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Science and Medical Dictionary and Reference

© Donald Reinhardt,2011
Dictionary for Science – A to Z – The "A" Definition and Reference Words
© Donald Reinhardt,2011
Specific Antibodies Recognize and React with Specific Antigens. Figure Credit: Jeanne Kelly, NIAID of NIH.


abiotic – non-living factors or components of biological or ecosystems. Water, minerals, oxygen and nitrogen gasses in air and temperature are examples of abiotic factors.

absolute temperature - developed by Lord Kelvin to express a condition where there is no molecular movement and the lowest temperature possible for matter (absolute zero = minus 273 Centigrade degrees). Studies in physics demonstrate that the volume of any gas maintained at a constant pressure is directly proportional to the absolute temperature. This is Charles' law and is expressed as V1/V2 = T1/T2. Boyle and Boyle's law show that gasses held at constant temperatures will vary in volume inverselyaccording to the pressure. Thus, more pressure equals less volume of a gas.
Boyle's law V1/V2 = P2/P1. Boyle's and Charle's laws have been combined into a single general gas law and formula as follows: P1V1/T1 = P2V2/T2

absolute zero – measured in degrees Kelvin and at absolute zero (the value is O Kelvin) there is no molecular movement and it is the lowest temperature possible for matter.  O Kelvin degree is equal to minus (-) 273 Centigrade degrees and minus (-) 460 Fahrenheit degrees.

acceleration – an upward change in the speed of an object or matter. Deceleration is a reduction in speed. Speed measurements often expressed as meters/second, km/second, feet/sec, miles/hr, etc. 

accretion disk – a rotating, disk-shaped mass in space formed by the force of gravity.

achromatic doublet, achromatic lens – a combination of two different glass lenses manufactured to eliminate color or chromatic aberration in microscopes and telescopes. 

acetylcholine (ACh) –
 is a neurotransmitter, a biochemical produced in neurons and released at the axon, which is active in the brain, spinal cord, muscles and glands. Acetylcholine is released at the ends of axons and can be captured by the dendrites of other neurons at the synapse region or directly bind to the cell membranes of muscles and glands. ACh can cause skeletal muscle contraction. ACh can slow the heart's rate of contraction, i.e., the heart beat.  

The enzyme acetylcholinesterase inactivates excess, unbound, or unused acetylcholine. 

activation energy – the minimum amount of energy needed to cause a reaction to occur. Heat may cause a reaction to be activated or a chemical additive such as ATP (see definition below) may cause a reaction to occur.

active transport – a process that requires energy in the form of ATP and that is assisted by proteins or enzymes. The movement of certain sugars and amino acids into a cell or the pumping out of excess sodium from a cell are examples of active transport. Example: glucose + ATP + hexokinase –> Gluose-6-phosphate + ADP = hexokinase.
Passive transport involves protein-assisted transfers that do not require ATP and in diffusion of water (osmosis) there is a free and unrestricted passage of water into and out of cells.

adenine – one of the four nitrogen bases which are found in DNA, RNA and certain compounds called nucleotides. 

Other nitrogen bases include thymine, guanine, cytosine and uracil. (Note:uracil is paired with adenine in RNA and thymine is paired with adenine in DNA). 

Base pairings in DNA are adenine to thymine and guanine with cytosine. In RNA the base pairings, when they occur, are adenine to uracil and guanine to cytosine.

adiabatic – describes the process of gas compression or expansion with no (or minimal) heat gain or loss. The adiabatic process follows the general gas law which says that the pressure (P) of a given quantity of gas (V= volume) is proportional to the absolute temperature (Kelvin degrees) and inversely proportional to the volume, i.e. PV/T= a constant value or constant factor. In an adiabatic process is the quanitity of heat in the gas remains unchanged during the process.  

aerate - to provide oxygen or air to an area, region or substance. 
Examples: the soil was aerated when it was plowed and turned over. The pump delivered air to the fish tank via tubing and a bubbler stone and as the air flowed out it oxygenated the water. A stream of air and oxygen flowing over the iron (Fe) rod soon turned it into rust (Fe2O3).

