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Transcript
Plasma Membrane: Models: In 1915, working with red blood cells by Evert Gorter and Grendel, it was discovered that the membranes consisted of lipids and proteins In 1935, Davson and Danielli proposed the first membrane model widely accepted, in which the cell membrane consisted of a lipid bilayer with hydrophilic coating of proteins on both sides which could also form pores • This model was called “sandwich” model of Davson and Danielli In 1957, Roberts by using the electron microscopy, proposed a modified version of the “sandwich” model named “unit • Under the electron microscope, he could identify a trilaminar structure of the membrane, which he interpreted as being an internal lipid bilayer with two flanking protein layers on the exterior. This model was accepted between 1960-1970 In 1972, Singer and Nicholson proposed proteins were embedded in the bilayer with only their hydrophillic ends exposed to the external aqueous solution and their hydrophobic regions embedded inside the membrane – Fluid mosaic model The "freeze-fracture" method of examining cells microscopically showed that the fluid mosaic model was accurate Figure down loaded from web site: https://www.boundless.com/biology/plasma-membrane/cell-membranes-and-fluidmosaic-model/development-membrane-models/ This model retains the bilayer structure proposed by the two Dutch scientists Gorter and Grendel • The proteins are, however, in the bilayer and can move due to the membrane fluidity • They proposed that the lipid bilayer is organized in such a way that the hydrophylic part of the phospholipids are on the exterior of the lipid bilayer in contact with the water • While their hydrophobic tails face inward • The proteins float in this bilayer and • can form pores and channels The fluid mosaic model continues to be refined. For example • The existence of membrane compartmentalization is also important for cell function • It is now well established that the plasma membranes contain: Protein-protein complexes Lipid rafts and pickets and fences formed by the actin-based cytoskeleton and can be polarized (apical and latero-basal compartmentalization in the epithelial cells) So this is the Fences and pickets model of plasma membrane - a concept of cell membrane structure suggesting that the fluid plasma membrane is compartmentalized by actin-based membrane-skeleton “fences” and anchored transmembrane protein “pickets” The fences and pickets model was proposed to explain observations of molecular traffic made due to recent advances in single molecule tracking techniques http://www.google.com search for membranes pickets and fences of plasma Plasma Membrane: Cell membranes are crucial to the life of the cell Plasma membrane • Encloses the cell • Defines its boundaries and • Maintains the essential differences between the cytosol and the extracellular environment Inside eukaryotic cells, the membranes of the endoplasmic reticulum, Golgi apparatus, mitochondria and other membrane-enclosed organelles • maintain the characteristic differences between the contents of each organelle and cytosol • Ion gradient across the membranes, established by activities of specialized membrane proteins, can be used to synthesize ATP to drive the transmembrane movement of selected solutes or as in nerve and muscle cells o to produce and transmit electrical signals In all cells, plasma membrane also contains proteins that act as sensors of external signals • allowing the cell to change its behavior in response to environmental cues including signals from other cells • These protein sensors or receptors transfer information rather than molecules across the membrane Despite their different functions, all biological membranes have a common general structure: • Each is very thin film of lipid and protein molecules held together mainly by non covalent interaction Fig 10.1 Alberts 3rd Ed / 5th Ed • Cell membranes are dynamic, fluid structures and most of their molecules move about in the plane of membrane • The lipid molecules are arranged as a continuous double layer of about 5 nm thick • This lipid bilayer provides the basic fluid structure of the membrane and • Serve as a relatively impermeable barrier to the passage of most water soluble molecules Protein molecules that span the lipid bilayer mediate nearly all of other functions of membrane. For example: • Transporting specific molecules across it or • Catalyzing membrane-associated reactions such as ATP synthesis • In the plasma membrane, some trans-membrane proteins serve as structural links that connect cytoskeleton through lipid bilayers to either the extracellular matrix or An adjacent cell while Others serve as receptors to detect and transduce chemical signals in the cell’s environment • Many different proteins to enable a cell to function and • Interact with its environment • It is estimated that about 30% of the proteins encoded in an animal cell’s genome are membrane proteins • We will discuss here the structure and organization of the two main constituents of biological membranes: The lipids and The proteins • Although we will focus mainly on plasma membrane but most concepts discussed apply to the various internal membranes in cells as well The Lipid Bilayer; • As discussed earlier, the lipid bilayer provides the basic structure for all cell