Biological Membranes - Bio201 4BiologicalMembranes (1) PDF

BIOLOGY tenth edition Topic 4 Biological Membranes © Cengage Learning 2015 SOLOMON MARTIN MARTIN BERG  Biological Membranes Separating a cell from its external envi...

BIOLOGY tenth edition Topic 4 Biological Membranes © Cengage Learning 2015 SOLOMON MARTIN MARTIN BERG  Biological Membranes Separating a cell from its external environment: essential to origin of life Membrane proteins are critical in cell membrane activities – Proteins associated with the plasma membrane transport materials: transmit information; serve as enzymes – Cell adhesion molecules connect cells to one another to form tissues © Cengage Learning 2015  5.1 The Structure Of Biological Membranes The cell is the smallest unit that can carry out all activities associated with life – Building blocks of complex multicellular organisms Every cell is surrounded by a plasma membrane that separates its internal environment from the outside world – Regulates passage of materials in and out of the cell; helps maintain homeostasis © Cengage Learning 2015  Phospholipids Form Bilayers in Water Amphipathic molecules with distinct hydrophobic and hydrophilic regions – Two fatty acid chains make up the nonpolar, hydrophobic portion of the phospholipid – Fatty acid tails are bonded (via a glycerol molecule) to a negatively charged, hydrophilic phosphate group, linked to a polar, hydrophilic organic group © Cengage Learning 2015  Phospholipids (cont’d.) Phospholipids form bilayers of two distinct regions, one strongly hydrophobic and the other strongly hydrophilic – Cylindrical shape allow them to associate with water most easily as a bilayer Amphipathic molecules with different shapes tend to form spherical structures in water © Cengage Learning 2015 Hydrophilic heads Hydrophobic tails (a) (b)  Figure 5-1 Properties of lipids in water (a)Phospholipids in water. Phospholipids associate as bilayers in water because they are roughly cylindrical amphipathic molecules. The exible hydrophobic fatty acid chains are not exposed to water, whereas the hydrophilic phospholipid heads are in contact with water. (b) Detergent in water. Detergent molecules are roughly cone-shaped amphipathic molecules that associate in water as spherical structures.  Phospholipid Bilayer The electron microscope revealed a three- layered structure, suggesting plasma membrane was uniform and no more than 10 nm thick J. Singer and G. Nicolson proposed the fluid mosaic model in 1972: embedded proteins loosely associate with the bilayer, like a dynamic mosaic – Positions of proteins are constantly changing © Cengage Learning 2015  Phospholipid Bilayer (cont’d.) TEM of red blood cell: Cell interior – Hydrophilic heads of phospholipids are the parallel dark lines Plasma – Hydrophobic tails are membrane the light zone between them Outside cell © Cengage Learning 2015  Figure 5-3 TEM of the plasma membrane of a mammalian red blood cell. The plasma membrane separates the cytosol (darker region) from the external environment (lighter region). The hydrophilic heads of the phospholipids are the parallel dark lines, and the hydrophobic tails are the light zone between them. © Cengage Learning 2015  Biological Membranes Are Two-Dimensional Fluids Phospholipid bilayers behave like liquid crystals – Form an ordered array but are in constant motion Phospholipid molecules are free to rotate and can move laterally within their single layer (a two- dimensional fluid) Molecules embedded in the membrane can move along the plane of the membrane, producing an ever-changing configuration © Cengage Learning 2015  Membrane fluidity The ordered arrangement of phospholipid molecules makes the cell membrane a liquid crystal. The hydrocarbon chains are in constant motion, allowing each molecule to rapidly move laterally on the same side of the bilayer. Flip-flop from one side of the bilayer to the other is a rare event and in cells it is facilitated by special membrane proteins. © Cengage Learning 2015  Biological Membranes Are Two-Dimensional Fluids (cont’d.) Membrane fluidity depends on its component lipids – When outside temperature is low, some organisms alter fatty acid content of their membrane lipids to increase relative proportions of unsaturated fatty acids – Cholesterol molecules in membranes act as “fluidity buffers,” keeping hydrocarbon chains fluid at low temperatures and stabilizing them at high temperatures © Cengage Learning 2015  Biological Membranes Fuse and Form Closed Vesicles Lipid bilayers: – Are flexible, allowing cell membranes to change shape without breaking – Are self-sealing, and spontaneously round up to form closed vesicles – Can fuse with other bilayers, allowing vesicles to transfer materials from one compartment to another, or secrete a product from the cell © Cengage Learning 2015  Membrane Proteins Include Integral and Peripheral Proteins Integral membrane proteins: amphipathic proteins firmly bound to the membrane Transmembrane proteins: integral proteins that extend completely through the membrane Peripheral membrane proteins: located on inner or outer surface of plasma membrane, bound to exposed regions of integral