The Mollusca (mollusks) are a large phylum of animals that includes the snails, slugs, clams, squids, and octopi, among others. Most are marine, many are freshwater, and some snails and slugs are terrestrial.
Monocots, or monocotyledons, are a class of the flowering plants, or angiosperms. Monocots are named for and recognized by the single cotyledon, or seed leaf, within the seed.
Monotremes are an ancient group of mammals in the order Monotremata, which probably split from the lineage leading to marsupials (those with no placenta and having a pouch in the abdomen) and placental mammals early in mammalian evolution. The earliest fossil occurrence of monotremes is in the lower Cretaceous, approximately 110 million years ago.
Muscle can be categorized into three types based on structure, function, and location in the body. The specific details of muscle, including structure, physiology of contraction, energy requirements, muscle conditioning, and disease, can be illustrated using skeletal muscle.
The musculoskeletal system includes bones, joints, skeletal muscles, tendons, and ligaments. Muscles generate force; tendons transfer it to bones; and the bones move if enough force is transmitted.
Mutations are physical changes in genes and chromosomes. They may be confined to a single cell or may be transmitted from one cell to another within a multicellular organism (somatic cell mutation), or may be transmitted from one generation to another through mutation in the gametes (germ-line mutation).
Symbioses are intimate associations between two unrelated organisms. Mycorrhizae are very common but largely unseen symbioses between plant roots and fungi that are important in plant nutrition, community structure, and nutrient cycling.
Natural selection is the process by which individuals with characteristics that are advantageous for reproduction in a specific environment leave more offspring in the next generation, thereby increasing the proportion of their genes in the population gene pool over time. Natural selection is the principal mechanism of evolutionary change, and is the most important idea in all biology.
Nematodes, also called roundworms, are members of the animal phylum Nematoda. These worms have a complete digestive system and are more complex than the flatworms (phylum Platyhelminthes) but lack a circulatory system and other advanced features found in the annelids (segmented worms).
The nervous system is a network of nerve cells and, in most animals, a brain. In vertebrates, it also includes a spinal cord.
Neurological disease is a structural disturbance or a malfunction of the central nervous system. Common neurological disorders include stroke, Alzheimer Disease, migraine headaches, epilepsy, Parkinson's disease, sleep disorders, multiple sclerosis, pain, brain and spinal cord injuries, brain tumors, and peripheral nerve disorders.
The neuron (nerve cell) is the fundamental unit of the nervous system. The basic purpose of a neuron is to receive incoming information and, based upon that information, send a signal to other neurons, muscles, or glands.
The nitrogen cycle is the series of biogeochemical transformations in which the element nitrogen is transferred among organisms and nonliving reservoirs such as the soil, the oceans, and the atmosphere. Nitrogen is an essential element for all living things because it is a principal component of proteins and nucleic acids.
Nitrogen fixation refers to the conversion of atmospheric nitrogen gas (N2) into a form usable by plants and other organisms. Nitrogen fixation is conducted by a variety of bacteria, both as free-living organisms and in symbiotic association with plants.
In animals, there are two types of defenses against foreign invaders: specific and nonspecific. Specific immune responses can distinguish among different invaders.
The distinguishing feature of eukaryotic cells is the segregation of ribonucleic acid (RNA) synthesis and deoxyribonucleic acid (DNA) replication in the nucleus, keeping it separate from the cytoplasmic machinery for protein synthesis. As a consequence, messenger RNAs, ribosomal RNAs, transfer RNAs, and all cytoplasmic RNAs of nuclear origin must be transported from their site of synthesis in the nucleus to their final cytoplasmic destinations.
The nucleolus is by far the most easily recognized substructure in the eukaryotic nucleus, and can be seen by using a variety of dyes as well as by phase contrast microscopy. Indeed, in budding yeast, the single nucleolus takes up nearly half of the nucleus.
Nucleotides are the subunits that are linked to form the nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), which serve as the cell's storehouse of genetic information. Free nucleotides play important roles in cell signaling and metabolism, serving as convenient and universal carriers of metabolic energy and high-energy electrons.
In eukaryotic cells, chromosomes are found in a special compartment called the nucleus. The nucleus is a defining feature of eukaryotic cells, which range from single-celled yeasts to plants and humans.
Nurses are health care professionals with direct responsibility for patient care. Nurses work in hospitals, clinics, long-term care facilities, schools, corporations, and many other settings.
Nurse practitioners are registered professional nurses who have completed a graduate education program in advanced practice nursing. They provide many of the same services as physicians.
Nutrition is the study of food, essential nutrients, and other substances in food and their effects on the body in relation to health and disease. It concerns how we eat, digest, absorb, use, and excrete the many components of food.