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Transcript
Chapter 5 Membrane Transport Mechanisms Membrane Permeability 1) lipid soluble solutes go through faster 1) smaller molecules go faster 1) uncharged & weakly charged go faster 2) Channels or pores may also exist in membrane to allow transport 1 2 How to get other molecules across membranes There are two ways that the molecules typically move through the membrane: passive transport and active transport •Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. •Passive transport does not require such an energy expenditure, and occurs spontaneously. 1 Passive Transport Net movement of material from hi to lo concentration • Diffusion • Osmosis • Facilitated Diffusion concentration gradient Passive Transport Simple Diffusion- simple movement from regions of high concentration to low concentration Osmosis- diffusion of water across a semi-permeable membrane Facilitated diffusion协助扩散- protein transporters which assist in diffusion Passive Transport Unaided movement through the phospholipid bilayer in response to concentration gradients. Large or charged molecules are unable to pass through the bilayer unassisted. Diffusion Movement generated by random motion of particles. Caused by internal thermal energy. Movement always from region of high free energy(high concentration) to regions of low free energy (low conc.) Diffusion If a concentration gradient exists, there will be a net flow of material across the membrane Osmosis Movement of water across a semipermeable barrier. Example: Salt in water, cell membrane is barrier. Salt will NOT move across membrane, water will. Text pg 87 Osmosis Refers to movement of water Across a semipermeable membrane • Permeable to water • Impermeable to dissolved materials Water always moves from lo to high solute • Hypotonic Hypertonic cell Osmosis in Hypertonic medium Hypertonic solutionsshrink cells Osmosis in Hypotonic medium Hypotonic solutions- swell cells Osmosis Hypotonic Hypertonic 简单扩散与协助扩散的比较 Osmotic pressure = Weight of water column Plant Turgor Hypertonic Hypotonic Central vacuole Water pressure inside cell lends support to plant structure Plasmolysis due to water deprivation Charged particle cloaked with water molecules, can not get through Na+ ClK+ H+ + Large molecules, also cloaked with water molecules, can not get through Nucleotides Sugars Amino acids Large molecules can not get through Facilitated Diffusion Sugars Transport Protein Channel Shape change resulting from solute interaction with transport protein Types of Protein Transporters A. Facilitated Diffusion Assist in diffusion process. Solutes go from High conc to Low conc. Examples: Glucose transporters Text pg 88 http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm A Transport Proteins Facilitated Diffusion & Active Transport move solutes faster across membrane highly specific to specific solutes can be inhibited by drugs Transport Proteins are specific glucose Na+ Cell Controls Movement by Number of Transport Proteins in Membrane High Moderate No transport Gated Channels Chemical messenger, e.g. insulin glucose Voltage Dependent K+ channel Model of protein shape change in response to change in voltage across a membrane Roderick MacKinnon, M.D., Howard Hughes Medical Institute Investigator, Youxing Jiang, Ph.D. Cystinuria Defective transport protein in urinary tract unable to absorb cystine from urine cysteine cystine Cystine crystal Facilitated Diffusion The Glucose Transporters Transport of glucose into cells mediated by proteins in the GLUT (GLUcose Transport) family of transporters. There are 7 different, but related, proteins. But, only four (GLUT1-4) are known to be involved in glucose transport. All GLUT proteins share a set of similar structural features and are all about 500 amino acids in length (giving them a predicted molecular weight of about 55,000 Daltons) Glucose uptake shows saturation and glucose uptake can be inhibited by drugs A classic Membrane Transport protein Glucose Transporter Characteristics: integral protein: spans the membrane 12 alpha helices woven into membrane 55,000 mol. wt. Text pg. 88 Glucose Transporter: How it works.. glucose binds to outside of transporter (exterior side with higher glucose conc.) glucose binding causes a conform. change in protein glucose drops off inside cell protein reassumes 1st configuration Types of Protein Transporters: Ion Channels work by facilitated diffusion No E! deal with small molecules... ions open pores are “gated”- Can change shape. • How? • How much gets in? important in cell communication B Ion Channels Work fast: No conform. changes needed Not simple pores in membrane: • specific to different ions (Na, K, Ca...) • gates control opening • Toxins, drugs may affect channels saxitoxin, tetrodotoxin cystic fibrosis Toxins…how they work