aerobe – an organism or cell that requires oxygen. Humans are aerobes. We require oxygen

aerobic – a condition, event, circumstance that uses or requires oxygen. Aerobic metabolism generates lots more energy in the form of ATP than anaerobic metabolism. Anaerobic metabolism does not require oxygen. The fermentation of sugar is an anaerobic event. Humans are aerobic organisms.

alga – a photosynthetic, chlorophyll a -containing, unicellular, or multicellular, organism that has a true nucleus (eukaryotic) and is a member of the kingdom called Protista. There are several classes of algae that include: diatoms (silica-walled algae), dinoflagellates (includes: photosynthetic and chemo-autotrophs or chemoheterotrophs), green, brown and red algae.

allele – is an alternate form of the same gene. This means that the same gene site may have variations or different types. For example, A, B and O are alleles of the same gene or site on the DNAS of humans. Therefore, the gene may be an A, B or O and since humnans have a pair of chromosomes (paternal, from the father and maternal, from the mother) a person may be of the following blood phenotypes: A (AO or AA genotype), B (BO or BB genotype) or O (OO gneotype is the only possibility).

allergens – are biochemicals or chemical substances that induce specific cell and host responses in vertebrate animals. Typically the allergen is a specific protein, carbohydrate that causes a special immune (allergic) response of mast cells to release histamine in the body. Histamine causes redness,swelling, sneezing, coughing and related signs and symptoms. Some people have no or few allergies, whereas other people have many allergies. Common allergies include those to: pollen, animal hair and skin, posin ivy, oak or sumac. Some people are allergic to peanut proteins or wheat glutens. An allergist does specific tests to determine a person's sensitivities to various allergens and recommends specific medications (antihistamines) and procedures (possible desensitization) to control these allergies.

alternation of generations
 – this is the concept and facts  of specific plant and algal types and phases where there is a gametophyte and sporophyte stage in the organism's life cycle. In the plant section this alternation of generations is clearly diagrammed and explained for the bryophytes (mosses and liverworts), ferns, gymmnosperms and angiospserms.

alveolus – a tiny or small sac in the terminal bronchioles of the lungs which permits exchanges of oxygen molecules and waste carbon dioxide molecules. There are millions of these alveoli in the lungs that make possible the oxygenation of tissues by red blood cells that pick up and transport oxygen to cells throughout the body. 

amniocentesis – a process which involves the use of a sterile needle to withdraw fluid and cells from a fetus' amniotic fluid that in utero. Amniocentesis allows genetic and biochemical analyses to be done to determine the sex of the baby, as well as normal and abnormal genes and biochemical features.
Amphibia (anphibians) – a class of vertebrate animals that live and inhabitat both land (terrestrial) and water (aquatic) environments for part of their lives. Amphibians include animals such as frogs, toads, newts, salamanders, skinks.

anabolic steroids  – synthetic variants of testosterone that have valid medical uses, but which may be used illegally by athletes to increase strength and body mass. Many sports prohibit the use of anabolic steroids since they usually give an unfair, competitive advantage to those who use them over those who do not. Futher, medical evidence clearly indicates that use of anabolic steroids can lead to harmful metabolic disorders such as diabetes, thyroiditis and certain types of cancer. 

anaerobe – a cell or organism that does not require oxygen to live, grow or metabolize. Some bacteria, like Clostridium, are anaerobes. The muscles of animal can ferment sugar in the absence of oxygen, but oxygen is needed to restore the muscle cells to normal aerobic metabolism.

anaerobic – means a condition or situation of no oxygen or the absence of oxygen. Anaerobic metabolism does not require oxygen. The fermentation of sugar is an anaerobic event. Humans are aerobic organisms.

