Biological Membranes-1 PDF

Ajilore B.S. (MBChB, PhD) them.  To stay alive, all living things need membranes  Biological membranes control movement of substances into & out of the cells  They regulate composition of fluid within the cell  They control the flow of information btw cells  They are involved in capture &...

Ajilore B.S. (MBChB, PhD) them.  To stay alive, all living things need membranes  Biological membranes control movement of substances into & out of the cells  They regulate composition of fluid within the cell  They control the flow of information btw cells  They are involved in capture & release of energy e.g photosynthesis & oxid. Phosphorylation take place on membranes  We can see that biological membranes are more than ordinary barriers or coverings but play an active part in life Brief History Models of membranes were developed long before membranes were first seen with electron microscopes in the 1950s  In 1915, membranes isolated from red blood cells were chemically analyzed and found to be composed of lipids and proteins  In 1925, E. Gorter and F. Grendel reasoned that cell membranes must be a continuous phospholipid bilayer – using extraction of Rbcs membranes in acetone, X-ray diffraction studies & Freeze-fracture electron microscopy, they conclude membranes are approx. 5-8nm thick  Freeze-fracture technique was the first evidence of integral membrane proteins In 1935, H. Davson and J. Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins Early images from electron microscopes seemed to support the Davson-Danielli model, and until the 1960s, it was widely accepted as the structure of the plasma membrane and internal membranes Furtherinvestigation revealed two problems: (i) Not all membranes were alike. Membranes differ in thickness, appearance when stained, and percentage of proteins (ii) Measurements showed that membrane proteins are not very soluble in water  Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions  If membrane proteins were at the membrane surface, their hydrophobic regions would be in contact with water  In 1970, L. Frye and M. Edindin demonstrated the bilayer mobility using: Cell fusion, Florescence photobleaching & Artificial bilayer In 1972, S. J. Singer and G. Nicolson presented a revised model and proposed that the membrane proteins are dispersed and individually inserted into the phospholipid bilayer In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water, and the hydrophobic regions are in a nonaqueous environment within the membrane Structural features  Membranes are fluid structures  They are sheet-like structures  Membranes mainly consist of proteins & lipids  They also contain carbohydrates which are attached to lipids & proteins  Membranes lipids are amphipathic- have both hydrophilic & hydrophobic moities  Membrane proteins are embedded in lipid bilayer  Membrane proteins serve as pumps, channels, receptors and enzymes  Membranes have 2 sides- extracellular & cytosolic which differ from each other i.e are not symmetric Fluid mosaic model  Proposed by Singer & Nicolson in 1972  Membranes are 2-dimensional fluids of lipids & proteins which aid the transverse movement of molecules  Membrane lipids are arranged as lipid bilayer  Proteins are inserted in the lipid bilayer  Some proteins float like icebergs in lipid bilayer sea & some proteins may span entire bilayer  On one surface, chains of sugar molecules stick out and wave about  Biological membranes are lipid-protein-sugar sheets in which the permeability barrier & structural integrity are provided by the lipids; specific functions are carried out by proteins; and distinctive appearance is provided by the sugars Biological membranes: Fluid mosaic model 3 Membrane components  Lipids  Proteins  Sugars (Carbohydrates) All membranes have a common general structure In which 2-layered sheets of lipid molecules have Protein embedded in them - Membranes are highly fluid so that lipids & protein Molecules move freely in their planes - Sugars are found only on one side of the membrane ▪ outer (extracellular) surface of plasma membrane 3 Types of lipids  Phospholipids  Glycolipids  Cholesterol - Each plays different roles in the membrane Phospholipids - Those that contain glycerol as their backbone - i.e Glycerophospholipids: most common Those that contain sphingosine as their backbone - i.e Sphingophospholipids 3 Major Glycerophospholipids  Phosphatidyl choline  Phosphatidyl serine  Phosphatidyl ethanolamine - Glycerol (Propan-1,2,3-triol) linked to ▪ 2 fatty acid chains - contain btw 14-24 C atoms - one chain is usually saturated with 1-4 cis double bonds: put bends in the fatty acid chains ▪ Phosphate ▪ Choline 1 Major phosphosphingolipids Sphingomyelin: contain choline attached to phosphate Glycolipids  Like phospholipids, glycolipids contain either glycerol or sphingosine linked to fatty acid chains  Unlike phospholipids, glycolipids have a sugar (glucose or galactose) instead of the phosphate-containing heads  Glycolipids in animal membranes almost always contain sphingosine  Glycolipids in bacterial & plant membranes principally contain glycerol  Glycolipids are found only on the outer surface Cholesterol: in a class of its own  Structure different from phospholipids & glycolipids  Structure contains a 4-fused ring steroid nucleus (cyclopentanoperhydrophenantrene or stearane or gonane ring) PLUS a short hydrocarbon side chain & a -OH group  Cholesterol is found mainly in animal membranes. But can also be found in a bacteria called mycoplasma  Not found in most bacterial membranes nor in any plant membranes 2-Faced membrane lipids Membrane lipids are amphipathic i.e they have a hydrophilic (water- loving or polar) head and a hydrophobic (water-fearing or non- polar) tail Membrane proteins  Are responsible for the specific functions of membrane  Amount varies from membrane to membrane, from cell to cell & from organism to organism  Variation depends on the specific function of the particular membrane  Myelin sheaths contain 18% protein & 82% lipid - serve as insulator around nerve cell  Plasma membrane: 50% protein + 50% lipid  Inner mitochondrial membrane: 75% protein - involves in energy transduction  Membrane proteins help to overcome permeability barrier created by lipid bilayer  They are responsible for the transport of molecules into & out of the cell while lipid component determines the type of molecules that pass into the cell -selective permeability Examples of membrane proteins Enzymes Carriers Channels Pumps Receptors 2 Types of membrane proteins Peripheral or Extrinsic proteins Integral or Intrinsic proteins Peripheral or Extrinsic proteins Are found on the surfaces of biological membranes (extracellular or cytosolic) Are weakly bound to the surface by non- covalent interactions (electrostatic & hydrogen bonds) They can be removed by mild treatments – e.g change in pH or ionic strength Examples of peripheral proteins G-protein:involves in signal transduction Glyceraldehyde-3-PO4 dehydrogenase Fibronectin: attaches cells to extracellular matrix Spectrin of Rbc Cytochrome c Integral or intrinsic proteins Are embedded within the membrane Some are completely embedded & firmly attached to the membrane They can be removed by treatments e.g detergents or organic solvents which disrupt the membrane The other type span the whole membrane extending from outside to inside of the cell i.e transmembrane proteins Examples of integral proteins G-protein & GPCR (G-protein coupled receptors): involve in sending messages across membrane Enzyme cholinesterase: found in synapses Glycophorin of Rbc Adenylate cyclase Anion channels in Rbc Membrane carbohydrates  Account for 2-10% of membrane weight in eukaryotes  They are present as glycoproteins or glycolipids  They are found only on the outer side of membrane  They give individual cell types distinguishing features or appearance  They are involved in cell to cell recognition  They vary from species to species, from individual to individual, & from cell type to cell type within the same individual  The difference in carbohydrates on Rbcs accounts for the different blood-group antigens (A,B,AB&O)  Cell surface differences are also responsible for the specificity of action of cells with hormones, drugs, viruses or bacteria  This also forms the basis of rejection of foreign cells by the immune system The 2 sides of all biological membranes are not symmetric or identical. WHY?  