Living cells can function only within a narrow range of such conditions as temperature, pH, ion concentrations, and nutrient availability, yet living organisms must survive in an environment where these and other conditions vary from hour to hour, day to day, and season to season. Organisms therefore require mechanisms for maintaining internal stability in spite of environmental change.
Hormones are molecules released by a group of cells in the body that influence the behavior of another group of cells. Hormones are the chemical signals of the endocrine system, the group of glands that, along with the nervous system, controls the body's responses to internal and external stimuli.
Plant hormones are chemical messengers that are produced in one part of the plant and have a physiological effect on a target tissue that may be distant from the site of production. When hormones reach the target tissue they can: (1) have a direct effect on the target tissue causing a rapid metabolic response; (2) involve the use of a second messenger within target cells; and/or (3) affect transcription of nuclear deoxyribonucleic acid (DNA).
Horticulturists find work in two distinct areas: agriculture and landscape design. The training for both of these specialties is the same but the day-today activities are different.
It is remarkable that the unique qualities of humans—language, advanced thought, and culture—evolved through the same processes that shaped the adaptations of all other creatures on earth: natural selection. How this came Sites of early civilizations.
The Human Genome Project (HGP), the determination of the complete nucleotide sequence of all of the more than three billion base pairs of deoxyribonucleic A researcher loading an automated DNA sequencer at the Joint Genome Institute in Walnut Creek, California. acid (DNA) in the nucleus of a human cell, is one of the greatest scientific undertakings in the history of humankind.
Nutrition is a broad topic that includes the components of food, food intake, what happens to the food once in the body, elimination of the residue, and how nutrients are related to health and disease. Research, education, and advertising combine to bombard the public with massive amounts of information or misinformation on what to eat, how much, and when.
Human population refers to the number of people living in a particular area, from a village to the world as a whole. A secondary meaning of population is the inhabitants themselves, but in most uses population means numbers.
Hybridization is a technique in which molecules of single-stranded deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) are bound to complementary sequences of either single-stranded DNA or RNA. Complementary base pairs are adenine (A) with thymine (T) or uracil (U) and vice versa, and guanine (G) with cytosine (C) and vice versa.
Hybridization is the process of interbreeding between individuals of different species (interspecific hybridization) or genetically divergent individuals from the same species (intraspecific hybridization). Offspring produced by hybridization may be fertile, partially fertile, or sterile.
The hypothalamus is a tiny part of the brain of vertebrate animals; in humans it weighs about four grams in a brain that weighs on average 1,400 grams (49 ounces). Despite its small size, the hypothalamus plays a pivotal role in an astounding number of functional and behavioral activities that are essential for day-to-day survival of the individual animal (or person) and for continuing survival of its species.
As recently as in the 1970s, the diagnosis of some diseases often required exploratory surgery, opening a body cavity to "have a look around" for visible disorders. The risks of infection, anesthesia, and imperfect healing weigh against exploratory surgery, but the diagnostic benefit may make the risk worth taking.
Among the many threats organisms face are invasion and infection by bacteria, viruses, fungi, and other foreign or disease-causing agents. All organisms have nonspecific defenses (or innate defenses) that provide them with some of the protection they need.
Jan Ingenhousz was a pioneer in plant physiology and demonstrated that oxygen is produced during photosynthesis. Born in the Netherlands, Ingenhousz practiced medicine in several European countries and served as a court physician to Empress Maria Theresa of Austria for twenty years.
Insects are a class of arthropods. Like other arthropods, they have exoskeletons made from the carbohydrate chitin, segmented bodies, and jointed appendages.
Animals, plants, and other organisms that are newly introduced into an area from another part of the world are sometimes referred to as "alien" or "exotic" species. These words are used to distinguish newly arrived species from the native species that have lived in the environment for very long periods of time.
Ions are charged particles such as Na+, H+, K+, Ca2+, and Cl-. Ions have a significant effect on many cell processes and also influence the amount of water in the cell.
The kidneys of vertebrates have the vital function of removing metabolic wastes from the blood and otherwise maintaining its normal composition. The two kidneys of a normal human adult produce 1 to 2 liters (about 30 to 70 fluid ounces) of urine each day that contain wastes, excess water, and other unneeded molecules.
Kingdom is the highest category in the hierarchical classification of organisms created by Carolus Linnaeus around 1750. Linnaeus recognized two kingdoms, plants and animals, a scheme that worked reasonably well for large multicellular organisms but failed as microscopes revealed diverse unicellular organisms.
When glucose is converted to pyruvate during glycolysis, two adenosine triphosphates (ATPs) are formed, but most of the energy in the original glucose remains in pyruvate. In most aerobic cells, the pyruvate formed by glycolysis is further degraded in a pathway called the Krebs cycle (also called the tricarboxylic acid cycle or citric acid cycle).
Laboratory technicians do almost all of the hands-on work in scientific research, development, and analysis. One of the benefits of being a laboratory technician is being the first to see experimental outcomes, whether they are prize-winning projects or more routine medical exams.