Membrane Structure & Function PDF

Membrane structure and function RADHIKA BHARDWAJ, PHD DEPARTMENT OF BIOTECHNOLOGY, AIU  Outlines STRUCTURE OF THE PLASMA MEMBRANE - The Phospholipid Bilayer - Cholesterol - Membrane Proteins - The Glycocalyx MOBILITY IN MEMBRA...

Membrane structure and function RADHIKA BHARDWAJ, PHD DEPARTMENT OF BIOTECHNOLOGY, AIU  Outlines STRUCTURE OF THE PLASMA MEMBRANE - The Phospholipid Bilayer - Cholesterol - Membrane Proteins - The Glycocalyx MOBILITY IN MEMBRANE - Mobility of Phospholipids - Mobility of Membrane Proteins  Outlines TRANSPORT OF MOLECULES - Passive Diffusion - Facilitated Diffusion (Carrier Proteins & Ion Channels) - Active Transport Driven by ATP Hydrolysis - Active Transport Driven by Ion Gradients ENDOCYTOSIS - Phagocytosis; Receptor-Mediated Endocytosis - Protein Trafficking in Endocytosis  Plasma membrane All cells (both prokaryotic and eukaryotic) are surrounded by a plasma membrane - Dynamic in nature (constantly changing; not fixed) - Defines the boundary of the cell - Separates its internal contents from the environment - Serve as a selective barrier to the passage of molecules  1. The Phospholipid Bilayer - The fundamental structure of the plasma membrane - Phospholipids account for more than half of the lipid in most membranes - In addition to phospholipids, also contains glycolipids and cholesterol Lipid components of the plasma membrane Phospholipid Composition of the Plasma Membranes of Animal Cells contain total five types of phospholipids 1. Sphingomyelin 2. Phosphatidylcholine The outer leaflet 3.Phosphatidylethanolamine 4. Phosphatidylserine The inner leaflet 5. Phosphatidylinositol (quantitatively minor membrane component) These phospholipids are asymmetrically distributed between the two halves of the membrane bilayer  2. Cholesterol Because of its rigid ring structure, cholesterol plays a distinct role in membrane structure. Inserts into a bilayer of phospholipids with its polar hydroxyl group close to the phospholipid head groups  2. Cholesterol Cholesterol prevents membranes from freezing and maintains membrane fluidity. Depending on the temperature, cholesterol has distinct effects on membrane fluidity. Rather than diffusing freely in the plasma membrane, cholesterol and the sphingolipids (sphingomyelin and glycolipids) form discrete membrane domains known as lipid rafts Move laterally within the plasma membrane and associate with specific membrane proteins.  3. Membrane Proteins -Responsible for carrying out specific membrane functions. -Membranes are viewed as fluid mosaics in which proteins are inserted into phospholipid bilayers.  3. Membrane Proteins Membrane proteins Peripheral proteins Integral proteins Transmembrane Anchored Single Pass Multi Pass Lipid anchored GPI anchored *Glycosylphosphatidylinositol (GPI) anchors 3. Membrane Proteins  4. The Glycocalax -The cell surface covered by a carbohydrate coat - Formed by the oligosaccharides of glycolipids and transmembrane glycoproteins - The role of the glycocalyx is to protect the cell surface -In addition, serve as markers for cell-cell recognition.  Structure of lipid rafts Lipid rafts are organized by the interactions of sphingomyelin, glycolipids, and cholesterol. GPI-anchored proteins are preferentially found in lipid rafts several other types of membrane proteins are present transiently in rafts to mediate cell signaling or endocytosis. Fluid mosaic model of the plasma membrane Why is membrane fluidity needed ? Why is lipid asymmetry important ? Unique Properties of Cell membrane 1. Phospholipid mobility 2. Lipid Asymmetry 3. Membrane fluidity - Fatty acid types - Temperature - Length of Phospholipid tails - Presence of Cholesterol  Mobility of phospholipids in a membrane Individual phospholipids can rotate and move laterally within a bilayer. a) Rotational movement (On its own axis) b) Lateral movement (Transition in the same layer; very fast; most common) c) Flip-Flop movement (Requires energy or help of other membrane proteins/enzymes; rare)  Phospholipid Asymmetry in a membrane In normal conditions, there is no flipping of lipids and no change in composition (Only when immunity gets disturbed)  Membrane fluidity in a membrane Parameters 1) Fatty acid types (Saturated & Unsaturated) Saturated More Rigid Unsaturated More fluid 2) Temperature High temp. More fluid/liquid Low temp. More rigid  Membrane fluidity in a membrane Parameters 3) Length of phospholipid tail/ fatty acid chains Long tails More compact / rigid Short tails More fluid 4) Cholesterol Present in lipid raft regions; Acts like buffer High temp makes more rigid Low temp makes more fluid  Mobility of Membrane Proteins - Proteins are free to diffuse laterally through the phospholipid bilayer - However, the mobility of some proteins is restricted by their associations with other proteins or specific lipids - In addition, tight junctions prevent proteins from moving between distinct plasma membrane domains of epithelial cells. Transport across the cell membrane  Permeability of phospholipid bilayers Small uncharged molecules can diffuse freely through a phospholipid bilayer. However, the bilayer is impermeable to larger polar molecules (such as glucose and amino acids) and to ions. Passive transport  Simple Diffusion ❖ Only small uncharged molecules can diffuse freely through phospholipid ❖ Small nonpolar molecules, such as 02 and C02, are soluble in the lipid bilayer and therefore can readily cross cell membranes ❖ Small uncharged polar molecules, such as H20, also can diffuse through membranes but larger uncharged polar molecules, such as glucose, cannot  Facilitated Diffusion Charged molecules, such as ions, cannot cross a lipid bilayer by free diffusion. Many such molecules (such as glucose) are able to cross via the action of specific transmembrane proteins, which act as transporters. Such transport proteins determine the selective permeability of cell membranes  Channel and Carrier Proteins (A) Channel proteins form open pores through which molecules of the appropriate size (e.g., ions) can cross the membrane. (B) Carrier proteins selectively bind the small molecule to be transported and then undergo a conformational change to release the molecule on the other side of the membrane.  Model for the facilitated diffusion of Glucose (A) Glucose binds to a site exposed on the outside of the plasma membrane. (B) The transporter then undergoes a conformational change and glucose is released into the cytosol. (C) The transporter then returns to its original conformation.  Model of an ion channel In the closed conformation, the flow of ions is blocked by a gate. Opening of the gate allows ions to flow rapidly through the channel. The channel contains a narrow pore that restricts passage to ions of the appropriate size and charge. Well studied in nerve and muscle, where their regulated opening and closing is responsible for the transmission of electric signals.  Three properties of ion channels are central to their function First, transport through channels is extremely rapid. Second, ion channels are highly selective Third, most ion channels are not permanently open.  Active transport Membrane proteins use the free energy stored as ATP to control the internal composition of the cell Transport against the concentration gradient  Active Transport Driven by ATP Hydrolysis Energy derived from ATP hydrolysis can drive the transport of molecules against their electrochemical gradients.  Model of active transport Energy derived from the hydxolysis of ATP is used to transport H+ against the electrochemical gradient (from low to high H+ concentration). Binding of H" is accompanied by phosphorylation of the carrier protein, which induces a conformational change that drives H+ transport against the electrochemical gradient. Release of H+ and hydrolysis of the bound phosphate group then restores the carrier to its original conformation.  The Sodium-Potassium Pump The sodium and potassium ion gradients across the plasma membrane are maintained by the Na+-K+ pump, which hydrolyzes ATP to fuel the transport of these ions against their electrochemical gradients.  Active Transport Driven by Ion Gradients Ion gradients are frequently used as a source of energy to drive the active transport of other molecules.  Active transport driven by the Na+ gradient is responsible for the uptake of glucose from the intestinal lumen. Active transport of glucose  ENDOCYTOSIS Include both the ingestion of large particles (such as bacteria) and the uptake of fluids or macromolecules in small vesicles. Phagocytosis (cell eating) Pinocytosis (cell drinking) Phagocytosis - Formation of a phagosome - Fusion with lysosome to form a phagolysosome within which the ingested bacterium is digested. Receptor-Mediated Endocytosis: - provides a mechanism for the selective uptake of specific macromolecules. Protein Trafficking in Endocytosis: - Molecules taken up by endocytosis are transported to endosomes, where they are sorted for recycling to the plasma membrane or degradation in lysosomes. Important functions 1. Structural- surround the cell cytoplasm; give a certain form to the cell and its organelles. 2. Barrier- secure the passing into and out of the cell only of necessary ions, low molecular compounds, proteins etc. 3. Contact- perform the contacts of cells between each other 4. Receptors – susceptible to different signals from surrounding environment by means of special protein structures incorporated into the membrane. These signals could be light, mechanical deformations, specificc substances etc. 5. Transport- provide the active and passive trans membrane transport of ions as well as transport of electrons in mitochondria and chloroplasts