微生物吸收营养物质的方式 Membrane Transport Mechanisms It is of seminal importance to the cell that it be able to transport molecules in and out of itself. Imagine that a protein having multiple transmembrane domains is structured so that these domains are arrayed in the plane of the membrane in a circle, thereby forming a cylinder, or, better yet, a barrel when viewed from the outside of the cell, with each of the staves of the barrel being one of the transmembrane domains. The center of the barrel could constitute a hole in the plasma membrane that is isolated from the lipid bilayer by an array of transmembrane domains around it. This hole could be used to transport substances into the cell or out from the cell. In fact, this hole can be a relatively hydrophilic environment if hydrophilic side chains from the membrane-spanning chains surrounding the hole protrude into the hole itself. In practice, given the structure of known membrane proteins , these holes are only large enough to allow the passage of small molecules through the plasma membrane, almost always simple ions like hydrogen, potassium or sodium. The ions may pass through the hole or orifice by passive diffusion, in which case the protein that allows this transport is called an ion channel. Alternatively, the transmembrane protein may invest energy, usually derived from ATP, to actively force ions from one side of the plasma membrane to the other, in which case it will be an ion pump. Given the importance of membrane transport, cells utilize a wide range of transport mechanisms. The mechanisms fall into one of three categories: simple diffusion, facilitated diffusion, and active transport.

1、单纯扩散(simple diffusion or passive diffusion) 被输送的物质，靠细胞内外浓度为动力，以透析或扩散的形式从高浓度区向低浓度区的扩散。 Simple diffusion means that the molecules can pass directly through the membrane. Diffusion is always down a concentration gradient. This limits the maximum possible concentration of the molecule inside the cell (or outside the cell if it is a waste product). The effectiveness of diffusion is also limited by the diffusion rate of the molecule. Therefore, though diffusion is an effective enough transport mechanism for some substances (such as H2O), the cell must utilize other mechanisms for many of its transport needs. 单纯扩散(又称被动扩散) 特点： 营养物质是以物理扩散方式而渗入(或渗出)细胞的.利用浓度差由高浓度向低浓度扩散,扩散的速度由细胞内外的浓度差来决定, 当细胞内外此物质浓度达到平衡时便不再进行扩散.扩散过程中,营养物质通过膜上小孔进出细胞体内外,是非特异性的,不消耗能量, 扩散速度较慢,运送前后营养物质分子不变. 运送的物质种类:水、二氧化碳、氧气、甘油、乙醇、少数氨基酸、盐类、代谢抑制剂等小分子物质。 由细胞质膜内外营养物的浓度 差而产生的物理扩散作用。 特点： 1. 非特异性； 2. 不需要载体； 3. 不需消耗能量； 4. 顺逆浓度梯度进行； 5.速度很慢 6.可运送的养料有限(水或溶于水的气体（氧）及小极性分子，如 尿素、甘油、乙醇等）

1、单纯扩散(simple diffusion or passive diffusion) 特点： ①扩散是非特异性的营养物质吸收方式：如营养物质通过细胞膜中的含水小孔，由高浓度的胞外环境向低浓度的胞内扩散； ②在扩散过程中营养物质的结构不发生变化：即既不与膜上的分子发生反应，本身的分子结构也不发生变化； ③物质运输的速率较慢：速率与胞内外营养物质的浓度差有关，即随细胞膜内外该物质浓度差的降低而减小，直到胞内外物质浓度相同； ④不需要载体参与； 扩散是一个不需要代谢能的运输方式：因此，物质不能进行逆浓度运输。 ⑤可运送的养料有限：限于水、溶于水的气体，及分子量小，脂溶性、极性小的营养物质。 Diffusion Simple diffusion means that the molecules can pass directly through the membrane. Diffusion is always down a concentration gradient. This limits the maximum possible concentration of the molecule inside the cell (or outside the cell if it is a waste product). The effectiveness of diffusion is also limited by the diffusion rate of the molecule (see Purves box 5.B). Therefore, though diffusion is an effective enough transport mechanism for some substances (such as H2O), the cell must utilize other mechanisms for many of its transport needs.