Carbohydrates are found only on the outer surface of plasma membrane  Lipid composition of the inner & outer surfaces is also different  Cholesterol is found mainly on the outer side of the membrane  Extrinsic membrane proteins e.g cytochromes are found only on the inner surface of the inner MT membrane  Integral membrane proteins e.g glycophorin has CHO groups on the external part of the protein & amino acid chains on the inner part  Glycolipids are found only on the outer surface of plama membrane Membrane fluidity Membranes are fluid – 2 dimensional fluid Molecules in biological membranes are not static Lipids & proteins move past each other 3 Types of movement Lateral movement or lateral diffusion - Movement along the plane of lipid bilayer - Very rapid Rotational movement - Movement along the longitudinal axis i.e head to tail axis & the flexible tail waves about Tranversion or “flip-flop” movement or Transverse diffusion - Movement of lipid molecules from one layer to the other - Rare, very slow & requires energy - Movement is achieved by proteins called flippases (phospholipid translocators) Factors that influence membrane fluidity  Temperature  Fatty acid or lipid composition  Cholesterol content Temperature  A higher temperature increases membrane fluidity, vice- versa. HOW?  - At low temperature, the hydrophobic carbon tails of bilayer lipids can pack closely to be in gel or fairly rigid state (ordered arrangement) - As temperature increases, lipid molecules vibrate more rapidly causing the bilayer to melt into a more liquid state (disordered arrangement) e.g Butter is solid when cool but liquid when warmed The temperature at which lipid bilayer melts is called “phase transition temperature”. Range is 10-40oC Fatty acid or lipid composition Transition temp. is lower i.e bilayer is more fluid if the lipid tails are short or have double bonds. HOW? Short chains will interact less with one another than will long chains, hence a lower temp. is needed to melt the bilayer  Double bonds put bends in the hydrocarbon tails, making it more difficult for the phospholipids to pack together, & bilayer fluidity is increased  e.g Olive oil is liquid @ room temp. whereas beef fat (lard) is solid. B/cos 80% of fatty acid chains in olive oil contain one or more double bonds compared to only 46% of beef fat acyl chains Cholesterol content ▪Cholesterol has variable effect on membrane fluidity  Animal membrane contains one cholesterol molecule per every 2 phospholipid molecules  If membrane contains mainly saturated f.a chains, cholesterol interdigitates with hydrocarbon chains making them more loosely packed & there/4 increasing fluidity  If the membrane consists mainly unsaturated f.a chains, the wedge of cholesterol rigid steroid ring fits into the gaps caused by bending @ the double bonds & stabilizes the membrane & makes them less fluid Transport across membrane  Materials continually pass in & out of cells  Ions move in & out of the cell in a controlled way so that ionic composition of the cell interior is different from that of the outside  Membranes are selectively permeable so that not all molecules can cross membrane equally  Hydrophobic lipid bilayer serves as a barrier to the passage of most polar molecules  This barrier is critical to the survival of the cell because it allows the cell to maintain conc. of solute between the cytosol & the ECF  Therefore conc. of solute in ECF is different from those in ICF Biological & medical importance of transport across membrane Regulation of cell volume Maintenance of intracellular pH required for optimum activity of cellular enzymes  Uptake & concentration of nutrients from the environment Removal of toxic substances Generation of ionic gradient across membrane which are essential for nerve impulse transmission and muscle contraction 3 Types of movement/transport across membrane  Transport of small molecules - To-ing & Fro-ing - Downhill & Uphill - Passive & Active  Transport of large particles(Bulk transport) - Exocytosis & Endocytosis - Pinocytosis & Reverse pinocytosis - Phagocytosis - Receptor-mediated endocytosis  Transport of messages - Fat soluble messages - Water soluble messages - Nerve impulses To-ing & Fro-ing Throughout the life of a cell, materials continually pass in & out. Ions move in & out in a controlled way Essential nutrients are taken in while poisonous wastes are taken out (excreted) Selectively permeable bilayers Not all molecules can cross membrane equally Membranes are selectively permeable so that not all molecules can cross membrane equally Hydrophobic lipid bilayer serves as a barrier to the passage of most polar molecules Small hydrophobic molecules can readily cross phospholipid bilayer by dissolving in the hydrophobic core- Simple/Passive diffusion  Small non-polar molecules e.g O2 & N2, and uncharged polar molecules e.g CO2, ethanol or urea can rapidly cross lipid bilayer  Large, uncharged polar molecules e.g glucose take hours to cross lipid bilayer  Charged molecules e.g Na+ & Cl- are much less likely to get across since they are impeded by hydrophobic core  Exception to this rule is water molecule: polar but can cross lipid bilayer rapidly 105x faster than gluc & 1010x faster than Na+& Cl- How do these molecules get across the membrane rapidly to meet cells’ needs? Special transport mechanisms are needed These mechanisms involve integral membrane proteins called transport proteins- Mediated transport 2 Types of transport proteins Channels Carriers Downhill & Uphill Molecules in solution move from a region of high concentration to a region of low concentration i.e DOWNHILL where the hill is the concentration gradient - Energy is obtained from downhill flow Conversely, energy is needed to reverse downhill flow i.e to make molecules move UPHILL, from a low conc. to a high conc., against their concentration gradient This also applies to charged molecules (ions) but an additional factor is involved Electrochemical gradient i.e conc. gradient plus electrical gradient  Many biological membranes are electrically +ve on one side & -ve on the other side because of potential diff. or membrane potential (voltage gradient) across them  This arises from differences in the distribution of +ve & -ve ions on the 2 sides  Cells use energy to maintain this membrane potential  Movement of ions is affected by this membrane potential  Inside of many cells is electrically –ve compared with the outside  So, entry of +ve ions is favoured, whereas that of – ve ions is opposed For +ve ions moving into the cell, the strength of inward attraction depends on:  Concentration gradient  Number of charges on the ion - ion with two +ve charges will be attracted more strongly than ion with one +ve  Size of potential difference - the more –ve the inside of the cell, the greater will be the attraction of +ve ion  Therefore, the hill for charged molecules is the combination of concentration gradient and electrical gradient  Getting things across cell membranes Channels They are water-filled pores across the bilayer through which inorganic ions move in single file They are built from 4-6 protein subunits which are assembled to form a pore Amino acid side chains that line the pore determine its selectivity - pores that admit +ve ions are lined with –ve side chains, vice-versa Size of ions admitted is determined by the diameter of the narrowest part of the pore  The shape of the protein subunits, & whether the channel is closed or open is affected by: 1. The binding of signal molecules to the subunit or; 2. Change in membrane potential  Channels are said to be gated: 1. Ligand / Chemically gated – if they are opened by the binding of signal molecule 2. Voltage / Electrically gated – if they are opened by change in membrane potential Membrane channels Carriers or transporters  They bind specific molecules or ions and transfer them across the membrane  Each carrier usually transport one type of molecule (e.g sugars, amino acid, ions) & often only one particular molecule of its class (e.g glucose transporter)  The binding of the substance to be transported causes its carrier to change shape, there/4 the bound substance is exposed first on one side of the membrane and then on the other  Also, affinity of the binding site for the bound substance decreases, so the transported molecule is released on the opposite side of the membrane  Types of carrier proteins  Uniports e.g glucose transporter - transport only one solute across memb.  Co-transpoters – transport two solutes at the same time: 1. Symport – if the two solutes are transported in the same direction e.g co-transport of amino acids & Na+ into the cell of the gut 2. Antiport – if the two solutes are transported in opposite directions e.g Na+/K+ pump which pumps Na+ out of cell & K+ into cells Passive & Active transport All channel proteins, and many carrier proteins, transfer molecules or ions across the membrane downhill – passive transport b/cos no input of energy is needed or facilitated diffusion b/cos the normal process of diffusion is helped by membrane proteins Facilitated diffusion is selective & saturable i.e when all carriers have bound a solute molecule, the rate of diffusion is not increased by increasing the concentration of solute  Cells also have transport proteins that transfer solutes across the membrane uphill i.e against their electrochemical gradient  This process is called active transport b/cos an input of energy is needed  This is always done by carrier proteins, not