Approximately 3.5 billion years ago, cellular life emerged on Earth in the form of primitive bacteria. Bacteria or "prokaryotes" organize their genes into a circular chromosome that lies exposed within the fluid environment of the cell.
Cell junctions can be divided into two types: those that link cells together, also called intercellular junctions (tight, gap, adherens, and desmosomal junctions), and those that link cells to the extracellular matrix (focal contacts/adhesion plaques and hemidesmosomes). These junctions play a prominent role in maintaining the integrity of tissues in multicellular organisms and some, if not all of them, are involved in signal transduction.
Cells exhibit a wide range of movement. These movements include migration of cells along a surface or through a tissue, or movement of components within cells.
With very few exceptions, all cells are enveloped by an extracellular matrix composed of proteins, carbohydrates, and other substances. Owing to its exceptional strength and its ability to control cell shape, the extracellular matrix of eubacteria, algae, fungi, and plants is called the cell wall.
The central nervous system (CNS) consists of a brain and spinal (nerve) cord. Most invertebrates and all vertebrates have sensory and motor neurons that are linked by way of a CNS.
The detection of chemicals by smell, taste, or other means is generally known as chemoreception. A phenomenon that occurs widely in nature, chemoreception is found in the simple chemotaxis of a motile bacterium toward food as well as the more complex interpretative pathways associated with an animal's ability to smell and taste.
Chloroplasts are the source of virtually all of the world's food and fuel and much of its oxygen supply, and as such life on Earth depends on them. They are a vital component of all photosynthetic cells in plants and algae, and are unique to them.
Chordata is a large and diverse group of animals, with roughly 50,000 living species included. The majority of chordates belong to a group called Vertebrata.
Chromosome aberrations are departures from the normal set of chromosomes either for an individual or from a species. They can refer to changes in the number of sets of chromosomes (ploidy), changes in the number of individual chromosomes (somy), or changes in appearance of individual chromosomes through mutation-induced rearrangements.
The deoxyribonucleic acid (DNA) of eukaryotic cells carries the blueprint for the biosynthesis of cellular proteins and the control of cellular assembly and regulation. If all the DNA in a single human cell were stretched out straight and the strands representing all the chromosomes laid end-to-end, they would extend for well over 1 meter (3 feet).
Animal circulatory systems consist of a blood or a bloodlike fluid, a system of tubular blood vessels, and one or more pulsating hearts that pump the blood through the vessels. Animals that are only a few cell layers thick do not need or possess circulatory systems, because they can rely on diffusion through the body surface to exchange materials with the environment.
A clinical trial is a prospective study of the effectiveness of a new treatment, such as a drug, surgical technique, or medical device. The term prospective indicates that there is a well-defined starting point from which the subjects are tracked for some definite period of time.
The word "clone" has several different meanings in biology. As a noun, a clone is an identical genetic copy of either a piece of deoxyribonucleic acid (DNA), a cell, or a whole organism.
The Cnidaria (pronounced ny-DARE-ee-ah) are a phylum of simple animals including the hydras, jellyfish, sea anemones, and corals. Any swimmer who has suffered a jellyfish sting has painfully encountered the feature for which the phylum is named: the venomous, stinging organelles called nematocysts or cnidae (pronounced NID-ee).
Coffee is made from the bean of the coffee plant, Coffea arabica or Coffea canephora, in the Rubiaceae family. It is native to the forest understory of the east African highlands.
College and university professors have satisfying careers because they work in an intellectually stimulating environment and with people who want to learn more about the world around them. Professors need to have many qualities and skills such as excellent teaching abilities, inquisitive minds, a love of learning, and a willingness to dedicate their lives to their profession.
An ecological community is a collection of organisms occurring together in a location and interacting to varying degrees. A community is often defined by the most common or prominent species found in it (a beech-maple forest) or by its environment (a wetland community).
Competition is a negative interaction that occurs among organisms whenever two or more organisms require the same limited resource. All organisms require resources to grow, reproduce, and survive.
The conifers are a group of about 588 species of trees and shrubs that include many of the best-known plants in the world. All conifers bear seeds inside cones, woody protective structures.
The human body is composed of just four basic kinds of tissue: nervous, muscular, epithelial, and connective tissue. Connective tissue is the most abundant, widely distributed, and varied type.
Biological diversity throughout the world is being threatened by human activity: species are being driven to the edge of extinction; biological communities are being degraded, fragmented, and destroyed; and the genetic variation within species is being lost as populations are reduced in size and lost. Conservation biology is a multidisciplinary science that has developed in response to this biodiversity crisis.
All cells contain a set of genes, which can be thought of as a set of instructions for making each of a very large number of proteins. The creation of a protein from its gene is called gene expression.