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habitat place where an organism lives out its life. hair follicle narrow cavities in the dermis from which hair grows. half-life halogen The time required for one-half of a radioisotope’s nuclei to decay into its products. A highly reactive group 7A element. haploid cell with one of each kind of chromosome; is said to contain a haploid or n, number of chromosomes. hardness Measure of how easily a mineral can be scratched, which is determined by the arrangement of a mineral’s atoms. harmonics Higher frequencies, which are odd-numbered multiples of the fundamental frequency; give certain musical instruments their own unique timbre. heat A form of energy that flows from a warmer object to a cooler object. hemoglobin iron-containing protein molecule in red blood cells that binds to oxygen and carries it from the lungs to the body’s cells. heredity heterogeneous mixture passing on of characteristics from parents to offspring. One that does not have a uniform composition and in which the individual substances remain distinct. heterotrophs organisms that cannot make their own food and must feed on other organisms for energy and nutrients. hibernation state of reduced metabolism occurring in animals that sleep during parts of cold winter months; an animal’s temperature drops, oxygen consumption decreases, and breathing rate declines. highlands Light-colored, mountainous, heavily cratered areas of the Moon, composed mostly of lunar breccias. homeostasis organism’s regulation of its internal environment to maintain conditions suitable for survival; a characteristic of all living things. hominids Group of upright, bipedal primates, including Homo sapiens. Homo sapiens Species to which humans belong. homogeneous mixture One that has a uniform composition throughout and always has a single phase; also called a solution. homozygous hormone host cell when there are two identical alleles for a trait. chemical produced in one part of an organism and transported to another part, where it causes a physiological change. living cell in which a virus replicates. hot spot Unusually hot area in Earth’s mantle that is stationary for long periods of time, where high-temperature plumes of mantle material rise toward the surface. hot spring Thermal spring with temperatures higher than that of the human body. human genome map of the approximately 80 000 genes on 46 human chromosomes that when mapped and sequenced, may provide information on the treatment or cure of genetic disorders. humidity Amount of water vapor in the air. hybrid offspring formed by parents having different forms of a trait. hydrate A compound that has a specific number of water molecules bound to its atoms. hydrocarbon Simplest organic compound composed only of the elements carbon and hydrogen. hydrogen bond weak chemical bond formed by the attraction of positively charged hydrogen atoms to other negatively charged atoms. hydrolysis hydrosphere Chemical reaction of water with other substances. All the water in Earth’s oceans, lakes, seas, rivers, and glaciers plus all the water in the atmosphere. hydrothermal vent hygrometer Hot-water, deep-sea vent that has the energy and nutrients needed for the beginnings of life. Weather instrument used to measure relative humidity. hypertonic solution in cells, solution in which the concentration of dissolved substances outside the cell is higher than the concentration inside the cell; causes a cell to shrink as water leaves the cell. hypothalamus portion of the brain that connects the endocrine and nervous systems, and controls the pituitary gland by sending messages to the pituitary, which then releases its own chemicals or stimulates other glands to release chemicals. hypothesis explanation for a question or a problem that can be formally tested. hypotonic solution in cells, solution in which the concentration of dissolved substances is lower in the solution outside the cell than the concentration inside the cell; causes a cell to swell and possibly burst as water enters the cell.