two Non-polar (Hydrophobic) fatty acid chains: Tails One polar (Hydrophilic) group: Head When in contact with water, the two non-polar chains are too bulky to fit into the interior of a micelle Stable structure for Phospholipids in water is a lipid bilayer Lipid bilayer: Phospholipid molecules are orientated with Hydrophobic chains in the interior of the structure Hydrophilic head groups on the surface Each layer in the lipid bilayer is referred to as Inner and outer leaflets Phospholipid bilayer  How is the structural integrity of biological membranes maintained? The major driving force for formation of lipid bilayer is the Hydrophobic effect Hydrophobic fatty acid chains avoid contact with water Lipid bilayer structure is maintained by multiple noncovalent interactions: Hydrophobic interactions Van Der Waals forces between the hydrocarbon chains Hydrogen bonding between Polar Head-groups Hydrogen bonding between Head-groups and surrounding water molecules  Components of Biological Membranes Cholesterol A steroid widely distributed in all living things Cholesterol and other steroids are found in most membranes; they do not form bilayers, but dissolve in the lipid layer Glycoprotein A protein containing carbohydrate chains In membranes, these proteins usually face the exterior of the cell. The sugars mannose & galactose are common in membrane glycoproteins. Many different combinations are possible, resulting in many different antigens, which are used as signals to distinguish different cells.  Components of Biological Membranes Integral protein At least one segment anchored within the lipid bilayer. Many integral proteins contain sequences of about 20 hydrophobic amino acids that fold into a hydrophobic α-helix that is embedded in the lipid bilayer. Peripheral proteins Located on the peripheral regions of the lipid bilayer of biological membranes Temporarily adheres to the biological membrane by a combination of hydrophobic, electrostatic, and other non-covalent interactions Fluid mosaic model of biomembrane Extracellular matrix The ECM Is the flexible and sticky layer of complex Carbohydrates, Proteins and Lipids that cover the surface of cells It is cell specific Serves in cell-cell recognition and communication Creates cell adhesion Provides a protective outer layer Extracellular matrix Biomembrane is selectively permeable An important feature of a biomembrane is that it is a selectively-permeable structure. Selective permeability is essential for effective separation of a cell or organelle from its surroundings.  Osmosis Diffusion Passive Transport Facilitated Diffusion Movement across the plasma membrane ATP-driven pump Active Transport Light-driven pump Coupled transport  Two types of transport: Passive - random molecular movement of substances either Transport through openings in the membrane though the or in combination with a carrier protein, without requiring energy. cell membrane Active - movement of substances through the membrane in combination with a carrier protein and additionally against an energy gradient.  Osmosis The passive diffusion of water across a semi- permeable membrane Water moves from a high concentration (low solute) to a low water concentration (high solute) Transport by simple diffusion A substance passes through a membrane unaided. The force that drives the substance from one side of the membrane to the other is the force of diffusion. The types of molecules that can do this are themselves substantially hydrophobic in nature such as CO2, O2 or ethanol.  Dependent on specific Integral Proteins called Uni-porters Uni-porter facilitates movement of molecules across membrane in the direction of the concentration gradient Facilitated without any energy Diffusion Molecule binds to protein on one side of the membrane, the protein then undergoes a conformational change, transports the molecule across the membrane and releases it on the other side Molecules transported in this way include: Hydrophilic molecules such as Glucose, Sugars, Amino Acids  Ion channels The transport of ions is mediated by ion channels Ion transport through these channels is an example of passive transport because energy is not required and the movement of ions is driven by their concentration gradient  The movement of molecules from site of low to higher concentration Active It requires both a Carrier Protein and Energy transport Energy for active transport can be derived either from direct coupling to hydrolysis of ATP or by coupling to the movement of an ion down its concentration gradient Active transport may involve: Translocation of a single molecule in one direction (Uni-port) Translocation of two molecules in opposite directions (Anti-port) Translocation of two molecules in the same direction (Symport) These processes are known as COUPLED ACTIVE TRANSPORT Recognition of Substrate Common features of all these models is Binding of Substrate to carrier protein the sequence of reaction steps: Translocation Release ATP driven Pumps ATP-driven pumps (ATPases) couple the hydrolysis of ATP with the movement of ions across a membrane against a concentration gradient ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane Transporter Subtypes Function Aquaporin Transport H2O Channel Voltage-gated Opens and closes in response to ligand binding, Protein Ligand-gated mechanical deformation or voltage. Mechanically gated Can transport polar, bulky or charged molecules. Carrier Protein Uniporter Proteins bind on one side, undergo a Symporter conformation change and released on other side. Antiporter Can transport polar, bulky or charged molecules. Active ATP-driven pump Uses energy to transport molecules against the Transport Light-driven pump electrochemical gradient Coupled transport Pumps transport molecules up an electrochemical gradient via ATP or light Coupled transport uses a molecule travelling down an electrochemical gradient to take another molecule up the gradient. Tying it all together THE RELATIONSHIP BETWEEN THE RESPONSE REQUIRES THE SEVERAL TRANSPORT MECHANISMS ACTION OF NUMEROUS ION CAN BE OBSERVED AT THE CHANNELS WHICH ARE NEUROMUSCULAR JUNCTION SEQUENTIALLY ACTIVATED WHEN A NERVE IMPULSE STIMULATES A MUSCLE CELL TO CONTRACT  Action Potential of Neurons When a neuron is inactive, the neuron is polarized The electrical difference across the membrane of the neuron is called its resting potential When a stimulus reaches a resting neuron, the neuron transmits the signal as an impulse called an action potential Nerve impulse stimulates a muscle cell to contract