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Notch-ligand interactions. Two Notch ligands, Delta and Serrate, are shown on the signal-sending cell (left). These ligands are distinguished by differing numbers of EGF-like repeats (indicated by the light boxes) and an extracellular cysteine-rich…

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from article Folic acid

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from article Digenea

Diagrams of various adult types of digenean trematodes. (After R. M. Cable, An Illustrated Laboratory Manual of Parasitology, Burgess, Minneapolis, 1940)

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from article Digenea

Digenean trematode life cycle. (After R. M. Cable, An Illustrated Laboratory Manual of Parasitology, Burgess, Minneapolis, 1940)

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from article Urinary bladder

Light micrograph of the healthy transitional epithelium tissue (stained by hematoxylin and eosin) of a urinary bladder. The transitional epithelium is a type of tissue consisting of multiple layers of epithelial cells, which can contract and expand.…

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from article Urinary bladder

Types of tubal bladders: (a) duplex; (b) bilobed; (c) simplex, with ureters united; (d) simplex, with separate ureters. (Credit: H. E. Walter and L. P. Sayles)

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from article Electrolysis

Electrolysis cell in which the electrolyte is a solution of sodium chloride.

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from article Learning

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from article Learning

Brain activity can be recorded in various ways, from single-cell activity (top tracing) to the familiar electrical patterns ("brain waves") of an electroencephalogram (bottom tracing). (Credit: Based on an original drawing by C. Cotman and J.…

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from article Ultracentrifuge

Diagram of an electrically driven analytical ultracentrifuge. (Beckman Instruments)

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from article Ultracentrifuge

Analytical rotor. (Beckman Instruments)

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from article Ultracentrifuge

Fixed-angle preparative rotor and tubes. (Beckman Instruments)

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from article Biofilm

Biofilms are highly structured communities of microorganisms that are surface-associated or attached to one another, and are enclosed within a self-produced protective extracellular matrix. Biofilm formation and actions involve a number of stages:…

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from article Ctenothrissiformes

A Cretaceous ctenothrissiform fossil, Ctenothrissa sp. (order Ctenothrissiformes), on a limestone matrix found in Lebanon. Size: 90 mm (3.5 in.). (Photo courtesy of The Virtual Fossil Museum)

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from article Lung

Human lungs. (a) The trachea bifurcates as the right and left primary bronchi that enter each lung. The bronchi subdivide further to form the bronchial tree. (b) Color-coded view of the major branches of the bronchial tree. (Copyright © McGraw Hill)

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from article Animal testing

Mice are the most commonly used vertebrate species in animal testing and research. (Credit: Dolores Bonaparte)

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from article Animal testing

The use of animals in biomedical research remains essential, but the handling of these sentient beings must be done humanely to minimize and prevent animal harm. (Credit: U.S. Department of Agriculture)

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from article Circadian clock (plants)

Three-module circadian clock system.

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from article Circadian clock (plants)

Model of the Arabidopsis circadian clock, showing light input (bold arrow), the central oscillator (large box) and output/response (tinted arrows). Light signals synchronize the clock via the cryptochromes (cry) and phytochromes (phy). These are…

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from article Acari

Parasitiformes body plan. Lateral view of a predatory adult female Mesostigmata showing major body regions and important structures. Scale bar = 0.1 mm (0.004 in.). (Scanning electron micrograph by D. E. Walter)

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from article Acari

Acariformes body plan. (a) Adult female Oribatida (Sarcoptiformes) in lateral view showing major body regions and important structures. (b) Hexapod larva (Trombidiformes): dorsal view of a chigger, a parasite of vertebrates. (c) Octopod deutonymph…

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from article Anthrax

A color-enhanced scanning electron micrograph showing splenic tissue from a monkey with inhalation anthrax. Rod-shaped bacilli of Bacillus anthracis (yellow) and an erythrocyte (red) are shown. (Credit: Arthur Friedlander/National Institute of…

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from article Anthrax

Micrograph (stained with hematoxylin and eosin) showing submucosal hemorrhage in the small intestine of a person fatally affected by gastrointestinal anthrax. (Credit: Marshal Fox/Centers for Disease Control and Prevention)

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from article Neuron

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from article Triple point

Phase diagram for water, showing gas, liquid, and several solid (ice) phases; triple points at A, B, and C. The pressure scale changes at 101 kilopascals from logarithmic scale at low pressure to linear at high pressure.