antibody – a unique type of large protein that is produced by specific human white blood cells (leukocytes) called B- lymphocytes. Antibodies are able to bind with certain kinds of chemical groups (epitopes of the antigen) and capture, neutralize or inactivate that antigen and protect the body from that specific factor. 
Vaccines, both living and dead as well as specific compounds or extracts, are able to cause a host or animal response and cause protective antibodies to be formed. Vaccine examples include: polio, tuberculosis, measles, mumps, chickenpox, influenza, hepatitis A and B as a few examples of the many available, protective vaccines available. Finally, there are different classes of antibodies. In humans and other mammals there are five different kinds: IgG, IgM, IgA, IgD and IgE. 

antigen – a chemical compound or substance – usually a protein, polysaccaharide (carbohydrate) – that is able to cause (elicit) an antibody response in a vertebrate (spinal-corded) animal. Examples of foreign antigens include include organisms or parts of organisms that gain entrance in or onto the body of the animal and are foreign to the animal such as bacteria, viruses or molds. These foreign organism contain factors that are strange, unusuall or non-self, therefore the animal reacts with specific cells (white blood cells, leukocytes) to protect itself against the invaders. During this counterattack the leukocytes interact and specialized cells begin to produce antiboy to tie up (i.e. bind, neutralize, lyse or inactivate) the foreign antigen. These are the classic antigen-antiboy reactions.

aorta – the main artery of the body which carries oxygenated blood (pumped outward by the heart) directly to other arteries that supply the general body circulation.  

aortic semilunar valve – heart valve that regulates blood flow. 

argon - one of the noble elements that exists mainly as a gas. The noble elements are generally non-reactive elements compared to many other elements. The noble elements include argon, neon, xenon.

arrythmia – a condition or state the heart not beating normally as it relaxes and contracts. There is a natural pacemaker site called the SA (sinoatrial) node located in the right atrium of the heart that helps regulate the heart beat or heart rate. The SA node has specialized nerve and muscle cells which convey electric impulses within the heart to make it work well and have a strong, normal and healthy contractions.   Arrythmias include atrial and ventricular fibrillation, and chronic bradycardia (slow heart rate) and tachycardia (fast heart rate). Arrythmias are related to electrical conduction problems in the heart. Artificial pacemakers are used for humans to set and establish normal heart rhythms by electrical, battery signals whenever an arrthymia starts or occurs.

 – used as an adjective to describe any process that does not involve the formation of sex cells (gametes). An example ofasexual nuclear division is called mitosis a process that involves doubling the number of a cell's chromosomes and then distributing them equally between two cells. Examples of asexual reproduction include binary fission (splitting of one cell into two cells) or the formation of buds from yeast cells or vegetative plant buds.

asteroids – are masses of rock, as large as 1,000 km in diameter, which are found mainly in an asteroid beltlocated between the planets Mars and Jupiter. Meteroids are of two types: 1. fragments of asteroids, 2. fragments of comet dust tails. If some of these fragments (meteroids) contact the earth's atmosphere they become heated and bright and are called meteors or meteor showers

astronomy – the study of the universe and all it components including stars (suns), planets, moons, asteroids, meteors, meteorites,  black holes.
astronaut - a person who travels into outer space via a rocket and an attached spaceship and then lives and operates equipment in outer space to study and understand  biology, chemistry, medicine and physics in weightless or specific outer space environments such as the moon.

atom - the smallest part of an element that retains or has the properties of that element. Each atom has a distinct nucleus and surrounding orbit(s) of one or more electrons. For example, an atom of hydrogen has a single proton in the nucleus and one orbiting electron. Deuterium is an isotope of hydrogen which contains a neutron and a proton in the nucleus.
Water Molecule Formed by  2Atoms of Hydrogen and 1 Oxygen
Diagram Credit: RST of NASA

atomic mass or atomic weight - the amount of material in the nucleus of an atom. Examples: hydrogen has a atomic mass of 1, deuterium, the isotope of hydrogen, has a mass of 2 (1 proton and 1 neutron), oxygen as a weight of 16 (8 protons and 8 neutrons) and uranium has a weight of 235 (92 protons and 143 neutrons). 

atomic number - the number of protons in the nucleus of atoms. Each element has a distinctive characteristic atomic number: hydrogen = 1, helium = 2, carbon = 6, nitrogen = 7, oxygen = 8.

atomic reaction - the interaction of two or more atoms which generates energy.

atomic reactor - a special container or reaction vessel that permits atomic reactions and the generation and capture of energy.