proteins © Cengage Learning 2015 Outside cell Lipid bilayer Cytosol  Proteins Are Oriented Asymmetrically Across the Bilayer A specific external domain of an intrinsic membrane protein is always found on the same side of the membrane – When sugars are added to proteins to make glycoproteins, only a part of the protein extends into the interior of the ER, Golgi complex and transport vesicle © Cengage Learning 2015  Synthesis and orientation of a membrane protein © Cengage Learning 2015  5.2 Overview Of Membrane Protein Functions Proteins associated with the membrane specialize for many essential activities – Anchor the cell to its substrate – Transport molecules across the membrane – Catalyze enzymatic reactions near the cell surface – Receive information from other cells in the form of chemical or electrical signals – Serve as identification tag © Cengage Learning 2015  6 Major Functions Of Membrane Proteins 1.Transport. (left) A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. (right) Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane 2.Enzymatic activity. A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a team that carries out sequential steps of a metabolic pathway. 3.Signal transduction. A membrane protein may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external messenger (signal) may cause a conformational change in the protein (receptor) that relays the message to the inside of the cell. © Cengage Learning 2015  6 Major Functions Of Membrane Proteins 4. Cell-cell recognition. Some glyco-proteins serve as identification tags that are specifically recognized by other cells. 5. Intercellular joining. Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions 6. Attachment to the cytoskeleton and extracellular matrix (ECM). Microfilaments or other elements of the cytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes © Cengage Learning 2015  5.3 Cell Membrane Structure And Permeability Biological membranes are selectively permeable membranes: allow some, but not all, substances to pass through them – A membrane may block a particular substance at one time and actively promote its passage at another time – By regulating chemical traffic across its plasma membrane, a cell controls its volume and its internal ionic and molecular composition © Cengage Learning 2015  Biological Membranes Present a Barrier to Polar Molecules Biological membranes are most permeable to small nonpolar molecules Water molecules are small enough to pass through gaps in the lipid bilayer, and slowly cross the membrane The lipid bilayer is relatively impermeable to charged ions and most large polar molecules © Cengage Learning 2015  Transport Proteins Transfer Molecules Across Membranes Transport proteins move ions, amino acids, sugars and other needed polar molecules through membranes Two main types of membrane transport proteins: carrier proteins and channel proteins – Each type transports a specific type of ion or molecule or a group of related substances © Cengage Learning 2015  Transport Proteins (cont’d.) Carrier proteins (transporters) bind the ion or molecule and change shape, moving the molecule across the membrane – Transfer of solutes by carrier proteins located within the membrane is called carrier- mediated transport – Two forms of carrier-mediated transport differ in capabilities and energy sources © Cengage Learning 2015  Transport Proteins (cont’d.) ABC transporters (ATP-binding cassette): use energy donated by ATP to transport certain ions, sugars and polypeptides – Mutations in the genes encoding these proteins contribute to many human disorders © Cengage Learning 2015  Transport Proteins (cont’d.) Channel proteins: form pores in membrane – Cells regulate passage of materials through the channels – Channels transport water and specific types of ions – Cells regulate the passage of materials through the channels by opening and closing the gates in response to electrical changes, chemical stimuli, or mechanical stimuli. © Cengage Learning 2015  Transport Proteins (cont’d.) Porins: transmembrane channel proteins ( rolled-up, barrel-shaped β-pleated sheets) that allow solutes or water to pass through membranes Aquaporins: gated water channels that allow rapid transport of water through the plasma membrane – In humans, aquaporins respond to hormonal signals that prevent dehydration by returning water from kidney tubules into the blood © Cengage Learning 2015  5.4 Passive Transport Many ions and small molecules move through membranes by diffusion – The random motion of particles resulting in net movement “down” their own concentration gradient Simple diffusion Facilitated diffusion © Cengage Learning 2015  Diffusion Occurs Down a Concentration Gradient If particles in a liquid or gas are not evenly distributed, then regions exist that have a higher or lower concentration of particles, forming a concentration gradient – The diffusion of particles is the movement of those particles from an area of high concentration to an area of low concentration © Cengage Learning 2015  Diffusion (cont’d.) A gradient across the membrane is a form potential energy capable of doing work – The stored energy of the