membrane • It is easily seen by electron microscopy and • Its structure is attributable exclusively to the special properties of the lipid molecules • Which assemble spontaneously in to bilayers even under simple artificial conditions Phosphoglycerides, Spingolipids and Sterols are the Major Lipids in Cell Membranes • Lipid molecules constitute about 50% of the mass of most animal cell membranes, nearly all of the reminder being protein • All the lipid molecules in cell membranes are amphiphilic/ amphiphatic i.e. They have a hydrophilic - water loving or polar end and a hydrophobic - water hating or non-polar end • The most abundant lipid are phospholipids. They have a polar head and hydrophobic hydrocarbon tails • In animals, plants and bacterial cells, the tails are usually fatty acids and they can differ in length - they normally contain between 14 and 24 carbon atoms • One tail typically has one or more cis - double bonds (unsaturated) creates a small kink in the tail while other tail does not (saturated) • Differences in the length and saturation of the fatty acid tails influence how phospholipid molecules pack against one another • thereby affecting the fluidity of the membrane • The main phospholipids in most animal cell membranes are the phosphoglycerides which have a three-carbon glycerol backbone Fig 10.2 Alberts 3rd / 5th Ed • By combining several different fatty acids and head groups, cells make many different phosphoglycerides Fig 10.3 Alberts 5th Ed/ Fig 10.10 Alberts 3rd Table 10.1 Alberts 3rd / 5th Ed • In addition to phospholipids, the lipid bilayers in many cell membranes contain cholesterol and glycolipids • Eukaryotic plasma membranes contain specially large amounts of cholesterol Fig 10.8 Alberts 3rd Ed / Fig 10.4 Alberts 5th Ed Fig 10.9 Alberts 3rd Ed / Fig 10.5 Alberts 5th Ed Phospholipids Spontaneously form Bilayers • The shape and amphiphilic nature of the phospholipid molecules cause them to form bilayers spontaneously in aqueous environment Fig 10.6 Alberts 5th Ed • The hydrophobic and hydrophilic regions of lipid molecules behave in the same way • Thus lipid molecules spontaneously aggregate to bury their hydrophobic hydrocarbon tails in the interior and expose their hydrophilic heads to water • Depending on their shape, they can do this in either of two ways; they can form spherical micelles with the tail inward or they can form double layered sheets or bilayers with hydrophobic tails sandwiched between the hydrophilic head groups Fig 10.7 Alberts 5th Ed Fig 10.8 Alberts 5th Ed The Lipid Bilayer is a Two-Dimensional Fluid • Around 1970, researchers first recognized the individual lipid molecules are able to diffuse freely within lipid bilayers • The initial demonstration came from studies of synthetic lipid bilayers • Two types of preparations have been very useful in such studies: Bilayers made in the form of spherical vesicles called liposomes Fig 10.4 Alberts 3rd Ed / Fig 10.9 Alberts 5th Ed Planar bilayers called black membrane formed across a hole in a partition between two aqueous compartments Fig 10.5 Alberts 3rd Ed / Fig 10.10 Alberts 5th Ed • Studies showed that phospholipid molecules in synthetic bilayers very rarely migrate from the monolayer (leaf let) on one side to that on the other - this process is called flip flop • These studies have also shown that individual lipid molecules rotate very rapidly about their long axis and have flexible hydrocarbon chains • Computer simulation show that lipid molecules in membrane are very disordered Fig 10.11 Alberts 5th Ed The Fluidity of a Bilayer Depends on its Composition • The fluidity of cell membranes has to be precisely regulated. For example: Certain membrane transport processes and enzyme activities cease when the bilayer viscosity is experimentally increased beyond threshold level • The fluidity of a lipid bilayer depends on both: Its composition and Its temperature as is readily demonstrated in studies of synthetic bilayers Fig 10.7 Alberts 3rd Ed / Fig 10.12 Alberts 5th Ed • Bacteria, yeast and other organisms whose temperature fluctuates with that of their environment adjust the fatty acid composition of their membrane lipids to maintain a relatively constant fluidity • As the temperature falls, the cells of those organisms synthesize fatty acids with more cis-double bonds and • They avoid the decrease in bilayer fluidity that would otherwise result from the temperature drop • Cholesterol modulates the properties of lipid bilayers • When mixed with phospholipids, it enhances the permeability barrier properties of lipid bilayer • It inserts in to the bilayer with its hydroxyl group close to the polar head groups of the phospholipids • So that its rigid, plate like steroid rings interact with and partly immobilize those regions of the hydrocarbon chain closest to the polar head groups Fig 10.9 Alberts 3rd Ed / Fig 10.5 Alberts 5th Ed • By decreasing the mobility of the first few CH2 groups of the hydrocarbon chains of the phospholipids molecules, cholesterol makes the lipid bilayer less deformable in this region and • thereby decreases the permeability of the lipid bilayer to small watersoluble molecules • Though cholesterol tightens the packing of the lipids in a bilayer, it does not make membrane any less fluid • At the high concentrations found in most eukaryotic plasma membranes, cholesterol also prevents