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from article Absorption
Incident sunlight is absorbed, reflected, and scattered by tree leaves. (Credit: Shutterstock / photolinc)
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from article Bioorganic chemistry
Vancomycin and an analog that is active against vancomycin-resistant bacteria.
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from article Bioorganic chemistry
Foldamers that mimic proteins.
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from article Bioorganic chemistry
The rate of this intramolecular lactonization, relative to that of intermolecular esterification, mimics the rate increase caused by an enzyme's binding and orienting substrates for a bimolecular reaction.
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from article Bioorganic chemistry
Intramolecular models of enzymatic general acid and general base catalysis. (a) Intramolecular general acid catalysis causes this acetal to hydrolyze at pH 7 more than 107 times faster than the corresponding acetal without the carboxylic acid group.…
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from article Bioorganic chemistry
An antibody raised against the phosphonate analog of a tetrahedral intermediate catalyzes the hydrolysis of cocaine to ecgonine methyl ester.
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from article Bioorganic chemistry
A functionalized cyclodextrin mimics the enzyme RNase by catalyzing the regioselective hydrolysis of a model phosphodiester.
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from article Bioorganic chemistry
Enzyme-catalyzed reactions that have been commercialized as industrial processes.
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from article Bioorganic chemistry
Mechanism of DNA scission by calicheamicin. Reductive or nucleophilic cleavage of the trisulfide triggers conjugate (Michael) addition to the enone, rehybridization of the bridgehead carbon, and Bergman cyclization of the enediyne, generating a…
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from article Negative temperature
Populations (n) of states of a thermodynamic system consisting of N magnetically interacting nuclei of spin 2 at absolute temperatures T = +D/k and T = −D/k, where k is Boltzmann's constant and D is the energy spacing between successive states.
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from article Polypodiales
Licorice fern (Polypodium glycyrrhiza), a member of the order Polypodiales, in Olympic National Forest, Washington. (Photo courtesy of the U.S. Department of Agriculture, Forest Service)
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from article Polypodiales
Christmas fern (Polystichum acrostichoides). (Photo by James H. Miller, U.S. Department of Agriculture, Forest Service)
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from article Polypodiales
The sensitive fern (Onoclea sensibilis), a representative of the Polypodiales. (After W. W. Robbins, T. E. Weier, and C. R. Stocking, Botany: An Introduction to Plant Science, 3d ed., Wiley, New York, 1964)
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from article Polypodiales
Types of steles in the Polypodiales: (a) Protostele. (b) Amphiphloic siphonostele. (c) Ectophloic siphonostele. (d) Dictyostele with central pith surrounded by xylem and phloem in the form of discrete vascular bundles; bands of parenchyma cells…
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from article Polypodiales
Life history of a fern: (1) the fern plant, or sporophyte, bearing sori, or clusters of sporangia, on its leaves; (2) a sporangium; (3) a tetrad of young spores; (4) the four mature spores that have come from the tetrad shown in part 3; (5) a spore…
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from article Polypodiales
Diagram of section through a fern leaf, showing details of a sorus. (After W. W. Robbins, T. E. Weier, and C. R. Stocking, Botany: An Introduction to Plant Science, 3d ed., Wiley, New York, 1964)
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from article Polypodiales
Eastern marsh fern (Thelypteris palustris): sori on the pinnules of a fertile frond. (Photo by Rob Routledge, Sault College, Sault Ste. Marie, Ontario, Canada)
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from article Polypodiales
Diagram of a fern prothallium. (After W. W. Robbins, T. E. Weier, and C. R. Stocking, Botany: An Introduction to Plant Science, 3d ed., Wiley, New York, 1964)
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from article Luminescence
Schematic representation of energy levels and electronic transitions in an atomic gas. E1, ground state; E2 and E3, excited states; 1, excitation; 2, emission of resonance luminescence; 3, radiationless transition to lower excited state; 4,…
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from article Luminescence
Configuration coordinate curves for a simple luminescence center. Transition A → B shows absorption, and transition C → D shows emission. Energy EQ is that necessary for the excited state to reach point E, from which a transition to the ground state…
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from article Luminescence
Variation of the brightness of a thallium-activated potassium chloride phosphor with temperature. °F = (K × 1.8) − 459.67.
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from article Luminescence
Variation of the width of the F-center absorption band in potassium chloride at its half-maximum points, FE, as a function of the square root of the temperature. °F = (K × 1.8) − 459.67.
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from article Luminescence
Emission of thallium-activated potassium chloride at 4 K (−452°F). In a plot of this particular form, a gaussian curve would consist of two straight lines which make equal angles with the abscissa.
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from article Luminescence
Graph showing the absorptance (100 minus percent of transmission) and edge emission of cadmium sulfide at 4 K (−452°F).
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from article Luminescence
Excitation spectra for emission of calcium carbonate phosphors, using manganese as an activator, with different impurity sensitizers: 1, no sensitizer; 2, thallium; 3, lead; and 4, cerium.
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from article Luminescence
Energy-band model for luminescence processes in zinc sulfide. Electron energy increases vertically.