smosis flows towards high salt concentrations

单纯扩散模式图 细胞膜外 细胞膜 细胞膜内 由细胞质膜内外营养物的浓度 差而产生的物理扩散作用。 特点： 被动扩散（passive diffusion) 由细胞质膜内外营养物的浓度 差而产生的物理扩散作用。 特点： 1. 非特异性； 2. 不需要载体； 3. 不消 耗能量； 4. 不能逆浓度梯度运输； 5. 可 运送的养料有限(水或溶于水的气体及小极 性分子，如 尿素、甘油、乙醇等） 单纯扩散模式图

促进扩散 (facilitated diffusion/transport) 三营养物通过与细胞膜上载体蛋白（也称作透过酶permease）的可逆性结合来加快其传递速度 Facilitated diffusion utilizes membrane protein channels to allow charged molecules (which otherwise could not diffuse across the cell membrane) to freely diffuse in a nd out of the cell. These channels comes into greatest use with small ions like K+, Na+, and Cl-. The speed of facilitated transport is limited by the number of protein channels available, whereas the speed of diffusion is dependent only on the concentration gradient. Facilitated Diffusion Facilitated diffusion utilizes membrane protein channels to allow charged molecules (which otherwise could not diffuse across the cell membrane) to freely diffuse in a nd out of the cell. These channels comes into greatest use with small ions like K+, Na+, and Cl-. The speed of facilitated transport is limited by the number of protein channels available, whereas the speed of diffusion is dependent only on the concentration gradient. 运送养料: 氨基酸、单糖、维生素、无机等

促进扩散(facilitated diffusion) 特点：在促进扩散过程中 营养物质本身在分子结构上也不会发生变化 不消耗代谢能量，故不能进行逆浓度运输 运输的速率由胞内外该物质的浓度差决定 需要细胞膜上的载体蛋白（透过酶）参与物质 运输 被运输的物质与载体蛋白有高度的特异性 养料浓度过高时, 与载体蛋白出现饱和效应 促进扩散的运输方式多见于真核微生物中，例如通常在厌氧生活的酵母菌中，某些物质的吸收和代谢产物的分泌是通过这种方式完成的。 三. 促进扩散 (facilitated diffusion) 协助扩散， 强化扩散 养料通过与细胞膜上透过酶（permease） 的可逆性结合来加快其传递速度 特点： 1. 养料与载体蛋白(诱导酶)具有特异性; 2. 不能逆养料浓度梯度运输; 3. 不消耗能量; 4. 养料浓度过高时, 与酶出现饱和效应 促进扩散 特点：利用浓度差由高浓度向低浓度扩散，当细胞内外此物质浓度达到平衡时便不再进行扩散. 与单纯扩散的主要差别是需要特异性载体蛋白，这种载体蛋白运送溶质的机制可能是由于其构象的改变，在膜的外侧时，它能与溶质分子结合，而在膜内侧可释放此溶质，不需要能量，运送速度快，运送前后营养物质分子不变. 运送的物质种类：SO42-、PO42-、糖（真核微生物）等。

Embeded protein:

Proteins that act as carriers are too large to move across the membrane. They are transmembrane proteins. They cycle between two conformations in which a solute binding site is accessible on one side of the membrane or the other. 三. 促进扩散 (facilitated diffusion) 协助扩散， 强化扩散

促进扩散模式图 细胞膜 细胞膜外 细胞膜内 恢复原构象 移位 再循环 结合 构象改变

or active ansport

主动运输(Active transport) 在代谢能的推动下，通过膜上特殊载体 蛋白逆养料浓度梯度吸收营养物质的过程 Active Transport Active transport requires the expenditure of energy to transport the molecule from one side of the membrane to the other, but active transport is the only type of transport that can actually take molecules up their concentration gradient as well as down. Similarly to facilitated transport, active transport is limited by the number of protein transporters present. We are interested in two general categories of active transport, primary and secondary. Primary active transport involves using energy (usually through ATP hydrolysis) at the membrane protein itself to cause a conformational change that results in the transport of the molecule through the protein. The most well-known example of this is the Na+-K+ pump. The Na+-K+ pump is an antiport, it transports K+ into the cell and Na+ out of the cell at the same time, with the expenditure of ATP. Secondary active transport involves using energy to establish a gradient across the cell membrane, and then utilizing that gradient to transport a molecule of interest up its concentration gradient. An example of this mechanism is as follows: E. coli establishes a proton (H+) gradient across the cell membrane by using energy to pump protons out of the cell. Then those protons are coupled to lactose at the lactose permease transmembrane protein. The lactose permease uses the energy of the proton moving down its concentration gradient to transport lactose into the cell. This coupled transport in the same direction across the cell membrane is known as a symport. E. coli uses similar proton driven symports to transport ribose, arabinose, and several amino acids.

