玉Microbial Nutrition 100 Pseudon ganisms that can produce large quantities of other vitamins. substance in a sample? 5.6 Uptake of Nutrients by the Cell t 5.1 Facitated dependence of acriliaicdicetorplt bove a s free pas ncIn view of the o The most important of these are facilitated diffusion.active trans ure5.1).Note that the diffusion rate levels offor reachesaplateau gradient value ecause the carner is saturate ssible.The res emyme-subsrae cve e s different fro Facilitated Diffusion substance to be erol,can cross the plasma mem no metabolic energy input is required.If the concentration gradi ound or by moving it to another membranous compartment uptake by p ve di cucaryotes. ese camiers are ated t the rate of uptake decreases as more nutrient i ngtothe MIP family of proteins.The two most widespread MIP sed imi diately.Very small mo an ndpolr substance do notcromm Although much work has been done on the mechanism of fa cell interio.The carrier would change sion es with the con shapc and be ready to pick up another mole he ne ve diffusion (fi sponse to its co centration gradient.Remember that the mecha
Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 5. Microbial Nutrition © The McGraw−Hill Companies, 2002 Pseudomonas), vitamin C (Gluconobacter, Erwinia, Corynebacterium), -carotene (Dunaliella), and vitamin D (Saccharomyces). Current research focuses on improving yields and finding microorganisms that can produce large quantities of other vitamins. 1. Briefly summarize the ways in which microorganisms obtain nitrogen, phosphorus, and sulfur from their environment. 2. What are growth factors? What are vitamins? How can microorganisms be used to determine the quantity of a specific substance in a sample? 5.6 Uptake of Nutrients by the Cell The first step in nutrient use is uptake of the required nutrients by the microbial cell. Uptake mechanisms must be specific—that is, the necessary substances, and not others, must be acquired. It does a cell no good to take in a substance that it cannot use. Since microorganisms often live in nutrient-poor habitats, they must be able to transport nutrients from dilute solutions into the cell against a concentration gradient. Finally, nutrient molecules must pass through a selectively permeable plasma membrane that will not permit the free passage of most substances. In view of the enormous variety of nutrients and the complexity of the task, it is not surprising that microorganisms make use of several different transport mechanisms. The most important of these are facilitated diffusion, active transport, and group translocation. Eucaryotic microorganisms do not appear to employ group translocation but take up nutrients by the process of endocytosis (see section 4.5). Plasma membrane structure and properties (pp. 46–48) Facilitated Diffusion A few substances, such as glycerol, can cross the plasma membrane by passive diffusion. Passive diffusion, often simply called diffusion, is the process in which molecules move from a region of higher concentration to one of lower concentration because of random thermal agitation. The rate of passive diffusion is dependent on the size of the concentration gradient between a cell’s exterior and its interior (figure 5.1). A fairly large concentration gradient is required for adequate nutrient uptake by passive diffusion (i.e., the external nutrient concentration must be high), and the rate of uptake decreases as more nutrient is acquired unless it is used immediately. Very small molecules such as H2O, O2, and CO2 often move across membranes by passive diffusion. Larger molecules, ions, and polar substances do not cross membranes by passive or simple diffusion. The rate of diffusion across selectively permeable membranes is greatly increased by using carrier proteins, sometimes called permeases, which are embedded in the plasma membrane. Because a carrier aids the diffusion process, it is called facilitated diffusion. 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 (figure 5.1). Note that the diffusion rate levels off or reaches a plateau above a specific gradient value because the carrier is saturated— that is, the carrier protein is binding and transporting as many solute molecules as possible. The resulting curve resembles an enzyme-substrate curve (see section 8.6) and is different from the linear response seen with passive diffusion. Carrier proteins also resemble enzymes in their specificity for the substance to be transported; each carrier is selective and will transport only closely related solutes. Although a carrier protein is involved, facilitated diffusion is truly diffusion. A concentration gradient spanning the membrane drives the movement of molecules, and no metabolic energy input is required. If the concentration gradient disappears, net inward movement ceases. The gradient can be maintained by transforming the transported nutrient to another compound or by moving it to another membranous compartment in eucaryotes. Interestingly, some of these carriers are related to the major intrinsic protein of mammalian eye lenses and thus belong to the MIP family of proteins. The two most widespread MIP channels in bacteria are aquaporins that transport water and glycerol facilitators, which aid glycerol diffusion. Although much work has been done on the mechanism of facilitated diffusion, the process is not yet understood completely. It appears that the carrier protein complex spans the membrane (figure 5.2). After the solute molecule binds to the outside, the carrier may change conformation and release the molecule on the cell interior. The carrier would subsequently change back to its original shape and be ready to pick up another molecule. The net effect is that a lipid-insoluble molecule can enter the cell in response to its concentration gradient. Remember that the mecha- 100 Chapter 5 Microbial Nutrition Concentration gradient Rate of transport Passive diffusion Facilitated diffusion Figure 5.1 Passive and Facilitated Diffusion. The dependence of diffusion rate on the size of the solute’s concentration gradient. Note the saturation effect or plateau above a specific gradient value when a facilitated diffusion carrier is operating. This saturation effect is seen whenever a carrier protein is involved in transport.