ATP – an energy molecule that is produced by cells during certain biochemical processes. ATP enables cells to do work and create cellular and organelle movement. ATP is known as adenosine triphosphate and it is a molecule composed of  the nitrogen adenine coupled to the sugar ribose and three phosphates. When ATP is converted to ADP one of the phosphates is lost and energy is released to do work or trapped by another molecule which then becomes energized. Example: glucose + ATP –> glucose-PO4 + ADP

atrium (auricle) – portion of the heart that receives blood and channels it to the ventricles. In the human heart there are two atria and two ventricles. At diastole (relaxed muscle phase of the heart) the right atrium receives blood from the the venae cavae (blood returns here from general body circulation) and the left atrium receives blood from the pulmonary veins.

autoimmune disease – a condition of the body where specific cells and antibodies begin to attack the cells, tissues, organs of the very organism they normally would protect. Lupus, Crohn's and Hashimoto's diseases are three significant autoimmune diseases of humans. Lupus involves a widespread and general attack in multiple organs, Crohn's is autoimmunity of the intestines and Hashimoto's disease involves attack against the thyroid gland.

autotrophic nutrition – photosynthetic and chemosynthetic
metabolism involves the use of inorganic carbon (typically, CO2 or carbon dioxide) as the carbon source. Green plants are autotrophs. Certain specific types of bacteria, such as photoautrophs and chemolithophs, are autotrophs.

autotroph – an organism that uses carbon dioxide (CO2) or inorganic carbon as a source of carbon and light or inorganic molecules (such as H2, N2, NO3-, N02-) as a source of energy. See above for further information on autotrophic nutrition. Heterotrophs and heterotrophic nutrition involves the use organic carbon as a source of carbon and energy. Animals are heterotrophs and green plants are autotrophs,

auxin – a plant growth hormone (biochemical molecule) which promotes cell division and elongation of roots downward into the soil (positive geotropism) and the growth of stems upward and curving toward a natural (sun) or artificial (incandescent or fluorescent) light source (as an example of positive phototropism). Auxins serve to control the rate of growth and the direction of growth as indicated for stems and for roots. One auxin is indole-3-acetic acid which is abbreviated as "IAA". Commercial plant growers uses auxins to promote the growth of roots from stems, shoots and branches of plants. 

Aves (avians) – this is class of animals commonly known as birds. These fascinating animals are warm-blooded, lay eggs and range in size from tiny hummingbirds to giant condors. Some birds are vegetarians or seed-eaters and others are carnivores that eat insects or other animals. Birds of prey include: eagles, hawks, ospreys, owls and other important, predatory species.

axon – a long filament or extension from a nerve cell (neuron) that sends electronic and chemical signals outward to other neurons and their extended receptor filaments called dendrites.. Axons typically are covered with myelin as a protective sheath that insulates and protects the axon and enables the axon to function well. Dendrites which receive signals are unmyelinated filaments. The site or place where the axon cummunicates with a dendrite is the synapse. Notice the neurotransmitter, which is produced within the neuron and packaged in a vesicle, is carried to the end of the axon and released from there to be captured by specific dendrite receptor molecules displayed on the surface of the dendritic membrane. Stimuli such as light, heat, pressure or chemicals cause nerves to receive those signals or cues and respond in a chracteristic fashion and to transmit that information to other nerves. The peripheral and central nervous system of higher animals reacts to and processes these environmental signals.

Two Neurons with Axons and Dendrites and Synapse Region. Nerotransmitter Chemical Release from Axon to Dendrite is Shown. Credit: NIAAA of NIH

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Donald Reinhardt is a Consultant in Medical and Industrial Microbiology and a Freelance Science writer. He is available for specific assignments for those who are interested – by contacting Other questions related to this teaching site should be directed to