the hydrocarbon chains from coming together and crystallizing • Besides major phospholipids (500-1000 different lipid species present in a cell), membranes also contain many structurally distinct minor lipids – some of which have important functions. For example: The inositol phospholipids are present in small quantities but have crucial functions in guiding membrane traffic and in cell signaling Their local synthesis and degradation are regulated by a large number of enzymes o which create both small intracellular signaling molecules and lipid docking sites on membrane that recruit specific proteins from the cytosol The Asymmetry of the Lipid Bilayer is Functionally Important • The lipid composition of the two monolayers of the lipid bilayer in many membranes are strikingly different. For example: In human red blood cell membrane, almost all the phospholipid molecules that have choline in their head group (phosphatidylcholine and spingomyelin) are in outer monolayer Whereas almost all that contain terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in inner monolayer Fig 10.11 Alberts 3rd Ed / Fig 10.16 Alberts 5th Ed • Lipid asymmetry is functionally important specially in converting extracellular signals in to intracellular ones • Many cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer Fig 10.17 Alberts 5th Ed • Animals exploit the phospholipid asymmetry of their plasma membranes to distinguish between live and dead cells • When animal cells undergo apoptosis - a form of programmed cell death, phosphotidylserine which is normally confined to the cytosolic monolayer of plasma membrane lipid bilayer, rapidly translocates to the extracellular monolayer • The phosphotidylserine exposed on the cell surface signals neighboring cells. Such as How membrane-bound phospholipid translocators generate and maintain lipid asymmetry? Macrophages to phagocytose the dead cell and digest it • The translocation of phosphatidylserine in apoptotic cells is thought to occur by two mechanisms: i. The phospholipid translocator that normally transports this lipid from the non cytosolic monolayer to the cytosolic monolayer is inactivated ii. A scramblase that transfers phospholipids non specifically in both directions between the two monolayer is activated Glycolipids are Found on the Surface of All Plasma Membranes • Sugar containing lipid molecules are called glycolipids, found exclusively in the non cytosolic monolayer of the lipid bilayer • They have the most asymmetry in their membrane distribution and • Have important roles in interactions of the cell with its surrounding Fig 10.12 Alberts 3rd Ed / Fig 10.18 Alberts 5th Ed • They may help to protect the membrane against the harsh conditions frequently found there. Such as Low pH and high concentrations of degradative enzymes • Charged glycolipids like gangliosides, may be important because of their electrical effects: Provide entry point for some bacterial toxins – acts as cell surface receptor for bacterial toxin that causes devastating diarrhea of cholera Their presence alters the electrical field across the membrane and the concentrations of ions especially Ca2+ at the membrane surface Glycolipids are also thought to function in cell-recognition processes in which Membrane bound carbohydrate binding proteins i.e. lectins bind to the sugar groups on both glycolipids and glycoproteins in the process of cell-cell adhesion Membrane Proteins; • As mentioned earlier lipid bilayer provides the basic structure of biological membranes, the membrane proteins perform the membrane specific task and • Therefore give each type of cell membrane its characteristic functional properties • Membranes Proteins can be Associated with the Lipid Bilayer in Various Ways Different membrane proteins are associated with the membranes in different ways: Fig 10.19 Alberts 5th Ed / Fig 10.13 Alberts 3rd Ed Anchored via oligosaccharide linker to lipid i.e. phosphotydylinositol Anchored to cytosol surface by amphiphilic α- helix β-Sheet, a β- barrel Single α helix Multiple α-helices Anchored by covalently attached lipid chain either fatty acid chain or a prenyl group Non-covalent interactions Fig 10.15 Alberts 3rd Ed / Fig 10.21 Alberts 5th Ed Fig 10.23 Alberts 5th Ed Fig 10.26 Albnerts 5th Ed Fig 10.14 Alberts 3rd Ed / Fig 10.20 Alberts 5th Ed • The Cell Surface is Coated with Sugar Residues As a rule, plasma proteins do not protrude naked from the exterior of cell but are: o Decorated o Clothed or hidden by carbohydrates which are present on the surface of all eukaryotic cells These carbohydrates occurs both as o Oligosaccharide chains covalently bound to membrane proteins called glycoproteins and o To lipids called glycolipids and o As polysaccharides chains of integral membrane proteoglycan molecules Proteoglycans, which consist of long polysaccharide chains linked covalently to a protein core, are found mainly outside the cell as part of the extracellular matrix The term coat or glycocalyx is often used to describe the carbohydrate rich zone on the cell surface Fig 10.28 Alberts 5th Ed / Fig 10.40 and 10.41 Alberts 3rd Ed The oligosaccharide side chains of glycoproteins and glycolipids are enormously diverse in their arrangement of sugars Although they usually contain fewer than15 sugar residues They are often branched and the sugars can be bonded together by a variety of