02-Topic 2 Biological Membranes PDF

TOPIC 2 BIOLOGICAL MEMBRANES SEM image shows a WBC (dyed red) in the act of destroying TB bacteria (yellow). The bacteria are surrounded by the cell membrane of the scavenger cell, then drawn inside and rendered harmless. Courtesy of MPI for Infection Biology/Volker Brinkmann  State the main...

TOPIC 2 BIOLOGICAL MEMBRANES SEM image shows a WBC (dyed red) in the act of destroying TB bacteria (yellow). The bacteria are surrounded by the cell membrane of the scavenger cell, then drawn inside and rendered harmless. Courtesy of MPI for Infection Biology/Volker Brinkmann  State the main components of membrane  Identify the structural organisation of membrane  Relate the structure and function of membrane  Illustrate the current model of membrane  Define and explain the characteristics of membrane nhi/bio411_Topic2/2013 2  2.1 2.2 Membrane 2.4 Organisation excitation and 2.3 Membrane Cell Signalling and fluidity of Special Transport and Cell membrane Membrane components Structures Recognition nhi/bio411_Topic2/2012 3  Lipids: phospholipids + glycolipids proteins Phospholipid bilayer Plasma membrane nhi/bio411_Topic2/2012 4  “A microscopic membrane of lipids and proteins that forms the external boundary of the cytoplasm of a cell or encloses a vacuole, and that regulates the passage of molecules in and out of the cytoplasm” Merriam-Webster Dictionary nhi/bio411_Topic2/2012 5 THE PLASMA MEMBRANE Phospholipid bilayer Hydrophobic Hydrophilic regions regions of of protein protein nhi/bio411_Topic2/2012 6 TECHNIQUE: FREEZE FRACTURE MICROSCOPY RESULTS Extracellular layer Proteins Inside of extracellular Knife layer Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer nhi/bio411_Topic2/2012 7  The plasma membrane is the boundary that separates the living cell from its surroundings  The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others  Biological membranes ▪ Provide separate compartments in the cell ▪ Are dynamic and fluid structures ▪ Consist of lipids and proteins. ▪ Vary in composition according to their site. ▪ The relative amounts and chemical properties of lipid and protein molecules affect the properties of the membrane. nhi/bio411_Topic2/2012 8  Compartmentalization-specialized internal activities  Scaffold for biochemical activities-effective interaction  Selectively permeable barrier  Transporting solutes  Responding to external signals- electrical and chemicals  Intercellular interaction- adhesion and communication nhi/bio411_Topic2/2012 9 nhi/bio411_Topic2/2012 10 nhi/bio411_Topic2/2013 11  Phospholipids are the most abundant lipid in the plasma membrane  Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions  RECALL: The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins and phospholipids embedded in it nhi/bio411_Topic2/2012 12  Other components of cell membrane are: ▪ Lipids ▪ phospholipid bilayer/Phosphoglycerides - Sphingolipids (amino+alcohol) - Glycolipids (CHO+lipids) - Cholesterols ▪ Carbohydrates ▪ Proteins Transmembrane proteins Interior protein network Peripheral protein Glycoproteins nhi/bio411_Topic2/2012 13 nhi/bio411_Topic2/2012 14  Plasma membrane is composed of both lipids and globular proteins. ▪ Membrane proteins are not very soluble in water. Molecules of cholesterol embedded in the membrane functions in breaking up the Van der Waals interactions and close packing of the phospholipid tails. This disruption makes the membrane more fluid. Therefore, one way for a cell to control the fluidity of its membrane is by regulating its level of cholesterol in the cell membrane. Q- What is the significance of a membrane which is fluid? nhi/bio411_Topic2/2012 15  Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity nhi/bio411_Topic2/2012 16 ▪ Types of membrane LIPIDS ▪ phospholipid bilayer/Phosphoglycerides ▪ Sphingolipids (amino+alcohol) ▪ Glycolipids (CHO+lipids) ▪ Cholesterols ▪ Phospholipids form the backbone of biological membranes ▪ The amount of lipid varies from 20% to 78% according to the site of the membrane. ▪ Phospholipid molecules can organize themselves to form stable membranes nhi/bio411_Topic2/2012 17 ▪ Function of lipid component ▪ Physical state of a membrane ▪ Influence the activity of particular membrane proteins ▪ Precursors for highly active chemical messenger ▪ Continuous interconnection, unbroken structures ▪ Facilitate the regulated fusion or budding of membranes ▪ Maintaining the proper internal composition of a cell- separating the electrical charges across PM ▪ Able to self assemble-eg liposomes nhi/bio411_Topic2/2012 18  Phospholipid has two fatty-acid chains attached to its backbone. ▪ One end is strongly nonpolar while the other end is strongly polar. ▪ POLAR HEAD oriented toward water (hydrophilic) and NONPOLAR TAILS oriented away from water (hydrophobic)-amphipathic lipids ▪ bilayer is stable because water’s affinity for hydrogen bonding never stops nhi/bio411_Topic2/2012 19  Alberts et al., 2002. Molecular Biology of the Cell (4th ed). NY: Garland Science. nhi/bio411_Topic2/2012 20   Amphipathic lipid molecule consists of  When suspended in aqeous solution- forms micelle structures (stable) 4-5 nm LIPID MOLECULE MICELLE PLASMA MEMBRANE nhi/bio411_Topic2/2012 21  Lateral movement Flip-flop (107 times per second) ( once per month) (a) Movement of phospholipids nhi/bio411_Topic2/2012 22  Cholesterol (c) Cholesterol within