Taxonomy, the field of biological classification, attempts to group types of organisms in meaningful ways. Modern taxonomy is based on similarities among organisms that reflect descent from recent shared ancestors, rather than similar solutions to environmental challenges.
Humans and other mammals are homeothermic, able to maintain a relatively constant body temperature despite widely ranging environmental temperatures. Although the average human body temperature is 36.7 degrees Celsius (98.2 degrees Fahrenheit), this temperature varies depending on individual differences, time of day, the stage of sleep, and the ovulatory cycle in women.
The study of ecology involves investigations of specific organisms and environments and the development of general conclusions about how the natural world works. These generalizations are called theories.
The thyroid gland, the largest of the endocrine glands, is located in the neck just below the thyroid cartilage of the larynx. It consists of two lobes, one on either side of the trachea, joined by a narrow band or isthmus.
A tissue is made up of a group of cells that usually look similar to one another and come from the same region in a developing embryo. The group of cells that make up a tissue have physiological functions that work together in a coordinated way to support special functions.
John Torrey was the preeminent botanist in the United States during the nineteenth century. Born in New York City and trained as a physician, chemist, and mineralogist, he taught at West Point, the College of Physicians and Surgeons in New York City, and Princeton University.
Touch is one of the five major sensory channels by which humans sample and experience their environment. The word "touch" describes the sensory experience resulting from gentle contact of the skin with the environment, including air moving over the skin and hairs.
Each gene on a chromosome can be thought of as the instructions for making a particular protein in a cell. However, the genes themselves cannot direct the synthesis of the proteins they encode but must first be converted into a form that can be recognized by the cellular protein-making machine, the ribosome.
During protein synthesis at the ribosome, the nucleic acid sequence of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) is translated into the amino acid sequence of a protein. Transfer RNA (tRNA) is an important adapter that "reads" the nucleic acid code in the messenger RNA (mRNA) and "writes" an amino acid sequence.
Transgenics describes the process of introducing foreign deoxyribonucleic acid (DNA) into a host organism's genome. The foreign DNA, or "transgene," that is transferred to the recipient can be from other individuals of the same species or even from unrelated species.
Translocation is the movement of materials from leaves to other tissues throughout the plant. Plants produce carbohydrates (sugars) in their leaves by photosynthesis, but nonphotosynthetic parts of the plant also require carbohydrates and other organic and nonorganic materials.
When conventional medical and surgical procedures are insufficient to help a patient, organ and tissue transplants are sometimes the only solutions. From a blood transfusion to multiple organ transplants, the procedures and mechanisms associated with these diverse medical interventions all fall under the common area of medical practice known as transplant medicine.
Transposons, also called transposable elements or jumping genes, are stretches of deoxyribonucleic acid (DNA) that can move around an organism's chromosome. These "transpositions" occur at a very low frequency.
Tropisms are growth responses of plants that result in curvatures of plant organs toward or away from certain stimuli. Tropisms can be positive, in which case the plant will bend toward a stimulus, or negative, in which case the plant will bend away from a stimulus.
Tuataras (class Rhynchocephalia) superficially resemble lizards (class Reptilia), but the two known species are actually members of the smallest terrestrial vertebrate class on Earth, the Rhynchocephalia, a unique and ancient evolutionary lineage whose fossils (from Asia, Europe, North and South America, and Africa) first appeared in the early Triassic more than 220 million years ago. Today, Tuataras are found only on about thirty islands off the coast of New Zealand; their ancestors on other continents became extinct around 65 million years ago.
Tundra is the global biome that consists of the treeless regions in the north (Arctic tundra) and high mountains (alpine tundra). The vegetation of tundra is low growing, and consists mainly of sedges, grasses, dwarf shrubs, wildflowers, mosses, and lichens.
On such surfaces as marine dock pilings, rocks, ships, offshore oil rigs, and coral reefs, one can often find humble blobs of jelly among the sponges and hydroids. Some are as small as sesame seeds and some as big as potatoes.
There are about 260 species of turtles, tortoises, and terrapins. They range in size from the leatherback, a marine species reaching an upper shell length of about 190 centimeters (6.2 feet) and a weight of over 900 kilograms (1,984 pounds), to small freshwater species that average around 10 centimeters (3.9 inches) in length and weigh less than 100 grams (a few ounces).
Vaccines are drugs used to increase the body's ability to combat disease organisms. Most vaccines are designed to help the body fight off a specific type of bacterium, protozoan, or virus.
A vacuole is a characteristic type of organelle found in plant and fungi cells and many single-cell organisms. The single large vacuole of the cell is surrounded by a membrane, called the tonoplast, and filled with a solution of water, dissolved ions, sugars, amino acids, and other materials.
Jan van Helmont was an early pioneer in the study of gases, and performed numerous chemical experiments, including an analysis of smoke, distinguishing it from ordinary air by the particles it contained. However, van Helmont is best known for a single experiment demonstrating that the weight a plant gains during growth is not due to absorption of an equal amount of soil, but instead is due (at least in part) to water.
Nikolay Vavilov is best known for attempting to apply the science of genetics to Russian agriculture, for his theories of the origin of crop plants, and for being persecuted during the Stalin Regime in the Soviet Union. Vavilov studied genetics in Moscow and in England.