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from article Polarized light microscope

Diagram of a high-sensitivity biological polarizing microscope. Polarizing microscopes for petrography may not require the use of the expensive prism polarizer and analyzer, which can be replaced by dichroic polarizing filters. Some petrographic…

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from article Atmospheric chemistry

The nitrogen cycle has important implications for air quality and climate in addition to fisheries and marine ecosystems. Anthropogenic activities have perturbed the natural nitrogen cycle, with major effects on the atmosphere and marine systems.…

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from article Atmospheric chemistry

Components of the carbon cycle showing the movement of carbon between land, the atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. (NASA:…

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from article Atmospheric chemistry

Processes that determine aerosol concentrations.

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from article Anemia

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from article Anemia

Aplastic anemia. This low-power photomicrograph of the bone marrow exhibits only a small amount of hematopoiesis (the process by which the cellular elements of the blood are formed). (Copyright © McGraw-Hill Education)

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from article Anemia

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from article Life forms of plants

Life forms of plants according to C. Raunkiaer. (After P. Dansereau, Biogeography: An Ecological Perspective, Ronald Press, 1957)

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from article Life forms of plants

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from article Homocysteine

Homocysteine metabolism. CBS, cystathionine ß-synthase; MTHFR, 5,10-methylene tetrahydrofolate reductase; MS, methionine synthase; BHMT, betaine:homocysteine methyltransferase; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; 5-methyl THF,…

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from article Echolocation

Echolocation is a physiological tool used by gray bats (Myotis grisescens). (Credit: Amareta Kelly)

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from article Echolocation

Sound spectrograms representing sonar signals of (a) Brazilian or Mexican free-tailed bats (Tadarida brasiliensis), (b) big brown bats (Eptesicus fuscus), (c) Wagner's mustached bats (Pteronotus personatus), and (d) greater horseshoe bats…

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from article Ovary

Anatomy of an ovary in an adult human female. The left inset shows the location of a pair of ovaries in relation to the uterus. The right inset is a close-up image of an oocyte. (Copyright © McGraw Hill)

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from article Ovary

Colored light micrograph of a section through a primary follicle in an ovary. The egg's nucleus is located at the center, surrounded by cuboidal follicular cells (yellow). (Credit: Steve Gschmeissner/Science Photo Library/Alamy Stock Photo)

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from article Ovary

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from article Ovary

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from article Ear (vertebrate)

Generalized diagram of a sensory hair cell found in the ear of all vertebrates.

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from article Ear (vertebrate)

Left-ear external view of the membranous labyrinths of (a) teleost, (b) frog, (c) bird, and (d) mammal. (After A. S. Romer, The Vertebrate Body, 3d ed., Saunders, 1962)

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from article Ear (vertebrate)

Dorsal view of the anterior part of the head and body of an otophysan fish showing the ear and its position relative to the brain. Also shown are the swim bladder and the Weberian ossicles. (From A. N. Popper and S. Coombs, Auditory mechanisms in…

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from article Ear (vertebrate)

Diagrammatic view of the middle and inner ears of a lizard. (After D. Webster and M. Webster, Comparative Vertebrate Morphology, Academic Press, 1974)

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from article Ear (vertebrate)

Schematic drawing of the human ear. (After M. Brödel, Three Unpublished Drawings of the Anatomy of the Human Ear, Saunders)

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from article Ear (vertebrate)

Otic region of the head of a mammal (showing the ear region only). (A. S. Romer, The Vertebrate Body, 3d ed., Saunders, 1962)

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from article Ear (vertebrate)

Inner ear of a mammal. (a) Cross section of a single turn of the cochlea. (b) Organ of Corti. (After D. Webster and M. Webster, Comparitive Vertebrate Morphology, Academic Press, 1974)

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from article Hydrosphere

Dissolved (a) sodium and (b) sulfate in rain (in parts per million) over the continental United States (After R. M. Garrels and F. T. Mackenzie, Evolution of Sedimentary Rocks, W. W. Norton, 1971)

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from article Hydrosphere

Accommodation of cations in aqueous solution. (After H. Blatt, G. Middleton, and R. Murray, Origin of Sedimentary Rocks, Prentice Hall, 1972)

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from article Hydrosphere

Distribution of dissolved gaseous oxygen (low values) and nutrient species (high concentrations) of orthophosphate ions at 26°22′.4N and 168°57′.5W in the Pacific Ocean. 1 m = 3.3 ft.