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from article Luminescence
Energy-band model for luminescence processes in zinc sulfide which uses copper as the activator and aluminum as the coactivator.
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from article Yeast
Colorized scanning electron micrograph (SEM) of Schizosaccharomyces pombe yeast. This single-celled fungus is studied widely as a model organism for eukaryotic cell division. It is a rod-shaped yeast that grows by elongation at its ends, and it…
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from article Yeast
Cell shapes of yeasts are extremely varied: (a) rectangular; (b) pear-shaped; (c) apiculate; (d) ogival; (e) spherical; (f) globose; (g) ellipsoidal (oval); (h) elongate; (i) tetrahedral.
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from article Yeast
Colorized scanning electron micrograph (SEM) of cells of brewer's yeast (Saccharomyces cerevisiae). This yeast consists of single vegetative cells. Small daughter cells can be seen budding off from the larger cells. Saccharomyces cerevisiae ferments…
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from article Yeast
Yeast mycelia. (a) A true septate mycelium showing a break to form an arthrospore. (b) A pseudomycelium.
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from article Yeast
Heterothallic (bifactorial) life cycle of a basidiomycetous yeast, Leucosporidium scottii.
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from article Yeast
A fluorescent antibody stain reveals the oval budding yeast cells of Candida albicans. This yeast is the most common human fungal pathogen. (Credit: Maxine Jalbert and Leo Kaufman/Centers for Disease Control and Prevention)
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from article Quantized electronic structure (QUEST)
Geometric configurations of (a) quantum well, (b) quantum wires, and (c) quantum dots.
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from article Quantized electronic structure (QUEST)
Electron energy levels and electron standing waves for n = 1, 2, 3, and 4 quantum states in a quantum well.
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from article Quantized electronic structure (QUEST)
Principles of operation of quantum-well devices. (a) Quantum-well laser. (b) Quantum-well infrared detector. (c) High-electron-mobility transistor (HEMT or MODFET). Electrons in the quantum well that came from donor atoms in the barrier are free to…
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from article Liquid chromatography
Chromatogram showing the separation of various ethoxylates (nonionic surfactants) from an environmental water sample by liquid chromatography. EO = ethoxylate. Cx = alkyl. OP = octylphenol. NP = nonylphenol. (Credit: U.S. EPA)
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from article Flow of granular solids
Bounds on mass flow and funnel flow. (a) Geometry of a transition hopper, showing slope angles θc and θp. (b) Range of kinematic angle of friction (φ′) and θc leading to mass flow and funnel flow. (c) Range of φ′and θp leading to mass flow and…
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from article Flow of granular solids
Flow patterns in a funnel-flow bin, showing segregation of solids. (a) Charge coming in is greater than draw coming out, resulting in draw with a preponderance of fine particles. (b) Draw is greater than charge, resulting in draw with a…
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from article Flow of granular solids
Section of bin with mass-flow conical hopper, together with graph showing relative values of pressure p, compressive strength f, and stress s, and illustrating criterion (f = s) for minimum outlet dimension B. (After A. W. Jenike, Quantitative…
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from article Cabbage
Cabbage (Brassica oleracea var. capitata). (Joseph Harris Co., Inc., Rochester, New York)
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from article Spray flow
Cross section of a vaporizing droplet, showing the relative gas-droplet motion and internal circulation.
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from article Atmospheric acoustics
Frequency dependence of attenuation coefficient in units of decibels per 100 m (dB/328 ft), temperature of 20°C (68°F), and a relative humidity of 70%. Total attenuation from air absorption and contributions from classical absorption (that due to…
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from article Atmospheric acoustics
Diagram of reflection of sound from a flat ground surface with impedance Z, showing direct (R1) and reflected (R2) paths from source to receiver, source height hs, microphone height h, distance of propagation d, and angle of incidence or grazing…
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from article Atmospheric acoustics
Relative sound pressure levels (SPL) for propagation from a source over mown grass for source height of 1.8 m (6 ft), microphone height of 1.5 m (5 ft), and distances of propagation indicated.
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from article Atmospheric acoustics
Curved ray paths. (a) Refraction downward, during temperature inversion or downwind propagation. (b) Refraction upward, during temperature lapse or upwind propagation.
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from article Atmospheric acoustics
Representative long-range ray paths of sound radiating from a source in the atmosphere and traveling from west to east in Northern Hemisphere in winter. 1 km = 0.6 mi. (After T. F. Malone, ed., Compendium of Meteorology, American Meteorological…
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from article Atmospheric acoustics
Contour plot of sound levels in the presence of upward refraction in a turbulent atmosphere. The source is very close to the ground at zero range. The white areas are regions of higher sound levels.
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from article Atmospheric acoustics
Facsimile record of acoustic echo sounding of atmosphere at the South Pole, February 9, 1975. 300 m = 984 ft, 600 m = 1969 ft. Dark regions above 300 m altitude correspond to temperature inhomogeneities associated with the passage of convective…
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from article Surface waves
Definition sketch for an oscillatory wave.