Cystic Fibrosis Fatal genetic disorder Mucus build-up results in lung and liver failure Patients die between 4 and 30 yrs. Single gene defect 1 in 25 Caucasians carry 1 bad gene copy 1 in 2500 kids has it in Canada Testing CF Cont… ~Proteins for diffusion of salt into the airways don't work. ~Less salt in the airways means less water in the airways. ~ Less water in the airways means mucus layer is very sticky (viscous). ~Sticky mucus cannot be easily moved to clear particles from the lungs. ~Sticky mucus traps bacteria and causes more lung infections. http://www.the-aps.org/education/lot/cell/HotT.htm 2 Active Transport Active transport- proteins which transport against concentration gradient. Requires energy input T Types of Protein Transporters: Active Transport carrier proteins go against the concentration gradients Low to High require Energy to function (ATP, PEP, light energy, electron transport) Membrane Transport Active Transport Move materials from lo to high concentration Requires cell to expend energy Equivalent to running diffusion in reverse Simulation 1- how can you put the particles back into the box? http://lewis.eeb.uconn.edu/lewishome/applets/Diffusion/diffusion.html Active Transport Sodium-Potassium Pump CA-ATP Pump proton Pump p- proton Pump v -proton Pump H- ATP Pump Cotransport symport aniport Active Transport: Sodium-Potassium Pump ATP-direct depend Na+ high Na+ low K+ low K+ high Balance of the two ions goes hand-in-hand ATP required for maintenance of the pump Electrochemical Gradient negative positive K+ Na+ Na+ Na+ K+ Na+ Na+ K+ Na+ ADP ATP Na+ K+ Na+ Na+ Na+ Na+ K+ pump Na+ K+ Na+ Na+ Na+ Na+ This gradient powers conduction of signals along nerves Stored Energy for Cotransport sucrose H+ H+ H+ H+ H+ sucrose H+ sucrose sucrose H+ H+ sucrose ATP sucrose H+ proton H+ H+ H+ H+ sucrose sucrose H+ ADP H+ pump H+ H+ H+ H+ H+ Voltage difference or membrane potential The sodium/potassium pump All nerve and muscle cells have a high internal potassium ion concentration and a low internal sodium ion concentration. [Ki=166 mM; Ko=5 mM; Nai=18 mM; Nao=135 mM]. Early on, it was thought that the nerve and muscle membranes were relatively impermeable to these ions and that the difference in ionic concentration was set up in early development of the cells. The membrane then became impermeable. The later availability and use of radioactive Na and K ions showed that this was not true and that there was a metabolic pump that pumped Na out of the cell and K in; the ratio being 3 Na pumped out of the cell for every 2 K pumped into the cell. Is a Protein Involved ? Experiments showed a dependency of both Na and K ions for pump to work Pump was inhibited by ouabain (a drug) 1957: an ATPase enzyme was found to be associated with Na/K pumping Studies showed this ATPase capable of pumping Na/K ions Sodium/Potassium ATPase Protein Made of 2 large and 2 small subunits 2 large units span membrane • inside region: contains ATP binding site • inside: binding sites for Na • outside: binding site for K How does it work?? Sodium-Potassium Pump http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/sppump.html Na-K Pump Model: Part I 3 Na+ bind to inner region of protein Na+ binding triggers phosphorylation of protein. ATP ADP + Pi Phosphorylation causes conformation change and Na+ binding site faces outside 3 Na+ released to outside Na-K Active Pump: Part II 2 K+ ions on outside are able to bind K+ binding causes dephosphorylation and new conformation change 2K+ ions exposed to inside and released Cyclic process uses ATP energy to drive Na & K ion transport against conc. Gradient 钙泵 Ca++浓度:胞外约10-3M,胞质小于或等 于10-7M(自由Ca++) 肌质网的Ca++泵 Transmembrane transport and membrane potential 静息电位 静息电位 动作电位 阳离子通道 突触前膜,突触后膜 乙酰胆碱受体(配体控制的闸门) 3 Transport of Macromolecules active transport Exocytosis & Endocytosis • Formation of vesicles around large molecules Receptor mediated Receptor proteins Coat proteins Endocytosis Pinocytosis phagocytosis Endocytosis Receptor –mediated endocytosis No-special endocytosis LDL endocytosis movie Low Density Lipoprotein http://www.blc.arizona.edu/courses/181gh/Lectures_WJG.01/LDL_F.01/atherosclerosis.html Endocytosis Transports macromolecules and large particles into the cell. Part of the membrane engulfs the particle and folds inward to “bud off.” • Fig. 5.16 Endocytosis Putting Out the Garbage Vesicles (lysosomes, other secretory vesicles) can fuse with the membrane and open up the the outside… Exocytosis (Cellular Secretion) Exocytosis Constitutive exocytosis pathway(default pathway) regulated exocytosis pathway Cellular Membranes REVIEW Importance of Membranes Membrane Structure Proteins Fluid Mosaic model Permeability Types of Transport • Passive and Active Assignment What is the role of NAD+ and the electron transport chain in extracting useful energy from food?