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from article Mutagens and carcinogens

Arsenic is a known carcinogen. In this graphical example, more than half of the mice exposed to arsenic in their drinking water [at concentration levels of 50 and 500 parts per billion (ppb)] showed increased incidences of lung tumors (compared to a…

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from article Mutagens and carcinogens

Proflavin is a chemical disinfectant agent against bacteria, causing mutagenic DNA changes (such as the insertion or deletion of a DNA base pair). However, it is carcinogenic in mammals. Thus, it is used only for the disinfecting of surfaces or for…

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from article Mutagens and carcinogens

Model of carcinogenesis. Changes often occur in the growth properties of normal cells, leading to cancer cells that are not responsive to normal regulatory controls and can eventually give rise to a visible neoplasm or tumor. The main stages of the…

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from article Autophagy

Modes of autophagy in yeast and mammalian cells. Macroautophagy involves the uptake of cytoplasm at a site away from the lysosome or vacuole. The origin of the sequestering membrane is not known, but it likely includes multiple sources such as the…

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from article Autophagy

Basic steps involved in macroautophagy, shown with the structural intermediates. See the text for full details.

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from article Autophagy

The cytoplasm-to-vacuole targeting (Cvt) pathway is an example of selective autophagy. The schematic illustrates the steps of the Cvt pathway and also shows electron micrographs of the sequestration event. Precursor aminopeptidase I (prApe1) forms a…

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from article Autophagy

Atg8 tagged with white fluorescent protein (GFP-Atg8) can be used to monitor autophagosome formation. Cells in growing conditions (for example, in the presence of nitrogen; +N) carry out a basal level of macroautophagy and typically display a single…

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from article Macrocyclic compound

Macrocyclic structures. (a) Two crown ethers. (b) A basic structure for three cryptands. (c) A lariat ether. (d) All-aliphatic spherand. (e) All-aromatic spherand.

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from article Macrocyclic compound

Macrocycles with various structural features. (a) Bicyclic cryptand (ditopic receptor). (b) Calix[6]arene; R represents a functional group. (c) Cavitand. (d) Carcerand. (e) Cyclophanes.

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from article Macrocyclic compound

Tetramers. (a) Resorcinarene having the rctt conformation. (b) Pyrogallolarene having the rccc conformation.

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from article Macrocyclic compound

The metal cation is the guest and the organic structure is the host in (a) 18-crown-6 complex of potassium cation (K+) and (b) sodium (Na+) complexed by a calix[4]arene.

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from article Macrocyclic compound

Complexing species. (a) Dibinaphtho-22-crown-6. (b) R,R-dibinaphtho-22-crown-6 complex of phenethylammonium chloride.

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from article Macrocyclic compound

Affinities for anions and cations shown for (a) pyrrole and (b) furan-derived cyclic tetramers.

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from article Macrocyclic compound

Amphiphiles. (a) Crown ether-steroid amphiphile. (b) Crown ether bolaamphiphile.

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from article Cell cycle

Phases of the eukaryotic cell cycle. Interphase is subdivided into three phases: gap phase 1 (G1), synthesis (S), and gap phase 2 (G2). M phase, which follows interphase, consists of mitosis (nuclear division) and cytokinesis (cell division). G0…

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from article Cell cycle

During cytokinesis (cell division), the cell pinches into two parts between the separated daughter nuclei. At the end of this process, the two daughter cells often remain tethered by a thin "midbody" of microtubules. Once this midbody breaks, the…

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from article Cell cycle

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from article Pituitary gland

The pituitary, also known as the hypophysis, is a pea-sized gland situated at the base of the brain. The hypothalamus is designed and anatomically positioned to receive a diversity of messages from external and internal sources that can be…

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from article Pituitary gland

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from article Salicylate

Chemical structures of (a) salicylate, the conjugate base of (b) salicylic acid, and the nonsteroidal anti-inflammatory drug (NSAID) (c) acetylsalicylic acid (aspirin).