the animal cell membrane HOW DOES IT EFFECT THE FLUIDITY??? nhi/bio411_Topic2/2012 23  Major types of membrane proteins are: Transmembrane proteins Interior protein network Peripheral protein Glycoproteins nhi/bio411_Topic2/2012 24  INTEGRAL/TRANSMEMBRANE PERIPHERAL PROTEINS- PROTEINS SURFACE  Embedded within the  Located on surface of PM membrane  Are loosely bound and may  May extend through the be removed with use of membrane and project at mild detergent both surfaces. Some only extend through one membrane layer and only project on one side. nhi/bio411_Topic2/2012 25  hydrophilic end interact with water soluble substances  large proteins contain interior channel – passageway through the PM ▪ single-pass anchors ▪ multiple-pass channels and carriers ▪ pores nhi/bio411_Topic2/2012 26  Functions: ▪ Form fibrillar networks-membrane skeleton- attachments to cytoskeleton ▪ Provide anchor for the integral proteins ▪ Enzymes ▪ Transmit transmembrane signals ▪ Lipid anchored proteins ▪ As receptors ▪ cell adhesion proteins nhi/bio411_Topic2/2012 27 nhi/bio411_Topic2/2012 28  Linked to lipids (glycolipids) or proteins (glycoproteins-90%) Its roles:  Mediating interactions btwn cell & its env.  Sorting of membrane proteins to diff cellular compartments  Antigenic markers – ABO blood groups nhi/bio411_Topic2/2012 29 BECKER PAGE MEMBRANE EXCITATION & SPECIAL MEMBRANE STRUCTURES nhi/bio411_Topic1/2013  2.1 2.4 Organisatio 2.2 Membrane excitation 2.3 Cell n and and Special Membrane Membrane Signalling fluidity of Structures Transport and Cell membrane components Recognition nhi/bio411_Topic2/2013 31  Neurons  Na/K pump  Saltutory conduction  Ion distribution across membrane  Special membrane structures: adherent junction, tight junction, gap junction, plasmodesmata nhi/bio411_Topic2/2013 32  The nervous system = CNS + PNS  CNS- command centre  PNS- processing centre; collects info carries out responses  2 types of cells in nervous system ▪ Neurons – send or receive electrical impulses ▪ Glial cells – most abundant type; provides interconnection  Neuronal highway connecting PNS to CNS PNS ▪ Interneuron –CNS ▪ Sensory neurons - PNS PNS CNS PNS ▪ Motor neurons - PNS PNS nhi/bio411_Topic2/2013 33 nhi/bio411_Topic2/2013 34  The membrane potential arises primarily from the interaction between the membrane and the actions of transmembrane proteins embedded in the plasma membrane.  Provides the basis of electrical potential difference across the plasma membrane. nhi/bio411_Topic2/2013 35 nhi/bio411_Topic2/2013 36  CONTRIBUTORS TO MEMBRANE POTENTIAL Na/K Pump Ion Channels  Pumps 2K+ INTO cell for  Membrane proteins 3Na+ pumped OUT forming pores; allows  Establishes & maintains diffusion of ions concentration difference  More numerous & higher ▪[K+]i ; [Na+]o permeability for K+ nhi/bio411_Topic2/2013 37  ELECTRICAL POTENTIAL CHEMICAL FORCE  Unequal DISTRIBUTION  Unequal of charges across CONCENTRATIONS of membrane ions across membrane nhi/bio411_Topic2/2013 38 READ ON RESTING MEMBRANE POTENTIAL  Learning Outcomes ▪ IDENTIFY ions involved in nerve impulse transmission and their relative concentration inside and outside the neuron ▪ DESCRIBE the production of the resting potential ▪ EXPLAIN how the action of voltage-gated channels produces an action potential nhi/bio411_Topic2/2013 40  When neurons are NOT stimulated it maintains resting potential ▪ Usually -40 to -90mV; use -70mV (-ve inside vs outside)  Formation of resting potential (equilibrium between 2 forces) caused by 2 factors nhi/bio411_Topic2/2013 41  CONTRIBUTORS TO MEMBRANE POTENTIAL Na/K Pump Ion Channels  Pumps 2K+ INTO cell for  Membrane proteins 3Na+ pumped OUT forming pores; allows  Establishes & maintains diffusion of ions concentration difference  More numerous & higher ▪ [K+]i ; [Na+]o permeability for K+ nhi/bio411_Topic2/2013 42  Arise due to action of Na+/K+ pump and differential permeability of membrane to Na+ and K+ due to ion channels ▪ Concentration gradient created by pump significant ▪ K+ inside higher; diffuse out thru open channels ▪ Membrane NOT permeable to –ve ions (organic phosphates, proteins, amino acids) nhi/bio411_Topic2/2013 43 ▪ so not able to counterbalance leads to build up of +ve ions outside; -ve ions outside ▪ This electrical potential is an attractive force pulling K+ back into cell ▪ Balance between electrical and diffusional force leads to equilibrium potential nhi/bio411_Topic2/2013 44 ▪ Nernst Equation EK= 58 mV log ([K+]out/ [K+]in Describe between membrane potential and ion concentration ▪ Goldman Equation Describes combined effects of ions on membrane potential nhi/bio411_Topic2/2013 45 READBECKER PAGE 367-379 ON GRADED POTENTIALS AND ACTION POTENTIALS  Neurons are unique as sudden temporary disruptions to resting membrane potential occurs in response to STIMULI  2 types of changes can be observed: ▪ Graded potentials ▪ Action potentials nhi/bio411_Topic2/2013 47  Graded potentials (fluctuations in potentials able to reinforce or negate each other)  mechanisms ▪ Ligand gated channels ▪ Depolarization & hyperpolarization nhi/bio411_Topic2/2013 48 Ligand gated channels  Temporary binding of ligands (hormones + neurotransmitters) changes the conformation of the membrane receptor proteins of channels nhi/bio411_Topic2/2013 49 Depolarization & Hyperpolarization  Effect of permeability changes; measured as depolarization or hyperpolarization  Depolarization: Vm becomes > +ve ▪ Eg change from -70mV to -65mV  Hyperpolarization: Vm becomes > -ve ▪ Eg change from -70mV to -75mV  Depolarizing and hyperpolarizing effects can combine to AMPLIFY or REDUCE their effects nhi/bio411_Topic2/2013 50  Classes of ion channels discussed so far ▪ Ligand-gated ion channels ▪ Voltage-gated ion channels: action potentials nhi/bio411_Topic2/2013 51  The action potential is “all-or-none”.  