Antidiuretic hormone (ADH) Also known as vaso-pressin, ADH is a nine-amino acid peptide secreted from the posterior pituitary gland. The hormone is packaged in secretory vesicles with a carrier protein called neuro-physin within hypothalamic neurons, and both are released upon hormone secretion. The single most important effect of antidiuretic hormone is to conserve body water by reducing the output of urine. It binds to receptors in the distal or collecting tubules of the kidney and promotes reabsorption of water back into the circulation.
The release of ADH is based on plasma osmolarity, the concentration of solutes in the blood. For example, loss of water (e.g., sweating) results in a concentration of blood solutes, so plasma osmolarity increases. Osmoreceptors, neurons in the hypothalamus, stimulate secretion from the neurons that produce ADH. If the plasma osmolarity falls below a certain threshold, the osmoreceptors do nothing and no ADH is released. However, when osmolarity increases above the threshold, the osmoreceptors stimulate the neurons and ADH is released.
Antigen A foreign substance, a macromolecule, that is not indigenous to the host organism and therefore elicits an immune response.
Antimetabolite A structural analog of an intermediate (substrate or coenzyme) in a physiologically occurring metabolic pathway that acts by replacing the natural substrate, thus blocking or diverting the biosynthesis of physiologically important substances.
Antisense molecule An oligonucleotide or analog thereof that is complementary to a segment of RNA (ribonucleic acid) or DNA (deoxyribonucleic acid) and that binds to it and inhibits its normal function.
Aphotic zone The deeper part of the ocean beneath the photic zone, where light does not penetrate sufficiently for photosynthesis to occur.
An antisense molecule is the noncoding strand in double-stranded DNA. The antisense strand serves as the template for mRNA synthesis.
Apical dominance Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth. It is thought to be caused by the apical bud producing a great deal of IAA (auxin), which is transported from the apical bud to the surrounding area and causes lateral buds to stay dormant.
Apical meristem Embryonic plant tissue (meristem-atic cells) in the tips of roots and in the buds of shoots that supplies cells via mitosis for the plant to grow in length.
Apomixis The ability of certain plants to reproduce clones of themselves, i.e., the scaly male fern group, Dryopteris affinis (Lowe) Fraser-Jenkins.
Apomorphic character A phenotypic character, or homology, in which the similarity of characters found in different species is the result of common descent, i.e., the species evolved after a branch diverged from a phylogenetic tree.
Two characters in two taxa are homologues if they are the same as the character that is found in the ancestry of the two taxa or they are different characters that have an ancestor/descendant relationship described as preexisting or novel. The ancestral character is termed the plesiomorphic character, and the descendant character is termed the apomorphic character. Examples are the flippers of whales and human arms. See also phenotype.
Apoplast The cell-wall continuum of an organ or a plant; in a plant it includes the xylem. The movement of substances via cell walls is called apoplastic transport.
Apoprotein A protein without its characteristic prosthetic group or metal.
Apoptosis Cells die by injury or commit "suicide." Apoptosis is a programmed cell death (PCD) brought about by signals that trigger the activation of a flood of "suicide" proteins in the cells destined to die. The destined cells then go through a number of molecular and morphological changes until they finally die. PCD is important in proper development in mitosis and cells that may be threatening to the host organism. It can be induced by a variety of stimuli, such as ligation of cell surface receptors, starvation, growth factor/survival factor deprivation, heat shock, hypoxia, DNA damage, viral infection, and cytotoxic/chemotherapeutical agents. Apoptosis is a word of Greek origin meaning "falling off or dropping off." There is a Web site devoted to the topic at http://www.celldeath.de/main-fram.htm.
Scanning Electron Micrograph (SEM) of human white blood cells (leucocytes) showing one cell undergoing apoptosis. Apoptosis is the process of "genetically programmed cell death." At upper right, an apoptotic white blood cell has shrunk and its cytoplasm has developed blebs (grapelike clusters). Normal white blood cells are seen beside it. These white blood cells are myeloid leucocytes, originating from bone marrow. The human myeloid cell line depends on growth factors to survive, and cells undergo apoptosis when deprived of growth factors. Research on apoptosis may provide genetic treatments for diseases such as cancer. Magnification: x7,500 at 8×10-in. size.
Aposematic coloration The bright coloration of animals with effective physical or chemical defenses that acts as a warning to experienced predators. The larvae of the monarch butterfly and Phymateus morbillosus, a foaming grasshopper from South Africa, are two examples. The warning coloration alerts the predator, who may have eaten a similar-looking animal and was sickened by it, to avoid it. This also helps those species that mimic others in appearance, such as the viceroy butterfly and the monarch butterfly.