covalent linkages In principal, both o the diversity and o the exposed position of these oligosaccharides on the cell membrane make them especially well suited to function in specific cell recognition processes Earlier it was believed that the role of the cell coat might be merely protect against o to mechanical and chemical damage and o to keep foreign objects and other cells at a distance o preventing undesirable protein-protein interaction Indeed , this probably is an important part of its function However, recently, plasma membrane bound lectins have been identified that recognize specific oligosaccharides on cell surface glycolipids and glycoproteins o to mediate a variety of transient cell-cell adhesion processes including those occurring in Sperm-egg interaction Blood clotting Lymphocyte recirculation and Inflammatory responses Fig 10.42 Alberts 3rd Ed Membrane Transport: Because of its hydrophobic interior, the lipid bilayer of cell membrane serves as a barrier to the passage of most polar molecules This barrier function is crucially important as it allows the cell to maintain the concentrations of solutes in its cytosol That are different from those in the extracellular fluid and In each of the intracellular membrane bounded compartments To make use of this barrier, cells have had to evolve ways of transferring specific water soluble molecules across their membranes in order to: • ingest essential nutrients • excrete metabolic waste products and • regulate intracellular ion concentrations Transport of inorganic ions and small water soluble organic molecules across the lipid bilayer is achieved by specialized transmembrane proteins Each of which is responsible for the transfer of specific molecule or a group of closely related ions or molecules Two main classes of membrane proteins that mediate the transfer are: • Carrier proteins / transporters - which have moving parts to shift specific molecules across the membrane • Channel proteins - which form a narrow hydrophobic pore, allowing the passive movement of small inorganic ions Carrier proteins / transporters can be coupled to a source of energy to catalyze active transport and A combination of selective passive permeability and Active transport creates large differences in composition of cytosol compared with that of either • the extracellular fluid or • the fluid with in membrane enclosed organelles Table11.1 Alberts 3rd / 5th Ed In particular, by generating the ionic concentration differences across the lipid bilayer, cell membranes are able to store potential energy in the form of electrochemical gradients: • Which drive various transport processes To convey electrical signals in electrically excitable cells and makes most of the cell’s ATP in • Mitochondria • Chloroplasts and Bactria Principles of Membrane Transport; • Protein-free lipid bilayers are highly impermeable to ions Fig 11.1 Alberts 3rd / 5th Ed Fig 11.2 Alberts 3rd / 5th Ed • Like synthetic lipid bilayers, cell membranes allow water and non polar molecules to permeate by simple diffusion • However, cell membranes also have to allow the passage of various polar molecules such as Ions, sugars, amino acids, nucleotides and Many metabolites that cross synthetic bilayer very slowly • As discussed earlier, carrier proteins / transporters and channels are the one who do the job Fig 11.3 Alberts 5th Ed / 3rd Ed • Although water can diffuse across synthetic lipid bilayers, all cells contain specific channel proteins called water channels or aquaporins • That greatly increase the permeability of these membrane to water • All channels and many transporters allow solute to cross the membrane only passively – downhill; a process called passive transport or facilitated diffusion • In case of transport of a single uncharged molecule, the difference in the concentration on the two sides of the membrane – its concentration gradient drives passive transport and determines its direction • Cells also require transport proteins that will actively pump certain solutes across the membrane against their electrochemical gradient – uphill; this process is called active transport mediated by transporters called pumps • In active transport, the pumping activity of the transporter is directional because it is tightly coupled to a source of metabolic energy. Such as ATP hydrolysis or An ion gradient • Thus transmembrane movement of small molecules mediated by transporters can be either active or passive • Whereas that mediated by channels is always passive Fig 11.4 Alberts 5th Ed / Fig 11.4 Alberts 3rd Ed Transporters and Active Membrane Transport; • The process by which a transporter transfers a solute molecule across the lipid bilayer resembles an enzyme-substrate reaction and • In many ways transporters behave like enzymes • In contrast to ordinary enzyme-substrate reactions, the transporter does not modify the transported solute • But instead delivers it unchanged to the other side of membrane Fig 11.5 Alberts 5th Ed Fig 11.9 Alberts 3rd Ed • While cells carry out active transport in three main ways: An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the electrical potential and a difference in the chemical concentration across a membrane. The difference of electrochemical potentials can be interpreted as a type of potential energy available for work in a cell. The energy is stored in the form of