It is always the same size.  Either it is not triggered at all - e.g. too little depolarization/the membrane is “refractory”; (time taken for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation)  Or it is triggered completely. nhi/bio411_Topic2/2013 52 Action potentials  Exists when depolarization reaches and exceeds threshold  Caused by ▪ voltage gated channels: either Na+ or K+ voltage- gated channels ▪ Used to establish action potential in neurons nhi/bio411_Topic2/2013 53  4 phases ▪ Resting phase ▪ Rising phase ▪ Top of curve ▪ Falling phase nhi/bio411_Topic2/2013 54  4 phases: Resting phase ▪ Voltage-gated ion channels are closed; partial diffusion of K+ ▪ +stimulus: depolarize until reach threshold; action potential produced nhi/bio411_Topic2/2013 55  4 phases: Rising phase ▪ Voltage-gated Na+ channel OPEN; influx of Na+ into axon ▪ Rapid depolarization (spike) nhi/bio411_Topic2/2013 56  4 phases: Top of curve ▪ Voltage-gated Na+ channel CLOSES; no more Na+ enters axon ▪ Voltage-gated K+ channel OPENS nhi/bio411_Topic2/2013 57  4 phases: Falling phase ▪ Repolarization occurs; K+ diffuses OUT from axon ▪ Hyperpolarization occurs before original resting potential is restored nhi/bio411_Topic2/2013 58 nhi/bio411_Topic2/2013 59 nhi/bio411_Topic2/2013 60  We have discussed how action potentials are ESTABLISHED.  Now, how are they PROPAGATED???? nhi/bio411_Topic2/2013 61  Factors affecting impulse transmission ▪ more or less nerve being stimulated ▪ type of nerve: High or low threshold nerve ▪ Different speed depending on: diameter of axon/strength of stimulus ▪ Achieved through SALTATORY CONDUCTION (120m/sec) nhi/bio411_Topic2/2013 62  Think of propagation of ACTION POTENTIALS as DOMINOS  Myelinated axon provides the dominos effect making SALTATORY CONDUCTION possible nhi/bio411_Topic2/2013 63  “rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of ranvier to another, skipping the myelin-sheathed regions of membrane.”  Saltatory conduction animation nhi/bio411_Topic2/2013 64 nhi/bio411_Topic2/2013 65  Most mammalian axons are myelinated.  The myelin sheath is provided by oligodendrocytes and Schwann cells.  Myelin is insulating, preventing passage of ions over the membrane. (80% lipid and about 20% protein) nhi/bio411_Topic2/2013 66 nhi/bio411_Topic2/2013 67 nhi/bio411_Topic2/2013 Source: https://www.inkling.com/read/medical-physiology-boron-boulpaep-2nd/chapter-7/propagation-of-action-potentials 68  Myelinated regions of axon are electrically insulated.  Action potentials occur only at unmyelinated regions: nodes of Ranvier. nhi/bio411_Topic2/2013 69 nhi/bio411_Topic2/2013 70  Communication between neurons achieved through transmission of signals using neurotransmitters.  Gap junction forms the synapse between 2 neurons nhi/bio411_Topic2/2013 71  SYNAPTIC CLEFT- the void  SYNAPTIC VESICLES- the vehicle  NEUROTRANSMITTERS- Ach, amino acids, epinephrine, norepinephrine, dopamine, serotonin nhi/bio411_Topic2/2013 72  Depolarization opens Ca+ channel; Ca+ influx into the terminal knob-neural transmitter (NT) vesicle fuses with PM  NT is then released into the synapse- bind to post synaptic receptors and triggers AP (influx of Na+) or inhibit AP (influx of Cl-) of the adjacent neuron. e.g Ach inhibits heart contraction but stimulate skeletal muscle contraction. nhi/bio411_Topic2/2013 73  LECTURE PLAN 2.2 SPECIAL MEMBRANE 2.4 CELL SIGNALLING & CELL STRUCTURES 2.3 MEMBRANE TRANSPORT RECOGNITION -DIFFUSION -PARACRINE -ADHERENS JUNCTION -FACILITATED DIFFUSION -ENDOCRINE -TIGHT JUNCTION -OSMOSIS -RECEPTORS (G-PROTEIN -GAP JUNCTION COUPLED; TYROSINE KINASE; -BULK TRANSPORT -PLASMODESMATA CYTOPLASMIC -ACTIVE TRANSPORT (1 @ 2) INTRACELLULAR) nhi/bio411_Topic2/2013 75  Adherens tight junction junction Gap junction plasmodesmata nhi/bio411_Topic2/2013 76 Junction between cells function: Hold cells together Form a barrier Prevent materials from moving between cells Cell to cell communication to integrate the life and growth of higher organisms 77  An anchoring junction that connects the actin filaments of one cell with those of adjacent cells or with the extracellular matrix nhi/bio411_Topic2/2013 78  Region of actual fusion of plasma membranes between two adjacent animal cells that prevent materials from leaking through the tissue nhi/bio411_Topic2/2013 79  barrier to water and solutes from ECM to cell maintain cell polarity by blocking the diffusion of integral proteins prevents water loss from skin (dehydration) blood-brain barrier (protect brain from unwanted solutes) nhi/bio411_Topic2/2013 80  Junction between adjacent animal cells that allows the passage of materials between the cells.  