The biological membrane prof. Dr. Pál Röhlich ÁOK 2013/2014, I. Semester: Introduction to cell biology 11. , 12. 09. 2013
Components of an idealized animal cell (light and electron microscopic „inventory”) microvilli kinocilium intermediate filaments stereocilium phagocytosis cortical actin filaments phagosome nuclear envelope lysosome peroxisome mitochondrium Zellkern secretory granule chromatin exocytosis microtubule nucleolus Golgi-apparatus lipid droplet pinocytosis coated pit glycogen granules centrosome smooth endoplasmic reticulum (s. ER) rough endoplasmic reticulum (r. ER)
The plasma membrane Microscopic morphology Each cell is covered by an extremely thin membrane that separates and protects the interior of the cell from its environment. At the same time, this so-called plasma membrane (or cell membrane) makes interaction of the cell with its environment possible and integrates the cell into its surroundings in multicellular organisms. In the light microscope the plasma membrane usually can not be seen due to its extreme thinness. Isolated erythrocyte membranes (after burst of the red blood cell and outflow of its contents in highly hypotonic solutions) can be just discerned as so-called „ghosts” in the phasecontrast microscope. In the electron microscope the plasma membrane can be seen as a fine black line, which can be resolved with high magnifications into a trilaminar structure (two dense layers with a light layer between them). Total thickness: 8 -10 nm. plasma membrane Low EM magnification High EM magnification
Intracellular membranes Membranes are present also in the cell interior (intracellular membranes), where they form imortant constituents of a number of cell organelles (endoplasmic reticulum, Golgi-apparatus, lysosome, mitochondria, etc. ). r. ER mitochondrium Golgi-apparatus
Basic structure of the membrane: the lipid bilayer I. Lipids are fat-like molecules with the largest part of the molecule being hydrophobic (water-repellent). However, these molecules also carry a hydrophilic (water-attractant) region or chemical group (amphiphilic molecules). Their main representatives in the membrane are: phospholipids, cholesterol and a few other lipid species. An example for phospholipids: phosphatidylcholin Cholesterol
Arrangement of lipids in an aqueous environment micelle bilayer Lipids occur in an aquous environment in two energetically stable supramolecular arrangements: the planar bilayer and the spherical micelle Arrangement of phospholipids and cholesterol in the lipid bilayer
Important physical properties of the lipid bilayer Continuous hydrophobic layer in the interior → diffusion barrier for hydrophilic molecules and ions across the membrane. However, it is not a barrier for hydrophobic molecules (e. g. O 2, CO 2, steroid hormones). The small water molecules can pass through the lipid bilayer with some difficulty. Dynamic structure: the bilayer behaves as a two-dimensional fluid. Lipid molecules are in continuous motion in the bilayer (mobility), they rotate around their long axis, their fatty acid chains are mobile, and they diffuse in the plane of the bilayer with high velocity: lateral diffusion. However, a flip-flop translocation of lipids from one lipid layer into the other one is very rare and is only possible with the aid of special enzymes in the living cell. Fluidity of the bilayer structure depends on various factors , like relative amount of cholesterol (rigid molecule!), unsaturated fatty acids in the phospholipid molecules and temperature. Due to its dynamic structure, the bilayer has a plasticity, the lipid bilayer is able to follow form and volume changes of the cell. Vesicles Due to energetic reasons sheet-like (planar) bilayers do not occur in an aquous environment. The lipid bilayer forms a spherical vesicle. Tears in the bilayer are quickly repaired by a spontaneous rearrangement of the lipids , vesicles are therefore quickly closed Fusion of lipid vesicles: vesicles can fuse with each other (role in vesicular transport between membrane -limited cell organelles). Lipid asymmetry in the plasma membrane. Various types of phospholipids are asymmetrically distributed in the two layers of the membrane (e. g. phosphatidylcholin in the outer layer, phosphatidylserine and phosphatidylinositol in the inner layer). Artificial lipid vesicles: liposomes (application in medicine). Liposomes visualized in a freezefracture sample. Electron micrograph.