chemical potential, which accounts for an ion's concentration gradient across a cell membrane, and electrostatic energy, which accounts for an ion's tendency to move under influence of the transmembrane potential. Fig 11.7 Alberts 5th Ed • Active transport can be driven by ion gradients Fig 11.8 Alberts 3rd and 5th Ed • Most animal cells, for example, take up glucose from the extracellular fluid by passive transport through glucose carriers that operate as uniporters • Where its concentration is high relative to that in cytosol • By contrast, intestinal and kidney cells take up glucose from the lumen of the intestine and kidney tubule respectively where the concentration of the sugar is low • These cells actively transport glucose across their plasma membrane by symport with Na+ Fig 11.11 Alberts 5th Ed /11.13 Alberts 3rd Ed • We will discuss active transport by considering a carrier protein/ transporter that plays a crucial part in generating and maintaining the Na+ and K+ gradients across the plasma membrane of animal cells • The concentration of K+ is typically 10-20 times higher inside cells than outside, whereas the reverse is true of Na+ Table11.1 Alberts 3rd / 5th Ed (Bacteria and Archaea) • These concentration differences are maintained by a Na+ - K+ pump that is found in the plasma membrane of virtually all animals Fig 11.14 Alberts 5th Ed / Fig 11.10 Alberts 3rd Ed Fig 11.15 Alberts 5th Ed / Fig 11.11 Alberts 3rd Ed • Na+ gradient produced by the Na+ - K+ pump drives the transport of most nutrients in to animal cells and • Also has a crucial role in regulating cytosolic pH • A typical animal cell devotes almost one-third of its energy to fueling this pump and • Pump consumes even more energy in electrically active nerve cells which repeatedly gain small amounts of Na+ and lose small amount of K+ during the propagation of nerve impulses • The Na+ - K+ pump does have a direct role in controlling the solute concentration inside the cell and • Thereby helps regulate osmolarity of the cytosol • All cells contain specialized water channel proteins called aquaporins in their plasma membrane to facilitate water flow across this membrane • Thus water moves into or out of cells down its concentration gradient – a process called osmosis • As discussed in Table 11.1, cells contain high concentrations of solutes including numerous negatively charged organic molecules that are confined inside the cell and their accompanying cations that are required for charge balance • This tends to create a large osmotic gradient that tends to “pull” water into the cell • Animal cells counteract this effect by an opposite osmotic gradient due to a high concentration of inorganic ions chiefly Na+ and Cl- in extracellular fluid • The Na+ - K+ pump helps maintain osmotic balance by pumping out the Na+ that leaks in down its steep electrochemical gradient • The Cl- is kept out by the membrane potential • In the special case of human red blood cells, which lack a nucleus and other organelles and • Have a plasma membrane that has an unusually high permeability to water, osmotic water movements can greatly influence cell volume and • Na+ - K+ pump plays an important part in maintaining red cell volume Fig 11.12 Alberts 3rd Ed / Fig 11.16 Alberts 5th Ed • Unlike carrier proteins/transporter, channel proteins form hydrophilic pores across membranes • One class of channel proteins found virtually all in animals forms gap junctions between two adjacent cell; Each plasma membrane contributes equally to the formation of the channels Which connects the cytoplasm of the two cells • Both gap junctions and porins – the channel forming proteins of the outer membranes of: Bacteria Mitochondria and Chloroplasts have relatively large permissive pores which would be disastrous if they directly connected the inside of a cell to an extracellular space • In actual many bacterial toxins do exactly that to kill other cells • In contrast, most channel proteins in the plasma membrane of animals and plant cells • that connect the cytosol to the cell exterior necessarily have narrow, highly selective pores that can open and close rapidly • Because these proteins are concerned specifically with inorganic ion transport, they are called ion channels • For transport efficiency, ion channels have an advantage over carrier is that up to 100 million ions can pass through open channel each second a rate 105 times greater than the fastest rate of transport mediated by any known carrier protein / transporter • However, channels can not be coupled to an energy source to perform active transport • So the transport they mediates always passive - down hill • Thus function of ion channels is to allow specific inorganic ions primarily Na+, K+, Ca2+ or Cl- to diffuse rapidly down their electrochemical gradient across the lipid bilayer • Two important properties distinguish ion channels from simple aqueous pores: i. They show ion selectivity, permitting some inorganic ions to pass but not others Fig 11.20 Alberts 5th Ed / Fig 11.17 Alberts 3rd Ed ii. Ion channels are not continuously open rather they are gated, which allows them to open briefly and than close again Fig 11.21 Alberts 5th Ed / Fig 11.18 3rd Ed Transport into the Cell from the Plasma Membrane: Endocytosis • Every cell must eat communicate with the world around it and quickly respond to changes in its