eg SA nodes impulses in heart, peristaltic waves of esophagus nhi/bio411_Topic2/2013 81 nhi/bio411_Topic2/2013 82  Cytoplasmic connections between adjacent plant cells nhi/bio411_Topic2/2013 83  2.1 Organisatio 2.4 2.2 Membrane n and Cell excitation and 2.3 Membrane fluidity of Signalling Special Membrane Transport membrane and Cell Structures component Recognition s nhi/bio411_Topic2/2013 84  Identify, explain, provide examples & state the importance of the different types of membrane transport  Distinguish passive from active transport  Explain the mechanism involved in the different types of active transport  Relate membrane components with transport types across membrane nhi/bio411_Topic2/2013 85 nhi/bio411_Topic2/2013 86  SELECTIVE PERMEABILITY OF PM ▪ Cells require high concentrations of the right chemicals in order to carry out the processes of life ▪ Cells need to be able to get rid of waste products that interfere with the processes of life ▪ small molecules (O2, CO2, ethanol, etc) can diffuse across the membrane but at differing rates ▪ depending upon their ability to enter the hydrophobic interior of the membrane bilayer. ▪ ions (Na+, Mg2+) have to be transported across the PM ▪ AMPHIPATHIC: having both hydrophobic and hydrophilic regions nhi/bio411_Topic2/2012 87  Lipid bilayer impermeable to: ▪ ions such as ▪ K+, Na+, Ca2+ (called cations because when subjected to an electric field they migrate toward the cathode [the negatively-charged electrode]) ▪ Cl-, HCO3- (called anions because they migrate toward the anode [the positively-charged electrode]) small hydrophilic molecules like glucose macromolecules like proteins and RNA nhi/bio411_Topic2/2012 88  Permeability of a molecule through a membrane depends on the interaction of that molecule with the HYDROPHOBIC CORE of the membrane.  Hydrophobic molecules, like hydrocarbons, CO2 and O2 can dissolve in the lipid bilayer and cross easily.  Ions and polar molecules pass through with difficulty. ▪ Includes small molecules (water) and larger critical molecules (glucose and other sugars). ▪ Ions, whether atoms or molecules, and their surrounding shell of water also have difficulties penetrating the hydrophobic core.  Proteins can assist and regulate the transport of ions and polar molecules → membrane transport proteins nhi/bio411_Topic2/2012 89  Facilitated Diffusion Osmosis Diffusion Active Bulk Transport (1° Transport and 2°) nhi/bio411_Topic2/2012 90 1) Uniport- (system in which one solute transported) 2) Passive Transport a) Simple Diffusion- (no assistance) b) Facilitated Diffusion- rate enhanced by carrier or channel, generally an integral membrane protein (transporter or permease) ▪ rapid diffusion "down" a concentration gradient ▪ saturable (reaches a maximum velocity that depends on transporter concentration ▪ specific (depends on interaction of solute with transporter) c) Gated Ion Channels nhi/bio411_Topic2/2012 91 3) Active Transport a) Primary active transporters i. Ion transporters: Na+/K+ ATPase, Plasma membrane Ca2+ ATPase, H+ pump b) Secondary active transporters i. Cotransport (system in which transport of one solute is coupled to transport of another) ▪ Symport- (different solutes transported in same direction) ▪ Antiport-(different solutes transported in opposite directions) nhi/bio411_Topic2/2012 92  PASSIVE TRANSPORT ▪ Translocation of molecules down their concentration gradient ▪ eg osmosis, diffusion & facilitated diffusion  ACTIVE TRANSPORT ▪ requires an investment of energy to move molecules against their concentration gradient. nhi/bio411_Topic2/2012 93 nhi/bio411_Topic2/2012 94   Defined as:  Movement of solutes down its concentration gradient (as potential energy)  From an area of higher concentration to an area of lower concentration  passive transport because it requires no energy from the cell nhi/bio411_Topic2/2012 95  a permeable membrane Molecules of dye Membrane separating a solution with dye molecules from pure water, dye molecules will cross the barrier randomly.  The dye will cross the membrane until both solutions have equal concentrations of the dye.  At this dynamic equilibrium as many molecules pass one way across the other direction. nhi/bio411_Topic2/2012 96  However, membranes are selectively permeable and the interactions of the molecules with the membrane play a role in the diffusion RATE.  Diffusion of molecules with limited permeability through the lipid bilayer may be assisted by transport proteins. nhi/bio411_Topic2/2012 97  Transports ions and other solutes across the plasma membrane.  Facilitate movement by physically binding molecules on one side of the membrane, and releasing them on the other.  essential characteristics ▪ specific ▪ passive ▪ Saturates- as limitting factor to rate nhi/bio411_Topic2/2012 98  Characteristics ▪ Movement is still passive (like diffusion), from high concentration to low. ▪ Saturates when substance reaches high concentrations due to lack of available protein ▪ Related substances can compete for the same carrier or pore. ▪ Maximum rate of transport (fully saturated) is called Tm, the transport maximum nhi/bio411_Topic2/2012 99 nhi/bio411_Topic2/2012 100  Transmembrane proteins create a water-filled pore through which ions and some small hydrophilic molecules can pass by diffusion.  The channels can be opened (or closed) according to the needs of the cell. nhi/bio411_Topic2/2012 101   Many transport proteins simply provide corridors allowing a specific molecule or ion to cross the membrane. ▪ These channel proteins allow fast transport. Channel Protein ▪ For example, water channel proteins and aquaporins, facilitate massive amounts of diffusion. nhi/bio411_Topic2/2012 102   The plasma membrane of human red blood cells contain transmembrane proteins that permit the diffusion of glucose from the blood into the cell.  