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from article Meningococcus

A photomicrograph reveals the presence of the bacterium Neisseria meningitidis, which can be pathogenic, leading to meningococcal meningitis. (Credit: James Volk/Centers for Disease Control and Prevention)

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from article Marcus equation

Free-energy plot for an electron-transfer reaction with zero standard free-energy change (an electron self-exchange reaction).

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from article Marcus equation

Free-energy plot for an electron-transfer reaction when the free-energy change (|Δ G °|) is less than the reorganization parameter (λ).

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from article Haplosporida

Spore of Urosporidium fuliginosum.

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from article Haplosporida

Stages in the development of Icthyosporidium giganteum.

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from article Lead

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from article Lead

Galena (lead sulfide, PbS) is an ore mined for lead. (Credit: USGS)

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from article Oxygen toxicity

The dual effect of oxygen on biologic activity. Organisms that avail themselves of the enormous metabolic advantages provided by dioxygen (O2) must defend themselves against its toxicity. (Copyright © McGraw Hill)

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from article Gibberellin

Carbon skeleton of ent-gibberellane showing the carbon numbering system and ring designations. (Credit: Peter Hedden)

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from article Gibberellin

The GA-biosynthetic pathway in higher plants via 2-methylerythritol-4-phosphate (MEP). The biologically active end products of the pathway are GA1, GA3, and GA4. The general diterpene precursor trans-geranylgeranyldiphosphate (GGPP) is formed in…

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from article Gibberellin

Mechanisms for GA inactivation. C19-GAs and C20-GAs may be oxidized on C-2 by different classes of GA 2-oxidases (GA2ox). In rice, the enzyme ELONGATED UPPER INTERNODE (EUI) oxidizes the 16,17-double bond of non–13-hydroxylated GAs, such as GA4, to…

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from article Gibberellin

Illustration of a DELLA protein showing conserved domains, and a model for DELLA degradation via the SCF E3 ubiquitin ligase, which consists of a DELLA-specific F-box, Skp1, Cullin1 (CUL), and Rbx1 subunits. Binding of the GID1-GA complex to DELLAs…

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from article Gibberellin

Model for GA-induced production of α-amylase in the aleurone of germinating cereal grains. GAs produced in the scutellum diffuse to the aleurone cells, where they stimulate transcription, processing, and secretion of α-amylase and other hydrolytic…

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from article Urodela

A typical member of the order Urodela: the longtail salamander (Eurycea longicauda). (Credit: Ryan Hagerty/U.S. Fish and Wildlife Service)

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from article Urodela

Southern dwarf siren (Pseudobranchus axanthus), which has undergone limb degeneration and retains only tiny forelimbs. (Credit: U.S. Geological Survey)

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from article Urodela

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from article Urodela

California tiger salamander (Ambystoma californiense). (Credit: John Cleckler/U.S. Fish and Wildlife Service)

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from article Urodela

The two-toed amphiuma salamander (Amphiuma means). (Credit: Southeast Ecological Center, U.S. Geological Survey)

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from article Urodela

The northern red salamander (Pseudotriton ruber ruber) is a member of the family Plethodontidae. (Credit: Alicia Lafever/U.S. National Park Service)

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from article Spinal cord disorders

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from article Thirst and sodium appetite

Regulation of cellular water. ADH = antidiuretic hormone.