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from article Red Sea

Division of the Red Sea into regions representing different stages in the formation of an ocean basin. (After F. Martinez and J. R. Cochran, Structure and tectonics of the northern Red Sea: Catching a continental margin between rifting and drifting,…

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from article Immunity

The immune response to an invading microbe can generate immunity via antibody production. Steps: (1) Antigen binds to a B cell receptor (antibody) on a B cell. (2) The B cell may also receive more information from a helper T cell about the presence…

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from article Immunity

Colored scanning electron micrograph of an activated human macrophage. Activation of macrophages is often an important aspect of the immune response. (Credit: Steve Gschmeissner/Science Photo Library/Alamy Stock Photo)

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from article Immunity

Colored scanning electron micrograph of B lymphocytes. These white blood cells are involved in the immune response. (Credit: Steve Gschmeissner/Science Photo Library/Alamy Stock Photo)

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from article Deforestation

View of an area of deforestation in Central Kalimantan, Indonesia. In this case, the land is being cleared to develop areas for intensive rice production. (Credit: Australian National University)

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from article Deforestation

The relationship between forest and nonforest (other) land. Preservation of forest lands includes reforestation and natural regeneration. However, deforestation and natural events can lead to nonforest land. Afforestation (the act or process of…

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from article Deforestation

Natural forest areas (in hectares; 1 hectare = 2.47 acres) in various climatic domains (boreal/polar, temperate, subtropical, and tropical) from 1990 to 2015. The data are derived from the Global Forest Resources Assessment 2015 by the Food and…

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from article Deforestation

Former rainforest in northern Queensland, Australia, that was deforested in the early 1900s and is now used for pasture. (Credit: Rodney Keenan)

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from article Deforestation

Aerial view of deforestation in the Amazon region of Brazil. Clearing of trees in the Amazon rainforest has resulted in the elimination of natural resources and numerous species. (Credit: Luoman/Getty Images)

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from article Basalt

Thin section of upper crust of lava about 1.2 in. (3 cm) from top; black is microcrystalline groundmass. 0.5 mm = 0.02 in.

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from article Basalt

Thin section of interior of lava shown in Fig. 1, but 26 ft (7.9 m) below the top of the flow. Clear plagioclase crystals are ophitically enclosed in large pyroxene grains up to 0.04 in. (1 mm) in diameter. Crystals of black magnetite are minor. A…

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from article Basalt

Growth of crystals as a function of temperature in basalt in Hawaiian lava. °F = (°C × 1.8) + 32. (After D. L. Peck, T. L. Wright, and J. G. Moore, Crystallization of tholeiitic basalt in Alae lava lake, Hawaii, Bull. Volcanol., 29:629–656, 1966)

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from article Basalt

Diagrams illustrating the compositions of basalts in terms of normative mineral formulas. Customarily, CaAl2Si2O8 (anorthite) is grouped together with NaAlSi3O8 (albite), and Fe2SiO4, FeSiO3, and CaFeSi2O6 are grouped with the corresponding Mg…

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from article Ordovician

Ordovician, the second period of the Paleozoic Era.

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from article Ordovician

Early Ordovician (Arenig) paleogeography, major rock suites and potential ocean surface currents; based on remanent magnetism data supplemented by plots of the positions of shelf sea carbonates and reefs.

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from article Lipidomics

Components of phosphoinositides. (a) General structure of phospholipids, showing positions sn-1 and sn-2, where various fatty acids may attach. X represents a phosphate-linked group; FA represents a fatty acid. (b) Space-filling models of four…

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from article Lipidomics

Selected MS ion chromatograms of synthetic 1,2-diacyl-sn-3-glycerol (DAG) molecular species from polymorphonuclear neutrophilic leukocytes (PMNs) of patients with local aggressive periodontal disease. DAG species were resolved and identified by…

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from article Lipidomics

Families of bioactive lipids. (a) Arachidonic acid is the precursor for many of the known bioactive mediators, such as EETs, prostaglandins, leukotrienes, and lipoxins. (b) Eicosapentanoic acids (C20:5 and C22:6) are precursors to potent new…

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from article Lipidomics

Functional-mediator lipidomics approach to evaluate and profile compounds during experimental inflammation and its resolution.

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from article Thrombosis

Organized thrombus in an artery with recanalization.

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from article Thrombosis

Organized thrombus in an artery with scar formation; the artery is completely occluded by the fibrotic mass.