The channels are selective; that is, the structure of the protein admits only certain types of molecules through. nhi/bio411_Topic2/2012 103   So, after all that..... What is the DIFFERENCE between DIFFUSION and OSMOSIS? Clue: solutes vs solvent nhi/bio411_Topic2/2012 104  Integral proteins function as gated ion channels  NOTE: transmembrane channels that permit facilitated diffusion can be opened or closed. They are said to be "gated".  types of gated ion channels: ▪ ligand-gated ▪ mechanically-gated ▪ voltage-gated ▪ light-gated nhi/bio411_Topic2/2012 105  Some channel proteins, ‘gated channels’, open or close depending on the presence or absence of a physical or chemical stimulus.  The chemical stimulus is usually different from the transported molecule.  For example, when neurotransmitters bind to specific gated channels on the receiving neuron, these channels open. ▪ This allows sodium ions into a nerve cell. ▪ When the neurotransmitters are not present, the channels are closed. nhi/bio411_Topic2/2012 106 Importance of Gated Ion Channels  Formation and propagation of impulses  Secretions of substances into extracellular space  Muscle contraction  Regulation of cell volume  The opening of stomatal pores in plant leaves nhi/bio411_Topic2/2012 107  Ligand is defined as a small signalling molecule which causes a response upon binding  conformational state depends on the binding of a specific molecule (the ligand)  Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens. ▪ External ligands: bind to a site on the extracellular side of the channel. ▪ Internal Ligands: bind to a site on the cytosolic side of the membrane nhi/bio411_Topic2/2012 108 Internal Ligands  bind to a site on the channel protein exposed to the cytosol.  Examples: ▪ "Second messengers", like cyclic AMP (cAMP) and cyclic GMP (cGMP), regulate channels involved in the initiation of impulses in neurons responding to odors and light respectively.  ATP is needed to open the channel that allows chloride (Cl-) and bicarbonate (HCO3-) ions out of the cell.  This channel is defective in patients with cystic fibrosis. Although the energy liberated by the hydrolysis of ATP is needed to open the channel, this is not an example of active transport; the ions diffuse through the open channel following their concentration gradient. nhi/bio411_Topic2/2012 109 nhi/bio411_Topic2/2012 110 Examples of Ligand gated ion channel:  Acetylcholin (ACh) ▪ The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction  Gamma amino butyric acid (GABA) ▪ Binding of GABA at certain synapses — in the central nervous system admits Cl- ions into the cell and inhibits the creation of a nerve impulse nhi/bio411_Topic2/2012 111  open by transmitting physical forces of stretch or pressure to the channels, causing them to undergo a conformational change to allow ions to pass through.  Opening the channels allows ions to permeate and flow down their concentration gradient; causing a change in membrane potential.  Example: nhi/bio411_Topic2/2012 112  conformational state depends on the difference in ionic charge of both sides of the membrane  In "excitable" cells like neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane.  Example: As an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane. This allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse. nhi/bio411_Topic2/2012 113  Definition: ▪ Group of transmembrane proteins forming pores  Pores responds to light stimuli by opening or closing  Eg of naturally occurring Light- gated ion channel is the Channelrhodopsin nhi/bio411_Topic2/2012 114  Active transport involves the expenditure of energy (ATP) to move substance against their concentration gradient.  involves highly selective protein carriers within the membrane, eg: ▪ sodium-potassium pump ▪ coupled transport - using energy stored in a gradient of a different molecule nhi/bio411_Topic2/2012 115 nhi/bio411_Topic2/2012 116  Proteins in the membrane act as pumps.  Move ions or small molecules from low concentration to high concentration (i.e. up their gradients).  Require cellular energy, usually as ATP  Saturates when substance reaches high concentrations due to lack of available protein  Example: Na-K ATPase - Present in nearly every cell in the body, Pumps 3 Na ions out in exchange for 2 K ions pumped in (cost=1 ATP)  Other pumps include the Ca-ATPase, and the H-ATPase. nhi/bio411_Topic2/2012 117 Ratio of Transport 3 Na: 2 K nhi/bio411_Topic2/2012 118  establishes a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior. This resting potential prepares nerve and muscle cells for the propagation of action potentials leading to nerve impulses and muscle contraction. The accumulation of sodium ions outside of the cell draws water out of the cell and thus enables it to maintain osmotic balance (otherwise it would swell and burst from the inward diffusion of water). The gradient of sodium ions is harnessed to provide the energy to run several types of indirect pumps. nhi/bio411_Topic2/2012 119  The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membrane. ▪ Typically, an animal cell has higher concentrations of K+ and lower concentrations of Na+ inside the cell. ▪ The sodium-potassium pump uses the energy of one ATP to pump three Na+ ions out and