主动运输(Active transport) 特点：物质在主动运输的过程中 需要消耗代谢能 可以进行逆浓度运输的运输方式 需要载体蛋白参与 对被运输的物质有高度的立体专一性 被运输的物质在转移的过程中不发生任何化学变化 不同的微生物在主动运输过程中所需的能量的来源不同，好氧微生物中直接来自呼吸能，厌氧微生物主要来自化学能，光合微生物中则主要来自光能 。 主动运输是微生物吸收营养物质的主要方式。 主动运送（主动运输） 在代谢能的推动下，通过膜上特殊载体 蛋白逆养料浓度梯度吸收营养物质的过程 特点： 1. 具有养料和载体蛋白的专一性; 2. 消耗能量(不同菌能量来源各异); 3. 可逆养料浓度梯度吸收营养; 4. 能改变运输反应的平衡点 Protein carrier, requires ATP Works against gradient 主动运输是微生物吸收营养物质的主要方式。 特点：运送时需特异性载体蛋白，需要能量，被运送的物质可以逆浓度梯度运送，由稀到高，内部浓度比外部高的多，运送速度快，运送前后营养物质分子不变。 运送的物质种类：氨基酸、乳糖等糖类，Na+、Ca2+等无机离子。

Comparison of passive and active transport.                                                                     Legend:If uncharged solutes are small enough, they can move down their concentration gradients directly across the lipid bilayer itself by simple diffusion. Examples of such solutes are ethanol, carbon dioxide, and oxygen. Most solutes, however, can cross the membrane only if there is a membrane transport protein (a carrier protein or a channel protein) to transfer them. As indicated, passive transport, in the same direction as a concentration gradient, occurs spontaneously, whereas transport against a concentration gradient (active transport) requires an input of energy. Only carrier proteins can carry out active transport, but both carrier proteins and channel proteins can carry out passive transport.

Figure 13. Operation of bacterial transport systems Figure 13. Operation of bacterial transport systems. Bacterial transport systems are operated by transport proteins (sometimes called carriers, porters or permeases) in the plasma membrane. Facilitated diffusion is a carrier-mediated system that does not require energy, but which does cannot concentrate solutes against a gradient. Active transport processes such as Ion-driven transport and Binding protein-dependent transport systems, use energy and concentrate molecules against a concentration gradient. Group translocation systems, such as the phosphotransferase (pts) system in Escherichia coli, use energy during transport and modify the solute during its passage across the membrane. Types of Transport Systems Bacteria have a variety of types of transport systems which can be used alternatively in various environmental situations. The elaborate development of transport processes and transport systems in procaryotes probably reflects their need to concentrate substances inside the cytoplasm against the concentration (gradient) of the environment. Concentration of solutes in the cytoplasm requires the operation of an active transport system, of which there are two types in bacteria: ion driven transport systems (IDT) and binding-protein dependent transport systems (BPDT). The definitive feature of an active transport system is the accumulation of the solute in the cytoplasm at concentrations far in excess of the environment. According to the laws of physical chemistry, this type of process requires energy. There are four types of carrier-mediated transport systems in procaryotes. The carrier is a protein (or group of proteins) that function in the passage of a small molecule from one side of a membrane to another. A transport system may be a single transmembranous protein that forms a channel that admits passage of a specific solute. Or a transport system may be a coordinated system of proteins that binds and sequentially passes a small molecule through membrane. Transport systems have the property of specificity for the solute transported. Some transport systems transport a single solute with the same specificity and kinetics as an enzyme. Some transport systems will transport (structurally) related molecules, although at reduced efficiency compared to their primary substrate. Most transport systems transport specific sugars, amino acids, anions or cations that are of nutritional value to the bacterium.