concentration gradient is released when ions or molecules move from high to low concentration Movement down a concentration gradient is spontaneous The rate of diffusion is determined by particles’ size and shape, their electric charges, and the temperature © Cengage Learning 2015  Diffusion (cont’d.) Diffusion moves solutes toward a state of dynamic equilibrium, in which particles are uniformly distributed and there is no net change in the system – In organisms, equilibrium is rarely attained In simple diffusion, small, nonpolar solute molecules move directly through the membrane down their concentration gradient © Cengage Learning 2015  Osmosis is Diffusion of Water The net movement of water through a selectively permeable membrane from a region of higher concentration of water to a region of lower concentration – Osmotic pressure of a solution is exerted on the side of the membrane containing the higher concentration of solute to prevent the diffusion of water from the side containing the lower solute concentration © Cengage Learning 2015  Pressure applied to piston to resist upward movement Water plus Pure water solute Selectively permeable membrane Molecule of solute Water molecule  Osmosis (cont’d.) Isotonic solution: – No net movement of water molecules occurs between cell and surrounding fluid Hypertonic solution: a higher concentration of solutes exists outside the cell – Cell will lose water and shrink Hypotonic solution: a lower concentration of solutes exists outside the cell – Cell will gain water and swell © Cengage Learning 2015  Responses of Animal Cells to Osmotic Pressure Differences © Cengage Learning 2015  Osmosis (cont’d.) Most prokaryotes, algae, plants, and fungi have rigid cell walls that can withstand a hypotonic solution without bursting – Water moves into the cells by osmosis, building up turgor pressure against the rigid cell walls – Plasmolysis occurs when a plasma membrane separates from the cell wall because it is in a hypertonic solution © Cengage Learning 2015  Facilitated Diffusion Occurs Down a Concentration Gradient In facilitated diffusion, a specific transport protein makes membrane permeable to a specific ion or polar molecule – Net movement is always from higher to lower solute concentration – Channel proteins and carrier proteins facilitate diffusion by different mechanisms © Cengage Learning 2015  Facilitated diffusion of potassium ions In response to an electrical stimulus, the gate of the potassium ion channel opens, allowing potassium to diffuse out of the cell. © Cengage Learning 2015  Facilitated Diffusion Occurs Down a Concentration Gradient (cont’d.) Channel proteins form hydrophilic channels through membranes – Transport specific ions down their electrochemical gradients – Ion channels are referred to as gated channels because they can open and close © Cengage Learning 2015  Facilitated Diffusion Occurs Down a Concentration Gradient (cont’d.) Carrier proteins bind to solutes and undergo a change in shape – Example: glucose transporter 1 (GLUT 1) transports glucose into red blood cells © Cengage Learning 2015  5.5 Active Transport A system across the cell membrane that pumps materials from a region of low concentration to a region of high concentration – Requires the cell to expend metabolic energy Indirect active transport: concentration gradient provides energy for the co- transport of some other substance © Cengage Learning 2015  Active Transport (cont’d.) The sodium–potassium pump uses ATP to pump Na ions out of the cell and K ions into the cell – 2 K ions move in for every 3 Na ions out – Membrane becomes polarized – Membrane potential is created because of the separation of charges – Electrochemical gradients store energy used to drive other transport systems © Cengage Learning 2015  The Sodium–Potassium Pump Higher Outside Lower cell Active transport channel concentration gradient concentration gradient Potassium Sodium Lower Cytosol Higher © Cengage Learning 2015  A model for the pumping cycle of the sodium-potassium pump © Cengage Learning 2015  The Sodium–Potassium Pump (cont’d.) Electrochemical gradients store energy that is used to drive other transport systems, the basis of major energy conversion systems in cells – Bacteria, fungi and plant cells create electrochemical gradients by using proton pumps to actively transport hydrogen ions out of the cell – Other proton pumps are used in “reverse” to synthesize ATP © Cengage Learning 2015  Carrier Proteins Can Transport One or Two Solutes Uniporters transport one type of substance in one direction – Example: proton pumps Symporters move two types of substances in one direction – Example: cotransported sodium and glucose Antiporters move two substances in opposite directions – Example: sodium–potassium pumps © Cengage Learning 2015  A model for the cotransport of glucose and sodium ions A carrier protein transports sodium ions down their concentration gradient and uses that energy to cotransport glucose molecules against their concentration gradient. This carrier protein is a symporter. © Cengage Learning 2015  Cotransport Systems Indirectly Provide Energy For Active Transport Movement of one solute