two K+ ions in. nhi/bio411_Topic2/2012 120  PRIMARY AT SECONDARY AT  energy is derived directly  energy is derived secondarily from energy that has been from the breakdown of ATP stored in the form of ionic concentration differences  utilizes energy in form of ATP between the two sides of a membrane to transport molecules across a membrane against their  energy comes from the electrochemical gradient concentration gradient created by pumping ions out of the cell  Pumps possess one or more  Co-Transport ATP binding sites present on ▪ symport cytosolic face of membrane ▪ antiport nhi/bio411_Topic2/2013 121  TWO TYPES OF CA 2+ ATPase PUMP Plasma Sarcoplasmic Membrane Reticulum Ca 2+ ATPase Ca 2+ ATPase nhi/bio411_Topic2/2012 122  resides in the sarcoplasmic reticulum (SR) within muscle cells. It is a Ca2+ ATPase that transfers Ca2+ from the cytosol of the cell to the lumen of the SR at the expense of ATP hydrolysis during muscle relaxation.  After muscle contraction, Ca 2+ is pumped back into the sarcoplasmic reticulum. This is done by a Ca 2+ ATPase that uses the energy from each molecule of ATP to pump 2 Ca 2+ ions.  SR: The endoplasmic reticulum of a muscle cell; it envelops the myofibrils. nhi/bio411_Topic2/2012 123  The plasma membrane Ca2+ ATPase (PMCA) is a transport protein in the plasma membrane of cells that serves to remove (Ca2+) from the cell.  It is vital for regulating the amount of Ca2+ within all eukaryotic cells  Large transmembrane electrochemical gradient of Ca2+ ▪ ~10,000-fold concentration gradient between cytosol and extracellular fluids (ICF: ECF, 10-20 mM: 150mM)  Ca2+ Concentration gradient drives the entry of the ion into cells  Propagation of cell signalling achieved by maintaining low [Ca2+]: necessary for the cell to employ ion pumps to remove the Ca2+.  The PMCA and the sodium calcium exchanger are together the main regulators of intracellular Ca2+ concentrations. Since it transports Ca2+ into the extracellular space, the PMCA is also an important regulator of the calcium concentration in the extracellular space. nhi/bio411_Topic2/2012 124  The parietal cells of the stomach use this pump protons (H+) into the gastric juice (giving it a pH close to 1).  In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell.  Protons pumps in the cristae of mitochondria and the thylakoids of chloroplasts, concentrate H+ behind membranes.  These electrogenic pumps store energy that can be harnessed for cellular work. nhi/bio411_Topic2/2012 125 Definition: energy from ATP hydrolysis is used to generate a gradient of another solute, and the transport of that solute "down" its concentration gradient is used to drive transport of a different solute against its concentration gradient Cotransport is a type of 2° Active Transport. Example: Symport- (different solutes transported in same direction) Antiport- (different solutes transported in opposite directions) nhi/bio411_Topic2/2012 126  Cotransporters restores [ion] using ATP nhi/bio411_Topic2/2012 127 Examples of cotransport  The human gut epithelial cells take in glucose and Na+ from the intestines and transport them through to the blood stream using the concerted actions of Na+-glucose symports, glucose permeases (a glucose facilitated diffusion protein), and Na+-K+ pumps.  Plants commonly use the gradient of hydrogen ions that is generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cell.  E. coli establishes a proton (H+) gradient across the cell membrane by using energy to pump protons out of the cell. It uses this system to transport ribose and several amino acids nhi/bio411_Topic2/2012 128  Osmosis is passive transport  Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. ▪ The solution with the higher concentration of solutes is hypertonic. ▪ The solution with the lower concentration of solutes is hypotonic. These are comparative terms. ▪ Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water. ▪ Solutions with equal solute concentrations are isotonic. nhi/bio411_Topic2/2012 129  Osmotic concentration - concentration of all solutes in solution ▪ Hyper-osmotic: solution with the higher solute concentration ▪ Hypo-osmotic: solution with the lower solute concentration ▪ Iso-osmotic: solute concentrations are equal nhi/bio411_Topic2/2012 130  Two solutions of differing concentration are separated by a membrane that will allow water through, but not solutes.  The hypertonic solution has a lower water concentration than the hypotonic solution.  More of the water molecules in the hypertonic solution are bound up in hydration shells around the solute molecules, leaving fewer unbound water molecules. nhi/bio411_Topic2/2012 131  Unbound water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.  This diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis.  Osmosis continues until the solutions are isotonic. nhi/bio411_Topic2/2012 132  Osmosis is not a problem for cells living in isotonic environment (eg marine invertebrates).  cells of most terrestrial animals are bathed in an extracellular fluid that is isotonic to the cells.  