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from article Thirst and sodium appetite

Hypovolemic thirst and sodium appetite are caused by altered sensory information from an underfilled circulation, reinforced by increases in circulating angiotensin II and mineralocorticoids. Angiotensin peptides generated from components of a…

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from article Mutation

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from article Mutation

The karyotype (complement of chromosomes) characteristic of an individual with Down syndrome. Note that there are three copies of chromosome 21; this mutation is known as trisomy 21. (Copyright © McGraw-Hill Education)

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from article Mutation

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from article Antibody

Computer-generated models of (a) an antibody molecule and (b) a metalloantibody molecule. The amino acid side chains have been omitted for clarity. In panel (a), blue indicates the constant regions of light chains and heavy chains; white, the…

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from article Antibody

Typical antibody molecule. There are two H and two L chains, held together by disulfide bonds, and the N-terminal variable domains are shown in dark color. The molecule is bilaterally symmetrical, so two identical epitopes are bound. The…

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from article Antibody

Domain structure of three typical light (L) and heavy (H) chains shows their similarities and differences. The amino- and carboxy-terminals of the chains are marked N and C, respectively. The flexible hinge region is also shown. At the N-terminals,…

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from article Chamois

Alpine chamois (Rupicapra rupicapra). (Photograph courtesy of Brent Huffman/Ultimate Ungulate Images)

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from article Bioassay

In conducting bioassays, researchers often employ laboratory culture tests, which use, for example, bacteria cultured in a petri dish (usually to model an effect on the organism of interest or on some basic cellular function). (Credit:…

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from article Surfactant

Foam blankets typically contain less than 1% surfactant to produce a stable foam that blocks oxygen for fire suppression. (Credit: U.S. Navy)

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from article Amniota

Basic types of vertebrate eggs.

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from article Amniota

Embryo developing in amniote eggs.

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from article Human genetics

A genetic map is a type of chromosome map that shows the relative locations of genes and other important features. The map is based on the idea of linkage (that is, the closer that two genes are to each other on the chromosome, the greater is the…

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from article Human genetics

Digital illustration of the human karyotype. The karyotype is the complete set of chromosomes in an organism. (Credit: Science Photo Library/Alamy Stock Photo)

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from article Human genetics

The double-helix structure of a DNA molecule. (Credit: Darryl Leja/National Human Genome Research Institute)

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from article Human genetics

Simplified diagram showing (a) the structure of a gene, (b) the mature messenger RNA (mRNA) after the two intervening sequences have been spliced out and the “cap” and “tail” added, and (c) the polypeptide translated from the mRNA. (Copyright ©…

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from article Human genetics

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from article Human genetics

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from article Human genetics

Typical pedigree pattern of a rare autosomal-dominant trait: classical achondroplastic dwarfism. Individuals with this anomaly, caused by a defect in bone growth, have extremely short arms and legs, but are otherwise normal. I–IV are generations;…

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from article Human genetics

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from article Human genetics

Typical pedigree patterns of a sex-linked recessive trait: hemophilia A. (a) Only males are affected and usually come from unaffected mothers, who often have affected fathers or brothers. (b) An affected female can result from an affected father and…

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from article Mecoptera

Male scorpion fly (Panorpa), order Mecoptera.

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from article Matrix isolation

Vacuum-vessel base cross section for matrix photoionization experiments. 10 K = −263°C or −442°F.

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from article Matrix isolation

Schematic plan view of the matrix assembly for pulsed-laser ablation.

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from article Matrix isolation

Infrared and Raman spectra of lithium superoxide, Li+O2−, using lithium-7 and 30% 18O2, 50% 16O18O, and 20% 16O2 at concentrations of Ar/O2 = 100. The Raman spectrum was recorded by using 200 mW of 488-nm excitation at the sample.

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from article Matrix isolation

Visible and ultraviolet absorption spectra of naphthalene samples in solid argon at 20 K. Upper curves show Ar/CCl4/N = 3000/4/1, and lower curves show the same sample after 15 s of irradiation at 220–1000 nm from a high-pressure mercury arc.

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from article Matrix isolation

Infrared spectra in the 2000–1200 cm−1 region for the products of the reactions of laser-ablated Al atoms with pure hydrogen co-deposited at 3.5 K. (a) Spectrum obtained from initial deposited sample. (b) Spectrum after >290 nm irradiation. (c)…

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from article Developmental genetics