Dr. Paul's Virtually Biology Show.

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H

Habituation. A decline in organism's response to a stimulus with repeated applications of the stimulus.
Habitat. The places where organisms of a species tend to live(e.g. stream bank, hard wood forest, prairie)
Hair cell. A mechanoreceptor that fires more frequently as sensitive tuffs at its end are bent. Hair cells are found in the inner ears of vertebrates and the lateral lines of fish. Hair cells are important in our ability to hear and maintain balance.

Haploid. Refers to a cell having one of each type of chromosome. Chromosomes(and genes) not in homologous pairs singly as in a gamete.

Haplodiploid. Organisms where one sex is diploid and the other sex is haploid. The females are diploid, the males haploid in all such known cases. Honeybees are a good example. The queen and workers are female and diploid, the males usually haploid.
Heat. A kind of kinetic energy carried by atoms or molecules in constant random motion. Be careful not to confuse heat with temperature. For instance, two rooms may both be at 27 degrees Celsius, but if the second room has twice the volume, the room will contain twice the heat of the first room.
Helper T cell. A lymphocyte in the immune system that stimulates both the cell mediated and the antibody mediated parts of the immune system, stimulating the production of both effector and memory T and B cells.
Heritability. In genetics heritability in the broad sense refers to the percentage of the total variation in phenotype in a population that is due to genetics as opposed to environmental influences.
Heterotroph. Refers to organisms that obtain their energy by complex organic compounds produced by other organisms. Animals and Fungi are technically heterotrophs.
Heterozygous. When both members of a gene pair in a diploid organism are not exactly identical but code for different expressions of the same characteristic.
Histamine. A local signaling molecule released by mast cells during an allergic reaction and as part of the the normal inflammatory responses of the body. Histamine acts on capillaries making them more leaky.
Histone. Any one of a number of histadine rich proteins. These proteins are important in the nucleus of eukaryotic cells where there serve as a scaffolding for the DNA and are involved in gene expression.

Homeostasis. The tendency of living things to actively maintain a constant internal environment.

Homeothermic. Refers to the ability to maintain a constant body temperature.
Homeotic genes. Genes involved in the overall organization of body plan and positioning of limbs in animals.
Homologous Chromosomes. A pair of chromosomes that have corresponding genes in the same locations along the chromosome and that pair together to form tetrads during prophase I of meiosis.

Homologous structures. Homologous structures are structures in different species that may have different functions and yet the structures have a common and deep developmental origin. Biologists infer that for organisms having homologous structures, the structures have a common evolutionary origin.

Homozygous. When both members of a gene pair in a diploid organism are identical and code for the same expressions of a characteristic. Contrast heterozygous.

Hormone. A chemical messenger that in animals is produced by endocrine glands and then travels through the blood stream to have an effect on target cells. The term is also used for plants to refer to chemical messengers in general.
Humoral immune system. That part of the immune system concerned with the production of free antibodies from effector B cells and also the production of memory B cells via clonal selection.
Hydration shell. A group of water molecules that collects around a charged or polar ion, atom or molecule in solution. Hydration shells help to keep oppositely charged ions in solution by preventing the ions from from recombining. They are also important in keeping large molecules such as some proteins in solution or in suspension.
Hydrocarbon. An organic compound consisting solely of carbon and hydrogen.
Hydrogen bond. A hydrogen bond is an electrostatic attraction between hydroxyl(OH) parts of a molecule and other charged parts of the same or different molecules. For example the negatively charged oxygen end of one molecule is attracted to the positively charged hydrogen ends of near by water molecules, hence the term hydrogen bond.
Hydrogen ion pump(proton pump). A protein that uses the energy from electrons being carried along an electron transport system to pump hydrogen ions from one side of a cell membrane to another. This sets up an electrochemical gradient necessary for the production of ATP.
Hydrophilic. Literally means water loving and refers to the fact that hydrophilic molecules are attracted to water. This is because hydrophilic molecules have charged or polar molecular groups as part of their structure. Hydrophilic molecules can be recognized because they either dissolve in water or attract water to them.
Hydrophobic. Literally water fearing. More precisely this term refers to non polar molecules that because of their lack of charge, are neither repelled by or attracted to water. The term arises from the tendency of oil to separate itself from water in a mixture of oil and water. But the oil is not repelled by the water, rather since it has no charge, it can only associate with itself.
Hydrostatic skeleton. A series of fluid filled compartments found in many soft bodied invertebrates that use water pressure to maintain the organism's form. In some invertebrates, such as the earthworm, the hydrostatic skeleton is essential for locomotion.

Hydroxyl group. A functional group consisting a hydrogen atom bonded to an oxygen atom. Hydroxyl groups are polar, and thus compounds with hydroxyl groups tend to dissolve in water.

Hymenoptera. An order of insects with two pairs of filmy wings and with biting mouth parts. The females have a well developed ovipositor and the males are usually haploid. In all cases hymenoptera have a larval stage which does not resemble the adult. Ants, bees, wasps are all in this order.
Hypertonic/Hypotonic. Hypertonic is a relative term referring to a solution with a greater concentration of solute(greater tonicity) than another solution. The second solution is considered hypotonic to the first one. These terms are important because in osmosis the diffusion of water across the cell membrane is going to be from the hypotonic side to the hypertonic side.
Hypha(ae). In fungi, a thread like structure consisting of cells one after the other. A mass of hyphae is called a mycelium and is the main part of most fungi.
Hypothalamus. A region of the brain involved in emotions, regulation of parts of the autonomic nervous system, and providing the link between the endocrine system and the nervous system by its influence on the pituitary gland.

Hypothesis. An hypothesis is an "educated guess" made by a scientist concerning the why or how of an observation made about the natural world. Creationists take note: an hypothesis is NOT a theory.