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1、Response to internal and external signals 3-1-2004 Control of transport (Chapter 748-750; 759-762)Symbiosis (Chapter 37, page 775-779).Cell cell communication (Chapter 39, page 802-816).Light (Chapter 39, page 817-819).Pathogen (Chapter 39, page 827-829)Transport system. Molecules tend to move down
2、their concentration gradient, and when this occurs across a membrane it is passive transport and occurs without the direct expenditure of metabolic energy by the cell. Facilitative transport: Transport proteins embedded in the membrane can speed movement across the membrane.Facilitative transport: S
3、ome transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side. Others act as selective channels, providing a selective passageway across the membrane. For example, the membranes of most plant cells have potassium channels that allow potassium ion
4、s (K+) to pass, but not similar ions, such as sodium (Na+). Some channels are gated, opening or closing in response to certain environmental or biochemical stimuli. In active transport, solutes are pumped across membranes against their electrochemical gradients. The cell must expend metabolic energy
5、, usually in the form of ATP, to transport solutes “uphill” - counter to the direction in which the solute diffuses. Transport proteins that simply facilitate diffusion cannot perform active transport. Active transporters are a special class of membrane proteins, each responsible for pumping specifi
6、c solutes. Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work. These are often used to drive the transport of many different solutes. For example, the membrane potential generated by proton pumps contribute
7、s to the uptake of potassium ions (K+) by root cells. The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3-or sucrose.Aquaporins affect the rate of water transport across membranes Until recentl
8、y, most biologists accepted the hypothesis that leakage of water across the lipid bilayer was enough to account for water fluxes across membranes. However, careful measurements in the 1990s indicated that water transport across biological membranes was too specific and too rapid to be explained enti
9、rely by diffusion.The Nobel Prize in Chemistry, 2003“ for the discovery of water channel”http:www.nobel.se/chemistry/laureates/2003/agre-lecture.html(around 45 minutes) Both plant and animal membranes have specific transport proteins, aquaporins, that facilitate the passive movement of water across
10、a membrane. Aquaporins do not affect the water potential gradient or the direction of water flow, but rather the rate at which water diffuses down its water potential gradient. This raises the possibility that the cell can regulate the rate of water uptake or loss when its water potential is differe
11、nt from that of its environment. If aquaporins are gated channels, then they may open and close in response to variables, such as turgor pressure, in the cell.Guard cell mediate the photosynthesis- transpiration compromise A leaf may transpire more than its weight in water each day. To keep the leaf
12、 from wilting, flows in xylem vessels may reach 75 cm/min. Guard cells, by controlling the size of stomata, help balance the plants need to conserve water with its requirements for photosynthesis.Proton gradient facilitates transport of K ion against its own gradient The K+fluxes across the guard ce
13、ll membranes are probably passive, being coupled to the generation of membrane potentials by proton pumps. Stomatal opening correlates with active transport of H+ out of guard cells. The resulting voltage (membrane potential) drives K+ into the cell through specific membrane channels. At least three
14、 cues contribute to stomatal opening at dawn. First, blue-light receptors in the guard cells stimulate the activity of ATP-powered proton pumps in the plasma membrane, promoting the uptake of K+. A second stimulus is depletion of CO2within air spaces of the leaf as photosynthesis begins. A third cue
15、 in stomatal opening is an internal clock located in the guard cells. Even in the dark, stomata will continue their daily rhythm of opening and closing. The opening and closing cycle of the stomata is an example of a circadian rhythm, cycles that have intervals of approximately 24 hours.Copyright 20
16、02 Pearson Education, Inc., publishing as Benjamin CummingsNitrogen Cycle in biosphere All life on Earth depends on nitrogen fixation, a process performed only by certain prokaryotes. The reduction of N2to NH3is a complicated, multi-step process, catalyzed by one enzyme complex, nitrogenase: N2+ 8e-+ 8H+ 16ATP - 2NH3+ H2+ 16ADP + 16Pi Nitrogen-fixing bacteria are most abundant in soils rich in
