Houghton Mifflin Trade and Reference DivisionHoughton MifflinHoughton Mifflin Trade and Reference Division

Detailed Search


Press Release


The American HeritageŽ Student Science Dictionary

G - L

gam·ete See reproductive cell.
Listen to Pronunciation

gene A segment of DNA, occupying a specific place on a chromosome, that is the basic unit of heredity. Genes act by directing the synthesis of proteins, which are the main components of cells and are the catalysts of all cellular processes. Physical traits, such as the shape of a plant leaf, the coloration of an animal's coat, and the texture of a person's hair, are all determined by genes. See also dominant, recessive. See Notes at DNA, Mendel.

Did You Know?
What makes a human different from a chimpanzee? Much of the answer lies in the genes, the basic units of heredity. Each gene is a specific segment of DNA, occupying a fixed place on a chromosome. Genes contain the chemical information needed to create different kinds of proteins. These proteins are used to repair cells and make new ones. The kinds of proteins, the amounts, and the order in which they are made all help determine how one type of cell differs from another and, ultimately, how one species of organism differs from another. Just how different are the genes making up different life forms? In the case of the human and the chimp, not much: about 98 percent of the DNA in a chimpanzee cell is identical to the DNA in a human cell. Because of this close similarity, scientists think that it is the sequence of genes, as well as the types of genes themselves, that account for most of the differences between the two species. However, not all differences between species can be explained by gene differences alone. How closely matching sets of genes can belong to entirely different species is one of the great mysteries of modern biology.

Listen to Pronunciation

grav·i·ta·tion The force of attraction that tends to draw together any two objects in the universe. Gravitation increases as the mass of the objects increases and as their distance from each other decreases.
Listen to Pronunciation

green·house effect The trapping of the sun's radiation in the Earth's atmosphere due to the presence of greenhouse gases.

gym·no·sperm Any of a group of plants that produce seeds that are not enclosed in a fruit or ovary. Most gymnosperms are cone-bearing trees or shrubs. Seeds develop next to the inside surface of the scales of female cones. Gymnosperms include the conifers, the cycads, and the ginkgo. Compare angiosperm.
Listen to Pronunciation

half-life The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.

he·mo·glo·bin An iron-containing protein in the blood of many animals that, in vertebrates, carries oxygen from the lungs to the tissues of the body and carries carbon dioxide from the tissues to the lungs. Hemoglobin is contained in the red blood cells of vertebrates and gives these cells their characteristic color. Hemoglobin is also found in many invertebrates, where it circulates freely in the blood. See Note at red blood cell.
Listen to Pronunciation

hor·mone A substance produced in one part of the body, especially in an endocrine gland, that has an effect on another part of the body, to which it is usually carried in the bloodstream. Hormones regulate many biological processes, including growth and metabolism.

Did You Know?
On the inside, humans are bathing in a sea of hormones, chemical compounds that regulate many essential activities in the body. A lot of hormones are produced in glands known as endocrine glands, such as the thyroid gland, pancreas, and ovaries, and travel from there through the bloodstream before arriving at their target sites of action. Specialized cells of the nervous system also produce hormones. Hormones are not found only in humans, but also in all other animals and plants. The variety of different functions hormones have is astounding. Insulin, secreted by the pancreas, regulates the absorption of sugars in the body. Thyroid hormones regulate the rate of cell metabolism and affect many other processes, including reproduction. Growth hormone, secreted by the pituitary gland, controls growth of the body. Estrogen and testosterone control sexual development. Some of the hormones released in the brain, known as endorphins, act as natural painkillers. When the amounts of these or other hormones are abnormal, disease can result. Too little insulin causes diabetes, too little estrogen weakens the bones of older women, and too much growth hormone causes people to grow without stopping. Fortunately, these diseases can usually be treated, either with hormones made artificially in laboratories or by operating on the affected gland.

Listen to Pronunciation

hy·dro·car·bon Any of numerous organic compounds, such as benzene, that contain only carbon and hydrogen.
Listen to Pronunciation

indicator species A species whose presence, absence, or relative well-being in a given environment is a sign of the overall health of its ecosystem. By monitoring the condition and behavior of an indicator species, scientists can determine how changes in the environment are likely to affect other species that are more difficult to study.
Listen to Pronunciation

in·duc·tion 1a. The process of deriving general principles from particular facts or instances. b. A conclusion reached by this process. See Note at deduction. 2a. The generation of an electric current in a conductor, such as a copper wire, by moving the conductor through a magnetic field or by moving or varying a magnetic field that already affects the conductor. b. The generation of an electric current in a conductor, such as a copper wire, by exposing it to the electric field of an electrically charged conductor. See magnetic induction.

Listen to Pronunciation

in·er·tia The tendency of a body at rest to remain at rest, or of a body in motion to continue moving in a straight line at a constant speed unless a force is applied to it. Mass is a measure of a body's inertia.
Listen to Pronunciation

in·ver·te·brate Adjective Having no backbone or spinal column. Noun An animal, such as a coral, insect, or worm, that has no backbone. Most animals are invertebrates.
Listen to Pronunciation

joule A unit used to measure energy or work. One joule is equal to the work done when a force of one newton acts over a distance of one meter.
Listen to Pronunciation

ki·net·ic energy The energy possessed by a body as a result of being in motion. Kinetic energy is dependent upon the mass and velocity of the object. Compare potential energy.
Listen to Pronunciation

Krebs cycle A series of chemical reactions in most aerobic organisms in which cells break down glucose and other molecules in the presence of oxygen to produce carbon dioxide and energy in the form of ATP. The Krebs cycle occurs in the mitochondria of all organisms except bacteria. Also called citric acid cycle.
Listen to Pronunciation

light-year The distance that light travels in a vacuum in one year, equal to about 5.88 trillion miles (9.48 trillion kilometers).

Usage
It is important to remember that a light-year is a measure of distance, not time. A light-year is the length of empty space that light can traverse in a year, close to six trillion miles. When scientists calculate how many light-years the stars are from Earth or from one another, they are calculating their distance, not their age.


lith·o·sphere The outer part of the Earth, consisting of the crust and upper mantle. It is approximately 62 miles (100 kilometers) thick. Compare asthenosphere, atmosphere, hydrosphere.
Listen to Pronunciation




Some of the words on this list include cross-references to entries in the dictionary that are not included in the list itself.




Booksellers Home | Trade Home | FAQ | Site Map
Privacy Policy | Trademark Information | Terms and Conditions of Use
Copyright © Houghton Mifflin Company. All rights reserved.