A New York tiger moth (Grammia virginiensis) exuding a toxic yellow froth from prothoracic glands. This is an example of lepi-doptera showing chemical defense and aposematic coloration.
Aquaporins (AQPs) The aquaporins are a family of proteins known for facilitating water transport. An aquaporin is a transport protein in the plasma membranes of a plant or animal cell that specifically facilitates the diffusion of water across the membrane (osmosis).
Aquaporin-1, or CHIP-28, discovered in 1992 by Peter Agre, is the major water channel of the red blood cells. In the kidneys, it is involved in the reabsorption of most of the waste filtered through the glomeruli. It is also thought to influence the movement of CO2 across the cell membrane, since it is present in most cells that have high levels of CO2. Aquaporin-2, or WCH-CD, is a water channel that makes the principal cells of the medullary collecting duct in the kidneys more permeable to water. Lack of a functional aquaporin-2 gene leads to a rare form of nephrogenic diabetes insipidus. There are many more aquaporins that have been discovered in the more water-permeable parts of the body, such as the moist surface tissues of the alveoli in the lung, the kidney tubules, the choroid plexus of the brain where cere-brospinal fluid is produced, the ciliary epithelium of the eye where aqueous humor is formed, and the salivary and lacrimal tear glands. Aquaporins are believed to be involved in mechanisms defending against brain edema, congestive heart failure, and many other clinical entities.
Aquation The incorporation of one or more integral molecules of water into another chemical species with or without displacement of one or more atoms or groups.
Aqueous solution A solution in which water is the solvent or dissolving medium, such as salt water, rain, or soda.
Archaea One of two prokaryotic (no nucleus) domains, the other being the Bacteria. Archaeans include organisms that live in some of the most extreme environments on the planet and resemble bacteria. They are single-cell organisms that, with bacteria, are called prokaryotes. Their DNA is not enclosed in a nucleus. Bacteria and archaea are the only prokaryotes; all other life forms are eukaryotes. Archaeans are among the earliest forms of life that appeared on Earth billions of years ago, and it is believed that the archaea and bacteria developed separately from a common ancestor nearly 4 billion years ago.
Some archaeans are "extremophiles," that is, they live near rift vents in the deep sea at temperatures well over 100°C (212°F). Others live in hot springs (such as the hot springs of Yellowstone National Park, where some of archaea were first discovered) or in extremely alkaline or acid waters. They have been found inside the digestive tracts of cows, termites, and marine life, where they produce methane. They also live in the anoxic muds of marshes and at the bottom of the ocean and in petroleum deposits deep underground. They are also quite abundant in the plankton of the open sea and even have been found in the Antarctic. They survive in these harsh conditions by using a variety of protective molecules and enzymes.
Three groups of archaeans are known and include the Crenarchaeota, those that are extremophiles; the Euryarchaeota, methane producers and salt lovers; and the Korarchaeota, an all-inclusive group that contains a number of types that are little understood today.
Archaeans produce energy by feeding on hydrogen gas, carbon dioxide, and sulfur and can even create energy from the sun by using a pigment around the membrane called a bacteriorhodopsin that reacts with light and produces ATP.
The archaeans were not discovered as a separate group until the late 1970s.
Archaezoa This group is believed to be the first to diverge from the prokaryotes. They lack mitochondria (converts foods into usable energy), though some archaezoans have genes for mitochondrial. They also lack an endoplasmic reticulum (important for protein synthesis) and golgi apparatus (important for glycosylation, secretion), have no peroxisomes (use oxygen to carry out catabolic reactions), and have small ribo-somes similar to bacteria.
Archaezoa has three known subgroups: diplomon-ads, microspoidians, and trichomonads. They are usually found with flagellas in moist/damp environments such as streams, lakes, underground water deposits, and in damp soil.
Some members have been found in harsh environments and can exist in bodies of water that can drop below -20° Fahrenheit and around ocean floor vents that exceed 320°F. These organisms can survive in a variety of environments as long as they are in water.
Many archaezoans are parasites and feed off their host. The species Giardia, which causes abdominal cramps and severe diarrhea, uses a ventral suction cup to attach to the human intestinal epithelium. Some species have chloroplasts that allow them to take in light energy and use it when needed. Some species contain hydrogenosomes, organelles that are similar to mitochondria but do not respire with oxygen. They convert pyruvate into acetate, CO2, and H2, allowing extra ATP synthesis without respiration.