„Intelligent” properties of the membrane depend on membrane proteins II. Proteins Some basic knowledge about protein structure. Proteins are the most important molecules of life. Amino acids, polypeptide chain, 20 different side chains, 3 D structure (folding), depending on the amino acid sequence, tertiary structure and function, conformational change and its significance. 25 -75% of the membrane mass consists of proteins. About 50 lipid molecules per one protein. Association of proteins with the lipid bilayer Integral and peripheral membrane proteins 1 -6 integral membrane proteins 7 -8 peripheral membrane proteins
1. Integral membrane proteins form integral constituents of the membrane, are bound firmly to the lipid bilayer and can not be removed without destructing the bilayer Transmembrane proteins extend across the bilayer, the polypeptide chain traversing the membrane once or many times („single-pass” and „multipass” proteins). The membrane-spanning portions are α-helixcal structures, where the hydrophobic side chains are turned outwards and are in hydrophobic interaction with the fatty acid chains of phospholipids. Most important membrane proteins (e. g. transport proteins, membrane receptors, cell adhesion proteins) belong to this group. Membrane proteins partially embedded into the bilayer These are similar to transmembrane proteins but do not span the bilayer. Their hydrophobic region is embedded into the cytosolic lipid monolayer (e. g. caveolin) caveolin Anchored membrane proteins are located on one side of the bilayer and are attached to it by a covalently bound lipid chain on the cytosolic side or by an oligosaccharide chain covalently bound to phosphatidyl inositol on the extracellular side (GPI anchor). The anchor can be detached from the protein (functionally important separation of the protein from the membrane) or, by binding back again to the anchor, reattached to the membrane. fatty acid anchor GPI-anchor
2. Peripheral membrane proteins peripheral membrane protein These are hydrophilic proteins on either the extracellular or cytosolic surface of the membrane and are attached by weak bonds to an integral membrane protein. They can be removed from the membrane without destruction of the lipid bilayer. III. Sugar constituents: Glycocalyx integral membrane protein oligosaccharide chains At the extracellular surface of the cell membrane there are short sugar chains (oligosaccharide chains), covalently bound to proteins or lipids (glycoproteins, glycolipids). Some proteins carry long chains consisting of specific sugar molecules (glycosaminoglycan chains, GAGs), they are called proteoglycans. This carbohydrate-rich layer on the external (extracellular) side of the membrane is the glycocalyx. Its functional significance is only partially known (protection of the cell against bacteria, binding sites of membrane receptors, blood group antigens in erythrocytes, binding sites to other cells, etc. ) The glycocalyx is rich in negatively charged (acidic) chemical groups and therefore can be visualized by applying positively charged colloidal stains (e. g. ruthenium red) as a dense layer in the electron microscope. peripheral glycoprotein GAG chains glycocalyx membrane
Membrane skeleton The membrane, consisting of the lipid bilayer and membrane proteins, is very thin and fragile and needs mechanical support. This consists of a network of filamentous proteins that is attached to the membrane by binding to integral membrane proteins. The network is called membrane skeleton. A further support for the membrane is the marginal, densely packed zone of the cytoskeleton (cell cortex, mainly consisting of actin filaments) that is also bound to the membrane, directly with adaptor proteins (like vinculin, talin, actinin) or indirectly. Membrane skeleton of red blood cells. A network of long spectrin and short actin filaments, bound to integral membrane proteins on the cytosolic surface of the cell membrane by specific proteins (ankyrin, band 4. 1 protein). It maintains the elasticity and biconcave shape of the red blood cells. (its detachment from the membrane leads to loss of the biconcave shape and fragmentation of the erythrocytes into spherical structures). Membrane skeleton in other cell types: variations of the membrane skeleton in red blood cells, where spectrin is substituted with other, spectrinrelated proteins, like fodrin. Cytosolic surface of the erythrocyte membrane with the membrane skeleton
Mobility of membrane proteins, membrane domains Lateral diffusion: proteins are usually mobile and move in lateral direction in the plane of the lipid bilayer. Exceptions: 1. Membrane macrodomains • proteins can be immobilized by binding to stable structures outside the membrane (on one or the other side) or when they are bound to each other. axonal domain apical domain • In the case of tight junction (see later) a linear arrangement of special membrane proteins in the lipid bilayer can form a barrier (fence) against lateral diffusion of proteins and can separate membrane regions with different protein and lipid basolateral compositions. domain Examples (important in the specific function of certain cell types): dendritic domain epithelial cell apical and basolateral membrane domains in eithelia Dendritic and axonal membrane domains in nerve cells 2. Membrane microdomains are small areas in the membrane with a molecular composition different from that of the surrounding membrane regions. lipid rafts: small membrane spots enriched in special lipids and proteins. caveola Caveolae: lipid rafts with the integral membrane protein caveolin on the cytosolic side of the membrane. Vesicle-like invaginations of the membrane with various functions. nerve cell
Functions of the cell membrane I. Diffusion barrier Basis of the barrier: the continuous hyrophobic layer in the interior of the lipid bilayer. It inhibits free diffusion of many substances across the membrane and makes thereby a controlled transport through the membrane possible. The membrane is a barrier against free diffusion of smaller or larger polar molecules as well as electrically charged particles (ions) It is not a barrier against hydrophobic molecules (e. g. gases, like O 2, CO 2, and steroid hormones). If the barrier is not functioning, e. g. by stripping, permeabilisation or poration of the membrane, the stable inner environment of the cell can not be maintained and the cell dies. Example: the natural killer cells of the immune system kill foreign cells by inserting pores into the membrane of the target cell.
