Glossary of Scientific Terms

H

Hemi-cellulose
A type of dietary fiber made up glucose and many other types of sugars and differs from cellulose, which only contains glucose.

Heterozygous
Identifies the two alleles of a diploid organism as being different from each other. If both alleles are genetically identical, the cell or organism is homozygous. Often, one of the two alleles is the dominant, the other the recessive allele. If a dominant allele is present, one copy is enough to establish the corresponding phenotype (e.g. dark eye color). The recessive allele for light eye color cannot be expressed, until two copies of the allele (homozygous) are present.

Homeostasis
In biology used to describe a condition where an organism maintains a stable structure where in fact a constant flux of molecules occurs. Although many organisms can live for years, all cellular components like proteins, membranes, sugars, and nucleic acids are constantly recycled while never compromising the integrity of the organism as a whole. This turnover processes can be characterized by specific half-life values that for most proteins, membranes, and RNA (but not DNA structures) are measured in hours. In a more narrow sense homeostasis refers to the maintenance of water and salt concentration in cells.

Homozygous
Identifies the two alleles of a gene in a diploid organisms as being identical to each other. In heterozygous individuals, the two alleles are different versions of the gene.

Hormones
Messenger substances synthesized in the body and secreted by the endocrine glands. Hormones regulate the digestive system, growth, hunger, thirst, blood glucose and cholesterol levels, fat burning and storage, absorption and excretion, internal clocks such as day and night cycles, menstrual cycles, and sex drive.

Hydrogenated fats (partially)
Partially hydrogenated fats are a the result of chemical addition of hydrogen units to polyunsaturated fatty acids. If hydrogenation is not complete, the process renders trans-fats (as compared to cis-fats found commonly by natural processes). The reason for hydrogenation is to gain optimal texture of fats and oils in processed foods. Trans-fats have been associated to increases in bad cholesterol (LDLs), but fully hydrogenated fats do not contain trans-fats and are not associated to clinical symptoms of heart disease.

Hydrophobic (op. hydrophilic)
Means water insoluble and refers to non-polar properties as opposed to polar properties of water. Water soluble, or hydrophilic, molecules do not easily mix with hydrophobic molecules or solutions. Oil is a hydrophobic substance and mixtures of oily and watery molecules tend to minimize their contact surface. Thus the observed separation of oil from water when left standing. However, oil can be partially mixed with water when shaking a container vigorously indicating that the separation or mixing of molecules is determined by the energy of molecular interaction. The energy to overcome these molecular interactions can be provided by mechanical force (shaking) or high temperature.  The latter is a well known observation; salts, sugars, and fats normally mix better in hot than cold water, and hot water is better in removing stains form dishes or cloths.

Hyperpolarization
A mechanism by which a membrane potential is made more negative inside with respect to the outside of the cell. Hyperpolarization causes neurons and muscle cells to be electrically silent (see action potentials) and stabilize at a resting potential. Hyperpolarization is the result of moving positive charges from in to out of a cell which is usually the result of K+ ions moving out, but can also be achieved by moving Cl- ions into the cell.

Hypothesis
A testable scientific idea that can be proved right or wrong with experiments. A hypothesis is a formulation of a question that lends itself to a prediction. This prediction can be verified or falsified. A question can only be use as scientific hypothesis, if their is an experimental approach or observational study that can be designed to check the outcome of a prediction.