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from article Clover

Red clover (Trifolium pratense). (Photo by Tom Heutte, U.S. Department of Agriculture Forest Service)

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from article Clover

White clover (Trifolium repens). (Photo by Pavel Šinkyřík)

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from article Clover

Crimson clover (Trifolium incarnatum). (Photo by Pavel Šinkyřík)

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from article Clover

White sweet clover (Melilotus albus). (Photo by Forest and Kim Starr)

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from article Clover

Leaves of clover affected by leaf spot, which is caused by the fungus Pseudopeziza trifolii. (Photo courtesy of the U.S. Department of Agriculture Cooperative Extension Slide Series, Clemson University)

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from article Mitochondria

A mitochondrion is a specialized compartment (organelle) of a eukaryotic cell (upper left). The electron micrograph (upper right) and the accompanying illustration (bottom) indicate the outer and inner membranes, cristae, and matrix. [Credit…

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from article Mitochondria

Three-dimensional reconstruction of a rat liver mitochondrion produced by electron microscopic tomography. During imaging, the sample is tilted around one or more axes to generate different projections that are later digitally combined to produce a…

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from article Mitochondria

Guard cells of a transgenic tobacco plant expressing green fluorescent protein (GFP) targeted to mitochondria. Chloroplasts (cell plastids) show red chlorophyll autofluorescence and mitochondria show green protein fluorescence. (Credit: Rainer…

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from article Mitochondria

Mitochondria are the powerhouses of the cell, being responsible for energy generation in the form of adenosine triphosphate (ATP). ATP is produced by the coupled processes of electron transport and oxidative phosphorylation in mitochondria.…

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from article Eye (invertebrate)

Pigment spot ocellus, the simplest invertebrate eye. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Simple open eye of the marine limpet Patella. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Large, complex eye of the marine snail Murex. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Simple pigment cup ocellus of flatworm. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Large, complex eye of the clam worm Nereis. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Simple eye of Ranzania, a marine annelid. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Highly developed eyes at anterior end of pelagic marine annelid Alciopidae. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Scallop Argopecten with one valve removed showing ocelli along the margin of the mantle. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Longitudinal section through the tubelike anterior median eye of jumping spider Salticus. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Octopus eye. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Two ommatidia from compound eye of crayfish Astacus. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Compound eye adapted for opposition image formation. Light rays from points a, b, and c. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed., Brooks/Cole Thomson, Belmont, CA, 2004)

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from article Eye (invertebrate)

Compound eye adapted for superposition image formation. The light from points a, b, and c is concentrated on an ommatidium. (After E. E. Ruppert, R. S. Fox, and R. D. Barnes, Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed.,…

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from article Buxbaumiidae

Morphological features of Buxbaumiidae. (a) Sporophyte of Buxbaumia aphylla (after W. H. Welch, Mosses of Indiana, Indiana Department of Conservation, 1957). Diphyscium foliosum (b) habit sketch, (c) leaves and (d) perichaetial leaf (after H. S.…

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from article Ferret

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from article Physiological acoustics

Schematic outline of the central auditory pathways. Numbers indicate the number of neurons from the cochlea. (After S. S. Stevens, ed., Handbook of Experimental Physiology, John Wiley and Sons, 1951)

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from article Physiological acoustics

Distribution of frequency maxima along the basilar membrane. Numbers give frequencies in hertz. Length of basilar membrane = 32 mm, width = 0.04 mm at base, 0.50 mm at apex. (After O. Stuhlmann, An Introduction to Biophysics, John Wiley and Sons,…

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from article Physiological acoustics

Isoamplitude curves of the dc receptor potential within one guinea pig inner hair cell of the basal region. Q10 is a measure of the width of each curve, equal to the center frequency divided by the bandwidth at 10 dB above threshold. (After I. J.…

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from article Physiological acoustics

Intensity/rate functions of two cat cochlear nerve fibers tuned to (a) 2.2 kHz and (b) 10.6 kHz. (After W. D. Keidel and W. D. Neff, eds., Handbook of Sensory Physiology, Springer, 1975)

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from article Physiological acoustics

Tuning curves of a number of nerve fibers in the cat cochlear nerve. (After N. Y.-S. Kiang, Discharge Patterns of Single Fibers in the Cat's Auditory Nerve, MIT Res. Monogr. 35, MIT Press, 1965)

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from article Physiological acoustics

PST histograms for a number of nerve fibers in a cat with various characteristic frequencies, showing response to click stimulation. Each histogram represents 1 minute of data (600 clicks at click rate of 10 per second). (After N.Y.-S. Kiang,…

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from article Physiological acoustics

Response patterns of cells in cat cochlear nuclei, with characteristic frequencies indicated. (a) Primarylike response, 3.5 kHz. (b) Chopper response, 7.0 kHz. (c) Pauser response, 6.6 kHz. (d) “On” pattern, 2.1 kHz. (After R. R. Pfeiffer,…

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from article Physiological acoustics

Ascending (afferent) and descending (efferent) auditory nerve systems. Number labels in the ascending system indicate the number of neurons from the cochlea, while those in the descending system indicate the number of neurons from the auditory…

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from article Granite

Classification of granitic rocks by the IUGS scheme. All proportions are by volume percentages.