by channels  The energy to drive active transport may come from: 1. Hydrolysis of ATP 2. Energy stored in ion gradient 3. Light  The original ion gradient is from a Io active transport process which uses a direct source of energy, such as ATP or light Transport driven by ATP  All animal cells actively pump Na+ ions out and K+ ions in  For every 3 Na+ that are pumped out of the cell, 2 K+ are pumped into the cell  These 2 processes are carried out by the enzyme Na+/K+ exchanging ATPase, also called sodium pump  Sodium pump is an integral membrane protein  It is a tetramer of 2 types of subunits – one large (α subunit) & one small (β subunit)  The large subunit contains the ATP binding site & is involved in ion transport  The small subunit has sugars on its extracellular surface Mechanism of sodium pump Complex but simplified as follows:  Binding of Na+ ions on cytosolic side triggers the phosphorylation of the large subunit by ATP  Phosphorylation changes the shape of the subunit so that the Na+-binding site faces the outside & Na+ ions are released  K+ ions then bind & trigger the dephosphorylation of the subunit  Dephosphorylation causes the subunit to change back to its original shape so that the binding site faces the inside  K+ ions are released and ATP can be bound again, ready to repeat the cycle Other examples of ATP driven active transport systems Proton pump or Na+/K+ ATPase found in the cells lining the stomach  - uses ATP hydrolysis to pump H+ ions out of the cells & into the stomach interior, in exchange for K+ ions Calcium pump or Ca2+ATPase found in the sarcoplasmic reticulum of muscle cells  - pumps Ca2+ ions from the cytosol into the sarcoplasmic reticulum during relaxation of muscle Transport driven by light READ UP: Retinal & Vision (Visual or Wald cycle)  When light (photon) strikes retinal membrane,rhodopsin (visual pigment), an integral glycoprotein present in rod of retina →→→→ closure of Na+ channels → Hyperpolarization → Nerve impulse → Perception of vision by brain Transport driven by ion gradient  Active transport can be driven by gradients of Na+ or H+ ions which have generated by another active transport system  This is called 2o active transport – the transfer of ion downhill drives the transport of other molecules uphill  Example is the transport of amino acids or glucose uphill into intestinal cells driven by co-transport of Na+ in the same direction, but downhill  This is applied clinically in the oral rehydration of patients with cholera  Patients are given a fluid which contains (among other substances) Na+ & Glucose  The presence of glucose allows uptake of Na+ by the intestinal Na+/glucose co-transport system How large particles cross membrane: Giving out & taking in Cells also transport across their membranes macromolecules such as protein, & even particles several micrometers in size such as bacteria Cells secrete macromolecules by exocytosis & take in macromolecules by endocytosis In both exocytosis & endocytosis, the macromolecules involved are contained within vesicles so that they do not mix with other components of the cytosol Exocytosis  Exocytosis may be constitutive or regulated  Constitutive exocytosis goes on all the time – it is the part of the cell constitution & occurs in all cells  Regulated exocytosis occurs when needed - proteins or molecules to be secreted are stored in secretory vesicles, which fuse with the plasma membrane in response to a signal outside the cell - it operates only in cells which secrete their products on demand e.g cells of the gut secrete digestive enzymes- (large proteins like pepsin), amylase & lipase; endocrine glands such as pancreas secretes peptide hormones like insulin & glucagon Endocytosis There are 2 types: Pinocytosis (cell drinking) - fluid or small particles are taken into small vesicles about 150nm in diameter Phagocytosis (cell eating) - large particles such as micro-organisms & cell debris are taken into large vesicles (vacuoles) about 250nm in diameter Most cells carry out pinocytosis while only specialized cells carry out phagocytosis Receptor-mediated endocytosis This is the internalization of large molecules after their attachment to a specific membrane receptor The receptors bound to the molecules to be transported gather in one area of the membrane and form a coated pit, which is then internalized into the cell Example: The hormone insulin enters cells in this way. It can enter only those cells that have the insulin receptors