Since they have no mitochondria or plastid, it is believed that they are the intermediate stage between prokaryotes and eukaryotes and are also used as evidence for the evolution of the nucleus before the organelles.
Archegonium In plants, the multicellular flask-shaped female gametangium (a moist chamber in which gametes develop in bryophytes, ferns, and gym-nosperms).
Archenteron The endoderm-lined gut (enteron) hollow cavity formed during the gastrulation process in metazoan embryos. The archenteron is formed by the infolding of part of the outer surface of the blastula and opening to the exterior via the blastopore. Also called the primitive gut, or gastrocoel in early embryonic development, it is the digestive cavity. The term is Greek for "primitive intestine."
Archipelago A group or chain of islands clustered in a body of water, e.g., the African Bazaruto Archipela go, consisting of five islands: Bazaruto, Magaruque, Santa Carolina, Benguera (Benguerra), and Bangue.
Aristotle (384 b.c.e.-322 b.c.e.) Greek Philosopher Aristotle, a Greek philosopher and scientist, has had more influence on the field of science than anyone. His influence, which lasted more than 2,000 years, was due to the fact that he was the first to depart from the old Platonic school of thinking by reasoning that accurate observation, description, inductive reasoning, and interpretation was the way to understand the natural world. Since he was the first to use this method, he is often called the "Father of Natural History."
Born in 384 b.c.e. in the Ionian colony of Stagirus (now Macedonia), Aristotle was the son of Nico-machus, a physician and grandfather of Alexander the Great. At 17, he became a student in Plato’s academy in Athens and stayed there for more than 20 years as a student and teacher. In 347 b.c.e., he moved to the princedom of Atarneus in Mysia (northwestern Asia Minor), ruled by Hermias, and who presided over a small circle of Plato followers in the town of Assos. Aristotle befriended Hermias, joined the group, and eventually married Hermias’s niece and adopted daughter Pythias.
Around 342 b.c.e., he moved to Mieza, near the Macedonian capital Pella, to supervise the education of 13-year-old Alexander the Great. Aristotle returned to Athens in 335 b.c.e. to teach, promote research projects, and organize a library in the Lyceum. His school was known as the Peripatetic School. After Alexander’s death in 323 b.c.e., Aristotle was prosecuted and had to leave Athens, leaving his school to Theophrastus. He died shortly after at Chalcis in Euboea in 322 b.c.e.
While his writings were immense, one of his works particularly influenced the field of meteorology for over 2,000 years. Meteorologica (meteorology) was written in 350 b.c.e. and comprised four books, although there are doubts about the authenticity of the last one. They deal mainly with atmospheric phenomena, oceans, meteors and comets, and the fields of astronomy, chemistry, and geography.
Aristotle attempted to explain the atmosphere in a philosophical way and discussed all forms of "meteors," a term then used to explain anything suspended in the atmosphere. Aristotle discussed the philosophical nature of clouds and mist, snow, rain and hail, wind, lightning and thunder, rivers, rainbows, and climatic changes. His ideas posited the existence of four elements (earth, wind, fire, and water), each arranged in separate layers but capable of mingling.
Aristotle’s observations in the biological sciences had some validity, but many of his observations and conclusions regarding weather and climate were wrong, and it was not until the 17th century—with the invention of meteorological instruments such as the hygrometer, thermometer, and barometer—that his ideas were disproved scientifically. However, he correctly reasoned that the earth was a sphere, recorded information regarding the bathymetry of seas, correctly interpreted dolphins and whales as mammals, separated vertebrates into oviparous and viviparous, and described and named many organisms, including crustaceans and worms, mollusks, echinoderms, and fish from the Aegean Sea.
Arrhenius, Svante August (1859-1927) Swedish Chemist, Physicist Svante August Arrhenius was born in Vik (or Wijk), near Uppsala, Sweden, on February 19, 1859. He was the second son of Svante Gustav Arrhenius and Carolina Christina (nee Thunberg). Svante’s father was a surveyor and an administrator of his family’s estate at Vik. In 1860, a year after Arrhenius was born, his family moved to Uppsala, where his father became a supervisor at the university. He was reading by the age of three.
Arrhenius received his early education at the cathedral school in Uppsala, excelling in biology, physics, and mathematics. In 1876, he entered the University of Uppsala and studied physics, chemistry, and mathematics, receiving his B.S. two years later. While he continued graduate classes for three years in physics at Uppsala, his studies were not completed there. Instead, Arrhenius transferred to the Swedish Academy of Sciences in Stockholm in 1881 to work under Erick Edlund to conduct research in the field of electrical theory.