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from article South America

Map of South America.

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from article South America

Andes Mountains, Chile. The Andes, which parallel the Pacific coast, constitute one of the Earth's most majestic features. Only the Himalayas exceed them in elevation and mass. (United Press International)

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from article South America

Representative view of the hilly uplands and low mountain country of the Guiana Highlands. One of five waterfalls on the Canaima River, Gran Sabana in southern Venezuela. (Venezuelan Information Service)

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from article South America

Uplifted beaches on desert coast, east side of San Juan Bay, Departmento de Ica, Peru. The sea cliff is about 100 ft (30 m) high. Each horizontal line represents a former beach. (Photograph by F. Atchley)

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from article South America

Port of Matarani on the desert coast of southern Peru. This oblique aerial view looks coastwise southward over one of the world's driest and most barren coastal zones. (Servicio Aerofotográfico Nacional)

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from article Plant physiology

The process of photosynthesis is a key feature of plant physiology. Chloroplasts (cell plastids that contain chlorophyll) use solar energy to manufacture carbohydrates and other large molecules. Mitochondria break down these molecules, thereby…

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from article Plant physiology

Adoption of genetically modified crops in the United States for the period of 1996 to 2016. Herbicide-tolerant (HT) soybeans, cotton, and corn have been modified genetically to survive the application of specific herbicides that previously would…

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from article Zosterophyllopsida

Examples of Devonian Zosterophyllopsida. (a) Reconstruction of the Early Devonian Zosterophyllum rhenanum, here depicted as an aquatic plant (after R. Kräusel and H. Weyland, Neue Pflanzenfunde im rheinischen Unterdevon, Palaeontographica B,…

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from article Sexually transmitted diseases

Acquired immune deficiency syndrome (AIDS) is a sexually transmitted disease caused by the human immunodeficiency virus (HIV). In this image, a human T cell (blue) is under attack by HIV (yellow), hampering the effectiveness of the body's immune…

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from article Sexually transmitted diseases

Prevalence of Chlamydia trachomatis (the most-common sexually transmitted bacterial pathogen) among individuals aged 14 to 39. The prevalence varies by sex, race/ethnicity, and age. Error bars indicate 95% confidence intervals. (Credit: U.S. Centers…

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from article Sexually transmitted diseases

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from article Bytownite

Bytownite, from Crystal Bay, Minnesota. (Specimen from Department of Geology, Bryn Mawr College)

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from article Excretion

Sequence of breakdown from nucleic acid to ammonia. Reverse pathways broken indicate either lack of knowledge of the mechanism or that other enzymes than those shown are involved. (After M. Florkin, Biochemical Evolution, Academic Press, 1949)

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from article Excretion

Chart of the principle of excretion of a common nitrogenous waste, whether derived from protein or nucleic acid. (After M. Florkin and G. Duchateau, Arch. Int. Physiol., 53:267–307, 1943)

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from article Tetanus

Photomicrograph showing endospores forming at the ends of Clostridium tetani bacterial cells, which are shaped like small tennis rackets. Clostridium tetani is the causative agent of tetanus. (Credit: Lisa Burgess; copyright © McGraw Hill)

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from article Syphilis

Syphilis is caused by Treponema pallidum, which is a corkscrew-shaped motile bacterium (spirochete). (Credit: National Institute of Allergy and Infectious Diseases, National Institutes of Health)

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from article Geraniales

A common eastern United States species of geranium (Geranium maculatum), which is characteristic of the order Geraniales. (Photo by Jennifer Anderson; USDA-NRCS PLANTS Database)

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from article Koji mold (Aspergillus oryzae)

Koji molds and related fungi. Left to right: Aspergillus oryzae, A. sojae, and A. flavus. Top: Colonies grown on agar plates. Bottom: Microscopic images of conidia (spores). (Credit: T. Okuda/Tamagawa University Research Institute, Japan)

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from article Koji mold (Aspergillus oryzae)

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from article Koji mold (Aspergillus oryzae)

Koji production as one of the processes of sake production in the Edo Era, Japan. Steamed rice is spread on straw mats for cooling (right) and then mixed with Tane-koji (next place to the left). The room at the back is the koji-muro. (Credit: Nihon…

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