Active transport is the movement of molecules up their concentration gradient, using energy.
To survive and reproduce, all living organisms must adjust to conditions imposed on them by their environments. An organism's environment includes everything impinging upon it, as well as everything that is affected by that organism.
The adrenal glands are located on the upper pole of each kidney. In fact, their name designates their location: the prefix ad means "adjacent," and renal refers to the kidney.
Human life span, or longevity, has two components: mean longevity (also called life expectancy) and maximum longevity. Mean longevity is the average age at death of all members of a population.
Agriculture is both an occupational practice and a subject to be studied. Farmers, horticulturists, and ranchers are examples of individuals who grow things for human use.
An agronomist is a professional who practices, or does research in the area of, agronomy, which is the art and science of managing field crops and the soils beneath them. Agronomy emerged early in the twentieth century when this component of agriculture involving the growing of plants was separated from animal husbandry.
AIDS (acquired immunodeficiency syndrome) is defined as the stage of infection with HIV-1, or HIV (human immunodeficiency virus), in which an infected person's immune system has become so weak that he or she is at risk of developing other infections or cancers (or has already developed them) that can potentially lead to death. Though all people with AIDS are infected with HIV-1, not all people with HIV-1 infection have AIDS, nor will all of them develop AIDS.
Ethanol, C2H5OH, also known as ethyl alcohol or grain alcohol, is the only common alcohol that humans are able to digest. Alcohol is readily absorbed by the body when consumed in an aqueous solution.
Algae are a diverse group of all photosynthetic organisms that are not plants. Algae are important in marine, freshwater, and some terrestrial ecosystems.
For sexually reproducing multicellular organisms such as plants and animals, the life cycle requires that diploid cells divide by meiosis to create haploid cells. Haploid cells then fuse to recreate the diploid number and a new organism.
Amino acids are molecules that have both an amino group (-NH2) and a carboxylic acid group (-COOH), hence the name. The most common amino acids are the α-amino acids, the building blocks of proteins.
The amniotic egg was an evolutionary invention that allowed the first reptiles to colonize dry land more than 300 million years ago. Fishes and amphibians must lay their eggs in water and therefore cannot live far from water.
The class Amphibia consists worldwide of nearly 4,700 species, contained in three major orders: Caudata (salamanders), Gymnophiona (caecilians), and Anura (frogs and toads). Salamanders are composed of about 415 species worldwide, and are typically characterized by their long tails and four limbs of nearly equal size.
Anabolic steroids are synthetic chemicals that mimic the effects of the male sex hormone testosterone. Some athletes seeking increased muscular strength and size abuse anabolic steroids.
Plants are the primary producers in Earth's ecosystem. Plants are autotrophic, meaning that they produce their own food (via photosynthesis), and as a result ultimately produce food for the ecosystem's consumers (such as humans).
The angiosperms, or flowering plants, are the largest and most species-rich phylum of plants, with more than 250,000 species estimated.
The kingdom Animalia, or Metazoa, includes all animals. Animals are multicellular, eukaryotic organisms, which are heterotrophic, meaning they obtain nutrition from organic sources.
Annelids, or true-segmented worms, are members of the animal phylum Annelida, the most complex of all wormlike groups of organisms. Annelids are commonly found in terrestrial, as well as marine, brackish, estuarine, and freshwater ecosystems worldwide.
Antibodies are proteins made by B cells, part of the body's immune system. The normal function of antibodies is to latch onto foreign substances (antigens) and flag them for destruction, thus helping to fight infection.
In 1890 scientists transferred blood from animals with diphtheria to animals never exposed to the disease. The second group of animals became resistant (or immune) to diphtheria.
Antisense nucleotides are either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) molecules that are complementary to a messenger RNA (mRNA) molecule. Because these molecules are complementary to given mRNA, they will bind to the RNA and form a free double-stranded molecule or double-stranded region of a chromosome.
Spiders, mites, ticks, and scorpions make up the class Arachnida. Arachnids are members of the phylum Arthropoda, which also includes crustaceans (such as crabs and shrimp), insects, and other animals with an exoskeleton and jointed legs.