Organisms without rigid walls experience osmotic fluctuations in either a hypertonic or hypotonic environment and require adaptations for osmoregulation to maintain their internal environment. nhi/bio411_Topic2/2012 133 For example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives. ▪ In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell. ▪ To solve this problem, Paramecium has a specialized organelle (contractile vacuole) that functions as a pump to force water out of the cell. nhi/bio411_Topic2/2012 134 nhi/bio411_Topic2/2012 135  Endocytosis (enveloping food) ▪ Phagocytosis: material taken in is in particulate form ▪ Pinocytosis: material taken in is in liquid form ▪ receptor-mediated endocytosis: transport of specific molecules  Exocytosis (discharge of material from vesicles at the cell surface) ▪ Vesicles produced by Golgi Apparatus nhi/bio411_Topic2/2012 136 nhi/bio411_Topic2/2012 137 nhi/bio411_Topic2/2012 138  This process is triggered when extracellular substances (ligands) bind to special receptors on the membrane surface, especially near coated pits.  This triggers the formation of a vesicle nhi/bio411_Topic2/2012 139 CLATHRIN nhi/bio411_Topic2/2012 140  Receptor-mediated endocytosis enables a cell to acquire bulk quantities of specific materials that may be in low concentrations in the environment. ▪ Human cells use this process to absorb cholesterol. ▪ Cholesterol travels in the blood in low-density lipoproteins (LDL), complexes of protein and lipid. ▪ These lipoproteins bind to LDL receptors and enter the cell by endocytosis. ▪ In familial hypercholesterolemia, an inherited disease, the LDL receptors are defective, leading to an accumulation of LDL and cholesterol in the blood. ▪ This contributes to early atherosclerosis. nhi/bio411_Topic2/2012 141 nhi/bio411_Topic2/2012 142  2.1 Organisa 2.2 Membrane tion and 2.4 excitation and 2.3 fluidity of Special Membrane Cell Signalling and membran Membrane Transport Cell Recognition e Structures compone nts nhi/bio411_Topic2/2013 143  Cell Communication- in multi-cell organisms cell-to-cell contact is critical.  CELL COMMUNICATION is most often done by CELL SIGNAL TRANSDUCTION  exogenous molecule is RECEIVED by a cell & CONVERTED into a RESPONSE by the receiving cell. 144 1. Synthesis and release of signaling molecule 2. Transport to target cells 3. Reception by target cells 4. Signal transduction 5. Response by the cell 6. Termination of signal 145  Overall process in which information carried by extracellular messenger molecules is translated into changes that occur inside a cell  Coordinating cellular activities  Communication through extracellular messenger molecules (ligand) and binds on receptors on target cells  Signal received is then relayed through: the second messenger – small substances that activate/inactivate specific proteins recruiting other cellular signaling proteins  To the activated target protein that result in various cellular activities 146  Transcription Survival Protein synthesis Movement Cell death Metabolic change Two protein domains – Kinases and phosphatases (add /remove phosphate groups) 147 148  local regulator chemical messengers are targeted to specific receptors  often includes: growth factor proteins that promote cell division & growth & neurotransmitters that move across synapses to other neurons 149  target cell is close to the signal releasing cell, and the signal chemical is broken down too quickly to be carried to other parts of the body. Examples of paracrine signaling include growth factor signaling and clotting factor. Growth factor signaling plays an important role in many aspects of development. In mature organisms paracrine signaling functions include responses to allergens, repairs to damaged tissue, formation of scar tissue, and clotting. Overproduction of some paracrine growth factors has been linked to the development of cancer 150  specialized cells release molecules (often hormones) into blood vessels of circulatory system  hormones move to distant target cells ▪ elicits response to target cells 151  Signaling can be more direct with CELL-TO-CELL CONTACT examples:  gap junctions & plasmodesmata results in cytoplasmic continuity favouring cellular interactions.  cell surface contacts receptor protein specificity  Eg: cytokines boost up immune response 152 ▪ Ion channel Receptors (ligand gated protein channels)  a protein PORE in a membrane opens in response to binding a signal molecule  ex: a neurotransmitter as acetylcholine (Ach) opens channel and lets Na+ ions flood into cell, changing that cell's electrical charge (potential) 153 154  Cascade of events in a cell upon receipt of a signal (ligand binding to receptor protein) which produces a cellular response to a signalling molecule. nhi/bio411_Topic2/2013 155  Reception - ligand molecules (usually water soluble) are recognized by only one receptor protein bound within a cell membrane  Transduction -leads to a conformation change in receptor shape; change results in receptor interacting with other intra-cellular molecules may result in multiple, conformational/structural changes in other cellular proteins ▪ inactive enzymes ---> active enzymes  Response - cellular activity, as enzyme catalysis, or the rearrangement of cytoskeleton (movement), or specific gene activity. 156 157 Extracellular messenger: Receptors:  amino acids and its  G-proteins (GpCR to derivatives – hormones GTP binding proteins) and neurotransmitter (Ach, Epinephrine,  Receptor Protein- thyroid hormones) Tyrosine kinases RTKs  Steroid hormones  Ligand-gated channel  Eicosanoids –pain  Steroid hormone signals receptor  Protein and  B- and T- cells receptors polypeptides 158 G Protein Coupled Receptor – Guanine nucleotides binding proteins 159  has 7 transmembrane -helicies  possess site for receptor molecule  a specific example of G-protein cellular responses: Fight of Flight Response...  