Any of the adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or peroxisomes). ABC lipid transporters consist of two hydrophobic transmembrane domains and two hydrophilic nucleotide-binding folds. These binding folds contain highly conserved sequence motifs (Walker A and B) and are separated by a linker sequence, also known as the signature (C) motif. ABC transporters are organized as full transporters or half transporters depending on the number of transmembrane domains and nucleotide-binding folds. Half transporters must form homodimers or heterodimers to be functional. See also: Adenosine triphosphate (ATP); Lipid; Lipid metabolism; Protein

A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C₁₅H₂₀O₄ (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various aspects of plant physiological function, including development, growth, and stress responses. Abscisic acid is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. In addition, abscisic acid is produced by mosses, certain algae, several phytopathogenic fungi, and various animals, ranging from sea sponges to mammals. Some abscisic acid signaling mechanisms appear to be common to plants and animals. See also: Plant; Plant development; Plant growth; Plant hormone; Plant metabolism; Plant physiology

A common medicine used to relieve mild to moderate pain and reduce fever. Acetaminophen (N-acetyl-p-aminophenol), or paracetamol, is a white crystalline compound with the molecular formula C₈H₉NO₂ (Fig. 1). It is a member of the aniline family of analgesics (acting to relieve pain) and antipyretics (acting to prevent or reduce fever). Three types of nonopioid analgesics exist: aniline derivatives (such as acetaminophen), nonsteroid anti-inflammatory drugs (NSAIDs, such as aspirin, ibuprofen, and naproxen sodium), and nonacidic pyrazole derivatives (such as phenylbutazone and dipyrone). NSAIDs and pyrazole drugs are both used to relieve pain, reduce fever, and reduce inflammation, while pyrazole drugs can also be used for their spasmolytic (acting to relieve muscle spasms) properties. Aniline derivatives exhibit analgesic and antipyretic, but not anti-inflammatory, properties. See also: Analgesic; Aspirin; Fever; Inflammation; Medicine; Pain; Pharmacology; Pharmacy

A naturally occurring quaternary ammonium cation ester, with the formula CH₃(O)COC₂H₄N(CH₃)₃⁺, which plays a prominent role in nervous system function as a neurotransmitter. Acetylcholine (Fig. 1) is an organic compound released from certain autonomic nerve endings and acts in the transmission of nerve impulses to excitable membranes. The great importance of acetylcholine derives from its role in physiology as a neurotransmitter for cholinergic neurons (nerves), which innervate many tissues, including smooth muscle, skeletal muscle, the heart, ganglia, and glands. The effect of stimulating a cholinergic nerve, for example, the contraction of skeletal muscle or the slowing of the heartbeat, results from the release of acetylcholine from the nerve endings. Other types of nerves release other transmitters, for example, norepinephrine. See also: Nerve; Nervous system (vertebrate); Neurobiology; Neuron; Synaptic transmission

Chemical compound that changes in physical appearance at a certain concentration of hydrogen cations, or pH. Acid-base indicators, or pH indicators, are used in various industries, such as chemistry labs and medical offices, to determine the concentration of hydrogen cations (H⁺), also known as pH range, of a solution. Acid-base indicators can be any chemical compound or material that changes in physical appearance, typically color, with a change in the concentration of H⁺ ions in solution (Fig. 1). This change occurs due to a large amount of H⁺ present in solution (a low pH) or a small amount of H⁺ in solution (a high pH). Indicators can therefore only reveal a pH range, rather than an exact pH value. Acid-base indicators may be natural or synthetic. Most indicators are weak organic acids used in medical, industrial, and laboratory settings. See also: Hydrogen; pH

A voltage or rapid sequence of changes in the movement of ions across a cellular membrane that propagates an electrical signal to other cells for purposes of communication and physiological functioning in the body. Cells that can experience action potentials are known as excitable cells. In animals, these cells include neurons, muscle cells (for instance, cardiac muscle cells), and certain endocrine cells. In neurons and, specifically, through nerves (bundles of neurons), action potentials serve as a nerve impulse: an electrical signal transmitted to send information via the nervous system to be received by bodily tissues. Action potentials can elicit physiological functions, such as movement coordinated by muscles. In this particular example, action potentials originating from motor neurons within the central nervous system are propagated to the specific motor nerves within the peripheral nervous system, where muscle cells cause contractions of the muscle fibers, generating movement. Neurons within the parasympathetic nervous system also communicate with the cardiovascular system via action potentials to control heart rate during rest. See also: Cell (biology); Cell membrane; Nerve; Nervous system (vertebrate)

A nucleotide, composed of ribose, adenine, and a triphosphate group, that is an important energy compound in metabolism. Adenosine triphosphate (ATP) [Fig. 1] is a vital energy-rich chemical found in all living cells. It has a chemical formula of C₁₀H₁₆N₅O₁₃P₃ and consists of three subunits—ribose (a 5-carbon sugar), adenine (a nitrogenous base composed of two carbon-nitrogen rings), and a triphosphate group (a chain of three phosphate groups, designated α, β, and γ). The oxidation of carbohydrates, fats (lipids), and proteins provides the chemical energy that is used to synthesize ATP, which then serves as the source of the chemical energy necessary for biosynthesis, ion transport, and muscle contraction. Originally discovered by Karl Lohmann in 1929, the idea that ATP serves as the common currency of energy exchange in all cells (an idea that is a cornerstone of biology) was first described by Fritz Lipmann in 1941. See also: Amino acid metabolism; Biological oxidation; Carbohydrate metabolism; Cell (biology); Cell biology; Energy metabolism; Lipid metabolism; Metabolism; Nucleotide

A plasma protein produced by the liver that maintains fluid balance in the blood and transports fatty acids in the plasma and interstitial fluid. Albumin is the most abundant protein in human serum (see illustration), as well as being one of the first discovered and earliest studied proteins. Serum albumin was precipitated from urine in 1500 by Paracelsus and was crystallized in 1894 by August Gürber. Probably no other protein has been studied as extensively as serum albumin, and knowledge of its structure and interactions with its ligands has come from many researchers, using a great variety of experimental approaches. Albumin's ability to bind many different ligands, most of which are hydrophobic anions, and several molecules of the same ligand (fatty acid) is well documented. Fatty acids, bilirubin (an orange-yellow bile pigment formed in the breakdown of red blood cells), and hematin (a blue to blackish-brown compound formed in the decomposition of hemoglobin) represent the endogenous ligands of albumin with highest affinity. See also: Bilirubin; Blood; Lipid; Liver; Protein

A cyclic organic compound that contains nitrogen in a negative oxidation state and is of limited distribution among living organisms. More than 10,000 alkaloids of many different structural types are known; and no other class of natural products possesses such an enormous variety of structures. The exact function of alkaloids in plants (see illustration) is not well understood, but they are often regarded as by-products of plant metabolism. They are sometimes considered to be reservoirs for protein synthesis; as protective agents to discourage attack by animals, insects, or microbes; as regulators of growth, metabolism, and reproduction; and as end products of detoxification of substances whose accumulation might be injurious to the plant. See also: Oxidation-reduction

A regulatory enzyme whose activity is modified by the binding of an activator molecule to an alternate (allosteric) site that is different from the active (catalytic) site. Allosteric enzymes (Fig. 1) are special enzymes involved in regulatory functions. These enzymes utilize end-product inhibition. In this control mechanism, the end product of a biosynthetic pathway can react with the first enzyme of the pathway and prevent its activity. Importantly, this end-product inhibition is a device through which a cell conserves its economy by shutting off the synthesis of building blocks when too many are present. See also: Cell (biology); Enzyme; Enzyme inhibition