down its concentration gradient provides energy for transport of some other solute up its concentration gradient An energy source such as ATP is required to power the pump that produces the concentration gradient © Cengage Learning 2015  5.6 Exocytosis and Endocytosis Larger molecules, particles of food and small cells are moved into or out of cells by exocytosis and endocytosis – Exocytosis: vesicles export large molecules by the fusion of a vesicle with the plasma membrane – Endocytosis: the cell imports materials by phagocytosis, pinocytosis, and receptor- mediated endocytosis © Cengage Learning 2015  Exocytosis 1 2 3 © Cengage Learning 2015  In Endocytosis, the Cell Imports Materials Phagocytosis: ingesting large solid particles (food or bacteria) – Folds of plasma membrane enclose the cell or particle, forming a vacuole, which may fuse with lysosomes, degrading ingested material Pinocytosis: taking in dissolved materials – Droplets of fluid are trapped by folds in the plasma membrane, which pinch off into the cytosol as vesicles, becoming smaller as their liquid contents are transferred slowly into the cytosol. © Cengage Learning 2015  Phagocytosis 1 2 3 © Cengage Learning 2015  Endocytosis (cont’d.) Receptor-mediated endocytosis: specific molecules combine with receptor proteins in the plasma membrane – Main mechanism by which eukaryotic cells take in macromolecules The endosomes have an acidic environment (pH 6.0-6.2) that facilitates the dissociation of ligand receptors from the ligands. Then, the ingested content is sorted out for either recycling to the plasma membrane or transport to lysosomes for degradation. © Cengage Learning 2015  Receptor-mediated Endocytosis: Uptake of low- density lipoprotein (LDL) particles, which transport cholesterol in the blood. © Cengage Learning 2015  5.7 Cell Junctions Junctions connect cells that form strong connections with one another, prevent the passage of materials, or establish rapid communication between adjacent cells – Anchoring junctions – Tight junctions – Gap junctions – Plasmodesmata © Cengage Learning 2015  Cell Junctions © Cengage Learning 2015  Anchoring Junctions Connect Cells of an Epithelial Sheet Strong junctions that tightly bind adjacent epithelial cells, such as those found in outer layer of mammalian skin – Cadherins are important components of anchoring junctions Two types: – Desmosomes – Adhering junctions © Cengage Learning 2015  Anchoring Junctions (cont’d.) Desmosomes: points of attachment that hold cells together at one spot – Allow cells to form strong sheets – substances still pass freely between membranes – Consist of regions of dense material associated with the cytosolic side of the two plasma membranes and protein filaments that cross intercellular space between membranes – Anchored to intermediate filaments inside the cells, which distributes mechanical stress © Cengage Learning 2015  Anchoring Junctions (cont’d.) Adhering junctions: cement cells together – Cadherins, transmembrane proteins, form a continuous adhesion belt around each cell, binding the cell to neighboring cells – Connect to microfilaments of the cytoskeleton Cadherins of adhering junctions are a potential path for signals from the outside environment to be transmitted to the cytoplasm © Cengage Learning 2015  Tight Junctions Seal Off Intercellular Spaces Between Some Animal Cells Connections between cell membranes that are so tight, no space remains between the cells – Seal off body cavities – Example: tight junctions between the cells that line capillaries in the brain and form the blood- brain barrier, which protects the brain from many substances in blood © Cengage Learning 2015  Gap Junctions Allow the Transfer of Small Molecules and Ions Allows communication between cells – Composed of connexin molecules that are grouped to form a cylinder that spans the plasma membrane – Groups of 6 connexin molecules form cylinders on adjacent cells. They join forming channel connecting two cells’ cytoplasm Cells control passage of materials through gap junctions by opening/closing channels Example: Cardiac muscle cells are linked by gap junctions that permit the flow of ions necessary to synchronize contractions of the heart. © Cengage Learning 2015  Rows of Plasma tight junction Protein Plasma Connexin Channel membranes proteins filaments membranes molecules Desmosome a b c Intercellular Intermediate space Intercellular filaments space Disc of dense protein material  Plasmodesmata Allow Movement Between Plant Cells Connections between plant cell walls that are functionally equivalent to gap junctions – Channels that pass through the cell walls of adjacent plant cells – Plasma membranes of adjacent cells are continuous with one another through plasmodesmata – Contain a desmotubule, which connects the smooth ER of adjacent cells © Cengage Learning 2015  Plasmodesmata Most plant cells have plasmodesmata that connect the cytoplasm of adjacent cells. Cytoplasmic channels through the cell walls of adjacent plant cells allow passage of water, ions and small molecules. The channels are lined with the fused plasma membranes of the two adjacent cells. © Cengage Learning 2015