Bacterial Plasma MembranesAfter learning this section,you should be able to:Describe the fluid mosaic model of membrane structure and identify the types of lipids typically found in bacterial membranes Cell envelopeFunctions of Plasma Membranes1.A primary role of all plasma membranes is that they are selectively permeable barriers:they allow particular ions and molecules to pass either into or out of the cell,while preventing the movement of others.Thus the plasma membrane prevents the loss of essential components through leakage while allowing the movement of other molecules.2.Bacterial plasma membranes play other critical roles.They are the location of several crucial metabolic processes:respiration,photosynthesis,and the synthesis of lipids and cell wall constituents.3.In addition to the plasma membrane,some bacteria have extensive intracytoplasmic membrane systems.These internal membranes and the plasma membrane share a basic design.However,they can differ significantly in terms of the lipids and proteins they contain.To understand these chemical differences and the many functions of the plasma membrane and other membranes,it is necessary to become familiar with membrane structure.fluid mosaic modelThe model is based on studies of eukaryotic and bacterial membranes,and was established using a variety of experimental approaches,including transmission electron microscopy(TEM)and atomic force microscopyCell membranes are very thin structures,about 5 to 10 nm thick,that look like two dark lines on either side of a light interior when imaged by TEM.Cell membrane by TEMFigure 3.8 The Structure of a Phospholipid.Phosphatidylethanolamine,a phospholipid often found in bacterial membranes.hydrophilic endhydrophobic endIntegral membrane proteins,like membrane lipids,are amphipathic;their hydrophobic regions are buried in the lipid while the hydrophilic portions project from the membrane surfaceProteinsBacterial Lipids Bacterial membranes are similar to eukaryotic membranes in that they are lipid bilayers and many of their amphipathic lipids are phospholipids.The plasma membrane is very dynamic:the lipid composition varies with environmental temperature in such a way that the membrane remains fluid during growth.For example,bacteria growing at lower temperatures have more unsaturated fatty acids in their membrane phospholipids;that is,there are one or more double covalent bonds in the long hydrocarbon chain.At higher temperatures,their phospholipids have more saturated fatty acids-those in which the carbon atoms are connected only with single covalent bonds.A questionHow the cell membranes help microbes adapt to their inhitats to face different temperatures?A wordlist1.fluid mosaic model 流動鑲嵌模型流動鑲嵌模型2.Cell contents 細(xì)胞內(nèi)容物細(xì)胞內(nèi)容物3.selectively permeable barriers 選擇性滲透屏障選擇性滲透屏障4.Molecule 分子分子5.metabolic processes 代謝過程代謝過程6.lipid bilayers脂雙層脂雙層7.Protein 蛋白質(zhì)蛋白質(zhì)8.transmission electron microscopy(TEM)透射電鏡透射電鏡9.atomic force microscopy 原子力顯微鏡原子力顯微鏡10.interior 內(nèi)部，里面內(nèi)部，里面11.characteristic 特征，特征性的特征，特征性的12.Appearance 現(xiàn)象，外觀現(xiàn)象，外觀13.freeze-etching冷凍蝕刻技術(shù)冷凍蝕刻技術(shù)14.microscopist 顯微鏡學(xué)家顯微鏡學(xué)家15.Phospholipids磷脂磷脂A WORD LIST16.Amphipathic 兩親的，兩性分子的兩親的，兩性分子的17.Asymmetric 不對稱的不對稱的18.polar and nonpolar ends 極性和非極性端極性和非極性端19.Hydrophilic 親水的親水的20.hydrophobic 疏水的疏水的21.Peripheral membrane proteins 外膜蛋白外膜蛋白22.integral membrane proteins整合蛋白整合蛋白23.electron transport chains電子傳遞鏈電子傳遞鏈24.saturated fatty acids飽和脂肪酸飽和脂肪酸25.unsaturated fatty acids不飽和脂肪酸不飽和脂肪酸