微生物学史微生物学史 (24).ppt

Nutrients uptakeAfter learning this section,you should be able to:Compare and contrast passive diffusion,facilitated diffusion,active transport,and group translocation,and provide examples of each What are the common features of nutrient uptake by bacteria?Bacteria can only take in dissolved molecules.Uptake mechanisms are specific;that is,the necessary substances,and not others,are acquired.It does a cell no good to take in a substance that it cannot use.Bacteria are able to transport nutrients into the cell even when the concentration of a nutrient inside the cell is higher than the concentration outside.Thus they are able to move nutrients up a concentration gradient.This is important because bacteria often live in nutrient-poor habitats.There are four major transport mechanisms for bacteria to uptake the nutrients.Passive diffusion Facilitated diffusionPrimary and secondary active transport Group translocationPassive diffusionPassive diffusion,often called diffusion or simple diffusion,is the process by which molecules move from a region of higher concentration to one of lower concentration;that is,the molecules move down the concentration gradient.The rate of passive diffusion depends on the size of the concentration gradient between a cells exterior and its interior.Cells exterior Cells interiorO2 and CO2 H2ODuring facilitated diffusion,substances move across the plasma membrane with the assistance of either channels or carriers.The rate of facilitated diffusion increases with the concentration gradient much more rapidly and at lower concentrations of the diffusing molecule than that of passive diffusion.Facilitated Diffusion SO what are channels or carriers?They are Transport Proteins.Bacterial cells employ a variety of transport proteins in their uptake mechanisms.These important proteins are embedded in membranes and can be classified into several major types are channels and carriers.Channels,as their name indicates,are proteins that form pores in membranes through which substances can pass;they are often involved in facilitated diffusion.Channels show some specificity for the substances that pass through them,but this is considerably less than that shown by carriers,which are far more substrate specific.Carriers are so named because they carry nutrients across the membrane.Facilitated diffusionWhen the transporter is a carrier,the diffusion rate reaches a plateau above a specific gradient value because the carrier protein is saturated;that is,it is transporting as many solute molecules as possible.Facilitated diffusion is truly diffusion,even though a transport protein is involved.A concentration gradient spanning the membrane drives the movement of molecules,and no metabolic energy input is required.Active Transport Active transport is the transport of solute molecules to higher concentrations(i.e.,against a concentration gradient)with the input of metabolic energy.Three types of active transport are observed in bacteria:Primary active transport,Secondary active transport,Group translocation.They differ in terms of the energy used to drive transport and on whether or not the transported molecule is modified as it enters.Primary active transportPrimary active transport is mediated by carriers called primary active transporters.They use the energy provided by ATP hydrolysis to move substances against a concentration gradient without modifying them.Primary active transporters are uniporters;that is,they move a single molecule across the membrane.ATP-binding cassette transporters(ABC transporters)are important primary active transporters.Secondary active transport Secondary active transport couples the potential energy of ion gradients to transport of substances without modifying them.Secondary active transporters are cotransporters.They move two substances simultaneously:the ion whose gradient powers transport and the substance being moved across the membrane.When the ion and other substance both move in the same direction,it is called symport.When they move in opposite directions,it is called antiport.Bacteria often have more than one transport system for a nutrient,as can be seen with E.coli.This bacterium has at least five transport systems for the sugar galactose,three systems each for the amino acids glutamate and leucine,and two potassium transport complexes.When several transport systems exist for the same substance,the systems differ in such properties as their energy source,their affinity for the solute transported,and the nature of their regulation.This diversity gives the bacterium an added competitive advantage in a variable environment.Group translocationThe distinguishing characteristic of group translocation is that a molecule is chemically modified as it is brought into the cell.The best-known group translocation system is the phosphoenolpyruvate:sugar phosphotransferase system(PTS),which is observed in many bacteria.A wordlist1.Macroelements 大量元素大量元素2.micronutrients(trace elements)微量元素，痕量元素微量元素，痕量元素3.passive diffusion 被被动扩散散4.facilitated diffusion 促促进扩散散5.active transport 主主动运运输6.group translocation集集团转运运7.Nutrients 营养养8.carbon,oxygen,hydrogen,nitrogen,sulfur,and phosphorus 碳、氧、碳、氧、氢、氮、硫、磷氮、硫、磷9.carbohydrates.碳水化合物碳水化合物10.potassium,calcium,magnesium,and iron 钾、钙、镁、铁11.manganese,zinc,cobalt,molybdenum,nickel,and copper 锰、锌、钴、钼、镍、铜12.Enzymes酶A wordlist13.nutrient-poor habitats 营养养贫瘠的瘠的环境境14.Transport Proteins运运输蛋白蛋白15.channels and carriers.通道蛋白通道蛋白、载体蛋白体蛋白16.Modified 被修被修饰的的17.Uniporter 单运运输蛋白蛋白18.Antiport逆向运逆向运输蛋白蛋白 19.E.coli大大肠杆菌杆菌20.phosphoenolpyruvate:sugar phosphotransferase system(PTS)磷酸磷酸烯醇醇式丙式丙酮酸酸糖糖磷酸磷酸转移移酶系系统21.Phosphate磷酸磷酸22.Salmonella沙沙门氏菌属氏菌属23.heat-stable protein(HPr)热稳定蛋白定蛋白24.symport 顺向转运蛋白顺向转运蛋白