environment • To help accomplish these tasks: cells continually adjust the composition of their plasma membrane in rapid response to need • They use an elaborate internal membrane system to add and remove cellsurface proteins embedded in the membrane. Such as Receptors Ion channels and transporters • • • • • • • • Through the process of exocytosis, the biosynthetic-secretory pathway delivers newly synthesized proteins, carbohydrates and lipids to either: plasma membrane extracellular space By the opposite process of endocytosis, cells remove plasma membrane components and deliver them to internal compartment called endosomes From where they can be recycled to the same or different regions of the plasma membrane or can be delivered to lysosomes for degradation Fig 13.1 Alberts 5th Ed Fig Downloaded Cells also use endocytosis to capture important nutrients. Such as vitamins lipids cholesterol and iron These are taken up together with the macromolecules to which they bind and are then released in endosomes or lysosomes and transported in to cytosol, where they are used in various biosynthetic processes Transport vesicle • The interior space or lumen of each membrane-enclosed compartment along the biosynthetic-secretory and endocytic pathways is topologically equivalent to the lumen of most other membrane-enclosed compartments and to the exterior • Protein can travel in this space without having to cross a membrane, being passed from one compartment to another by means of numerous membraneenclosed transport containers • Some of these containers are small spherical vesicles • While other are large irregular vesicles or tubules formed from the donor compartment • All containers are generally called transport vesicles • Within a eukaryotic cell, transport vesicles continually bud off from one membrane and fuse with another, carrying membrane components and soluble molecules • Which are referred as cargo Fig 13.2 Alberts 5th Ed • This membrane traffic is highly organized directional routes • Which allows the cell to: secrete eat and remodel its plasma membrane • The biosynthetic-secretory pathway leads outward from the endoplasmic reticulum (ER) toward the Golgi apparatus and cell surface with a side route leading to lysosomes • While the endocytic pathway leads inward from the plasma membrane Fig 13.3 Alberts 5th Ed • To perform its function, each transport vesicle that buds from a compartment must be selective • It must take up only the appropriate molecules and must fuse only with appropriate target membrane. For example: Must exclude proteins that are to stay in the Golgi apparatus and It must fuse only with the plasma membrane and not with any other organelle • The routes that lead inward from the cell surface to lysosomes start with the process of endocytosis by which cells take up : * * * * macromolecules particulate substances and in specialized cases even other cells • In this process, the material to be ingested is progressively enclosed by a small portion of plasma membrane which first invaginates and then pinches off to form an endocytic vesicle containing the ingested substance or particles • Two main types of endocytosis are distinguished on the basis of the size of endocytic vesicles formed; i. Phagocytosis (cell eating) - large particles are ingested via large vesicles called phagosomes which are generally >250 nm in diameter ii. Pinocytosis (cell drinking) - fluid and solutes are ingested via small pinocytic vesicle which are about 100 nm in diameter • Most eukaryotic cells are continually ingesting fluid and solutes by pinocytosis while • Large particles are ingested most efficiently by specialized phagocytic cells Specialized Phagocytic Cells can Ingest Large Particles; • Phagocytosis is a special form of endocytosis in which a cell uses large endocytic vesicles called phagosomes to ingest large particles such as: microorganisms and dead cells • In protozoa, phagocytosis is a form of feeding: Large particles taken up into phagosomes end up in lysosomes and The products of the subsequent digestive processes pass in to the cytosol to be used as food • However, few cells in multicellular organisms are able to ingest such large particles efficiently. For example: Extracellular processes break down food particles and Cells import small hydrolysis products • Phagocytosis is important in most animals for purposes other than nutrition and • It is carried out mainly by specialized cells - professional phagocytes. Which are: Macrophages and Neutrophils • These cells develop from hemopoietic stem cells and • They ingest invading microorganisms to defend us against infection • Macrophages also have am important role in scavenging senescent cells and • Cells that have died by apoptosis • In quantitative terms, the clearance of senescent and dead cells is by far the most important. For example: Our macrophages phagocytose more than 1011 senescent red blood cells in each of us every day • Whereas the endocytic vesicles involved in pinocytosis are small and uniform i.e. phagosomes have diameters that are determined by the size of the ingested particle and • They can be almost as large as the phagocytic cell itself Fig 13.26 Alberts 3rd Ed / Fig 13.46 Alberts 5th Ed • The phagosomes fuse with lysosomes inside the cell and • Ingested material is then degraded • Any indigestible substances will remain in