1 signal molecule gives multiple- enhanced response. 160  G-Protein Receptors are receptor proteins that bind GTP/GDP INSTEAD OF ATP  convert between active & inactive forms a signal molecule binds to a receptor --> conformational change -->  an inactive G-(GDP)-protein now binds GTP (replacing GDP)...  and active G-(GTP)-protein stimulates other inactive enzymes.  Elicits Cellular response 161 162 G-Protein has its GTP hydrolyzed -- inactivates G- protein 163  second messenger are small substances that activate or inactivate specific protein- cyclic-AMP (cAMP )  signal molecule (hormone epinepherine) leads to activation of membrane bound inactive enzyme Adenyl Cyclase  ATP → active adenyl cyclase → cyclic-AMP → Cellular processes ▪ eg: Glycogen breakdown  cAMP is inactivated by phosphodiesterase.  Other example of second messengers: Ca 2+, Nitric Oxide , cGMP (relaxation of muscle), phosphoinisitides 164 Epinephrine for glycogen breakdown Glucagon & epinephrine 165  Vibrio cholerae grow in fecal infected waters, infect small intestine, produce toxin, which binds to a G-protein, prevents conversion of GTP (active) --> GDP (Inactive)  thus G-protein remains active, causing cAMP to remain active....  net result: small intestine secretes water and salts = perfuse diarrhea = fatal. 166  mouse embryos lacking one G-protein... show no blood vessel development  some human embryos w/o G-proteins... decreased senses (esp: vision & smell) 167  receptor proteins that have kinase activity.  they can add a -PO4 group to tyrosine residues of inactive proteins  making them active → elicit cellular response  many growth factors (stimulate cell division & growth) function via tyr-kinases ▪ binding of growth factor (signal ligand) causes 2 single tyr-kinases to aggregate ▪ the tyr-dimers, now each phosphorylate the others tyr residues via ATP kinase activity ▪ the activated-phosphorylated dimer binds relay proteins, activating them which in turn (by cascade effect) can active up to 10 others, etc.  1 signal molecule can trigger many proteins and multiple pathways (cascade effect) 168 Activation: Epidermal growth factor, insulin, Platelet-derived growth factor etc. 169  signal molecules (usually lipid soluble) diffuse through membrane, where binding to an intracellular receptor protein initiates a cellular response.  ex: steroid hormones 170  Transcription Translation Protein synthesis 171  Ca ions as second messenger cells maintain a low (10 -7 M) cytoplasmic [Ca] by active transport of Ca out of cytoplasm pumps keep a low cytoplasmic [Ca], but high ER cisternae & mitochondria [Ca] (10,000 X greater)  Ca itself functions as a 2nd messenger, like cAMP, as [Ca] in cytoplasm goes up, activates Ca+ pumps (active transport), & other responses ensue.  Eg. muscle contractions, neurotransmitter release, etc... 172  Calcium channel in pm and ER membranes are kept closed ATP-driven ca ions transport it out of cytosol Calcium ions as the second messenger IP3 opens the calcium gate to release calcium into the cytosol – muscle contraction, cell division, secretion, fertilization, synaptic transmission, metabolism, transcription and cell movement Calcium-binding protein - calmodulin 173 174 175  Eg: Guard cell of the stomata  Opening and closing of the stomata – ionic concentrations/turgor pressure  Adverse conditions, stimulate the release of abscisic acid that open calcium channel, cause Ca influx into the cell and release calcium from internal stores.  The K+ influx channel close but efflux open followed by Cl- efflux  Lead to a drop internal ion solute osmotic loss of water 176  most are like dominos... one activates another, then another, then another...producing a significant cascade multiplication effect.  enzyme activation is by protein phosphorylation by protein kinase enzymes, that transfer P from ATP to another enzyme protein, thereby activating it. 177  Signal-receptor – second messenger- transduction  Then the final molecules has to enter the nucleus to activate the gene.  The last step involves the activation or deactivation of a process.  For example an enzyme may be turned on or off, a gene may be activated or not, or ions may or may not be transported. 178 179  plant phytochrome action via G protein & Ca+ channels  Function during process of greening plant pigment in responses to light ▪ seed germination, photoperiodism, flowering, etc. 180 181  Downregulate cell surface receptors.  Endocytosis to reduce the number of cell surface receptors  Phosphorylation to inactivate cell surface receptors  Decreased sensitivity between receptor and ligand  separate receptor from ligand 182  Programmed cell death is as needed for proper development as mitosis is. Programmed cell death is needed to destroy cells that represent a threat to the integrity of the organism.  What makes a cell decide to commit suicide? the withdrawal of positive signals; that is, signals needed for continued survival, eg. growth factors for neurons Interleukin-2 (IL-2), an essential factor for the mitosis of lymphocytes the receipt of negative signals-increases levels of oxidants within the cell damage to DNA by these oxidants or other agents Signals –intrinsic or extrinsic factors 183