Arrhenius studied electrical conductivity of dilute solutions by passing electric current through a variety of solutions. His research determined that molecules in some of the substances split apart, or dissociated from each other, into two or more ions when they were dissolved in a liquid. He found that while each intact molecule was electrically balanced, the split particles carried a small positive or negative electrical charge when dissolved in water. The charged atoms permitted the passage of electricity, and the electrical current directed the active components toward the electrodes. His thesis on the theory of ionic dissociation was barely accepted by the University of Uppsala in 1884, since the faculty believed that oppositely charged particles could not coexist in solution. He received a grade that prohibited him from being able to teach.
Arrhenius published his theories ("Investigations on the Galvanic Conductivity of Electrolytes") and sent copies of his thesis to a number of leading European scientists. Russian-German chemist Wilhelm Ost-wald, one of the leading European scientists of the day and one of the principal founders of physical chemistry, was impressed and visited him in Uppsala, offering him a teaching position, which he declined. However, Ostwald’s support was enough for Uppsala to give him a lecturing position, which he kept for two years.
The Stockholm Academy of Sciences awarded Arrhenius a traveling scholarship in 1886. As a result, he worked with Ostwald in Riga with physicist Friedrich Kohlrausch at the University of Wurzburg, with physicist Ludwig Boltzmann at the University of Graz, and with chemist Jacobus Van’t Hoff at the University of Amsterdam. In 1889, he formulated his rate equation that is used for many chemical transformations and processes, in which the rate is exponentially related to temperature, known as the "Arrhenius equation."
He returned to Stockholm in 1891 and became a lecturer in physics at Stockholm’s Hogskola (high school) and was appointed physics professor in 1895 and rector in 1897. Arrhenius married Sofia Rudbeck in 1894 and had one son. The marriage lasted a short two years. Arrhenius continued his work on electrolytic dissociation and added the study of osmotic pressure.
In 1896, he made the first quantitative link between changes in carbon dioxide concentration and climate. He calculated the absorption coefficients of carbon dioxide and water based on the emission spectrum of the moon, and he also calculated the amount of total heat absorption and corresponding temperature change in the atmosphere for various concentrations of carbon dioxide. His prediction of a doubling of carbon dioxide from a temperate rise of 5-6°C is close to modern predictions. He predicted that increasing reliance on fossil fuel combustion to drive the world’s increasing industrialization would, in the end, lead to increases in the concentration of CO2 in the atmosphere, thereby giving rise to a warming of the Earth.
In 1900, he published his Textbook of Theoretical Electrochemistry. In 1901 he and others confirmed the Scottish physicist James Clerk Maxwell’s hypothesis that cosmic radiation exerts pressure on particles. Arrhenius went on to use this phenomenon in an effort to explain the aurora borealis and solar corona. He supported the Norwegian physicist Kristian Birkeland’s explanation of the origin of auroras that he proposed in 1896. He also suggested that radiation pressure could carry spores and other living seeds through space and believed that life on earth was brought here under those conditions. He likewise believed that spores might have populated many other planets, resulting in life throughout the universe.
In 1902, he received the Davy Medal of the Royal Society and proposed a theory of immunology. The following year he was awarded the Nobel Prize for chemistry for his work that originally had been perceived as improbable by his uppsala professors. He also published his Textbook of Cosmic Physics.
He became director of the Nobel Institute of Physical Chemistry in Stockholm in 1905 (a post he held until a few months before his death). He married Maria Johansson and had one son and two daughters. The following year he also had time to publish three books, Theories of Chemistry, Immunochemistry, and Worlds in the Making.
He was elected a foreign member of the Royal Society in 1911, the same year he received the Willard Gibbs Medal of the American Chemical Society. Three years later he was awarded the Faraday Medal of the British Chemical Society. He was also a member of the Swedish Academy of Sciences and the German Chemical Society.
During the latter part of his life his interests included the chemistry of living matter and astrophysics, especially the origins and fate of stars and planets. He continued to write books such as Smallpox and Its Combating (1913), Destiny of the Stars (1915), Quantitative Laws in Biological Chemistry (1915), and Chemistry and Modern Life (1919). He also received honorary degrees from the universities of Birmingham, Edinburgh, Heidelberg, and Leipzig and from Oxford and Cambridge Universities. He died in Stockholm on October 2, 1927, after a brief illness, and is buried at uppsala.