lysosomes forming residual bodies • Which can be excreted from cells by exocytosis • Some of the internalized plasma membrane components never reached to lysosomes • Because they are retrieved from the phagosome in transport vesicles and • Returned to the plasma membrane • Particles must bind to the surface of the phagocyte to be phagocytosed • Phagocytes have a variety of specialized surface receptors that are functionally linked to the phagocytic machinery of the cell • Phagocytosis is a triggered process • It requires the activation of receptors that transmit the signals to the cell interior and initiate the response • The best characterized triggers of phagocytosis are antibodies • Which protects us by binding to the surface of infectious microorganisms to form a coat that exposes the tail region on the exterior of each antibody molecules • This tail region is called Fc region • The antibody coat is recognized by specific Fc receptors on the surface of macrophages and neutrophils • Whose binding induces the phagocytic cells to extend pseudopods that engulf the particle and fuse at their tips to form a phagosome Fig 23.20 Alberts 3rd Ed Fig Down Loaded Fig 13.47 Alberts 5th Ed • In this way, the ordered generation and consumption of specific phosphoinositides guide sequential steps in phagosome formation • Several other class of receptors that promote phagocytosis have been characterized: Some recognize complement components which collaborate with antibodies in targeting microbes for destruction Other directly recognize oligosaccharides on the surface of certain microorganisms Yet others recognize cells that have died by apoptosis Apoptotic cells lose the asymmetric distribution of phospholipids in their plasma membrane As a consequence, negatively charged phophotidylserine (normally confined to the cytosolic leaflet of lipid bilayer) is now exposed on the outer side of the cell Polymerization, initiated by Rho family GTPase, shapes the pseudopods Where it helps to trigger the phagocytosis of the dead cell • Macrophages will also phagocytose a variety of inanimate (lifeless) particles. Such as: glass or latex bead and asbestos fibers • Yet they do not phagocytose live animal cells • Living animals seems to display “do not eat me’ in the form of cell surface proteins that bind to inhibiting receptors on the surface of macrophages • Thus, like many other processes, phagocytes depends on a balance between positive signals that activate the process and negative signals that inhibit it • Therefore, apoptotic cells gain “eat me” signals (extracellularly exposed phosphatidylserine) and lose “do not eat me” signals causing them to be very rapidly phagocytosed by macrophages - Slides 31 and 33 Pinocytic Vesicles form from Coated Pits in the Plasma Membrane; • Virtually all eukaryotic cells continually ingest bits of plasma membrane in the form of small pinocytic (endocytic) vesicles which are later retuned to cell surface • The rate at which the plasma is internalized in this process of pinocytosis varies between cell types but it is usually large. For example: A macrophage ingests 25% of its own volume of fluid each hour This means that it must ingest 3% of its plasma membrane each minute or 100% in about half an hour Fibroblasts endocytose at a somewhat lower rate i.e. 1% of their plasma membrane per minute Whereas some amoebae ingest their plasma membrane even more rapidly • Since a cell’s surface area and volume remain unchanged during this process, it is clear that same amount of membrane being removed by endocytosis is being added to the cell surface by the converse processes of exocytosis • In this sense, endocytosis and exocytosis are linked processes that can be considered to constitute an endocytic-exocytic cycle • The coupling between endocytosis and exocytosis is particularly strict in specialized structures characterized by high membrane turnover. Such as • Neuronal synapse • The endocytic part of the cycle often being at clatherin-coated pits • These specialized region typically occupy about 2% of the plasma membrane area • The life time of a clatherin-coated pit is short: Within a minute or so of being formed, it invaginates in to the cell and pinches off to form a clatherin-coated vesicle 13.28 Alberts 3rd Ed / Fig 13.48 Alberts 5th Ed • It has been estimated that about 2500 clatherin-coated vesicles leave plasma membrane of a cultured fibroblast every minute • The coated vesicle are even more transient than the coated pits: Within seconds being formed, they shed their coat and are able to fuse with early endosomes • Since extracellular fluid is trapped in clatherin-coated pits as they invaginate to form coated vesicles, any substance dissolved in the extracellular fluid is internalized - a process called fluid-phase endocytosis Not All Pinocytic Vesicles are Clatherin-Coated; • In addition to clatherin-coated pits and vesicles, there are another, less well understood mechanisms by which cells can form pinocytic vesicles • One of these pathways initiates at caveolae (in Latin means little cavities) originally recognized by their ability to transport molecules across endothelial Which form the inner lining of blood vessels • Caveolae are present in plasma membrane of most cell types and in some of these they are seen in the electron microscope as deeply invaginated flasks Fig 13.49 Alberts 