细胞膜外 细胞膜 细胞膜内 恢复原构象 再循环 结合 构象改变 ADP+Pi ATP 移位 主动运输模式图

Na+-K+-ATP酶系统 功能: Na+-K+-ATPase是存在于原生质膜上的一种重要离子通道蛋白 该酶由大小两个亚基组成（MW: 12万, 5.5万）

作用步骤: 1. ATP酶(E)在细胞内侧与3个Na+结合,同时消耗能量; 2. 磷酸化ATP酶(E+)构象变化将Na+排除胞外,并与2个K+ 结合; 3. K+激发E+脱磷酸化恢复为E, 同时将K+运入细胞.

基团转位(Group translocation) 基因转位是一种特殊的主动运输，与普通的主动运输相比，营养物质在运输的过程中发生了化学变化（糖在运输的过程中发生了磷酸化）。其余特点与主动运输相同。 基因转位主要存在于厌氧和兼性厌氧型细菌中，也主要是用于单（或双）糖与糖的衍生物，以及核苷与脂肪散的运输 基团移位（基团转移） 特点：运送时需特异性载体蛋白，需要能量，被运送的物质可以逆浓度梯度运送，由稀到高，内部浓度比外部高的多，运送速度快，运送前后营养物质分子改变。 运送的物质种类：葡萄糖、果糖、甘露糖、嘌呤、核苷、脂肪酸等。 运送机制:是依靠磷酸转移酶系统,即磷酸烯醇式丙酮酸-己糖磷酸转移酶系统. 运送步骤: 1.热稳载体蛋白(HPr)的激活 细胞内高能化合物磷酸烯醇式丙酮酸（PEP）的磷酸基团把HPr激活。 酶1 PEP+HPr 丙酮酸+P-HPr HPr是一种低分子量的可溶性蛋白，结合在细胞膜上，具有高能磷酸载体的作用。 2、糖被磷酸化后运入膜内 膜外环境中的糖先与外膜表面的酶2结合，再被转运到内膜表面。这时，糖被P-HPr上的磷酸激活，通过酶2的作用把糖-磷酸释放到细胞内。 酶2 P-HPr+糖 糖-P +HPr 酶2是一种结合于细胞膜上的蛋白，它对底物具有特异性选择作用，因此细胞膜上可诱导出一系列与底物分子相应的酶2。 基因转位主要存在于厌氧和兼性厌氧型细菌中，也主要是用于单（或双）糖与糖的衍生物，以及核苷与脂肪散的运输

在酶Ⅱ的作用下P-HPr将磷酸转移给糖 在酶Ⅰ的作用下HPr被激活 细胞膜具有磷酸转移酶系统(PTS),能使糖 在进入细胞膜的同时发生磷酸化 (厌氧及兼性厌氧细菌) PTS系统 结构: PEP, 酶I, 酶II (a,b,c), Hpr (小分子量热稳定蛋白) 在酶Ⅰ的作用下HPr被激活

运送机制:是依靠磷酸转移酶系统,即磷酸烯醇式丙酮酸-己糖磷酸转移酶系统. 运送步骤: 1.热稳载体蛋白(HPr)的激活 细胞内高能化合物磷酸烯醇式丙酮酸（PEP）的磷酸基团把HPr激活。 酶1 PEP+HPr 丙酮酸+P-HPr HPr是一种低分子量的可溶性蛋白，结合在细胞膜上，具有高能磷酸载体的作用。

2、糖被磷酸化后运入膜内 膜外环境中的糖先与外膜表面的酶2结合，再被转运到内膜表面。这时，糖被P-HPr上的磷酸激活，并通过酶2的作用将糖-磷酸释放到细胞内。 酶2 P-HPr+糖 糖-P +HPr 酶2是一种结合于细胞膜上的蛋白，它对底物具有特异性选择作用，因此细胞膜上可诱导出一系列与底物分子相应的酶2。

基团移位模式图 细胞膜外 细胞膜内 S 细胞膜 Enz2 HPr P ~ Enz1+ PEP 丙酮酸

四种运输营养物质方式的比较 比较项目 单纯扩散 促进扩散 主动运输 基团转位 特异载体蛋白 运输速度 物质运输方向 胞内外浓度 运输分子 能量消耗 运输后物质的结构 无 慢 由浓至稀 相等 无特异性 不需要 不变 有 快 特异性 由稀至浓 胞内浓度高 需要 改变