5.6 Uptake of Nutrients by the Cell 101 2 ATP ADP ABC a er complex..(②)Tm ngpwoteinaaC will move outward.Because the cell metabolizes nutrients upor ilar solute molecules can compete for the same carrier protein in does not seem to be im yotes because nutrient con trations（佰gure5..Nevertheles. fomfaciitatoddisionniusecofmctaboicsnegyand onas,Bacillus,and many other ba energy prod faci m ATP-binding transporters(ABCtransporters)are active in bac Active Transport two nucltid-inding cannot take up solutes that are al- bran and the nucleouide-binding domains bind and h eady more con rces,and,too rish.thev ram-r amma ese nu s.Thus xternal face o transpo he (s).bind the molecule to situations are active transport and group Active transportis the transport of solute molecules gars (arab maltose,galactose,ribose)and amino acids ort in gm by th volves protein carrier activity,it resembles facilitated diffusion in
Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 5. Microbial Nutrition © The McGraw−Hill Companies, 2002 nism is driven by concentration gradients and therefore is reversible. If the solute’s concentration is greater inside the cell, it will move outward. Because the cell metabolizes nutrients upon entry, influx is favored. Facilitated diffusion does not seem to be important in procaryotes because nutrient concentrations often are lower outside the cell so that facilitated diffusion cannot be used in uptake. Glycerol is transported by facilitated diffusion in E. coli, Salmonella typhimurium, Pseudomonas, Bacillus, and many other bacteria. The process is much more prominent in eucaryotic cells where it is used to transport a variety of sugars and amino acids. Active Transport Although facilitated diffusion carriers can efficiently move molecules to the interior when the solute concentration is higher on the outside of the cell, they cannot take up solutes that are already more concentrated within the cell (i.e., against a concentration gradient). Microorganisms often live in habitats characterized by very dilute nutrient sources, and, to flourish, they must be able to transport and concentrate these nutrients. Thus facilitated diffusion mechanisms are not always adequate, and other approaches must be used. The two most important transport processes in such situations are active transport and group translocation, both energy-dependent processes. Active transport is the transport of solute molecules to higher concentrations, or against a concentration gradient, with the use of metabolic energy input. Because active transport involves protein carrier activity, it resembles facilitated diffusion in some ways. The carrier proteins or permeases bind particular solutes with great specificity for the molecules transported. Similar solute molecules can compete for the same carrier protein in both facilitated diffusion and active transport. Active transport is also characterized by the carrier saturation effect at high solute concentrations (figure 5.1). Nevertheless, active transport differs from facilitated diffusion in its use of metabolic energy and in its ability to concentrate substances. Metabolic inhibitors that block energy production will inhibit active transport but will not affect facilitated diffusion (at least for a short time). Binding protein transport systems or ATP-binding cassette transporters (ABC transporters) are active in bacteria, archaea, and eucaryotes. Usually these transporters consist of two hydrophobic membrane-spanning domains associated on their cytoplasmic surfaces with two nucleotide-binding domains (figure 5.3). The membrane-spanning domains form a pore in the membrane and the nucleotide-binding domains bind and hydrolyze ATP to drive uptake. ABC transporters employ special substrate binding proteins, which are located in the periplasmic space of gram-negative bacteria (see figure 3.23) or are attached to membrane lipids on the external face of the gram-positive plasma membrane. These binding proteins, which also may participate in chemotaxis (see pp. 66–68), bind the molecule to be transported and then interact with the membrane transport proteins to move the solute molecule inside the cell. E. coli transports a variety of sugars (arabinose, maltose, galactose, ribose) and amino acids (glutamate, histidine, leucine) by this mechanism. Substances entering gram-negative bacteria must pass through the outer membrane before ABC transporters and other 5.6 Uptake of Nutrients by the Cell 101 Inside the cell Outside the cell Plasma membrane Figure 5.2 A Model of Facilitated Diffusion. The membrane carrier can change conformation after binding an external molecule and subsequently release the molecule on the cell interior. It then returns to the outward oriented position and is ready to bind another solute molecule. Because there is no energy input, molecules will continue to enter only as long as their concentration is greater on the outside. ATP 1 2 Solutebinding protein Transporter Nucleotidebinding domain Cytoplasmic matrix Periplasm ADP + Pi ATP ADP + Pi Figure 5.3 ABC Transporter Function. (1) The solute binding protein binds the substrate to be transported and approaches the ABC transporter complex. (2) The solute binding protein attaches to the transporter and releases the substrate, which is moved across the membrane with the aid of ATP hydrolysis. See text for details