5th Ed • The major structural proteins in caveolae are caveolins which are a family of unusual integral membrane proteins • These are thought to invaginate and collect cargo proteins Cells Use Receptor-Mediated Endocytosis to Import Selected Extracellular Macromolecules; • In most animals cells, clathrin-coated pits and vesicles provide efficient pathway for taking up specific macromolecules from the extracellular fluid • This process is called receptor-mediated endocytosis • In this process: macromolecules bind to complementary transmembrane receptor proteins accumulate in coated pits and Then entered the cell as receptor-macromolecule complexes in clathrincoated vesicles 13.28 Alberts 3rd Ed / Fig 13.48Alberts 5th Ed • Because ligands are selectively captured by receptors receptor-mediated endocytosis provides a selective concentrating mechanism that increase the efficiency of internalization of particular ligand more than a hundred fold • In this way, even minor components of the extracellular fluid can be specifically taken up in large amounts without taking in a large volume of extracellular fluid • A particularly well understood and physiologically important example is the process that mammalian cells use to take up cholesterol Many animal cells take up cholesterol through receptor-mediated endocytosis and In this way, acquire most of the cholesterol they require to make new membrane If the take up is blocked, cholesterol accumulates in the blood and Can contribute to the formation of atherosclerotic plaques in the blood vessel walls more specifically artery • • • • • Atherosclerotic plaques are actually deposits of lipid and fibrous tissue that can cause strokes and heart attacks by blocking arterial blood flow Most of the cholesterol is transported in the blood as cholesterol esters in the for of lipid-protein particles known as low density proteins (LDLs) Fig 13.29 Alberts 3rd Ed / Fig 13.50 Alberts 5th Ed When a cell needs cholesterol from membrane synthesis, it makes transmembrane receptor proteins for LDL and insert them in to its plasma membrane Once in plasma membrane, the LDL receptors diffuse until they associate with clathrin-coated pits that are in process of forming Fig 13.51A Alberts 5th Ed / Fig 13.30A Alberts 3rd Ed Since coated pits constantly pinch off to form coated vesicles, any LDL particles bound to LDL receptors in the coated pits are rapidly internalized in coated vesicles After shedding their clathrin coats, the vesicles deliver their content to early endosomes which are located near the cell periphery Once the LDL and its receptor encounter the low pH in the endosomes, LDL is released from its receptor and is delivered to lysosomes via endosomes • If too much cholesterol accumulates in a cell, the cell shuts off both its own cholesterol synthesis and • synthesis of LDL receptors so that it ceases either to make or to take up cholesterol • This regulated pathway for cholesterol uptake is disrupted in individuals who inherit defective genes encoding LDL receptors • The resulting high levels of blood cholesterol predispose these individuals to develop atherosclerosis prematurely and • Many would die at an early stage of heart attacks resulting from coronary artery disease if they were not treated with drugs that lower the level of blood cholesterol • In some cases; the receptor is lacking altogether In others, the receptors are defective in either; o the extracellular binding site for LDL or o the intracellular binding site that attaches the receptor to the coat of a clathrin coated pit Fig 13.51B Alberts 5th Ed / Fig 13.30B Alberts 3rd Ed Endocytosed Materials that are not Retrieved from Endosomes end up in Lysosomes; • The endosomal compartment can be made visible in the electron microscope by adding a readily detectable tracer molecules Fig 13.31 Alberts 3rd Ed • The endosomal compartment is kept acidic i.e. ~ pH 6 by ATP-driven H+ pumps in the endosomal membrane that pump H+ in to the lumen from the cytosol • Late endosomes are more acidic than early endosomes • This acidic environment plays a crucial part in function of these organelles Specific Proteins are Retrieved from Early Endosomes and Returned to the Plasma Membrane; • Early endosomes form a compartment that acts as the main sorting station in the endocytic pathway • Just as cis and trans Golgi networks serve this function in the biosynthetic-secretory pathway Fig 13.52 Alberts 5th Ed / Fig 13.32 Alberts 3rd Ed • LDL receptors follow the first pathway Golgi Apparatus Endosomes ER Fig 13.53 Alberts 5th Ed / Fig 13.33 Alberts 3rd Ed Multivesicular Bodies form on the Pathway to Late Endosomes; • As discussed earlier, many of the endocytosed molecules move from early to late endosomal compartment • In this process, early endosomes migrate slowly along microtubules toward the cell interior • while shedding membrane tubules and vesicles that recycle materials to plasma membrane • At the same time, the membrane enclosing the migrating endosomes forms invaginating buds that pinch off and • form internal vesicles – called multivesicular bodies Fig 13.56 Alberts 5th Ed / Fig 13.34 Alberts 3rd Ed • The multivesicular bodies carry those endocytosed proteins that are to be degraded Fig13.57 Alberts 5th Ed x -------------------------- x ----------------------------- x ------------------------------x (Acidic environment)