Epithelium is a tissue composed of sheets of cells that are joined together in one or more layers. Epithelia cover the body surface, line body cavities and hollow organs, and form glands.
Estuaries are partially enclosed bodies of water that occur where the land meets the ocean. The world's largest estuaries are at the ocean ends of rivers that deliver freshwater from surrounding and sometimes remote upland areas.
Ethnobotany is a field of study that combines botany (the study of plants), anthropology (the study of human cultures), and medicine. Plants have been the original sources of many medicines used in all past and current societies.
Bacteria are microscopic organisms that comprise the domain Eubacteria. A domain is the highest grouping of organisms, superseding the level of kingdom in the classical Linnaean system of biological classification.
The eudicots are the largest group of flowering plants (angiosperms). The term eudicots derives from the term "dicotyledons." Historically, dicots were the group of flowering plants characterized by having two seeds leaves upon germination, presence of woody or secondary growth, tap root system, reticulate (netlike) venation in the leaves, and flower parts in groups of four or five.
The remains of stadiums, temples, and aqueducts indicate as clearly as any ancient document that the Roman Empire once existed. Likewise, fossils speak eloquently of a time when dinosaurs and not humans dominated Earth.
Evolution is the unifying principle in biology. It explains the overwhelming diversity of life on Earth, as well as the constancy of molecular and morphological attributes observed in diverse assemblages of plants and animals.
Modern classification systems, based largely on molecular evidence, divide living organisms into three domains: Bacteria (also called Eubacteria), Archaea, and Eukarya. Plants are classified as a kingdom (Plantae) within the Eukarya; organisms that possess a nucleus, mitochondria, an internal cytoskeleton, and, in photosynthetic species, chloroplasts.
Most animals require some system to excrete the waste products of metabolism from the body fluids. Kidneys are the major organs of the excretory systems of humans and other vertebrates, but several other kinds of excretory organs occur in other kinds of animals.
Exocytosis is the cellular process in which intracellular vesicles in the cytoplasm fuse with the plasma membrane and release or "secrete" their contents into the extracellular space. Exocytosis can be constitutive (occurring all the time) or regulated.
Extinction is the termination of evolutionary lineage. The most common extinction event is the loss of a species.
The extracellular matrix is a meshwork of proteins and carbohydrates that binds cells together or divides one tissue from another. The extracellular matrix is the product principally of connective tissue, one of the four fundamental tissue types, but may also be produced by other cell types, including those in epithelial tissues.
The environments of Earth include conditions in which physical and chemical extremes make it very difficult for organisms to survive. Conditions that can destroy living cells and biomolecules include high and low temperatures; low amounts of oxygen and water; and high levels of salinity, acidity, alkalinity, and radiation.
The human eye is an amazing instrument. It is the body's camera, capturing images of the world with striking clarity in a virtual instant.
All animals are heterotrophic, meaning they must eat other organisms, living or dead, to acquire organic nutrients. A large percentage of an animal's life is occupied with acquiring food.
Like the male reproductive system, a primary function of the female reproductive system is to make gametes, the specialized cells that contribute half of the total genetic material of a new person. The female reproductive system has several additional functions: to be the location for joining of the male and female gametes, to protect and nourish the new human during the period of gestation, and to nourish the newborn infant for some time after birth, though lactation and nursing.
The miracle of the renewal of human life takes place in well-defined stages, from the union of the egg and sperm to the birth of the baby. Fetal development is the longest and clinically the most important phase of this process.
Field studies of animals help scientists understand the complexities and causes of animal behavior. Wild animals interact with their physical surroundings and the biological world while breeding, eating, and moving within their habitat.
Some of the most important ecological research taking place today is in field studies in plant ecology. These are studies undertaken to answer such important questions as: How much carbon dioxide do plants take up from the atmosphere?
Fire is one of the leading natural forces that has shaped nearly all land-based ecosystems for several thousand years. Fire is especially important in regulating the species composition of vegetation.
Flying organisms include insects, birds, and bats, all of which evolved the ability to fly (and the wings that flight requires) independently. Flying squirrels, flying fish, and other animals that only glide are not considered capable of true flight.
Flowers are the typically showy reproductive organs of angiosperms (flowering plants). Their diverse blooms generate countless horticultural products; functionally flowers are essential for sexual reproduction.