II. Controlled uptake and release of substances through the membrane A completely closed system is not viable, the cell must have an interaction with its environment to survive. It has to take up nutrients and waste products have to be released. Comparison with a medieval town : city wall with controlled gates Two mechanisms for uptake and release: 1. Membrane transport: Transport of small molecules and ions across the membrane. 2. Endocytosis and exocytosis: Uptake and release of macromolecules or colloidal particles with vesicles (see later).
Transport across the membrane by transmembrane proteins. 20 -30% of proteins in the cell are membrane transport proteins! Two large groups of transport proteins: carrier proteins and channel proteins. kinn 1. Carrier proteins A carrier protein transports a molecule or an ion by conformational change (binding of the transported molecule on one side, conformational change, release of the molecule on the other side of the membrane). Cotransport: 2 molecules or ions are simultaneously transported. lipid bilayer concentration gradient molecule ion A. Facilitated diffusion Molecules (or ions) are transported along their concentration gradient (from higher concentration towards lower concentration). This transport does not need energy (passive transport). COTRANSPORT
B. Active transport (pump) The carrier protein transports a substance (e. g. ion) against its concentration gradient. This transport needs energy (active transport). Two energy sources: ATP-hydrolysis or potential energy of a concentration gradient. 1. ATP-driven pumps. Examples: Na+-K+-pump. Na+ are transported outwards and K+ inward, both against their concentration gradients, using the energy of ATP-hydrolysis. ABC transporter: pumps hydrophobic molecules from the cell into its environment with the hydrolysis of 2 ATP molecules. Example: in cancer chemotherapy the cancer cell can pump anti-cancer agents out from the cell. 2. Secondary active transport. Concentration gradient as energy source. Example: potential energy of water in a water tower. The energy for the transport comes from backflow of molecules or ions along their concentration gradient. Example: Na+ glucose transporter. electro-chemical gradient of Na glucose gradient
2. Channel proteins a) Ion channels These proteins form a hydrophilic channel across the membrane, through which ions can flow from one side of the membrane to the other side. Ion selectivity (e. g. K-channels, Na-channels). Passive transport. Rapid ion flow through the channel: 105 - times faster than with a carrier (in one second up to 1 million ions can flow through a channel). Significance: primarily in uptake and conduction of electric impulses in nerve and muscle cells. The open state of the channel is usually regulated. Various channels can be opened upon specific stimuli by conformational changes: 1. Ligand-gated channels: a signal molecule is bound to a specific binding site of a channel protein (e. g. neurotransmitters, nucleotides, ions). 2. Mechanically-gated channels react to mechanical stimuli (e. g. pressure on or stretch of the membrane). 3. Voltage-gated channels react to changes of the electric field in the immediate vicinity (important in nerve conduction). b) Water channels Aquaporin proteins with a water channel in their interior facilitate an intensive water flow through the membrane (e. g. kidney cells). Specific for water, no ions can flow through. c) Non-specific channels Porin-channels in the outer mitochondrial membrane, Connexons in gap junction between two cells.
III. Contact between the cell and its environment The cell surface is a contact surface between the cell and its environment (important in the adaptation of the cell to its environment and in its integration in a multicellular organism). The cell surface is the „face” of the cell to the outside world. 1. Signal registration. Membrane receptors (tansmembrane proteins) that by binding and reacting to specific (chemical, physical, etc. ) signals switch on signal transduction pathways that lead informations into the interior of the cell. Receptor specificity. 2. Cell-cell recognition. Adhesion molecules (transmembrane proteins) in the membrane are responsible for specific cell associations or for binding cells to the extracellular matrix. Important in the development of tissues and organs. „Homing” of lymphocytes in lymphatic organs. Cell-cell contacts in the immune response. MHC complex (cell identity). 3. Stable binding between cells: intercellular junctional structures (desmosome, tight junction, gap junction, … see later).
Textbook: Essential cell biology, 3 rd edition, p. 70 -73, 363 -400 + lecture, (summarized in the present ppt presentation). Sources of figures Röhlich: Histology, 3 rd. edition, Semmelweis Publishing House, Budapest Alberts – Johnson – Lewis – Raff – Roberts – Walter: Molecular biology of the cell. 5 th edition, Garland Science Own specimens, micrographs and schematic drawings
Скачать презентацию The biological membrane prof Dr Pál Röhlich ÁOK
26125a68ecb2771cdaa0c3c16cc3ec4e.ppt